FRP Pipe Installation | Successful Pipe System Design | Material Selection | Pressure Design of Piping Components
Fittings and Joints | Flanges | System Analysis | Pipe Stress Analysis | Strength Properties | Stress Intensification and Flexibility
How Pipe Stress Analysis is Carried Out | Properly Functioning Support System
FRP Pipe Installation
Installation of FRP pipe systems needs to be carefully considered as part of a robust system design. Without doubt, more issues arise in FRP piping systems due to improper installation than due to any other single cause. There are a number of steps that must be followed to provide assurance that problems won’t arise later. These steps include preparation of a suitable bonding procedure, qualification of the bonding procedure, qualification of the bonders, and hydrotesting of the system. Let’s look at these steps in more detail.
For any FRP piping system, the manufacturer should prepare jointing instructions that apply to the specific piping system. This would include general instructions that apply to almost any FRP jointing operation, and should also include specific instructions for each type of joint and each size of pipe to be installed. General instructions would typically address issues such as safety consideration, required tooling, appropriate ambient conditions, surface preparations, etc. Specific instructions would include laminate sequences and reinforcement dimensions. Collectively, these instructions are referred to as the Bonding Procedure Specifications or “BPS”. This is the terminology used in the ASME piping codes, i.e. B31.1 “Power Piping” and B31.3 “Process Piping” and is completely analogous to the Welding Procedure Specifications used for metallic piping systems.
While all of the instructions must be followed for proper jointing, it is worthwhile emphasizing the need for proper surface preparation. It is probably safe to say that no single step in the jointing process is more important than the surface preparation. If this is not done properly, there is a very good chance there will be problems later.
Once the BPS has been prepared it needs to be qualified. There are a number of differences between B31.1 and B31.3 with respect to the requirements for procedure qualification, but in both cases, a sample spool must be assembled, examined for quality, and pressure tested. In the case of B31.3, the test spool must include each type of joint to be qualified, and the spool must survive a pressure test of 3 times the design pressure for 1 hour. A record is prepared to document the specifics of the jointing process and the results of the qualification testing. This record is referred to as the Procedure Qualification Record, or “PQR”.
Once the BPS has been qualified, the bonders themselves must be qualified to make joints using that procedure. The first step in this process is to ensure the bonders are properly trained. This would typically include a 2 – 3 day training course by the pipe manufacturer, and would address all aspects of bonding from proper storage and handling of materials, to joint preparation, and laminating techniques. RPS regularly conducts these types of courses for installation contractors and owners’ maintenance staff. RPS can also provide further installation services including quality control of jointing operations, supervision of installation crews, and full responsibility of jointing and installation activities.
After a bonder has received appropriate training for the type of joint to be made, he/she assembles a similar test spool to that used for the procedure qualification. Again, the spool has to be examined for quality and pressure tested. The same requirements apply for the bonder qualification as for the procedure qualification. Each qualified bonder is assigned a unique identifier so that each joint made by that bonder can be traced to the bonder. A bonder remains qualified to the procedure for as long as they continue to make joints using that procedure. A bonder would have to be re-qualified if he/she did not use the procedure for more than 6 months, or if there were any reason to question the bonder’s ability make joints to the BPS.
A record is prepared to document the results of the bonder qualification. This record is referred to as the Bonder Qualification Record, or “BQR”.
After examination of the completed joints, the final step in ensuring the joints have been properly made is the performance of a hydrotest. ASME B31.1 and B31.3 both require piping systems to be tested prior to being put into service. This is usually accomplished with a hydrotest at a pressure of 1.5 times the design pressure for the system. The pressure is held for a minimum of 10 minutes after which the joints must be examined for leaks.
Bonding operations for FRP piping are not difficult to do well. But it is necessary to adhere to the steps discussed above to provide assurance that joint issues won’t arise later.
Successful Pipe System Design
Composite pipe systems, both Fiberglass Reinforced Plastic (FRP) and Dual Laminate (Thermoplastic Lined FRP) offer many advantages over other piping materials including Rubber Lined Carbon Steel (RLCS), Stainless Steels (SS), and Alloys. Composite pipe systems are used in a broad range of tough industrial services from acidic to alkaline environments and they are unaffected by chlorides. In most applications Composite Piping offers lower installation and life cycle costs than RLCS, SS, and Alloys. As with any piping material good pipe system design is critical to ensure you realize the full benefits of Composite Piping. Following are the key components of RPS Composites pipe system design and the questions that need to be addressed which we will be answering in more detail in other Technical Center articles.
What are the process conditions – chemical concentrations, normal operating temperature, and upset conditions? Are Composites suitable for the application? Which is the best material for the application – Dual Laminate or FRP? What is considered in selecting materials?
This includes pipe, fittings, and joints. Which are the applicable Codes and Standards? Do the specifications include requirements beyond the Codes and Standards? Are there conflicts between the specifications and Codes and Standards and best practices? Which Codes and Standards apply to Composites?
When is a stress analysis required? What is unique about analyzing Composite piping?
Who is supplying the supports? Have the supports been specifically designed and fitted for Composite piping? Are weight supports, guides, anchors, etc. properly utilized?
Who will be installing the pipe system? Do they have relevant Composite pipe installation experience? Do they have qualified bonders?
Material selection, component design, support design, system stress analysis, and installation – all critical to a successful pipe system design.
Composite piping systems are manufactured with a corrosion barrier and a structural laminate. The corrosion barrier provides the chemical resistance needed to protect the structural laminate so that the piping system can perform its intended function. There is a wide range of corrosion barriers commonly used in the Corrosion Industry, everything from just a surfacing veil in a light duty resin system, to corrosion barriers containing multiple layers of surfacing veil and chopped strand mat manufactured using premium grade resins, to those made using fluoropolymer materials. For long term, trouble free service of your piping system, you need to have a corrosion barrier that will stand up to the challenge of your application. But there can be significant cost differences between the various corrosion barrier configurations. Choosing the right configuration for the application will ensure you get the performance you want without paying for more than you need.
The first step in selecting the right corrosion barrier configuration is accurately defining the environment to which the piping will be exposed. Items that need to be identified are:
- Chemical environment
- Concentrations of chemicals (normal and maximum)
- Process temperatures (normal operating and upset)
It’s important to consider not only the individual chemicals that will be present in the process stream, but also the combinations of chemicals that might be present. The particular combination of chemicals could pose additional corrosion challenges that need to be addressed. It’s also very important to accurately define the temperatures of the process stream as chemical attack can be significantly accelerated at higher temperatures.
