What is a Pressure Relief Valve?

A pressure relief valve (PRV) is a safety valve that controls or limits system pressure. It is designed to automatically release pressure when it exceeds a predetermined level, preventing potential damage or catastrophic failure. When the pressure reaches the set limit, the valve opens, releasing fluid or gas to a safe location, such as the atmosphere or a containment system.

Pressure relief valves are used in many residential, commercial, and industrial applications, including water heaters, hot water tanks, boilers, hydronic heating systems, hydraulic systems, pumps, pressure vessels, compressed air systems, piping, and process equipment.

In simple terms, a pressure relief valve is the system’s last line of defense against unsafe pressure buildup.

How Does a Pressure Relief Valve (PRV) Work?

A pressure relief valve opens to release fluid or gas when the pressure inside a vessel or system exceeds a predetermined level. The valve is typically spring-loaded and designed to remain closed under normal operating conditions.

When the pressure inside the vessel or system exceeds the valve’s set point, the spring's force is overcome, and the valve begins to open. As the valve opens, fluid or gas is released from the system, and the pressure inside the vessel or system decreases. When the pressure drops below the valve’s set point, the spring force closes the valve.

The released fluid or gas is typically vented to a safe location, such as the atmosphere or a containment system. Once the fluid has been exhausted from the system and the pressure has fallen to its pre-set level, the pressure relief valve will close. The size of the valve determines its discharge capacity, set pressure, and the characteristics of the fluid or gas being relieved.

Pressure relief valves usually work in the sequence below:

1. Pressure builds inside the system.
2. Pressure reaches the valve’s set point.
3. The valve opens automatically.
4. Fluid, gas, steam, vapor, or another fluid is discharged to the air.
5. System pressure drops.
6. The valve closes once pressure returns to a safe level.

The valve’s discharge capacity must match the system. A valve that is too small may not relieve pressure fast enough, while a valve with the wrong set pressure may open too early, too late, or fail to protect the system properly.

How does a PRV control or limit pressure in a system?

A pressure relief valve (PRV) does not control or limit pressure; instead, it is a safety device that prevents pressure from exceeding a predetermined level. In other words, a PRV is a reactive device that responds to an overpressure situation and prevents further pressure buildup. The valve is typically set to open at a specific pressure level, known as the set point or blow-off pressure, and is designed to release fluid or gas from the system until the pressure drops below this set point.

PRVs are typically installed with other pressure control devices, such as pressure regulators or control valves, that actively control and maintain the pressure within a desired range. Together, these devices help ensure the safe and reliable operation of pressure systems.

This distinction is important: a pressure regulator manages normal operating pressure, while a pressure relief valve protects against abnormal or unsafe pressure.

Why Use a PRV?

The release of fluid or gas from the system through a pressure relief valve reduces the pressure to a safe level, thereby preventing potential damage or catastrophic failure of the equipment or system. PRVs help ensure personnel and environmental safety and minimize the risk of production loss or property damage. Using pressure relief valves is critical to pressure system safety and can prevent costly and dangerous accidents.

Pressure relief valves are used to help prevent:

• Tank rupture
• Pipe failure
• Boiler overpressure
• Water heater damage
• Pump damage
• Hydraulic system failure
• Heat exchanger damage
• Production downtime
• Unsafe operating conditions

In many applications, properly rated pressure relief valves are required by codes, standards, equipment manufacturers, insurance requirements, or facility safety programs.

Benefits of Using a Pressure Relief Valve (PRV)

• Safety – PRVs prevent overpressure situations that can lead to damage or catastrophic failure of equipment or systems. The release of fluid or gas from the system through a PRV reduces pressure to a safe level, ensuring personnel and environmental safety.
• Compliance – Many regulatory bodies require PRVs in specific applications to ensure compliance with safety regulations and standards.
• Reliability – PRVs are designed and tested to deliver consistent performance across a range of operating conditions.
• Cost-Effective – PRVs can prevent costly damage or downtime from overpressure situations, leading to cost savings and increased efficiency.
• Versatility – PRVs are available in a wide range of sizes, designs, and materials, making them suitable for use in a variety of applications and industries.

