A common application problem when designing piping systems is the issue of pipe expansion. All piping materials, regardless of installation location or practice, will expand and contract as they experience different temperatures. While this change in length can be large in some cases and small in others, it is important to take an analytical look at the piping system to see if it needs expansion mitigation and if so what type of mitigation should be used. This is especially true for systems utilizing plastic pipes.
While all materials expand and contract in response to temperature changes, some materials change more than others. Plastics are one of those materials. It is not uncommon for plastics pipes to exhibit expansive and contractive behaviors that are up to ten times greater than their metal counterparts. This makes even short runs of plastic pipes an issue when it comes to designing for thermal expansion. Larger runs of plastic pipes can experience extreme snaking or buckling due to the compounding effect of thermal expansion in relation to overall pipe length. Below is a table listing several values from ASME B31.3 for thermal expansion coefficients for various metallic and plastic pipe materials.
As a general rule of practice, plastic pipes should be avoided for applications where there will be large changes in pipe temperatures. If plastic pipe must be used in this form of application, careful consideration should be taken to mitigate the effects of thermal expansion.
It is necessary to evaluate all piping systems for their total expansion and contraction for the operations and applications of the system. Any piping system that is found to have a considerable change in length will need to have some form of expansion mitigation as well as additional pipe supports to constrain pipe movement. What is a “considerable change in length” changes from engineer to engineer, and should be considered for the specific application, but a general rule of thumb I use is that any piping system or segment of a piping system that changes in length more than ¼” will need some form of expansion mitigation. Expansion mitigation comes in a couple of forms, but I will discuss two common ones in this blog; expansion loops and expansion joints. I will also briefly touch on pipe anchoring considerations for thermal expansion.
A common means of mitigating thermal expansion is an expansion joint. Expansion joints work for both metal and plastic piping systems but often have a limited range of operability. The maximum amount of lateral expansion and contraction that an expansion joint can manage is often less than 2”. This is dependent on the make, model, design, and joint material of the expansion joint, with some expansion joints having more maximum movement and some having less maximum movement. Expansion joints come in several common types; rubber, hose and braid, and stainless steel. These style of expansion joints typically come with flanged or threaded connections. Additionally, there are some less common types such as long stroke telescoping and short stroke telescoping. Telescoping style expansion joints often come with socket connections and can be affixed directly to the plastic pipe.
Expansion joints are a good solution for systems that have a small amount of contraction or expansion. Additionally, they can be an easy solution for any piping system retrofit that needs new expansion mitigation due to their prefabricated nature. When designing a system with expansion joints for thermal expansion mitigation, careful consideration needs to be taken to the joint material as the pipe medium and environment may be incompatible with those conditions.
While expansion joints are versatile due to the many different solutions for a piping system application, their limited movement and construction can limit their usage in larger systems or outdoor environments.
Figure 1 – Examples of expansion joints
Another common means of mitigating thermal expansion is an expansion loop. Expansion loops are best utilized in piping systems with very long runs, piping systems that experience extreme temperature swing, piping systems that reside outdoors, or piping systems with relatively large expansion/contraction. Expansion loops can either be field manufactured or prefabricated, both serving the same relative function.
Field manufactured expansion loops are often manufactured from the same piping material as the piping system they are being installed on, and can be insulated in a similar fashion to that system if insulation is required. There are three general types of manufactured expansion loops; Loop types, offset types, and change of direction types. See the figure below for a diagram of each type.