With the process conditions properly defined, we can determine whether an FRP corrosion barrier will be suitable for the conditions, or whether it will be necessary to use a thermoplastic corrosion barrier, i.e. a Dual Laminate piping system.
For most corrosion applications, the standard FRP corrosion barrier is 0.100” – 0.120” (2.5 – 3.0 mm) thick, consisting of 1 or 2 layers of surfacing veil and 2 layers of chopped strand mat. Surfacing veils are usually composed of “C” glass or polyester; both are commonly used and both are resistant to a wide range of process conditions. There are a few applications where one would be preferred over the other such as hydrochloric acid where “C” veil would be preferred, or hydrofluoric acid where polyester would be preferred, but both types of veil have been used successfully in many applications.
Chopped strand mat is either “E” or “ECR” glass. “E” glass has been used successfully in corrosion barriers for over fifty years. “ECR” glass, which is a boron-free form of “E” glass, has been gaining wide acceptance in recent years. “ECR” glass displays improved chemical resistance in some services (particularly acids), and it also displays moderately better mechanical properties.
While the “100 mil” corrosion barrier is the most widely used configuration, thicker corrosion barriers, e.g. 0.200” (5.1 mm) are often specified for more aggressive services.
At least as important as the choice of thickness of corrosion barrier is the choice of resin. There are resins available in the Industry such as Isophthalic and Terephthalic polyester resins that will provide good chemical resistance to many dilute chemicals at moderate temperatures. But for most corrosion applications, it is necessary to choose higher grades of resins. There are specialty resins available that are particularly well suited to specific applications such as the bis-A fumarate polyesters in alkaline environments, but the workhorses in the Industry today are the vinylester resins (VE resins). The bisphenol epoxy VE resin is the most commonly used VE resin and it is suitable for a very wide range of chemical services in temperatures up to approximately 200°F (93°C). For higher temperatures, VE resins based on novolac epoxy can be used. These resins are more expensive, but they can provide much better chemical resistance and mechanical properties retention at elevated temperatures.
RPS manufactures several standard FRP piping systems using 0.11” (2.8 mm) corrosion barriers and bisphenol epoxy VE resins (ABCO P-150 and A-150). For higher temperature applications, RPS also manufactures a standard piping system using a 0.11” (2.8 mm) corrosion barrier and novolac epoxy VE resin (ABCO H-150). However, if there is no RPS standard FRP piping system that is suitable to meet your needs, we would be happy to recommend and supply a custom FRP piping system that will.
For the particularly aggressive environments such as concentrated acids or alkalines at elevated temperatures, the best choice for chemical resistance is Dual Laminate piping, i.e. a composite piping system with a thermoplastic corrosion barrier and an FRP structural laminate. RPS’s MAXAR piping for example is manufactured with an FEP (fluorinated ethylene-propylene) corrosion barrier. This corrosion barrier provides the very best chemical resistance available in composite piping systems.
RPS also offers other options in Dual Laminate piping systems that can provide more cost effective solutions for less aggressive services. These include corrosion barriers consisting of PVC, CPVC, PE, PP, and PVDF.
RPS has been supplying corrosion-resistant piping systems for well over forty years. With our extensive experience and our regular consultation with raw materials suppliers, we can offer assistance in selecting the best material for your application. And by offering a wide range of products, we can provide you with the best choice of piping system for that application. Please contact us; we would be happy to help you select the right composite piping material to meet your needs.
Pressure Design of Piping Components
Composite piping components are manufactured with a corrosion barrier and a structural laminate. The corrosion barrier provides the chemical resistance needed to protect the structural laminate, and the structural laminate provides the structural integrity needed for the piping component to perform its intended function. Structural design of piping components starts with pressure design, i.e. determining the thickness of the component necessary to provide the required pressure rating. There are a number of different methods used to determine the required the thickness of components, and we are going to consider the most commonly used methods as we begin our discussion on pressure design by first looking at pipe.
Straight lengths of pipe are by far the simplest component to design in any piping system. The geometry is easy to analyze (i.e. a cylinder) and samples are relatively easy to test. The stresses in a pipe when exposed to pressure are:
Sh = P · Dm / (2 · Ts) and Sax = P · IDs / (4 · Ts) (this equation is approximate)
Sh = Hoop stress.
Sax = Axial stress.
P = Pressure.
Dm = Mean diameter of structural wall.
Ts = Thickness of structural wall.
IDs = Inside diameter of structural wall.
The above equations can be arranged to:
Ts = P · Dm / (2 · Sh) and Ts = P · IDs / (4 · Sax)
From the first equations, it can be seen that the hoop stress due to pressure is approximately twice that of the axial stress. So it is generally sufficient to determine the wall thickness required by simply satisfying the hoop stress equation. This assumes that the axial tensile strength of the pipe is at least half that of the hoop tensile strength, which is true for most commercially available FRP pipe.
To determine the required structural wall thickness for a particular size of pipe and for a particular pressure rating we need to know the value of the allowable hoop stress. The most common methods for determining allowable hoop stress for FRP pipe are:
Long term pressure testing in accordance with ASTM D2992 to determine the long term hoop tensile strength, and application of a suitable Design Factor (not less than 2 for static test method). This testing requires about 1.5 years to complete, so it has generally only been used for commodity FRP piping or for standard products. For example, RPS A150 pipe and P150 pipe have been tested using this test method.
Short term pressure testing in accordance with ASTM D1599 to determine the short term hoop tensile strength, and application of a suitable Design Factor (typically in the range of 6 – 10). This testing can be completed in a relatively short period of time making it ideal for custom-designed pipe.
Determination of the elastic properties of the pipe using either laminate analysis or testing, and then calculating the allowable stress from a pre-determined maximum allowable strain. Maximum allowable strains are typically in the range of 0.0010 – 0.0200 depending on the severity of the service.
