Pipe fitting | very important for piping design

pipe fitting
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I will explain the types and functions of pipe fitting, which are important in piping design.

Elbows, cheeses, reducers, and other parts are routine parts, but it is essential for mechanical and piping engineers to know the functions and options for each part.

Even if a problem arises when designing faithfully to the basics, it is possible to apply it with just one knowledge of fitting.

I tried to summarize various ingenuity that can be done using fittings in a chemical plant.

Since piping design has a high degree of freedom, let’s understand it from the basics.

Piping knowledge is required for mechanical engineers!

butt weld joints

Fittings are parts used to adjust the diameter and direction of piping.

There are two types of connection methods for straight pipes: screwing and welding, but welding is common in chemical plants.

It’s a butt weld joint.

Threaded joints have a high risk of leakage.

Welding includes socket welding and butt welding , butt welding is common.

Socket welding is used for high-pressure lines, and butt welding is used for low-pressure lines, but in batch-type chemical plants where low-pressure is the majority, butt welding is used.


Let’s explain the types of fittings and how to use them.

Let’s start with the elbow.

The elbow is like the image below.

elbow (pipe fitting)

Elbows are sometimes abbreviated as E and sometimes abbreviated as L.

Elbo means elbow.

Similar to elbows, this is a fitting that allows you to change the angle of the piping arbitrarily.

90° elbow

However, more than 99% of the time I use a 90° elbow.

This is for a batch-type chemical plant with a large number of pipes, so that the pipes can be arranged efficiently.

If the quantity of piping is small, a 90° elbow may be used in limited locations.

Between the method of changing the direction using two 45° elbows and the method of changing the direction using one 90° elbow, using a 90° elbow is cheaper and more efficient as it requires fewer welds.

Even so, there are times when it is not possible to pass the pipe through without using two 45° elbows.

This can be said to be an ingenuity in piping design.

There should be almost no opportunity to use the 180° elbow in the first place.

As a designer, I think using a 180° elbow is a “lose”.

In some cases, it can’t be helped, but my personal preference is not to use a 180° elbow.

45° elbow

I will explain about the 45° elbow.

Although it is limited, it has clear uses.

Avoid interfering piping to a minimum

The first is to avoid interfering piping as much as possible.

At chemical plants, interference with piping is often discovered only after on-site construction begins.

I had to reroute the piping to avoid interference, but doing so with a 90 degree elbow would take up more space than I expected.

If you want to minimize damage, consider a 45 degree elbow.

Specifically, the image is below.

pipe fitting

This is an image of a plan piping diagram.

This is a case where there is a vertical pipe in the middle of a new pipe that is being installed horizontally, and it interferes with the pipe.

We will add an elbow to the newly installed horizontal pipe to avoid the problem, but you will notice that it is much easier to avoid the problem with a 45 ° elbow than with a 90° elbow.

take atmospheric legs

Atmospheric legs refer to “seals that use water injection within the vacuum system.”

It is used to separate mist and fumes during the process while maintaining a vacuum inside the process system.

In batch-type chemical plants, air legs are often used for vacuum systems that use steam ejectors and vacuum systems for drying processes.

pipe fitting

The atmospheric leg connects the process piping and seal water.

If it consists of completely vertical piping as shown in the diagram above, there will be no problem.

In general, there is a horizontal misalignment between the process pipe and the seal pot, so the direction of the pipe must be changed with an elbow.

If you use a 90° elbow , there will be horizontal piping in the middle.

Horizontal piping always collects liquid, which may adversely affect operation.

Therefore, by using a 45° elbow, you can completely avoid sealing the piping due to liquid pooling.

Use a 45° elbow to avoid unnecessary worries.

45 degree pipe fitting

Drop powder and slurry liquid with the head

A 45° elbow is also used when dropping powder or slurry liquid with the head.

The way of assembling the piping and the way of thinking is the same as that of the Atmokikyaku.

Powder and slurry liquids are likely to accumulate in horizontal piping.

Vertical piping should be used whenever possible, but horizontal piping cannot be eliminated.

In that case, the 45° elbow comes into play.

short elbow

There are two types of elbows.

Short elbow and long elbow.

