Vent line | rules for correctly design at chemical plant

vent line
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I will explain the design points of the vent line of a batch chemical plant.

They tend to be confused with gas lines, but they are handled slightly differently.

In terms of piping design, it can be managed somehow even if it is neglected, but it is very important in terms of safe driving management.

Failure to do so may lead to disasters such as abnormal pressure rise and explosion.

As a part of piping design, it is likely to be neglected because there are not many, but I would like to be conscious of it when designing.

The vent line is a safety design point

  • Make the diameter of the vent line larger than the receiving line
  • The shortest piping distance to the open port
  • Keep the opening away from the manufacturing site

What is a vent line?

First, let’s briefly explain the definition of the vent line.

Vent lines in batch chemical plants refer to the following locations.

vent line

Vent is one of the terms that has become famous in nuclear power.

An emergency release of pressure will be called a vent.

If we apply this image to a chemical plant, the release port of the safety valve is exactly the vent.

However, among the gas lines of chemical plant reactors, tanks, heat exchangers, etc., the lines that are open to the atmosphere are sometimes called vent lines.

A vent in the sense that it is a line that keeps the device from holding pressure.

This line is extremely important for safe driving.

Here are some things to think about when designing your vents.


The vent line caliber design is generalized.

Annex B of JIS B 8501 Structure of Steel Oil Storage Tanks (All Welded) has a standard, “Standards for determining the set pressure of the air valve and the capacity of the air valve and the air vent”.

This is the base.

It’s OK if you don’t think too hard.

Basically, it is OK just to make the diameter of the pipe that goes into the tank or the pipe that comes out of the tank, whichever is larger, larger.

For example, if the pipe entering the tank is 40A and the pipe leaving the tank is 50A , the vent line should be 50A or more.

Although it is a standard that considers volume change due to temperature change, at least batch chemical plants do not consider temperature change.


The material of the vent line is where the thoughts are quite divided.

Batch-type chemical plants generally have the following three cases.

  • SUS304
  • FRP
  • Fluororesin lining

There is a pattern of using SUS304 for cost purposes, but I think FRP is more common.

Considering versatility, fluororesin lining seems to be better, but it is not used much because the piping is heavy and it is difficult to fine-tune.

Flange standard

JIS 10k is fine for the flange standard of the vent line if there is no particular preference.

Normally, it is used only under slight pressure to slight pressure, so a flange thickness equivalent to 5k is sufficient.

However, if you use different flange standards, you may end up in an unexpected pitfall, so be careful.

  • The equipment nozzle was JIS 10k, but the piping flange was changed to JIS 5k and it did not fit.
  • When the vent line was designed with JIS10k, the existing vent line was actually JIS5k.

Increasing the number of choices causes confusion.

In piping design, where volume is the enemy , it is important to keep things simple.


We summarized the design points for the location of the vent line.

Leave the exit away from the mill

It is basic to keep the outlet of the vent line away from the manufacturing site.

It looks like the image below.

Outside the factory (vent line)

The idea is that it is OK if the exit of the vent line protrudes even a little from the hazardous materials manufacturing site.

This comes from the idea of ​​preventing dangerous goods from staying in a specific place.

exit on the roof inside the railing

Vent line exits are increasingly being installed inside handrails on rooftops.

This is a different way of thinking about the retention of dangerous substances.

away from the factoryDisperse the leaked gas in the atmosphere
installed on the roofCatch the leaked liquid inside the factory

In the case where the vent line exit is away from the manufacturing site, there is an implicit understanding that what comes out of the vent exit is gas.

Hazardous substance Class 4 gases have a lighter specific gravity than air, so once they are released into the atmosphere, they will diffuse downward.

The separation from the factory here makes it easier to spread with the wind in the atmosphere.

However, in the case of separating from the manufacturing site, the existence of mist is ignored.

The mist contained in the gas exists in a very small amount.

This mist will fall as it is if you do poorly.

Falling mist is like light rain.

But even if it’s raining lightly, it’s scary if dangerous things fall from above.

The means to solve this is to install the vent inside the handrail on the roof.

Since it is located on the roof, it is easily diffused by the wind , and even if the mist falls, it can be caught on the roof.

