Combination design of Safety valve and Reduce valve

safety reducing
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Explains the basic concept of pressure design related to safety valve/reduce valve .

A safety valve is one of the main safety devices in a chemical plant, and is a very important valve.

If it is a continuous plant with a large amount of single-pressure and high-pressure gas, safety valves should be considered quite seriously.

However, batch plants with relatively loose regulations are relatively loose with respect to safety valves.

After the piping diagram was completed, it was only at the stage of purchasing the necessary materials that I had no choice but to consider it.

This pattern is often seen.

If it’s true, it’s just right to decide the specifications at the same time as designing the equipment.

All you have to do is ask a specialized manufacturer, so the only point is whether or not you notice the request early.

Therefore, we have summarized the basic concepts that you should know at a minimum when talking to manufacturers about safety valves and pressure reducing valves, and the basics about guarantees for the target equipment.

Safety valve/Reduce Valve flow

Regarding the series of safety valves , pressure reducing valves , and vessels , the following flow is targeted.

Safety valve/Reduce Valve
  • Reduce pressure from 0.5 MPa to 0.1 MPa with a pressure reducing valve
  • Safety valve operates at 0.18MPa
  • Vessel withstands up to 0.2MPa

I would design it like this.

Safety Valve

There are three concepts of set pressure for safety valves :

  • start pressure
  • blowing pressure
  • Blow-off pressure

This idea is the same for boiler safety valves.

Images of these pressures and blowout volumes are shown below.

Fukidashi Fukidari (Safety valve/Reduce Valve)

start pressure

The blow start pressure is the “inlet pressure when a very small amount of fluid is detected on the outlet side “.

The minute leak is also subtle.

The starting pressure is specified for gases and liquids .

Steam is not covered .

In batch plants, safety valves are only interested in steam, so surprisingly few people are aware of the pressure at the start of blowing.

blowing pressure

Blow-out pressure is “the inlet pressure when the valve body rises greatly and the fluid blows out vigorously”.

Intended for steam or gas .

It is this pressure that is commonly recognized as the pressure at which safety valves operate.

It is recognized as the same as the set pressure , but it is different.

  • The set pressure is used as a general term for the specifications related to the pressure of the safety valve. It’s a design number.
  • Blow-out pressure is the pressure when the safety valve actually operates.

Blow-out pressure has a tolerance against the set pressure .

For example, for safety valves for steam, the tolerance is ±3% except for boilers .

In the example above, the relationship between set pressure and blowout pressure is as follows.

  • Specification that 0.18MPa is the set pressure
  • Adjust the blowing pressure within the range of 0.175 to 0.185MPa

If the blowing pressure cannot exceed the set pressure, adjust to the minus side such as 0.178 MPa.

Blow-off pressure

The blow-off pressure is “the pressure at which the inlet pressure drops and the valve re-seals after the safety valve operates.”

Similar to blowout pressure, this is the actual operating pressure for each safety valve.

Blow-out pressure > blow-off pressure

There should be a relationship of (blowing pressure) – (blowing stop pressure) is also called fukidari .

  • Downward 0.03MPa
  • Set pressure 0.180MPa

Under these conditions, how much should the blow-stop pressure be?

(Set pressure) – (downward flow) = 0.180 – 0.03 = 0.177 MPa

It looks like it’s going to be.

Note that the set pressure here is a specification.

When blowing pressure is set to 0.175MPa,

Blow-out pressure 0.175 MPa < blow-off pressure 0.177 MPa

That’s a funny thing.

In this case, considering blow-down by the blow-off pressure instead of the set pressure, by setting the blow-off pressure to 0.172 MPa,

Blow-out pressure 0.175MPa > blow-off pressure 0.172MPa

becomes. This is safe, isn’t it ^^

Reduce Valve

The pressure reducing valve should distinguish between the following pressures.

  • Set pressure
  • cut-off pressure

If you thought that a pressure reducing valve can be defined only by the primary and secondary pressures, you are wrong.

Safety valve/Reduce Valve

Set pressure

Set pressure is a generic term for specifications, just like safety valves.

Some users may refer to the downstream pressure as the set pressure.

In the flow example above, the set pressure is 0.10MPa .

I would like to clearly indicate the primary side pressure and secondary side pressure. Such as 0.50/0.10MPa .

cut-off pressure

Shutoff pressure is the pressure that rises on the secondary side when fluid is not flowing through the pressure reducing valve.

Since the pressure reducing valve is open when it is closed, we tend to think that the secondary side pressure will reach the primary side pressure , but this is not the case.

This area is about the mechanism itself of the pressure reducing valve ^ ^

The pressure reducing valve is designed with the concept of shut-off pressure rise .