To get a better idea for how each of these methods would be used, let’s look at an example of 12” diameter, 150 psi pipe using typical results from the three different methods:
In the long term pressure test, a minimum of 18 pipe samples are tested at various pressure levels, and the time to failure is measured for each sample. The test method requires at least one sample to survive at least 10,000 hours (1.14 yrs). The failure pressures (or stresses) and hours to failure for all samples are plotted on a Log-Log scale, and a best-fit curve is calculated for the data. The long term pressure strength is then determined by extrapolating the data to the design life (typically 100,000 hours (11.4 yrs)). A typical value of long term pressure strength for filament wound vinylester pipe would be 13,500 psi (93.1 MPa). Applying a Design Factor of 2 (as required by ASME B31.1 and B31.3) results in an allowable hoop stress of 6750 psi (46.6 MPa). The required wall thickness would be (assuming a Corrosion Barrier thickness of 0.11”):
Ts = 150 · 12.36 / ( 2 · 6750) Ts = 0.14” (Note: Ts must actually be known to calculage Dm)
In the short term pressure test, a minimum of 5 pipe samples are tested to failure in approximately 1 minute. The results are averaged, with a typical hoop tensile strength for filament wound vinylester pipe being 40,000 psi (276 MPa). Applying a Design Factor of 6 results in an allowable hoop stress of 6667 psi (46.0 MPa). Similar to above, the required wall thickness would be:
Ts = 150 · 12.36 / (2 · 6667) Ts = 0.14”
If we were to model a typical Filament Wound laminate using Laminate Theory, we would find that the hoop tensile modulus of the pipe would be approximately 2.5 x 106 psi (17,240 MPa). Multiplying the modulus by an allowable strain of 0.0020 would yield an allowable stress of 5000 psi (34.5 MPa). The required wall thickness would be:
Ts = 150 · 12.41 / (2 ·5000) Ts = 0.19”
As we can see from these values, the wall thicknesses based on test results are somewhat less than that based on laminate analysis. This is as it should be given that the test results are based on demonstrated properties.
It may appear that we have completed our design of the wall thickness for the 12” diameter 150 psi pipe, but we still need to consider the pipe’s ability to carry axial loads other than just pressure. Pipe, like all other piping components, will be exposed to axial loads such as bending between supports due to the weight of the pipe and its contents, bending due to constraint of thermal expansion, bending due to wind loads, etc. So the pipe must be capable of carrying axial loads other than just pressure. If the pipe is made by contact molding using layers of chopped strand mat and woven roving, the mechanical properties will be essentially the same in all directions. This type of composite is referred to as “isotropic”. Designing the wall thickness for hoop stress due to pressure will result in more than enough strength in the axial direction for the pressure stress, exactly double as a matter of fact. This provides significant additional capacity for those other axial loads.
However, FRP pipe is more commonly manufactured by filament winding with the fibers oriented at + 55° to the pipe axis. This is the ideal winding angle for resisting pressure as it results in a hoop strength that is twice that of the axial strength, exactly matching the applied pressure stresses in a cylinder where the hoop stress is twice that of the axial stress. Such a laminate is referred to as “orthotropic”. While this type of construction is ideal for resisting pressure, if we only design for pressure, there will be no axial load-carrying capacity left over for the other axial loads. It is therefore necessary to provide additional axial load-carrying capacity so that the pipe will have an adequate design factor when all axial loads are considered. One such method for accomplishing this is simply to use a higher design factor when designing for pressure. For example, RPS uses a design factor of 10 when applied to the short term hoop tensile strength rather than the 6 used in the example above. This higher design factor results in us using an allowable hoop stress of 4,000 psi (27.6 MPa), and it yields a structural wall thickness of 0.24” for the 12” diameter, 150 psi pipe. This additional wall thickness, while not required for pressure containment, will provide a significant additional axial load-carrying capacity for the non-pressure axial loads.
Pressure Design of Piping Components - Fittings and Joints
In our third article, we took our first look at pressure design of piping components by looking at pipe. We are going to continue that discussion, looking at pressure design of fittings and joints.
Pipe is the simplest component in a pipe system to design for pressure. The stresses in a pipe when exposed to pressure are:
Sh = P · Dm / (2 · Ts) and Sax = P · IDs / (4 · Ts)
where: Sh = Hoop stress. Sax = Axial stress. P = Pressure. Dm = Mean diameter of structural wall. Ts = Thickness of structural wall. IDs= Inside diameter of structural wall.
The situation isn’t quite so simple for most fittings. The geometry of fittings gives rise to local stresses within the fitting that can be significantly greater than those calculated using the equations above. The most common method for dealing with these higher stresses is to include a “fudge factor” with one or both of the above equations. This fudge factor is called the pressure stress multiplier. It is usually designated by the letter “m”, and it represents the ratio of the maximum pressure stress in the fitting to the pressure stress in an equivalent pipe.
The equation for hoop stress would then become:
Sh = m · P · Dm / (2 · Ts)
For example, in a long radius elbow of uniform thickness, the maximum hoop stress in the elbow is 1.25 times that in a pipe, so m = 1.25. This means that a long radius elbow should be at least 1.25 times as thick as a pipe to provide the same factor of safety against burst. (This statement is only true if the pipe and elbow are manufactured using the same type of laminate; a situation that is most often not the case. The elbow could be thinner or thicker than the 1.25 times depending on the relative strengths of the elbow and pipe laminates).
If we looked closer at the distribution of the pressure stresses around the circumference of the elbow, we would learn that the stress is greatest on the inside radius of the elbow, and least on the outside radius. This can be seen in the Finite Element Analysis (FEA) graphic below, where the red areas represent the higher hoop stresses and the green areas represent the lower hoop stresses (the blue areas are at the joints where the total thickness has been increased by the joint laminate).
This non-uniform distribution of stresses means that the thickness need not be uniform to address the higher stresses. By applying the additional thickness only where the stresses are higher, it is possible to manufacture a more cost effective elbow. This approach would allow us to avoid wasting material by applying the additional thickness only where it’s required.
Like the pipe, we still need to know the strength of the laminate before we can determine what the thickness needs to be. Very few fittings are tested under long term pressure testing (ASTM D2992). It is more common to conduct short term burst testing of fittings to determine laminate strength (ASTM D1599), but testing 5 samples (as required by ASTM D1599) of any fitting can become very expensive. It is more cost effective and much more common to conduct short term testing of coupons cut from typical laminates (ASTM D638). This latter method is possible in fitting laminates as the materials are such that they can usually be applied on a flat sheet (and absence of coupon curvature is usually a requirement for the relevant tensile test methods). The same can’t be said for the filament wound laminates used in pipe. Once the laminate strength is known, a design factor is applied to the laminate strength to calculate an allowable stress, and the required fitting thickness is calculated using the modified equation above.
This approach of applying a pressure stress multiplier to the basic pipe equations can be used for almost any fitting. We just have to use appropriate values for the pressure stress multiplier (m). As we’ve already seen, “m” for an elbow is 1.25. But some fittings, tees for example, have a much larger “m”. A typical value for “m” of a tee is about 2.5, so a tee has to be significantly thicker than a pipe to provide the same factor of safety.
