Piping Flexibility – Thermal Expansion in Piping

One major requirement in piping design is to provide adequate flexibility for absorbing the thermal expansion of the pipe. However, due to lack of quick method of checking, pipings are often laid-out to be either too stiff or too flexible. In either case, valuable time and material are wasted.

This article presents some of the quick methods for checking piping flexibility. These methods include visual, hand calculation, and micro computer approaches. They are all quick and easy for designers to use in planning their layouts. Once the designers have taken care of the flexibility problem, the iterative procedure between the stress engineers and the designers become simpler. The project schedule can also be improved.

Piping Flexibility Requirement

Piping is used to convey a certain amount of fluid from one equipment to another. It is obvious that the shortest straight path for the pipe seems to be most economical and viable in the first sense. There can be many reasons;

• Shorter the pipe, lesser the capital expenditure required in procurement, welding and erection.
• Shorter the pipe, lesser will be the pressure drop making it more suitable for the proper operation.
• Shorter the pipe, lesser will be the number of supports required to support the pipe.

Still, as a piping engineer, we hardly see pipe routing following shortest straight path. Why ??

The biggest reason for that is that the direct shortest layout generally is not acceptable for absorbing the thermal expansion.

As the pipe temperature changes from the installation / ambient condition to the operating / design condition, it expands or contracts depending upon the difference between installation and operating temperature. In the general term, both expansion and contraction are called thermal expansion.

When a straight pipe connected end to end with equipment’s expands, it has the potential of generating enormous force and stress in the piping system. However, if the pipe routing is flexible enough, the expansion can be absorbed without creating undue force or stress.  Let us understand this with the help of an example.

Figure 1 shows what will happen when a straight pipe directly connected from one point to another is subjected to change in temperature. First, consider that only one end is connected and the other end is loose. The loose end will expands an amount equal to  ΔL = α L ΔT

where,

ΔL = change in length or thermal expansion (in)

α = linear expansion coefficient (K¯¹)

L = original length of pipe (in)

ΔT = change in temperature (K)

Figure 1

However, since the other end is not loose, this expansion is to be absorbed by the piping. This is equivalent to squeezing the pipe to move the free end back an ΔL distance. This amount of squeezing creates a stress of the magnitude  S = E (ΔL/L) and the force required to squeeze this amount is  F = A S

For checking the magnitude of such stress and force, lets take a real life example. Consider a pipe of standard wall thickness with,

Material =  ASTM A53

outer diameter (O.D) = 6 in

L = 100 ft = 1200 in

T1 =70 F (Installation), T2 = 270 F (Operating)

α = 6.33 x 10^-6 in/in-°F

E = 27.5 x 10⁶ lbf/in²

Then, ΔL = (6.33 x1 0^-6 in/in-°F)(1200 in)(270°F-70°F) = 1.52 in

F = AEα (ΔT) = (5.581 in²)(27.5 x 10⁶ lb_f/in²)(6.33 x 10^-6 in/in-°F)(270°F-70°F) = 194,315 lbf

Now, one can imagine the magnitude of force produced in pipe following shortest straight path. The result will likely be failed anchors, a buckled pipe or both. If the pipe routing is flexible enough, the stresses will remain well below the yield point of the steel. It is clear that the straight line direct layout is not acceptable to most of the piping and flexibility has to be provided.

Pipe’s Natural Flexibility

Providing the proper flexibility is one of the major tasks in the design of piping system. Piping flexibility are provided in many different ways. The simplest method is to take advantage of the pipe’s natural flexibility.

Pipes bend, even under their own weight. The longer the pipe, the easier it is to bend. If a pipe is bent within its elastic limit (no permanent deformation), it will behave like a spring and return to its original shape after the load is removed. If the elbows and anchors on a pipe system are arranged to allow free movement of pipe under effect of thermal expansion, the forces will be much less than a straight run. Figure 2 shows how thermal expansion of horizontal pipe leg is accommodated in the deflected shape of vertical pipe.

Figure 2

The anchor loads and stresses are much less than in the straight pipe case,but there are some restraints for this approach.

• This layout introduces moment (torque) loads on the anchors.
• The pipes also move in one direction, which may not be acceptable due to space constraints.
• Geometry can affect this arrangement. If thermal expansion accommodating leg is shorter, the forces and moments will be higher.

Expansion Loop and Expansion Joint

This pipe’s natural flexibility may or may not be sufficient depending on the individual cases. Additional flexibility can be provided by adding expansion loops or expansion joints. In the straight line example discussed above, the stress can be reduced by loops installed as shown below. The idea is to provide some pipe perpendicular to the direction of expansion. In this way when the pipe expands it bends the loop leg first before transmitting any load to the anchor. The longer the loop leg the lesser the force will be created.

 

But expansion loop also have some limitations.

• Require more space to accommodate the loop.
• Routing length increases. This results in excess material procurement (pipe and elbows) and more pressure drop.
• Difficult arrangement where free drain requirement is there.

In such cases the better method is to use expansion joint. Expansion joints are more sophisticated than the pipe loops which are just extra lengths of the same piping. For this and other reasons, engineers tend to favor piping loops over expansion joints.

However, expansion joints can be used effectively in many applications when they are properly designed. One of the major requirements in the design of expansion joint system is to install sufficient restraints for maintaining the stability.

Advantages of an “Expansion Joint” versus a “Expansion Loop”

• Space is inadequate for a pipe expansion loop with sufficient flexibility.
• A minimum pressure drop throughout the pipe line is required and the absence of flow turbulence from the elbows and piping is required by process flow conditions.
• The fluid is abrasive and flows at a very high velocity.
• There is no adequate support structure to support the size, shape, and weight of a pipe loop.
• The pipe loop is impractical as in an application of low pressure or large diameter.
• Construction schedule does not allow for the man-hours required to install the pipe loop and the piping loop support structure.
• In most cases it is more economical to use an expansion joint instead of pipe loops.