Valves

What is a pressure relief valve?

What is a safety valve and relief valve?

We can see safety and relief valves in many industrial applications in order to protect the equipment from overpressure. These valves are either used as a safety valve or a relief valve according to the applications. By using a safety valve, we could ensure the safety of a process. These valves would act as a protective device in order to protect a vessel or a process system in case of overpressure. These valves would open for accidents like overpressure and it would reduce the overpressure and after that, the valve would reset. These valves would be useful in case of power failure, during a power failure the system controls won’t be functional, at this time these valves could come in handy. The major input of a pressure relief valve is the process fluid.

Types of pressure relief valve

Pressure relief valve

This is a spring-loaded valve that would open in case there is an excess pressure and it would reclose while the normal pressure is restored. So if the pressure increases more than the set pressure then this valve would open. These valves can be used for compressible or for incompressible fluids.

What are the major types of pressure relief valves?

Safety valve

This is a type of pressure relief valve and it would be actuated by the inlet static pressure and it has a quick opening or pop action. These valves are mostly used for compressible gas, steam, and air.

Applications

It is used in refineries, power plants, and also in heat exchanger systems

Relief valve

In this valve, actuation would also happen due to the inlet static pressure and it would open gradually. The valve opening would be proportional to the increase in pressure over the set pressure. These valves are mostly used in liquid systems mostly for lower capacities and also for thermal expansion duty, these valves are also used in pumped systems.

Applications

It is used in oil and gas industries, petrochemical, and also in power generation using liquid or gas.

Safety relief valves

This type of pressure relief valve could either have a gradual opening according to the increase in pressure or it could have a quick opening/pop opening characteristics depending on the application in which it is used.

So this valve would act as a safety valve when used with the incompressible gas system and it would act as a relief valve when used with the liquid systems.

Applications

Chemical industry, power generation which utilizes steam or water and also in oil and gas industries.

What are the major parts of the pressure relief valve?

Parts of pressure relief valve

Guide

This part is used to control the lateral movement of the disc or the disc holder.

Disc

This would prevent the flow through the nozzle under normal operating conditions.

Spindle

This part is connected to the spring in the valve and this would transmit the motion. The axial orientation of this part is parallel to the travel of the disc.

Adjusting ring

This can be used to control the pressure build-up in the chamber, this part is assembled to the nozzle and it is used to control the opening characteristic and the reset pressure.

Adjusting ring pin

The load-carrying element of the adjusting ring

Base

This would contain the operating element.

Nozzle

This would allow the media to move into the valve chamber. This is the primary pressure-containing component.

Primary orifice and Secondary annular orifice

This would create the flow restriction and it would create pressure which is required for the operation of the valve.

What are the major difference between, safety valves and relief valves?

The relief valve would only operate or open when it is required to relieve the overpressure conditions. These valves are mostly used for overpressure conditions and it is used with non-compressible fluids. A safety valve is also a type of pressure relief valve with additional safety features. A safety valve would rapidly open in case the set pressure is reached and it would stay open till the pressure is lowered below the reset pressure. Reset pressure will be lower than the set pressure.

Relief valves are used with incompressible fluids such as water and oil, while the safety valves are used with compressible fluids such as gases or steam. A safety valve has a lever on top of the valve body and it can be used in case of emergency. So the major difference between these valves would be the setpoint and the capacity. The relief valve would relieve the pressure during an over-pressurized condition, there could be an operator to operate the relief valve according to the received control signal. The safety valve would relieve the pressure all by itself without the help of an operator and these valves are capable to relieve more than the energy input to the volume being protected.

Pressure safety valvePressure relief valve
It can be used for compressible fluid systemIt can be used for compressible and non-compressible fluid system
These valves are fully opened after their set pressure is reachedThe opening is directly proportional to the pressure rise
Sudden valve openingGradual opening
It could vent out to the atmosphereThe vent could be connected to the upstream of the equipment
Automatic operationOperator is required
This valve would actuate automatically to reduce the excess pressureThis valve would reduce the system pressure within a specified set level
Mostly used in gas serviceIt can be used for liquid or gas service
Difference between a safety valve and relief valve

How does a pressure relief valve operate?

Pressure relief valve operation
Pressure relief valve

These valves are operated by the inlet system pressure to surpass a spring load and based on that the valve could relieve a defined capacity. This valve will be in closed condition during normal pressure. The spring force would resist the vessel pressure that acts against the seating surface. When the vessel pressure increases then the pressure at point A would be equalized to the spring force.

Pressure relief valve

When the vessel pressure comes near to the set pressure then the media would move to the chamber. The secondary annular orifice would create flow restriction and this would create pressure over a large area and this force would be utilized to overcome the spring force. We can vary the opening in the secondary, annular orifice by using the adjusting ring, and thus we can control the pressure build-up in the chamber. This pressure in the chamber would overcome the spring force and thus the disc would move away from the nozzle valve would open.

Opening of pressure relief valve

What are the major types of the safety relief valve?

Conventional valve

Conventional valve

In this type of pressure relief valve, the spring housing is vented to the discharge side of the valve. The operational characteristics of this valve like the opening and closing pressure and relieving capacity are directly affected by the changes in the backpressure of the valve. These valves are actuated by the static pressure upstream of the valve. The valve opening would be proportional to the pressure increase over the opening pressure.

Advantages

  • These valves are very reliable
  • It can be utilized for many services

Disadvantages

  • Relieving pressure is affected by back pressure
  • These valves are susceptible to chatter in case the build-up backpressure is really high

Applications

These valves are used in systems where the discharge is really simple, these valves are used when the back pressure is less than ten percent of the set pressure.