This same approach can also be applied to butt joints. Although butt joints are similar in shape to that of pipe (i.e. cylindrical), the discontinuity in the geometry where the pipe ends meet results in stress concentrations within the joint. Applying a pressure stress multiplier “m” of 1.67 will result in an acceptable factor of safety to the joint design. This is equivalent to saying that a contact molded butt joint laminate should be 1.67 times the thickness of a contact molded pipe of the same size to achieve the same burst strength.
The design approach just described is an appropriate method for designing fittings and joints, but we haven’t yet actually proven that the component is adequate. Recall that the approach utilized strength properties from flat sheet laminates and application of a “fudge factor” (i.e. the pressure stress multiplier). For example, we haven’t yet demonstrated that the fudge factor was adequate, or that we applied the reinforcements appropriately. To provide evidence that the component actually achieves the factor of safety required, testing of the actual component is necessary. This is the purpose of proof testing (also called proof of design testing). There are several methods that can be used for proof testing. The first of these is to actually burst the component in accordance with ASTM D1599. While this is the most direct approach, it requires specialized equipment – particularly for larger diameter components, and it carries with it some safety concerns. Unlike filament wound pipes, which typically fail by weeping, fittings often fail by actually bursting. A lot of energy can be released during these sometimes “exciting” events, so appropriate precautions need to be taken to avoid damaging equipment and facilities, and more importantly, for ensuring the safety of the personnel involved.
Proof testing is more commonly carried out by pressurizing to lower levels, but holding the pressure for longer periods of time. For example, ASTM D6041 requires that the component be pressurized to 4 times its rated pressure for 1 hour without leaking. This is the method that RPS uses to verify adequacy of its fittings and joints.
Pressure Design of Piping Components - Flanges
Flanges present the greatest design challenge of all of the standard piping components. The theory underlying design of flanges is very complex, and the design methods are typically trial and error, i.e. a flange configuration is first assumed, and then it is checked for stresses. If the stresses are not acceptable, another configuration is assumed, and the stresses checked again.
Flange design theories are generally complex for several reasons:
A flange is not just simple component like that of an elbow or a reducer; it is actually composed of several components acting together, i.e. the flange ring, the flange hub, the bolts, and the gasket. The individual components of the flange are typically composed of materials with significantly different properties. For example, the bolts are metallic and hence very stiff; the gasket is typically elastomeric and hence very flexible. As a result of these issues, most flange design approaches are either “simplified” elastic analyses, or even empirical in nature.
The most commonly used design methods for contact molded, full-face FRP pipe flanges (see Figure 1) are those found in ASME RTP-11 and ASME Section X2 (The details of these design methods are somewhat beyond the scope of this article, but interested readers are encouraged to consult the references). The two methods are very similar, and for the same inputs, will yield virtually identical results. Both methods are derived from the design method for metallic flanges found in the ASME Section VIII3 for metallic pressure vessels. The Section VIII design method applies to raised-face configurations where there is no contact between the flanges out beyond the bolt circle. Such a flange configuration results in high longitudinal hub stresses arising due to bolt tightening and operating loads, which would be difficult for a material like FRP to accommodate. For this reason, FRP flanges are most often designed with flange contact over the entire flange face. This configuration substantially reduces the longitudinal hub stresses. But given the greater contact area of the gasket, the initial bolt loads must be much higher to effectively seat the gaskets. To limit the bolt torques to reasonable levels requires the use of soft gaskets. Unfortunately, the soft gaskets limit the use of this configuration to relatively low pressures, e.g. 150 psi.
Figure 1 – Contact molded, full face flanges
Adequacy of the design of contact molded flanges can be verified by demonstrating compliance to the performance requirements in ASTM D5421 “Contact Molded “Fiberglass” (Glass-Reinforced-Thermosetting Resin) Flanges”. These requirements include a sealing test, a bolt torque test, and a burst test. The flange must withstand a pressure of 1.5 times the pressure rating for 1 week without leakage; the flange must withstand a bolt torque of 2 times the recommended torque without visible signs of damage; and the flange must withstand a pressure of at least 4 times the rated pressure without failure.
There are instances where an FRP flange must be exposed to a raised-face configuration. These instances would include mating an FRP flange to a metallic pipe flange with a raised-face, or mating an FRP flange to a valve with a flat face but with a lining that covers the flange face only out to the bolt circle. Using a full face flange directly in these types of situations is not recommended due to the likelihood of over-stressing the flange during bolt-up or operation. If a full face FRP flange must be used, then a spacer ring should be inserted between the flange faces to fill the gap and hence eliminate the raised-face condition. An alternative solution would be to use an FRP Lap Joint flange in lieu of the full face flange (see Figure 2). An FRP Lap Joint flange has a stub OD that is the same diameter as the inside of the bolt circle. As long as the mating raised-face has the same OD, the bolt-up and operation stresses in the FRP Lap Joint flange will be much lower than they would be in a full face flange.
Design of contact molded Lap Joint flanges follows essentially the same approach as for the full face flanges, but with the contact between flanges being entirely within the bolt circle, the design method in ASME Section VIII3 can be used directly.
Figure 2 – Contact molded, Lap Joint flange
The design methods discussed so far are appropriate for contact molded flanges. But FRP flanges are often manufactured by other than contact molding. One such method is filament winding (see Figure 3). In this manufacturing method, the flange mold is rotated about the flange axis, and continuous strands of glass are applied in the hoop direction. This type of construction results in very high hoop strength and stiffness of the flange. Although the longitudinal strength of this type of laminate is relatively low, the stiffness provided by the hoop reinforcements and flange ring geometry, along with the greater thickness of these flanges often results in the filament wound flanges being capable of withstanding higher bolt-tightening loads and operation loads than contact molded flanges.
Due to the non-standard geometry of the flange and to the highly orthotropic properties of the laminate, standard design methods have not yet been developed. These flanges are typically designed by empirical methods, i.e. configurations that have been proven to work are modified as required to satisfy the design conditions. Filament wound flanges should be verified to comply with the performance requirements in ASTM D4024 “Machine Made “Fiberglass” (Glass-Reinforced-Thermosetting Resin) Flanges”. Like ASTM D5421, the performance criteria of ASTM D4024 include a sealing test, a bolt torque test, and a burst test.
