Understanding a Pressure Relief Valve

Discovering the safety device used in most industry for pressure flow control, pressure relief valve it’s called. It’s an equipment that must be found in the industry to control an over-pressurized vessel.

So, a pressure relief valve is a safety device designed to secure a pressurized system during an overpressure occurrence during operation. The system is widely available today as an electronic, pneumatic and hydraulic system. The various types serve the same purpose in different applications and are called pressure relief valve, relief valve or safety valve.

Depending on the type of system, the power source could be electricity or compressed air for operation. Since its purpose is to control the pressure so that life and properties can be safe. A pressure relief valve must be capable to serve for a long period of time and capable of operating at all times. And a professional operator with experience must be in charge to ensure proper working as there is no room for error.

pressure relief valve

Today we’ll be looking at the definition, functions, applications, working, types, components, considerations, benefits and limitations of a pressure relief valve.

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What is a pressure relief valve?

A pressure relief valve or relief valve is a special type of safety valve system used to control or limit the pressure in a system. It can be manually or automatically controlled from a pressurized vessel or piping system. The pressurized fluid or gas is discharged to a reservoir or atmosphere to relieve pressure in excess of the maximum allowable working pressure (MAWP).

The primary purpose of a pressure relief valve is to protect pressure vessels or system from catastrophic failure. Catastrophic failure could be disastrous during an overpressure event, could either be liquid or gaseous.

This device is widely used in petrochemical, petroleum refining, chemical manufacturing industries. Industries where natural gas processing occurs and power generation, as well as water supply industries, also make good use of pressure relief valve. Though it’s generally known as relief valve depending on its field of application it can be called pressure relief valve (PRV), pressure safety valve (PSV), or safety valve. You should note the design and operation of these valves are slightly different.

Working Principle

The working of a relief valve is quite complex but can be easily understood. It consists of ball, poppet or spool opposed by a spring which is placed into a cavity or ported body. A hydraulic system is often used to limit fluid pressure in the part of the circuit they are installed.

The poppet is in the form of a disc or cone shape object that is mounted within an opposite machine seat. If the part is forced closed by spring pressure, very low leakage will be providing. The spool is a cylindrical, machined steel rod with metering grooves or notches that’s also stopped by spring pressure. It leaks more than a poppet valve but offers superior metering effects.

A relief valve is often installed in a hydraulic system after the pump because it provides direct, effective

and rapid response when the device opens to bleed excess fluid into a reservoir. The fluid could be liquid, gas or liquid-gas. This reduces the pressure equal to its spring setting. the device open as the pressure caused by backpressure or downstream is high enough to force the poppet or spool to open against its spring.

In the working of a relief valve, excessive pressurized fluid is provided from an open path to a tank with the purpose of reducing work port pressure. As soon as the fluid pressure begins to rise, the force is applied to the bottom of the spool or poppet. This allows the valve to open modestly at first, bleeding little fluid as required to maintain the downstream pressure. But if the downstream pressure continues to rise, the force acting upon the poppet or spool will be pushing it further towards the spring until the point spring force is balanced by the hydraulic force.

watch video to easily learn how a relief valve works:

The pressure rise is as a result of load pressure combination, backpressure, and energy required to flow through the valve itself. The initial fluid force overcomes the seated force of the spring by the help of the cracking pressure. As the valve flows more fluid to the tank, the rate of pressure rise is safe because the forces of the pressurized fluid counteract the compression rate of the spring. If the valve is nearly fully open, pressure rise increases again as the valve bottom opens due to the flow forces.

As the operation or backpressure decreases, the valve begins to close. This is done at differing rates than the opening. The difference between the opening and closing curve is called its hysteresis, it’s indicative and instruction of the quality of its construction. This is because higher quality valves with advanced construction allow lower pressure rise with better hysteresis.

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Types of Pressure Relief Valve

There are three basic types of pressure-relief valves such as;

Conventional Spring Loaded:

the conventional spring-loaded types of relief valve that contained the bonnet, spring, and guide in the released fluids. Its relief pressure backpressure decreases the set pressure if the bonnet is vented to the atmosphere. But, if the bonnet is vented internally to the outlet, the relief-system backpressure increases the set pressure.

These types of pressure relief valve are used in noncorrosive services and where backpressure is not up to 10% of the set point.

Balanced Spring Loaded:

The design of a balanced spring-loaded is to protect the bonnet spring, and guide from the released fluids and reduce the effects of backpressure. Its disk area is vented to the atmosphere and another disk is exposed to backpressure. The vented disk is equal to the exposed disk.

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Balanced spring-loaded is used in corrosive and dirty service and with variable backpressure.

Pilot Operated:

The pilot-operated valve is controlled by an auxiliary pressure pilot. The resistance force on the piston in the main valve is achieved by the pressure during the operation through an orifice. The net seating force on the piston actually rises as the process pressure nears the set point.

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Various types of relief valve with a different design:

Safety Valve:

a safety valve is a pressure relief valve that works by inlet static pressure and design to rapidly open with a pop action. It is widely used for air and stream services. A safety valve is available in two forms:

  • Low-lift safety valve: in a low-lift safety valve, the disc lifts automatically in a way that the disc position determines the actual discharge area.
  • Full-lift safety valve: in this safety valve the disc is also automatically lifted but the actual charge area is not determined by the position of the disc.

Relief Valve:

A relief valve is a pressure relief device design with an inlet static pressure for its working. It has a gradual lift generally proportional to the increase in pressure over opening pressure. It’s suitable for close discharge system as it is enclosed in a spring housing.

Safety Relief Valve:

A safety relief valve is a pressure relief valve characterized by its rapid opening or pop action. Also known for its opening in proportion to the increase in pressure over the opening pressure base on its application. Safety relief valves are used either for liquid or compressible fluid. The common type is the conventional and balanced safety relief valve which were earlier explained.

Power-Actuated Pressure Relief Valve:

A power actuated pressure relief valve contained a relieving device that is joined with and controlled by a device requiring an external source of energy.

Temperature-Actuated Pressure Relief Valve:

A temperature actuated pressure relief valve may be actuated by external internal temperature or by pressure on the inlet side.

Vacuum Relief Valve:

A vacuum relief valve is a pressure relief device that admits fluid to prevent an excessive internal vacuum. It’s designed to reclose so as to prevent further flow of fluid after normal conditions have been restored.

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Components of a relief valve

components of a pressure relief valve

Things to Consider Before Selecting a Pressure Relief Valve

Below are things an engineer or professional in this field consider while selecting a relief valve:

  • Design Considerations which include the size, material, product, complexity, etc.
  • The individual relief load should be determined.
  • Causes of overpressure.
  • Various types of relief devices and their characteristics should consider seeing the best option.
  • Selecting appropriate relief devices to handle the imposed loads. Several components must be put into consideration. Parts like set pressure, backpressure, dual relief valve, multiple relief valves must suit the capacity of the relieving device.
  • Installation considerations
  • Inlet piping and discharge piping
  • Reactive forces
  • Tailpipe considerations
  • Rapid Cycling
  • Resonant chatter which occurs when the inlet piping produces excessive loss at the valve inlet.
  • Liquid discharge considerations.

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In conclusion, a pressure relief valve is a safety device used in the industrial world to save lives and properties. We saw the various types and components of pressure relief valve, how it works and also selection consideration.

I hope you enjoyed the reading, if so, kindly comment, share, and recommend the site to other technical students. Thanks!

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