For example, if the secondary pressure is 0.1 MPa and the cut-off pressure is 0.125 MPa, the cut-off pressure will be 0.225 MPa.

A word of caution here.

Closing pressure of pressure reducing valve > Blowing pressure of safety valve

If it is, it will blow out with a high probability in batch operation.

This is because the object to be decompressed may stop or move.

At a certain timing, the cut-off pressure is reached.

If you only look at the set pressure of the pressure reducing valve, it tends to be overlooked . Note.

Other terms

There are some other terms for pressure reducing valves.

You can leave the technical content to the valve maker, but as a concept, I would like you to know the following two things.

  • Differential pressure: (Primary side pressure) – (Secondary side pressure). Flow-dependent valve resistance.
  • Secondary side pressure adjustment range: Secondary side pressure that can be set with a pressure reducing valve

The pressure reducing valve seems to be designed to specialize in a specific pressure, and the adjustment range of the secondary side pressure is quite suitable.

I think that there are many maintenance personnel who think whether it is possible to adjust the existing pressure reducing valve if they want to adjust the secondary pressure for various reasons.


Here is a list of terms you should know about the pressure of the vessel in which it is used .

The following indicators are used to measure the pressure resistance of containers.

  • Design pressure (maximum working pressure)
  • tightness test pressure
  • hydraulic test pressure

Design pressure (maximum working pressure)

Normally, safety valves and pressure reducing valves are set for the design pressure (maximum working pressure) .

For this example, it looks like this.

Since 0.10 MPa steam is used and there is a 0.18 MPa safety valve, let’s set the design pressure to about 0.20 MPa.

In terms of design,  it tends to be a container → a pressure reducing valve → a safety valve , but there is a trap.

  • Blowout pressure of safety valve > Design pressure of container
  • Closing pressure of pressure reducing valve > Blowing pressure of safety valve

If you ask the valve manufacturer to set the pressure reducing valve and the safety valve as a set, they will adjust it properly.

The problem is that the container side and the design are separated.

At the stage of designing the vessel, ask the valve manufacturer to confirm that there is no mistake in the combination of the safety valve and the pressure reducing valve .

If the standard design pressure of the vessel is determined, safety valves and pressure reducing valves will not cause large deviations.

However, there are cases in which manufacturers of safety valves and pressure reducing valves change their specifications, so it is dangerous to think that once they have been standardized, they will not be reviewed.

Standardization is not everything.

tightness test pressure

It is a good idea to separate the pressure of the airtightness test into legal regulations and internal regulations.

Examples of legal regulations include:

  • High pressure gas: more than design pressure
  • Pressure vessel: when water is not allowed
  • Fire Service Law: 1.25 times the design pressure 

While looking at these regulations, choose how much pressure you want to apply airtightness to piping and equipment.

Your company may have strict regulations.

Most importantly, don’t put too much pressure on yourself .

It’s super important.

Airtightness tests are dangerous .

Low pressure is fine. It’s okay because I’m going to the plumbing on a daily basis.

Such pampering may become a trap.

If the airtightness test fails, the damage is very large.

Even within the PV = constant law, if airtightness fails and it diffuses into the atmosphere, the volume will expand 5 to 10 times.

The purpose of the tightness test is leakage .

It is easy to check for leaks by using gas .

Gases are less dense than liquids and leak easily.

Proof test pressure

As with the airtightness test pressure, it is good to separate the pressure test pressure into legal regulations and internal regulations. Examples of legal regulations include:

  • High pressure gas: 1.5 times the design pressure
  • Pressure vessel: 1.5 times the design pressure
  • Fire Service Law: 1.5 times the design pressure 

While looking at these regulations, choose how much pressure you want to apply airtightness to piping and equipment.

Strictly speaking, the conditions are subdivided.

However, it is quite troublesome in practice.

Even if the pressure is 1.5 times the design pressure, it is permissible because it is a batch plant with low pressure.

You can recognize that the water pressure is 1.5 times the design pressure , but it is dangerous to have a fixed idea that anything other than 1.5 times is absolutely unacceptable.

This is good for young people.

Balance is very important in chemical plant engineering, and extreme swings can be dangerous.

Fundamentals of Pressure Vessel Design

When studying pressure vessel concepts, let’s start with the basics of pressure vessel design.

This book is useful.


I explained the basic concept of setting safety valves, pressure reducing valves, and pressure vessels.

A safety valve has a blow-off pressure, a blow-off pressure, and a set pressure, and a pressure-reducing valve also has a shut-off pressure and a set pressure.

At the stage of deciding the set pressure of the vessel to be used, the specifications of the safety valve and pressure reducing valve must be decided, so it is necessary to pay attention to the order of design.

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.