Overall, using PRVs is a critical component of pressure system safety and can provides benefits, including safety, compliance, reliability, cost-effectiveness, and versatility.provide

Because pressure relief valves are available in many pressure ratings, materials, end connections, and designs, they can be selected for residential plumbing, commercial heating, industrial processing, hydraulic power units, boiler rooms, pump systems, and other pressure applications.

Common Applications of Pressure Relief Valves

Pressure relief valves are used wherever pressure must be limited for safety, typically in closed-loop systems. The correct valve size and type depend on the system type, media, pressure, temperature, capacity, and applicable code requirements. Common applications include:

• Water heaters
• Hot water tanks
• Boilers
• Hydronic heating systems
• Hydraulic systems
• Pumps
• Pressure vessels
• Compressed air systems
• Process piping
• Industrial equipment
• Heat exchangers

The following topic will present the PRV types and where they are best suited, given the system's nature and constraints.

Types of PRVs

Several types of pressure relief valves (PRVs) are available, each designed to meet specific application requirements. The most common types of PRVs are:

Spring-Loaded PRVs

These are the most common types of PRVs and are typically used in applications with relatively constant pressure. They operate by using a spring to maintain the valve in the closed position until the system pressure exceeds a set point, at which point the valve opens to relieve pressure.

Common applications:

• Boiler systems that use a valve to protect the boiler from excessive pressure.
• Water heater systems where a temperature and pressure relief valve protects the tank.
• Compressed air receivers where the valve releases air if pressure exceeds the tank rating.
• Pump systems where the valve protects piping and equipment from pressure spikes.
• Hydronic heating loops using water or glycol, where overpressure protection is required.

Pilot-Operated PRVs

These valves use a pilot valve to control the opening and closing of the main valve. They are typically used in applications with fluctuating pressures, as the pilot valve can be adjusted to maintain a specific pressure range.

Pilot-operated pressure relief valves are often used in systems with higher pressures, larger flow rates, or more demanding performance requirements. They can provide higher capacity, precise operation, and improved performance in certain industrial applications.

Common examples include:

• Oil and gas pipelines that may need to relieve large volumes of gas or liquid.
• Chemical processing systems require accurate pressure protection.
• High-pressure steam systems require fast and controlled pressure relief.
• Large industrial pressure vessels are used that require high flow capacity.
• Process plants with changing operating conditions or pressure fluctuations.

Balanced Bellows PRVs

These valves use balanced bellows to sense pressure and control valve opening and closing. They are typically used in applications where the process fluid is corrosive or a tight shut-off and high accuracy is required.

Balanced bellows pressure relief valves can also help reduce the effect of back pressure on valve operation. This makes them useful in systems where discharge piping creates variable or built-up back pressure.

Common examples include:

• Chemical processing lines handling corrosive or aggressive fluids.
• Refinery systems where discharge piping may create variable back pressure.
  
• Process vessels where accurate relief pressure is important.
  
• Systems with closed discharge headers.
• Applications involving vapors or fluids that should be isolated from the valve spring.

Liquid Relief Valves

These valves are specifically designed for liquid applications and are typically used to prevent overpressure in pipelines or tanks. They allow a small amount of liquid to escape when the pressure exceeds a set point.

Liquid relief valves may be used with water, oil, glycol, process liquids, hydronic heating fluids, and other compatible media.

Common examples include:

• Closed-loop hydronic heating systems using water or glycol.
• Pump discharge lines where a blocked outlet could cause pressure buildup.
• Storage tanks and piping systems where liquid expansion may increase pressure.
• Heat exchangers where trapped liquid can expand as the temperature rises.
• Oil circulation systems that require pressure protection for pumps and piping.