Generally, long elbows are used, so some plant engineers are not aware of the existence of short elbows.

Elbow main dimensions

First, let’s check the main dimensions of the elbow.

elbow (pipe fitting)

The main shape of the elbow is determined by the diameter D and radius F.

Long elbow and short elbow have the same diameter D but different radius F.

Long elbow = short elbow x 1.5

short elbow

If you look at the places where short elbows are used, you can guess that there’s something wrong with it.

The designed piping probably interferes with something.

This is necessary when checking piping diagrams or creating spool diagrams.

If you do not follow the drawings or sketch the site properly before construction, the piping may not be able to pass through.


The biggest risk of short elbows is that they can easily break.

Since the bending radius is large and the direction of bending is forced, you can imagine that it is easy to break.

The thin wall on the outer diameter side tends to be uneven.

That’s why I usually use a long elbow.


The image of cheese is as follows.

cheese (pipe fitting)

It is sometimes expressed as T-tube or T.

Cheese is used to divide the direction of fluid in a pipe into two directions.

Same diameter T

Same diameter T is often used in headers.

Be careful if the main pipe of the header is 50A and the branch pipe is 40A.

Different diameter T (pipe fitting)

It would be better to extend the branch with the same diameter T of 50A and narrow it down to 40A with a reducer.

This is because in batch chemical plants, there are cases where you later decide you want to use the line at 50A.

Different diameter T

Different diameter Ts are not often used in batch chemical plants.

The next step is to use different diameter T-shirts with glass lining or fluororesin lining.

This is because in many cases, the distance between the surfaces does not change whether the diameter T is the same or the diameter T is different.

In the previous example, in the case of main 50A/branch 40A, it is possible to reduce unnecessary reducers by using a different diameter T.

Cost reduction, isn’t it?

Different diameter T may be used to drain the liquid from the piping, but this is an idea.

There may be cases where the punch diameter is set to about 20A for the purpose of cost reduction.

pipe fitting

In this example, which one do you think will reduce costs: the different diameter T on the left or the same diameter T on the right?

It’s a rather delicate story.

This is determined by the sum of “material cost + welding cost + installation cost”.

The smaller the diameter, the better! You can’t make a quick decision.

As an engineer, I want to think about this carefully, as it may change at each company.

In my workplace, the same diameter T is more advantageous.


The image of the reducer is below.

reducer (pipe fitting)

Reducer is sometimes expressed as R.

Reducers are used to change the pipe diameter.

It is rare to use a reducer in the middle of the piping line, and it is often used when the connection nozzle of the equipment and the pipe diameter are different.

They appear at the starting and ending points of piping lines.

A concentric reducer is used when aligning the piping before and after the reducer at the center , and an eccentric reducer is used when aligning the end faces of the piping.

The purpose of using the eccentric reducer is to minimize liquid and gas puddles.

I don’t care about such details, so all reducers are ” concentric “, right?

There are a certain number of people who think so.

The same applies to engineers, piping designers, and construction companies.

This is the part where you can see the philosophy of each company.

In chemical plants , we would like to demonstrate the “specialty” of the eccentric reducer for cleaning chemicals in piping lines and safe operation of equipment.

Around the pump

First of all, I will explain the problem that occurs around the pump.

This is a typical example of the concentricity / eccentricity problem of reducers.


Check the intake type.

The suction type has the following shape.

Around the pump (pipe fitting)

The suction type refers to a shape in which the height of the liquid suction port is lower than that of the pump.

If the surface of the liquid to be sucked is under atmospheric pressure, the piping at the pump suction port will be at negative pressure.

If cavitation occurs, you cannot drive.

The pump suction port often has a pattern in which the pipe diameter and the pump nozzle diameter do not match.

If the pipe diameter and the pump nozzle diameter are different, you must attach a reducer to adjust the diameter.

If an eccentric reducer is used here, gas will accumulate at the top of the pipe if the structure is such that the bottom of the pipe is horizontal.

Accumulation of gas increases pressure loss, induces cavitation, and causes vibration in the pump.

This will reduce the efficiency of pump operation.

For the suction type, install the eccentric reducer so that the top of the pipe is horizontal.