Figure 3 – Filament wound, full face flange
The piping system Designer is responsible for ensuring that the selected piping components are suitable for his/her piping system’s design conditions. In the case of metallic piping, the Designer will often carry out pressure design calculations to demonstrate the adequacy of the selected components, particularly non-standard components. But given the specialized nature of FRP piping, the task of pressure design of the piping components is typically left to the manufacturer. For example, RPS has developed several standard piping product lines (e.g. P150, A150, & Maxar), typically with a pressure rating of 150 psi (10 Bar). RPS ensures that all components within those product lines are properly designed for the stated design conditions of pressure and temperature. If none of the RPS standard product lines are suitable for some reason for a specific application, RPS engineers will custom-design piping components that will meet the needs of that application.
Once the piping components have been properly designed for pressure, it is necessary to ensure that the piping system can safely carry the other loads to which it will be exposed.
This aspect of piping design is sometimes referred to as “axial” design as the loads of interest usually give rise to just axial stresses.
This step in the piping design process is usually referred to as “pipe stress analysis”. In addition to pressure, the loads that are typically considered in the pipe stress analysis are the operating loads of weight and temperature, and occasional loads such as wind and/or seismic. The primary purpose of the pipe stress analysis is to ensure that the applied stresses do not exceed the piping allowable stresses. The pipe stress analysis can serve other functions as well including determination of loads on supports, determination of loads on connected equipment, and assessment of piping movements to help avoid potential clashes with other equipment and structures.
There are several levels of analysis that are commonly undertaken for FRP piping systems. These include using rules of thumb, carrying out hand calculations, and full blown analyses using purpose-built computer software. The choice of which approach to take depends on several factors including the level of experience of the Designer, the severity of the design conditions of the system, and the criticality of the line.
It is very common to utilize the kind of rule of thumb information as provided in design guides produced by the piping manufacturer for designing piping systems such as vents and drains and other low pressure/temperature systems. For example, documents such as RPS’s Design Manual provide recommended maximum support spans and minimum offset legs to help the designer lay out standard piping systems. If the piping designer takes these recommendation into account as the piping system is laid out, the Designer can be assured that the applied stresses will be kept within safe levels.
If there are specific portions of a piping system for which the Designer requires a little more accuracy (e.g. stress at a specific location), or a little more information (e.g. loads on a particular support), he/she can carry out hand calculations using formulas from widely recognized resources such as Roark’s Formulas for Stress & Strain, Kellogg’s Design of Piping Systems, etc.
With the ease of use of today’s pipe stress analysis software, more and more FRP piping systems are being modelled and analyzed using full blown analyses. And this is certainly recommended for all systems for which the design pressures and temperatures are high, and also for systems for which the criticality is high, e.g. those systems that contain hazardous or toxic fluids, or those for which unplanned outages must be avoided. Pipe Stress Analysis programs such as CAESAR II are easy to use and provide a very powerful tool for designing and analyzing any piping system, FRP included.
Regardless of the level of analysis of the piping system, the primary goal of the task is to ensure the piping has enough supports to safely carry the weight loads and other “primary” loads, while at the same time providing sufficient flexibility to ensure the “secondary” loads do not become unmanageable. Primary loads are those such as pressure, weight, wind, and seismic loads, and they give rise to “primary stresses” in the piping. Primary stresses are those that are required within the piping to balance the applied primary loads. They are characterized by being able to cause ultimate failure of the piping system should they become too large. Secondary loads are those caused by constraint of displacement of the piping such as restriction of thermal expansion or by settlement of a support or connected equipment. Secondary loads give rise to secondary stresses. Secondary stresses typically result in excessive distortion should they become too large, but they rarely result in ultimate failure. In fact, in ductile piping systems, secondary stresses are permitted to exceed the yield strength of the material, and they are then relieved by local yielding. Due to the fact that the displacement associated with secondary loads is typically limited to the extent that the piping tries to expand or contract, the secondary stresses are often referred to as being “self-limiting”.
The analysis of FRP piping systems is not hugely different from that of metallic piping systems, but perhaps the most important difference between the analysis of the two types of systems is in the way that secondary stresses are handled. In metallic piping systems, secondary stresses are permitted to be much higher than primary stresses. This is in recognition of the ability of the ductile material to locally yield without inducing ultimate failure. The ASME B31.1 and B31.3 piping codes treat secondary stresses as “Expansion” stresses, and they permit the Expansion stresses (or more correctly, the expansion stress range) to be as much as 2.5 times the allowable stress for primary loads. The expansion stress range is the stress induced in the piping during a change in temperature from the highest temperature to which the piping will be exposed to the lowest temperature.
This same treatment for Expansion stresses is not appropriate for FRP piping. FRP does not display significant ductile behavior, so Expansion stresses should be treated essentially the same as primary loads. This fact has given rise to the practice of analyzing FRP piping systems for “Operating” loads, i.e. the combined effects of pressure, weight, and thermal loads, and comparing the resulting stresses to the allowable stress for sustained loads. In this case, the thermal stresses are calculated for the change in temperature from the minimum installation temperature to the highest temperature the system will be exposed to, and in some cases, from the maximum installation temperature to the lowest temperature.
Efforts are currently underway within ASME to permit somewhat higher stresses in FRP piping for expansion stresses given the self-limiting nature of these stresses, but this increase should be modest, say 10% – 20%, not 150% as in the case for metallic piping.
Pipe Stress Analysis
To carry out any pipe stress analysis, there are a number of piping system properties that must be known in advance. Some of these properties, such as fluid pressure, temperature, and density, are dictated by the process and would be the same regardless of what piping material was used. Other properties are dependent on the piping material and include physical properties (e.g. density), thermal properties (e.g. coefficient of thermal expansion), and mechanical properties (e.g. modulus of elasticity and strength). Of these properties, we’re going to focus on the mechanical properties, which are typically divided into the elastic properties and the strength properties (from which we can determine the allowable stresses).
The elastic properties include the modulus of elasticity values and the Poisson ratios. The modulus of elasticity is a measure of the material’s resistance to deformation when a force is applied to it. The higher the modulus, the less the material will deform, i.e. the “stiffer” the material is. The Poisson ratio is a measure of how much a material will deform in one direction when a load is applied in another direction. For example, if an axial load was applied to a pipe, the tensile modulus of elasticity will tell us how much the pipe elongates in the axial direction, and the Poisson ratio will tell us how much the pipe will shrink in the hoop direction. For isotropic materials such as steel, there are 3 elastic properties that must be known:
Young’s modulus of elasticity (or tensile modulus of elasticity, usually given the symbol E). Shear modulus of elasticity (usually given the symbol G).