Figure 2 – Diagrams of the Types of Field Manufactured Expansion Loops
Each type has its advantages and disadvantages in terms of layout and implementation. The loop type expansion loop allows for axial expansion while remaining coaxial to the run of the pipe but requires a space adjacent to the pipe for loop. This can cause coordination issues on busy pipe pathways and trestles. Additionally, loops oriented vertically can cause fluids to pool within the loop or piping system when there is no active flow. Depending on the design of the system and the medium flowing through the pipe, this may not be a desired outcome. Offset type expansion loops can be easier to place within a piping system as they can be strategically placed within parts of the piping system where the pipe needs to change direction twice. A disadvantage to this type of expansion loop is it requires more space and clearance to implement correctly. This type of expansion loop cannot be used for coaxial application (for obvious reasons) and can also experience the same fluid pooling issues depending on orientation. The easiest type of expansion loop to implement is the change of direction type expansion loop. This type requires the most clearance and also requires that the change of direct be free to move to absorb expansion. This type of expansion loop is also the most likely to have issues as unintentional changes to the system design during construction and maintenance can lead to an anchor point that restricts movement and invalidates the expansion loops efficacy. Careful attention needs to be made to how this type of expansion loop is anchored to ensure it works appropriately. Additionally, in piping systems with large amounts of movement, the pipe and associated fittings within the expansion loop may experience heightened fatigue and may fail sooner than other regions of the piping system.
In order to calculate the length of the expansion loop, you need to know the modulus of elasticity at maximum temperature for the pipe material, the outside diameter of the pipe, the working stress at maximum temperature for the pipe material, and the change in length of the piping system. The equation below (provided by Corzan material & piping solutions) can be used to calculate the loop length required for your system parameters.
Figure 3 – Expansion Loop Formula from Corzan material & piping solutions
Prefabricated expansion loops work in a similar manner to field manufactured expansion loops but may require less space to implement. Similar to expansion joints, prefabricated solutions come in a variety of materials and end connections but are most commonly constructed of braided metal and hose with flanged or threaded connections. Prefabricated solutions may require special anchoring techniques and may have a maximum permissible amount of movement. As such, prefabricated solutions may not always be the best solution for plastic piping systems, as the total amount of expansion/contraction may exceed what can be provided by a prefabricated solution.
In the previous section I briefly touched on pipe anchor considerations. Pipe anchoring is an important part of the design of any thermal expansion mitigation. Inappropriate anchoring techniques can result in out of plane expansion and movement, or in worst case scenarios a partial or complete invalidation of the thermal expansion mitigation. Both expansion loops and expansion joints will require a mixture of lateral anchors, axial anchors, and pipe guides to ensure expansion is constrained to the expansion joint/loop. The pipe anchor directly before the expansion joint/loop should be a guide type support, allowing for axial movement and restraining lateral movement. This prevents out of plane movement and snaking, allowing for the pipe to expand towards the mitigation solution. Pipe anchors with both axial and lateral constraints should be strategically placed on the piping system to restrict movement towards the mitigation solution. These types of pipe anchors can also be used to break a long run of pipe into smaller constrained segments that can expand towards a dedicated mitigation solution for that pipe segment. When designing hangers and supports for the pipe system, careful consideration should be taken towards the supports for the system outside the expansion mitigation regions to make sure the pipe is not constrained in a manner that would prevent the expansion mitigation from absorbing the expansion (and associated stresses) within the system.
Beyond the considerations for the type of expansion mitigation to be applied to a plastic piping system, there are several additional system and environmental factors that should be accounted for when determining the overall thermal expansion or contraction. Ambient conditions, temperature of pipe contents, and solar gain all contribute to the total thermal expansion experienced by the piping system.
The ambient conditions surrounding a piping system can drastically affect the overall thermal expansion and contraction of a piping system, even more so in plastic piping systems. The change of temperature for any piping system should be calculated from the difference between the maximum or minimum ambient temperature experienced by to the piping system and the installation temperature of the piping system. It is important to use the ambient temperature and conditions at installation rather than the average temperature of the system for a piping system as it can result in more drastic changes in pipe length. Using the average temperature of the piping system may yield a thermal expansion value that is bilateral in the change of length when expanding and contracting, but this may not be what the piping system is truly experiencing. A pipe installed in the coldest month of the year will exhibit more expansion than a pipe installed in the warmest month of the year, and vice versa. As such, the calculation for determining the overall change in length should look at the largest change in temperature between the installation conditions and the maximum/minimum conditions.
When designing your piping systems, you may need to assume an installation temperature. It is good practice to go back and revise the thermal expansion calculations for the estimated installation temperature as you approach construction of the system. This is more important for systems that are installed outdoors or in outdoor conditions and may require usage of local climate data to make more accurate estimates for the actual thermal expansion. When the installation conditions cannot be easily estimated, it may be best to assume that the piping system will be installed in the warmest or coldest month and perform the calculation for the full ambient temperature change for the system.