When using a concentric reducer, a gas pool is forced and there is no easy way to avoid it.

It’s a bit of an aside, but the reason the pump nozzle diameter is smaller than the piping diameter is due to the pump manufacturer’s circumstances.

To increase pump efficiency, even at the expense of pressure loss at the pump nozzle.

In order to increase pump efficiency, we want to minimize the nozzle size.


Just like the suction type, check the push type.

Around the pump (pipe fitting)

In the push-in type, the suction liquid level is above the pump.

Since the suction port’s own weight can be used, the possibility of cavitation is low.

In this example, placing the eccentric reducer so that the top of the pipe is horizontal creates a pool inside the pipe.

If the eccentric reducer is installed so that the bottom of the pipe is horizontal, liquid will not accumulate.

However, each time you want to disassemble and inspect the pump, you have to pull out the plug.

This is quite troublesome.

In that case, the surest way is to install a drain valve.

If a drain valve is attached, there is no need to specify the orientation of the eccentric reducer.

pipe fitting

horizontal draining

Let’s take an example of draining horizontal piping.

horizontal pipe fitting

Attach this using an eccentric reducer so that the bottom of the pipe is horizontal.

Using a concentric reducer will create a puddle.

In batch chemical plants, slurry liquids may be sent intermittently.

If the liquid is not drained thoroughly at this point, crystals will accumulate inside the pipe.

After using the pump, drain the liquid.

Horizontal draining is used to minimize liquid puddles and blockages in pipes.

If you are satisfied with just looking at the P&ID, this is the part that tends to be overlooked in the piping diagram.

vertical flow

Finally, let’s look at vertical flow.

vertical flow (pipe fitting)

In the case of vertical flow, there is no difference between concentricity and eccentricity.

Either is fine.

Concentric will be useful in terms of ease of construction.

Concentric is better because it is cheaper and does not cause sudden changes in flow.

cross fittings

It is best not to use cross joints often.

In particular, it is best to avoid welding iron and stainless steel.


The first reason is that cross joints are “homemade.”

Homemade means that it is not sold as a generic product from a manufacturer.

Purchase two cheese joints and make a cross joint.

It looks like the image below.

cross (pipe fitting)

It doesn’t matter, but cross joints are sometimes represented by an “X”.

Cheese joints are “T” and cross joints are “X”. It’s easy to understand.

Well, hand-made cross joints have the following problems.

  • Welding man-hours increase
  • Welding that is not usually done (usually girth welding)
  • Unnecessary material costs

Both are “margin of error” problems.

Still, as a mechanical engineer, I think that cross joints are a loss when it comes to designing piping.

high risk of leakage

The second problem is the high risk of leakage.

Because it is not a manufacturer’s genuine fitting but a handmade one, it tends to increase the risk of leakage.

It is a welding method different from usual girth welding, and there is a risk of failure.

Even if the soundness is confirmed by welding, PT inspection and airtightness inspection,

The risk of leakage increases during the welding stage .

As a chemical plant engineer who wants to minimize leakage, I would like to avoid cross joints as much as possible.

There is a problem with the piping design in the first place

The third is that there is a problem with the piping design in the first place.

  • Why are cross joints necessary?
  • Is this a piping layout that can only be done with cross joints?

Instead of using cross fittings, you should be able to achieve the same function by connecting two tee fittings.

The following cases can be considered as cases in which the use of cross joints is unavoidable.

  • There is not enough piping height to connect two branch pipes.
  • Due to lack of space in the header, we have no choice but to use cross joints instead of tee joints.
  • To minimize the scope of modification of the branched piping, the cheese part is modified to a cross joint.

In a case like this, it means that we are nearing the limit of expansion and renovation of the plant.

It’s not good to overdo it.


Knowledge of piping design is essential for mechanical engineers in chemical plants.

In addition to this site, you can use the following books to further your studies.

created by Rinker
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Related information

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We explained fittings for chemical plant piping.

Elbow cheese reducer cloth.

Elbows are available in 45° and 90°, cheeses are available in the same diameter and different diameters, and reducers are available in concentric and eccentric types.

If you can use them differently, the range of piping design will be greatly expanded.

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.)

*We will read all comments received and respond seriously.