Poisson ratio (typically given the symbol ν, i.e. Greek letter “Nu”).
For isotropic materials, these three properties are related to each other by the following equation:
G = E / [ 2(1+ν)] So it is only necessary to know two of the values, as the other property can be determined by this relationship. Appropriate values for these properties are listed in the ASME Piping Codes such as B31.1 “Power Piping” and B31.3 “Process Piping”.
For steel, typical values of these properties are:
E = 200,000 MPa (29 x 106 psi) G = 77,000 MPa (11 x 106 psi) ν = 0.3
For orthotropic materials like FRP, there are 5 elastic properties that must be determined:
Axial modulus of elasticity (usually given the symbol Eaxial or Ea). Hoop modulus of elasticity (usually given the symbol Ehoop or Eh). Shear modulus of elasticity (usually given the symbol G). Major Poisson ratio (usually given the symbol ν1 or νx). Minor Poisson ratio (usually given the symbol ν2 or νy).
The axial and hoop modulus values and the Poisson ratios are related by:
ν1 X Eaxial = ν2 x Ehoop
So only 4 of the 5 properties must be known, as the fifth can be determined by this relationship.
For 55 deg. filament wound pipe, typical values of these properties would be:
Eaxial = 9,655 MPa (1.4 x 106 psi) Ehoop = 15,860 MPA (2.3 x 106 psi) G = 10,345 MPa (1.5 x 106 psi) ν1 = 0.72 ν2 = 0.44
As you can see, the modulus values are significantly less than those for steel. In particular, the axial modulus of elasticity is less than 1/20th that of steel. This results in FRP pipe being significantly more flexible than steel pipe. That fact can be quite helpful in that it makes offset legs in piping systems very effective at absorbing thermal expansion of the piping. But we also have to be careful to limit support spans to more modest values than for steel pipe to ensure the deflection between supports is not excessive.
The elastic properties can vary widely depending on the construction of the FRP laminate. The format of the reinforcement and the orientation of the fibers in particular can have a huge effect on these properties. The properties can be determined by testing samples of the material, and this is often done for “standard” laminates. That is, for pipe products that are manufactured using a specific and fixed construction (resin system, glass type, fiber format, fiber orientation, and laminate sequence), determination of these properties by testing is a practical approach. However, one of the distinct advantages of using a composite material like FRP, is the ability to custom-design a laminate to suit the specific application. This could mean anything from selecting a different resin, to changing the corrosion barrier construction, to adding directional reinforcements, to changing the entire laminate sequence. It would be impractical and cost-prohibitive to conduct testing on all such potential solutions to a given problem. Fortunately, there are mathematical procedures available that allow us to calculate the elastic properties of these various constructions. These procedures are addressed by the field of Mechanics of Composites.
The mathematics associated with Mechanics of Composites can appear a bit daunting to those not familiar with it, e.g.
But many manufacturers and designers of FRP equipment use these principles on a daily basis, and are well qualified to take full advantage of them.
Mechanics of Composites includes two subjects of direct relevance to our current discussion, i.e. Micromechanics and Lamination Theory. Micromechanics provides a method of determining the properties of individual layers (or “lamina”) from the properties of the constituent materials (resin and glass) and from the configuration of the layer, i.e. thickness, glass content, fiber format, and fiber orientation. The field of micromechanics is largely empirical, and there have been several micromechanics theories developed over the years to attempt to accurately predict lamina elastic properties. One of the most widely accepted theories is that presented in Military Handbook 17, and it is this theory that has been (or will be) adopted in ASME RTP-1, ASME Section X2, and ASME NPPS NM-2.
Once the elastic properties of each layer are known, we can combine the properties of all the layers using Lamination Theory to determine the overall elastic properties of the laminate. Lamination Theory is well developed and will yield very good predictions of elastic properties of laminates. We can then use those elastic properties as inputs to a pipe stress analysis.
The mechanical properties that are required to conduct a pipe stress analysis are the strength properties, or to be more specific, the allowable stresses. As we discussed in our article on pressure design of components, we need an allowable hoop stress to determine the component thickness required for any particular pressure capability. For pipe stress analysis, we are more concerned with allowable axial stresses. For isotropic materials such as steel, the allowable stresses are listed in the ASME Piping Codes, so we can just select the appropriate value for the given design temperature. It is not quite so simple with FRP piping, as allowable stresses have not been standardized to the same extent as metallic materials. However, it is possible to list appropriate values for a few standard constructions, and that is what will be done in the new ASME Standard for FRP Piping (ASME NM-2) scheduled to be published in December of 2016. That Standard (via its companion standard NM-3) will provide both elastic properties and allowable stress values for three types of construction: Type I, Type II, and Type III laminates. Type I and II are contact molded laminates as defined in ASTM C582. A Type III laminate is a 55 deg filament wound laminate as will be defined in the new FRP Piping Standard. For other types of laminates, or for values that are more appropriate to a manufacturer’s constructions, it is necessary to consult with the manufacturer. It is not possible to rely on Mechanics of Composites for allowable stresses to the same extent as is true for elastic properties. The allowable stresses must be determined through testing of the specific laminate of interest, or by using conservative estimates based on previous testing of similar laminates.
The strength values must be reduced by appropriate design factors to arrive at suitable allowable stresses. ASME NM-2 will provide minimum design factors that must be used, and these design factors will be based on the type of testing that was used to determine the laminate strength, i.e. short term versus long term testing. For example, for laminate strengths based on short term testing, a minimum design factor of 6 will be required for sustained loads.
For “quasi-isotropic” laminates such as Type I and Type II laminates, it is usually sufficient to determine the allowable axial stress by conducting a uniaxial strength test of the laminate and dividing the strength by a suitable design factor such as 6. For orthotropic laminates such as Type III (55 deg filament wound laminate), the axial strength can depend on the magnitude of the coincident hoop stress. It is therefore necessary to determine the axial strength under at least two different loading conditions:
• Hoop stress that is twice that of the axial stress.
• Hoop stress that is zero.
During a pressure test, the hoop stress is twice that of the axial stress, so a pressure test is used to determine the axial strength under “biaxial” loading. A simple tensile test can be used to determine the uniaxial strength of the laminate. Two allowable stresses are determined by dividing the two axial strengths by suitable design factors. These allowable stresses are then used to create the “allowable stress envelope”, i.e.