The temperature of a plastic piping system’s contents may drastically affect the temperature for the piping system. While under fluid flow, the temperature of the pipe substrate will change the overall temperature of the pipe material. When evaluating the maximum and minimum temperatures of the pipe, you will need to calculate the average pipe temperature based off the ambient conditions, temperature of the substrate, thickness of the pipe, and the thickness of the thermal insulation. It can be assumed that the interior of the pipe material is at the same temperature as the fluid in the pipe, and that the outermost material is at the temperature of the ambient conditions. Given these conditions, you need to determine the temperature of the outer surface of the pipe. Once you know the outer surface temperature and inner surface temperature, you can estimate the average temperature of the pipe material from the two values. The figure below shows a generalize form for heat transfer through a cylindrical wall consisting of several concentric cylindrical layers. If you do not know how much heat will be lost through the pipe system, you may need to take an iterative approach calculating the total heat transfer from the interior of the pipe to the exterior of the pipe, then calculating the surface temperature of the pipe from that heat transfer value.
Figure 4 – Heat Transfer Through Composite Cylindrical Layers
For outdoor piping, some additional considerations are required when determining the total thermal expansion. Similar to how the temperature of the pipe fluid affects the average temperature of the pipe material, solar gain from the sun can affect the average temperature of the pipe material. This should be considered for any pipe that has direct or partial exposure to sunlight, as this exposure will lead to a surface temperature greater than the ambient temperatures around the pipe. There are several ways to estimate the surface temperature of a pipe, but ASTM E1980 is a good means of estimating the surface temperatures. You can use equations (2) and (3) from ASTM E1980 to determine the surface temperature, depending on what information you have access to for the piping system.
Figure 5 – Equation (2) from ASTM E1980
Figure 6 – Equation (3) from ASTM E1980
In the equations above, α is solar absorption (α = 1 – a), a is solar reflectance, ϵ is thermal emissivity, and hc is the convective coefficient for the system. To calculate Tb and Tw, use equation (2) with the following values for solar reflection and thermal emissivity.
Black Surface:
White Surface:
For standard solar and ambient conditions, equation (4) and equation (5) can be used to estimate SRI and then calculate Ts.
Figure 7 – Equation (4) from ASTM E1980
Figure 8 – Equation (5) from ASTM E1980
Once you have calculated the surface temperature of the outer layer of the piping system, you can follow the same procedure laid out previously in the “Temperature of Pipe Contents” section to calculate the adjusted maximum and minimum temperatures of the piping system. You can use these adjusted values to determine more accurate values for the total expansion and contraction of the piping system.
While thermal expansion in pipes is a common problem, there are many techniques to calculate the total thermal expansion/contraction and many techniques to mitigate the effects caused by the change in pipe length. It is important to correctly determine what the pipe material reference temperature, pipe material minimum temperature, and pipe material maximum temperature to appropriately calculate the total thermal expansion or contraction. Once you have considered all the different conditions contributing to the thermal expansion of a plastic piping system, you can select an appropriate mitigation strategy and apply it to your design. As your piping system design evolves and changes, it is necessary to review your thermal expansion calculations and piping system design to ensure that any changes to the system design have not affected the estimated thermal expansion or affected the mitigation strategy.
About the author
Siegfried Boyd Isidro-Cloudas is a Mechanical Engineer working out of Burlington, VT, who joined Hallam-ICS in April of 2022. He graduated from RPI with a B.S. in Mechanical and Electrical Engineering in 2017, and has been working in the field of mechanical MEP design since 2017.
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About Hallam-ICS
Hallam-ICS is an engineering and automation company that designs MEP systems for facilities and plants, engineers control and automation solutions, and ensures safety and regulatory compliance through arc flash studies, commissioning, and validation. Our offices are located in Massachusetts, Connecticut, New York, Vermont and North Carolina Texas, Florida and our projects take us world-wide.