Balanced valve

This is a spring-loaded pressure relief valve and it utilizes bellows or other means to reduce the effect of back pressure on the operational characteristics of the valve. In this type, a bellow is attached to the disc holder with an effective bellow area equal to or greater than that of the nozzle area. This would isolate the top part and thus remove the back pressure effect.

Balanced valve

Advantages

  • Relieving pressure is not affected by back pressure
  • Capable to handle high built-up backpressure
  • Spring would be protected from corrosion

Disadvantages

  • The rupture could happen in due course of time

Applications

  • It can be used with complex discharge systems, like common discharge manifolds and flares

Pilot operated valve

Pilot operated valve

In this type of pressure relief valve, the major relieving device or the main valve is combined with and controlled by a self-actuating auxiliary pressure relief valve(pilot). This is an automatic operating valve and it would relieve by the remote command from a pilot on which the static pressure is connected. This type of valve doesn’t use any external source of energy and it is balanced when the auxiliary pressure relief valve is vented to the atmosphere

Advantages

  • Relieving pressure is not affected by the backpressure
  • It can operate up to 98% of the set pressure
  • Less susceptible to chatter

Disadvantages

  • Susceptible to plugging
  • Chances of backflow are high
  • Can’t be used for chemical or for high-temperature applications

Power actuated pressure relief valve

In this type of pressure relief valve, the major relieving device is combined with and controlled by a device that requires an external source of energy. The valve movement to fully open or close is done by a power source such as electricity, water, or steam. These valves could discharge to the atmosphere or to a container that is at lower pressure. Discharge capacity could be affected by the downstream pressure.

Applications

  • These valves are mostly used for forced flow steam generators with no fixed steam or waterline
  • These valves are also used in nuclear power plant

Temperature actuated pressure relief valve

This type of pressure relief valve could be actuated by the external or the internal temperature or by the pressure on the inlet side. These valves can be considered as dual-purpose valves, this valve would prevent the temperature within a vessel from rising above a specified limit and it would also prevent the rise in pressure in the vessel.

Advantages

  • It can be used to maintain the pressure and temperature in a system

Applications

  • These valves are mostly used for portable water heaters and heating systems

How to install a pressure relief valve?

The pressure relief valve for the vapor application must be connected to the vessel in the vapor space above any contained liquid or to piping which is connected to the vapor space in the vessel which is to be protected. The area of the inlet piping must be equal to the area of the valve inlet. These valves should be installed after checking the set pressure, blowdown, and seat tightness.

The pressure relief valve for the liquid service application must be connected below the normal liquid level. There shouldn’t be any intervening stop valves between the vessel and the pressure relief valve.

How to select a safety valve or relief valve?

These are the criteria that are needed for a pressure relief valve

  • Fluid phase, state, capacity, viscosity at relief temperature
  • Maximum operating pressure
  • Set pressure
  • Maximum operating temperature and relief temperature
  • Overpressure in the percentage of set pressure
  • Maximum relief pressure

Maximum back pressure is seen by relief valve while discharging

How to do the inspection and maintenance of a pressure relief valve?

  • The exit of the relief valve should stay unobstructed
  • We must do the visual inspection of the relief valve and the discharge piping every six months
  • Before conducting the maintenance we should shut down the gas supply, drain gas  from the relief valve, and the system
  • Atmospheric discharge line should have proper rain and moisture protection
  • We should not use a relief valve for more than five years
  • These valves should not discharge during the installation or start-up
  • Make sure there is no trapped ice buildup between the valve and the pipeline
  • The inlet pressure must be reduced to zero before installing or in the case of replacement
  • Check for corrosion and leaks

Why should we install a pressure relief valve?

By installing a safety or pressure relief valve we could make sure the protection of the people, property, and the environment in case of a blowout from sabotage or human error, collision, equipment failure, fire, or in case of leaks.

Operating pressure

It is the gauge pressure to which the equipment is normally subjected in service.

Set pressure

This is the inlet pressure in which the pressure relief valve is set to open while it is in the service. So when the system pressure reaches this valve then the PRV would open.

Overpressure/Accumulation

This is the pressure increase over the set pressure of the relieving device during discharge and it is expressed as a percentage of set pressure

Relieving pressure

It is the pressure that is obtained by adding the set pressure with the overpressure/accumulation.

Built-up back pressure

This is the increase in pressure at the outlet of the pressure relief device, which is formed as a result of flow after the pressure relief device opens.

Backpressure

This is the pressure that is in the discharge side of the pressure relief valve, total back pressure would be the sum of superimposed and built-up backpressure.

What are the major relief events where the pressure relief valve is required to be operated?

  • External fire
  • Flow from a high-pressure source
  • Heat inputs from associated equipment
  • Ambient heat transfer
  • Control failure
  • Chemical reaction
  • Tube rupture
  • Liquid expansion in pipes and surge
  • Line leaks

Where should we mount a pressure relief valve?

  • Blocked in sections of cool liquid lines which is exposed to heat
  • Vessel steam jackets
  • The discharge side of the positive displacement pumps, compressors, and turbines

What are the different types of testing which is done for a pressure relief valve?

Bench testing

This type of testing is done by using an external pressure source, with or without an auxiliary lift device to determine some or all of its operating characteristics.

Flow capacity testing

By doing this test we could determine the operational characteristics, including the measured relieving capacity.

In-service testing

Testing of pressure relief valve which is installed on and protecting a system using system pressure, or by an external pressure source. It would be done with or without an auxiliary lift device to determine the operational characteristics.

In-place testing

Testing of the pressure relief valve which is installed on but not protecting a system using an external pressure source with or without an auxiliary lift device to determine some or all of its operating characteristics.

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