In the allowable stress envelope, the hoop stress is plotted along the X axis, and the axial stress is plotted along the Y axis. The allowable axial stress for the biaxial stress state is represented by SA(2:1), and the allowable axial stress for the case where the hoop stress is zero is represented by SA(0:1). A straight line is drawn between these two points to provide the allowable stress at any intermediate hoop stress, i.e. at pressures lower than the maximum allowable design pressure. The distance between the allowable stress line and the 2:1 stress line (shown as a dashed line in the above figure) represents the allowable axial stress that is available for loads other than pressure. It should be noted that if the applied hoop stress is equal to the maximum allowable hoop stress, then there is no distance between the allowable stress line and the 2:1 stress line, and there would be no allowable stress available for axial loads other than pressure. This goes back to our earlier discussion on pressure design of filament wound pipe, and why we said it was necessary to provide additional capacity in some laminates. For example, for design of filament wound pipe, RPS uses a design factor of 10 when applied to the short term hoop tensile strength, rather than the 6 that would be expected under combined loading. This higher design factor will result in a greater wall thickness than that required just for pressure containment, and will provide a significant additional axial load-carrying capacity for the non-pressure axial loads.
One further point to note about the allowable stress envelope: When the axial stress is compressive rather than tensile, as would typically be the case for rigidly restrained systems, the allowable stress is based on the axial compressive strength, i.e SA(0:-1). It is acceptable to simply use the negative of the axial tensile strength (SA(0:1)) for this value as the compressive strength is always greater than the tensile strength, but it is typically worth testing to determine this value as the compressive strength can be much greater than the tensile strength.
Stress Intensification and Flexibility
When engineers conduct a pipe stress analysis, they construct a mathematical model of the piping system. The basic elements of the model are beam elements because, as far as loads other than pressure are concerned, a piping system is essentially a combination of connected beams. That’s fortunate because the equations for stresses in a beam are simple and exact; no approximations are required. Unfortunately, only pipe itself behaves as a beam element. The other components in a piping system, i.e. the fittings such as elbows, tees, reducers, etc., display more complex stress behaviors compared to the pipe. So in order to correctly predict the stresses in the fittings, we need to apply “fudge factors” to the stresses calculated for pipe. These fudge factors are called stress intensification factors or “SIFs”. For FRP fittings, SIFs are defined as the ratio of the maximum stress in the fitting to the maximum stress in a pipe when exposed to the same loading. For example, let’s assume that a section of pipe is exposed to a bending moment equal to M. This could be the bending moment due to the weight of the pipe and contents midspan between two supports. The maximum bending stress in the pipe is simple to calculate and is equal to:
Sb = M / z
Sb = Bending stress
M = Bending moment
Z = Section modulus of the pipe. This is a geometrical property of the pipe dependent only on the diameter and thickness of the pipe.
Now, if there were a fitting such as an elbow or a tee at the midspan between the supports and therefore exposed to the same bending moment, the maximum stress in the fitting would likely be different (and probably higher) than in the pipe. The Finite Element Analaysis (FEA) image below of a reducing tee illustrates how the local stresses in a fitting can be much higher than in the adjacent pipe.
To predict the maximum stress in a fitting, we apply an SIF to the equation above, i.e.
Sb = i x M/z
i = Stress intensification factor (SIF)
Where do the values for SIFs come from? If we were analyzing metallic piping systems, we could find SIFs for a wide range of fittings in the ASME piping codes, i.e. B31.1 and B31.3. But these values typically do not apply to FRP fittings due to differences in geometries and differences in definitions of SIFs between FRP and metallic fittings. To obtain appropriate SIFs for FRP fittings, it is necessary to generate values by testing or by using FEA such as that illustrated in the image above. In some cases, FRP piping manufacturers have utilized empirical values that have proven to be conservative over many years, and these too can be helpful. In any case, the FRP piping manufacturer should be consulted for appropriate SIF’s for their fittings.
A second factor that should be considered in a pipe stress analysis is the flexibility factor, or “k”. This factor is used to predict how much greater the rotational deformation of a fitting will be compared to that of an equivalent length of pipe. The k factor for many fittings such as tees and reducers is simply taken to be 1.0 as the deformation of the fittings is essentially the same as the pipe. The same is not generally true of elbows. When a bending moment is applied to an elbow, the elbow will ovalize as illustrated in the FEA image below:
This ovalization results in the rotation of one end of the elbow relative to the other being different than would be the case for pipe, and in fact, is typically several times that of the pipe. This is an important characteristic when the piping system is being analyzed for displacement loadings such as temperature differential. The presence of an elbow in a section of piping can provide a significant amount of flexibility in the piping run, thereby keeping the thermal loads and hence stresses, to manageable levels.
Where do we find values for k? Again, if we were analyzing metallic piping systems, we could find k’s in the ASME piping codes, i.e. B31.1 and B31.3. But these values are typically not appropriate for FRP fittings due to differences in geometries, constructions, and properties of FRP fittings compared to metallic fittings. So as with SIF’s, we need to utilize testing or FE analysis to determine k values. FRP piping manufacturers should be consulted for appropriate k’s for their fittings.
How Pipe Stress Analysis is Carried Out
In pipe stress analysis there are inputs that would be the same regardless of the choice of piping material. These include pressure, temperature, and density of the fluid, which are dictated by the process conditions. They also include the occasional loadings such as wind and seismic. The Owner (or Owner’s agent) will typically define the basic occasional load parameters such as wind speed and seismic accelerations, as well as the building code to which all plant equipment and systems must be designed. The Designer will then utilize the provisions of a building code standard such as ASCE 71 to determine the actual loads that can be included in the pipe stress analysis. The Designer creates a computerized model of the piping system with a pipe stress analysis program such as CAESAR II or AutoPIPE. This task involves a number of steps including:
Breaking the piping system into discrete elements, each of which represents a small portion of the piping system. The elements are defined by the “nodes” at each end of the elements. The choice of node locations is up to the Designer, but for accurate representation of the piping system, nodes are required at each change of direction or material, at all in-line equipment, and at all restraints (supports and terminal points). Additional nodes would be included at any location where the Designer requires output information. An example of this would be at the midpoint between supports so that the deflection between supports can be determined.
• Defining the appropriate geometric properties of each element, i.e. diameter and thickness of the pipe, and length and orientation of the element.
• Defining the design conditions/loadings including pressure, temperature, weight of the pipe and contents, and seismic and wind loadings.
