The role of Expansion joint | piping design more free

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Describes expansion joint in piping.

This is a highly important part of piping design and is prone to problems.

It is a strong element behind the scenes.

Although it is a part that is installed somehow, piping trouble may occur due to the lack of this part.

As a mechanical and electrical engineer, if you can properly understand and use them properly, you will be able to improve your level.

Types of expansion joints

First, let’s look at the types of expansion joints.


The basis of an expansion joint is an elbow. One of the fittings.

Elbows, which we use casually, have the role of absorbing the expansion and contraction of piping.

Elbows (Expansion joints)

When a pipe stretches, it usually means that it stretches in the longitudinal direction.

It also extends somewhat in the circumferential direction, but it is within the error range.

In the case of a long and narrow shape like piping, the longer it is, the greater the amount of elongation.

If the pipe does not have an elbow and both ends are fixed, thermal stress may cause the pipe to break.

To avoid this problem, use elbows.

Using an elbow makes it possible to prevent the effect of elongation from being transmitted to the end by deforming the R-bent part even if the pipe is stretched.

We refer to this as absorbing expansion and contraction.

Since many elbows are used in chemical plants, it can be said that they unconsciously absorb expansion and contraction to some extent.

taco bend

A taco bend looks like this: Also called an expansion bend.

Octopus bends (Expansion joints)

It looks like an octopus, so it’s an octopus bend.

A octopus bend is constructed by bending a straight pipe three times.

A convex shape composed of four elbows is also sometimes called a taco bend.

If you see a octopus bend and recognize it as a steam pipe (steam pipe), it is almost OK.

Steam piping is piping that exceeds 150°C, and if it is a long-distance piping in a common stand outside the factory, the effect of elongation will be large.

The temperature difference Δt exceeds 100°C because the temperature is 150°C, which is the usage condition, compared to the normal temperature of 20 to 30°C.

You can calculate the thermal stress by multiplying this by the thermal expansion coefficient and the pipe distance.

In terms of experience, one octopus bend is attached to 100m.


Bellows is also called bellows .

It has the following shape.

Bellows (Expansion joints)

It literally looks like the belly of a snake.

This is also a combination of elbow and octopus bends.

While elbows and octopus bends use metal pipes themselves, bellows are sometimes made of resin instead of metal.

  • The amount of metal expansion and contraction is large
  • Resin The amount of expansion and contraction is small

If expansion and contraction of piping is a particular concern with bellows, use metal bellows made of the same material as the piping.

However, batch-type chemical plants use glass lining pipes and fluororesin lining pipes as highly corrosion-resistant materials.

In this case, the bellows are covered with PTFE.

Flexible Tube

flexible tube is also a type of expansion joint.

Many people recognize it as a version with a longer bellows.

The longer the length, the greater the tolerance for expansion and contraction, but the higher the probability of breakage.

As with bellows, they are divided into single metal ones and PTFE-coated ones.

High temperature and long distance piping

Octopus Bend is famous for pipe expansion joints.

Some of you may notice that out of many pipes on a pipe rack, only some pipes suddenly have strange shapes.

What is going through this?

A hot fluid is flowing. Specifically steam.

Consider passing steam through a pipe with fixed ends.

Thermal stress (Expansion joints)

The steam heats the pipes and causes them to expand.

I want to inflate it, but it’s fixed with a fixed end, so I can’t inflate it.

At this time, a compressive force is applied to the piping to suppress expansion. This is called thermal stress.

It can be calculated from the relationship between coefficient of thermal expansion and volumetric compression.

In this case, the pipe can burst, so an expansion joint is used.

Expansion joints

There are many octopus bends in long-distance piping such as on piping stands . In some cases, an elbow is used to create a pseudo taco bend.

Easy to understand.

The plant’s internal piping probably won’t have room for taco bends.

In this case, you have to create a mechanism that allows only one side to move so that both ends are not fixed.

Expansion joints

In general, the port that draws in steam from outside the plant should be on the fixed side, and the end should be on the free side.

Connection with high vibration equipment

Attach expansion joints to vibration equipment.

Typical examples of vibrating equipment are centrifuges and vibrating fluidized bed dryers.

Flexible tubing is generally used.

Bellows is not a problem.

Since it will shake firmly, it is preferable to use a flexible tube that can take proper measures.

Even with this, if you make a mistake in designing the flexible tube, cracking will normally occur.

It is not necessary for a type such as a conical dryer in which the piping is manually disconnected each time.

In this case, a labor-saving connection method such as a ferrule joint will be adopted.

It’s obvious, isn’t it?

Connection with lining system equipment

Expansion joints are sometimes used when connecting to lining equipment.

  • Between lining pipe and pump
  • Between lining equipment and high-temperature piping

Install bellows between the lining pipe and the pump.

Expansion joints

Typical examples are fluororesin lined pipes and fluororesin lined pumps.

Originally, glass lining equipment was the starting point, but the same goes for fluororesin lining equipment.

In the case of glass, there is a problem of cracking due to the vibration of the pump, and in the case of fluorine resin, the vibration of the pump may add a creep effect.

It is safer to recognize that the lining of both cases is vulnerable to vibration.

In some cases, bellows is installed between the reactor and the heat exchanger.

If the piping is a lining system and the heat exchanger is a lining system, it should be attached.

If you’re worried about it, you can think of attaching a bellows to the gas line.

In the case of a batch-type chemical plant, even if the temperature is normal during operation, distillation may be performed during cleaning, and the gas line will be at a high temperature.

If it’s piping here, but if the equipment side is damaged, the damage will be enormous.

By attaching a bellows, the pipe diameter of the gas line is large, and there is no worry that the pipe load will be applied to the nozzle and cause it to break.

Instead, the piping cut off with bellows must be properly supported.

Not only are the bellows related to the operating conditions of the installation, but so are the supports.

Support is important.

Connection with load cell equipment

Expansion joints are also used to connect load cell equipment.

There are many flexible tubes.

The purpose of this is not to put the pipe load on the load cell.

The load applied to the load cell changes even if there is liquid in the pipes before and after the load cell device.

If you want to strictly control dripping in a chemical reaction, you definitely want to avoid changing the weight value of the tank.

Using a flexible tube is convenient because it can also block the effects of pump vibration.


You may wonder which method is best for expansion joints.

Even if you follow the manufacturer’s advice, be able to consider multiple methods instead of just one.

Related information

Related information

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piping diagram

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liquid expansion


We introduced expansion joints used for piping in chemical plants.

Summarizes the features of elbows, taco bends, bellows, and flexible tubes.

We introduce four cases of use: long-distance high-temperature piping, high-vibration equipment, lining systems, and load cells.

Please feel free to post your worries, questions, and questions about the design, maintenance, and operation of chemical plants in the comments section. (The comment section is at the bottom of this article.)

*I will read all the comments and answer them seriously.