The mist that falls on the roof is gathered in the manufacturing plant’s reservoir along with rain and other rain.

If it is strangely separated from the factory, the mist will remain on the ground or permeate the soil, causing environmental deterioration.

suck the exit

There are not many cases where the outlet of the vent line is left open to the atmosphere.

In practice, the centralized abatement is used to enclose the exit of the vent line.

There are many cases where the piping is opened immediately outside the factory and the piping is immediately returned to the factory.

vent line

In this case, it is likely that the flame arrestor will be installed at the outlet of the vent line itself.

The flame arrester contains a mesh that prevents the spread of fire.

But it can also cause clogs.

Regular cleaning inspection is required.

As a result, there is a need for a vent line exit at a location where the flame arrestor can be removed.

If you move away from the factory, you will be working with your body leaning over the handrail, which is a little dangerous.

Rather than that, it is a design that is easier to attach to the inside of the roof railing.

Precautions for piping

This is a summary of points to note in vent line piping design.

Keep the vent line short

Pump up the vent line as short as possible.

Here’s why

  • If the vent line is long, the pressure loss will increase, and it is dangerous to have pressure inside the equipment.
  • Gas flows through the vent line, so the diameter is large and the cost is high.

It’s a matter of course when you say it, but when you draw up and check P&IDs and piping diagrams, there are many cases where ideas are not reflected.

shortest line (vertical) (vent line)

avoid headers

Avoid vent line headers as much as possible.

Even if a header is used, the vent line should be connected to the main line, and the lines for other purposes should be connected to the sub line.

This is the concept of “pressure loss” .

The more bent the header, the greater the pressure loss.

In the case of the rooftop, you can open it immediately, so it’s not much of a problem.

Shortest in horizontal direction

The horizontal route of the vent line should also be minimized.

There are designers who have a mysterious idea of ​​detouring because there are existing pipes and obstacles , but let’s have an iron will there (laughs)

shortest line (horizontal) (vent line)

Adding pressure loss in the vent line is a problem for safe operation.

It should be designed with a higher priority than other piping and equipment.

not too low

When lowering the vent line close to the floor, be careful not to lower it too much.

Do not lower too much (vent line)

This actually goes against the idea of ​​”minimizing the vent line”.

If you really want to minimize the vent line, you have to open it directly above the equipment.

This makes maintenance difficult, so we often receive requests to have the vent line directly connected to the floor.

This is acceptable without any major problems, but if it is lowered too much, unexpected problems will occur.

A typical example is a vent line in a dike

Drainage ditches are often dug in dikes.

It is natural for the designer to want the outlet of the vent line to be pulled directly above the drain so that the liquid in the vent line can be drained properly.

Some designs even dip the vent line into the drain.

This is dangerous.

Without considering the drainage capacity of the drain, it is very possible that the drain will be flooded when it rains heavily.

Attaching the outlet of the vent line into the drain will complete the “water seal”.

Even though a vent line is installed to keep the equipment at atmospheric pressure, if the equipment is pressurized due to the presence of a water seal, it will act in a dangerous direction.

If it is an atmospheric pressure tank, it is a very dangerous state that can cause the tank to deform greatly or even be destroyed.

In the past, there were many tanks that were destroyed by this. . . Scary, isn’t it?

make an inspection hole

The vent line may have an inspection port.

The image is as follows.

inspection port (vent line)

I think it’s a special case to some extent, but there are many cases where it is used unexpectedly.

  • for dryers that handle powder
  • where salt is released in the reaction
  • solidify when cooled or crystallize

For example, the precipitation of crystals can also occur with caustic soda.

You have to be more careful than general-purpose liquids.

It tends to be overlooked in batch-type chemical plants.


It is difficult to record individual thoughts in piping design, but the impact of making a mistake is very large.

In particular, the vent line is closely related to the safety of the plant itself, so let’s have a firm design concept.

Of course, it is also important to study with books such as the following.

created by Rinker
¥3,850(2023/09/27 23:53:29時点 Amazon調べ-詳細)

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I explained what to think about when designing vent lines for chemical plants.

Diameter/Material/Flange Standard/Position

I want the exit to be inside the handrail on the roof of the factory.

You can also inspect and clean the flame arrester, killing two birds with one stone.

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.