• Assigning the appropriate material properties to each element including modulus values and allowable stresses.
• Applying the appropriate Stress Intensification Factors and Flexibility Factors to the fittings.
• Defining the support types and locations.
• Defining any imposed displacements on the piping system, e.g. thermal growth of a vessel to which the piping is attached.
Creating the appropriate load cases to ensure all load combinations of interest are analyzed.
An example of the input screen (CAESAR II) defining the properties of a particular element is shown below:
When all the inputs have all been entered into the model, the analyses are run. The Designer reviews the results including the piping stresses, piping deflections, and loads on restraints to determine if the design is acceptable.
If the stresses due to pressure plus weight or occasional loads (seismic and wind) exceed the allowable stresses, or the deflections between supports exceed the allowable deflections, the Designer will typically add restraints to the system. The additional restraints will often be sufficient to solve the stress/deflection problems due to these primary loads, but there may still be stress problems due to constraint of thermal expansion. The piping will try to expand when exposed to an increase in temperature, and some means must be provided to accommodate that expansion. The methods that are typically used include:
• Utilizing the flexibility inherent in the piping layout due to changes in direction
• Introducing expansion loops
• Introducing expansion joints
Another method that is commonly used to control thermal expansion in FRP piping systems is to rigidly restrain straight runs of pipe. Due to the relatively low axial modulus of FRP pipe, the thermal compressive stresses in the pipe and the loads at the anchors are often quite manageable, but this method of support is typically limited to smaller sizes of pipe or to systems where the change in temperature is modest.
While it is necessary to include sufficient restraints to limit the stresses or deflections due to weight and occasional loads, too many restraints can result in flexibility problems with the piping. In fact, the process of arriving at a suitable piping system design often comes down to finding an acceptable balance between providing enough supports to handle the primary loads (weight, seismic, and wind), and providing sufficient flexibility to accommodate the thermal expansion. This process typically requires several attempts at defining an acceptable configuration for the number, types, and locations of the supports.
Apart from the stresses and deflections, the pipe stress analysis also provides the loads on the restraints. It is these loads that are used to design the pipe supports. This task is a necessary part of any pipe system design, but there are particular aspects of the support design for FRP piping that require special attention.
1 – ASCE 7 Minimum Design Loads for Building and Other Structures
Properly Functioning Support System
In any piping system, it is necessary that the support and support system be configured to behave as intended in the piping design. The support must be physically configured to provide the type of restraint that was assumed in the design. For example, if the particular support was intended to act as a guide, i.e. to provide lateral restraint for loads such as wind or seismic, then it must be strong enough and stiff enough in the lateral direction to provide the required restraint. The photo below indicates a situation where the guides were not properly configured. Being on tall posts like this results in the supports having very little lateral stiffness. The piping will therefore be allowed to deflect laterally to a much greater extent than would likely have been assumed in the design.
With an FRP piping system, there are additional support considerations that must be borne in mind. These include the following:
- FRP piping should never be supported directly on steel structure.
- Supports should be configured to avoid point loads on the FRP pipe.
- Supports must be stiff enough to keep the pipe round.
- Supports must be sized to properly fit the pipe.
- In-line equipment should be independently supported.
- Supports should be located to avoid interference with pipe joints.
Let’s look at these points in a little more detail.
FRP is a non-ductile material. That means it won’t yield and redistribute local high stresses in the same way as a ductile material would. It is therefore necessary to ensure that the pipe is not exposed to point loads, or even to line loads.
Supports should include elastomeric liners to act as cushioning to soften any point loads. FRP pipe typically has an irregular outside surface, so resting the pipe directly on steel, or even using steel saddles without the elastomeric linings can result in point loads on the pipe.
Apart from the need to avoid point loading, FRP pipe should never be supported directly on steel simply because any relative movement can lead to chafing damage of the FRP pipe.
The photo below illustrates what can happen when FRP pipe is supported directly on a steel beam. The pipe experienced a very high stress along the line of contact between the pipe and steel, and it actually failed along this line.
Supports should also provide sufficient bearing area to distribute the load over a reasonable area of the pipe. It is common to specify a minimum length of a support to be ¼ – 1/3 of the diameter of the pipe. A good rule of thumb is to ensure the bearing stress is limited to not more than 50 psi.
A commonly over-looked requirement for FRP pipe supports is the need for adequate support stiffness. When the pipe is under loads, the job of the support is to transfer the loads to the support structures, but in doing so it must also hold the pipe round. FRP pipe is much less stiff than steel pipe, so it could ovalize if the support were not sufficiently stiff. This would lead to high local stresses in the pipe, and a reduction in the ability of the pipe to act as a beam, i.e. to span between supports. Small diameter pipe is proportionately stiffer than large diameter pipe, so a minimum saddle angle of 120 deg is usually adequate. For larger diameter pipes, a minimum saddle angle of 150 deg is recommended, and supports should be properly stiffened to hold their shape. If supports are not designed to provide adequate stiffness, the support can deform significantly under load as illustrated in this computer graphic.
The image below illustrates the configuration of a properly designed weight support. Note the webbing and gusseting that ensures the support will retain its round shape.
FRP pipe typically does not have the same outside diameter as steel pipe, so it shouldn’t be assumed that a support intended for steel pipe will properly fit FRP pipe. This is particularly true in larger diameters where the outside diameter of the FRP pipe does not match the nominal size of the pipe (as it does for steel pipe). In FRP pipe, the nominal size of the pipe typically matches the inside diameter of the pipe, so the mismatch between the outside diameter of the pipe and the inside diameter of a steel pipe support can be significant. The supports should be designed to match the outside diameter of the FRP pipe. RPS provides a complete line of pipe supports designed specifically with this in mind, thereby eliminating any concern about proper fit of the supports.
FRP pipe has a relatively low allowable axial stress, so it is important that supports be included at or near heavy in-line equipment. It is good practice to include supports at all valves as illustrated in this photo.
It is important to locate supports to avoid pipe joints. The overall width of an FRP pipe joint is typically in the range of about one pipe diameter. Piping layout and support locations should be chosen with this in mind. If there is no other choice, the piping can be configured to accommodate a support on or at least near a joint, but this usually increases the cost significantly.
Adhering to these basic principles when designing and fabricating the supports for an FRP piping system will go a long way to ensuring trouble-free performance of the piping system, and purchasing the supports from RPS will provide peace of mind that these principles have been properly addressed.