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What is a Pressure Relief Valve? Pressure Relief Valve Explained

Pressure Relief Valve

In liquid and gas systems, pressure overloads can cause damage. A pressure relief valve helps prevent that, making it an essential fitting in the piping systems of waterworks, boilers, compressors, and similar equipment. Join us as we delve into the world of these valves, explaining everything about them.

What is a Pressure Relief Valve?

A pressure relief valve (PRV) is a valve that, as its name suggests, releases excess pressure in flow systems to prevent damage to components. In other words, it helps maintain the system pressure within safe levels.

You can also call the PRV a pressure-release valve, pressure-reducing valve, or simply a relief valve. However, it’s not to be confused with a safety relief valve, which has a slightly different design and operates differently.

Pressure Relief Valve Function

Pressure-reducing valves allow fluid to escape in the event of pressure buildup, thereby preventing an overload and saving various system parts from damage.

It can reduce pressure downstream, for example, or upstream, depending on the type of system and the required protection level.

These devices find use in water supply lines, irrigation systems, air compressors, and boilers. They discharge media during a pressure rise, protecting system parts from getting damaged.

Pressure relief valve components
Pressure relief valve components
Resource: https://youtu.be/vfz_CM0ygs4?

Pressure Relief Valve Parts

A PRV comprises a flow-control component, a pressure-sensing element connected to a spring, and the valve body or housing. Let’s take a brief look at the function of each part.

Flow Control Mechanism

The flow control mechanism is the section that opens to release fluid pressure and closes when the pressure drops to normal levels. In most cases, it’s a poppet or ball.

Pressure Sensing Element

The pressure-sensing element is the part that detects the flow pressure and responds by moving to open the valve. It can be a piston or diaphragm.

Valve Body

The valve body encloses all parts of this valve. Depending on the required performance, such as impact or pressure and corrosion resistance, it can be a brass, aluminum, stainless steel, or plastic housing.

The pressure relief valve working principle illustrated
The pressure relief valve working principle illustrated
Resource: https://youtu.be/PkQAyF4bmqY?

How Does a Pressure Relief Valve Work?

The pressure relief valve operation depends on the force exerted by a fluid under pressure. It has a pressure-sensing mechanism that opens it when pressure exceeds a preset value, allowing it to vent media through an auxiliary opening.

  • The valve is closed when pressure levels are within safe levels, and the valve element (a disc or poppet) presses against its seat by the force of a spring.
  • If the pressure rises above a set limit (cracking pressure), the valve starts to open, venting out or discharging media through a port and out of the system.
  • A further increase in pressure causes the valve to continue opening until the maximum pressure limit.
  • A drop in pressure results in the valve closing; the force of the spring is now higher than that of the system.

A PRV can directly use flow pressure to open or rely on pilot pressure to perform the same function.

Direct Acting Pressure Relief Valve

The direct-acting relief valve directly relies on the system pressure to work. It opens when pressure rises to excess levels and closes when it drops to safe limits.

Controlled Pressure Relief Valve

A controlled relief valve has its operation controlled by another valve. The other valve is either a direct-acting type that connects to it through a hose or an electric PRV that uses a solenoid valve.

Direct-acting pressure release valve
Direct-acting pressure release valve
Resource: https://www.youtube.com/watch?v=VpNf6NWTOL4

Pressure Relief Valve Types

Three variations of pressure-reducing valves are available today, classified by their designs and operation methods. They’re direct-acting, pilot–operated, and solenoid-based PRVs. Let’s see more about these types of these valves below.

Direct Acting Pressure Relief Valve

A direct-acting type consists of inlet and outlet ports, the on/off component, and a mechanical spring. The on/off element allows the valve to open or close under the action of pressure and can be a poppet, ball, or diaphragm.

On the other hand, the spring connects to the flow control component and is usually either adjustable or non-adjustable. Direct-acting valves with adjustable springs can be tuned to respond to specific pressure levels.

Pilot Operated Pressure Relief Valve

The pilot-operated PRV has two poppets, the main and the pilot poppet. A soft spring supports the main poppet, while the pilot poppet has a stiff spring.

In the closed state, the pressure exerted by media on both poppets is similar. During a pressure load, the pilot poppet opens to discharge media, reducing the pressure level above the main poppet and causing it to open.

That way, the pilot valve controls the operation of the main valve or poppet. These PRV types are best for larger systems than the direct-acting valves.

Solenoid Operated Pressure Relief Valve

A solenoid operates this type of pressure release valve, opening and closing it to meet set pressure levels for the media flowing through it.

A solenoid-operated PRV offers the advantage of precision when releasing excess pressure. It also makes cycling through different pressure levels using multiple valves easier.

Another advantage of the solenoid-operated relief valve is its ability to support remote operation. As such, it’s most valuable in the automated systems of manufacturing plants and other industrial environments.

Pressure relief and pressure safety valves
Pressure relief and pressure safety valves
Resource: https://youtu.be/-R5kBKn1BZc?

Pressure Relief Valve vs. Pressure Safety Valve

Many people use the terms pressure relief valve and pressure safety valve to mean the same thing. However, the two valves differ in design, purpose, and working mechanisms.

1. The relief valve is more of a pressure-regulating fitting, helping to protect system components from pressure damage. A PSV is more of a safety fitting, helping to prevent dangerous explosions.

2. The relief valve opens gradually with each increase in pressure. A PSV opens suddenly when the system pressure reaches a specific level and closes when all pressure discharges.

3. A pressure-reducing valve opens slowly and silently. Safety relief valves open quickly with a pop followed by a hissing noise or swoosh.

The pressure relief valve on a hot water tank
The pressure relief valve on a hot water tank
Resource: https://youtu.be/z5ItEW_Ubgw?

Pressure Relief Valve Applications

PRV valve application spans many industries where hydraulic or pneumatic pressures can damage sensitive components. The valve prevents overpressure in these systems by venting fluids upon sensing a buildup. Common uses include:

Pressure Relief Valve for Water

In pumping systems where a centrifugal pump operates in a closed circuit, low pressure can be a problem, causing cavitation. Installing a PRV keeps the pressure level within tolerable levels and protects the pump from damage.

Pressure Relief Valve for Compressor

Pneumatic systems such as compressors use pressurized air and other gas to work. Despite that, excessive pressure can cause them to fail. The PRV prevents pressure from exceeding set levels in air compressors and similar applications.

Pressure Relief Valve for Hot Water System

The flow of heated fluids causes increased pressures. If left unregulated, this pressure may rise to dangerous levels. Pressure relief valves and fittings keep this pressure down.

Pressure reducing valve
Pressure reducing valve
Resource: https://youtu.be/n3xcv1BKbrc?

Sizing of Pressure Relief Valve

If your application requires a PRV, you want to know what to look for when buying one or how to size it. Based on your application requirements, the following points should help you find the best pressure relief valve.

  • Find a size that is compatible with your system or equipment. It ensures the fitting can vent or discharge the required amount of fluid.
  • Consider the minimum and maximum pressure levels of your system. Your preferred valve should have ratings that strictly fit these low and high-pressure limits.
  • Look for a material that will hold up in your application environment. Your options include brass, bronze, stainless steel, cast iron, or plastic.

Conclusion

The pressure relief valve is a crucial component in pressure systems. It opens in overpressure conditions and closes if the pressure returns to normal. Its action prevents situations where overloads may damage delicate parts of your hydraulic or pneumatic equipment.

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