Safety Relief Valves

These are high-capacity PRVs typically used in applications where there is a potential for large-scale fluid or gas release, such as in the oil and gas industry. They are designed to open quickly and thoroughly, enabling rapid pressure release.

Safety relief valves are commonly used in steam systems, boilers, pressure vessels, industrial gas systems, chemical processing, oil and gas systems, and high-pressure equipment.

Common examples include:

• Steam boilers have a valve that protects against dangerous steam pressure buildup.
• Pressure vessels storing air, gas, steam, or process media.
• Oil and gas production equipment is high-capacity.
• Chemical reactors where pressure can rise quickly during a process upset.
• Industrial steam headers where rapid overpressure protection is critical.

T&P Relief Valves

Temperature and pressure relief valves, or T&P valves, are commonly used on water heaters. They respond to both excessive pressure and excessive temperature.

This type of valve is essential for water heater safety and should be selected in accordance with the manufacturer’s specifications.

Common examples include:

• Residential water heaters that use a valve to protect the tank from excessive pressure or overheating.
• Commercial water heaters in restaurants, schools, apartments, hotels, and mechanical rooms.
• Hot water storage tanks, where stored heated water can expand and increase system pressure.
• Indirect water heaters connected to boiler systems, where heat transfer can raise tank temperature and pressure.
• Solar water heating systems, where water temperature may rise significantly during periods of high solar gain.
• Domestic hot water systems where a T&P valve provides safety protection for potable hot water equipment.

How to Select a Pressure Relief Valve (PRV)

Selecting a PRV depends on the specific application requirements, including the type of fluid or gas, pressure range, flow rate, and operating conditions. Various pressure relief valves are available, including spring-loaded, pilot-operated, and bellows-type valves. Each type has its advantages and disadvantages.

To select the best fit for your application, determine the maximum flow rate the pressure relief valve needs to handle. This will depend on the size of the system and the amount of pressure that needs to be relieved. Maximum flow can be calculated using the system's maximum operating pressure and flow rate. Then determine the set pressure at which the pressure relief valve must open to relieve pressure; it should be set below the system's maximum operating pressure to ensure safe operation.

The valve body and trim material should be compatible with the fluid or gas in the system to prevent corrosion or contamination. Your application will decide the valve's size, end connections, and other features. Consider the system's temperature, pressure, flow rate, and other parameters to ensure the valve can handle the conditions.

The valve you select must also comply with relevant standards, such as ASME Boiler and Pressure Vessel Code, API, or ISO. If you need clarification on any aspect of the selection process, consult experts or manufacturers for guidance. They can help you select the best valve for your application.

It is essential to ensure that pressure relief valves are correctly installed, maintained, and tested to ensure their reliability and effectiveness in preventing overpressure situations.

To make the selection process easier, review the following factors before choosing a pressure relief valve to ensure the correct valve type for the right usage:

• Application
• Media Type
• Set Pressure
• Temperature Rating
• Discharge Capacity
• Valve Size and Connection Type
• Body and Seal Materials
• Code and Certification Requirements
• Installation and Discharge Piping

Pressure Relief Valves by Manufacturer

Bell & Gossett Pressure Relief Valves

Bell & Gossett pressure relief valves protect fired and unfired hot water vessels against hazardous operating pressures.

• Highest BTUH ratings available for valves in their class (790,000 to 5,999,000 BTUH)
• EPDM diaphragm-operated (cast iron models) and diaphragm-assisted (bronze models) have an effective area approximately five times greater than conventional “pop-type” relief valves to help overcome the effects of fouling
• A small differential between opening and closing pressures helps prevent conditions in which system water might flash to steam, causing hammering.
• Engineered in accordance with Section IV of the ASME boiler and pressure code for heating boilers with capacities certified by the National Board of Boiler and Pressure Vessel Inspectors

Shop Bell & Gossett Pressure Relief Valves

McDonnell & Miller Pressure Relief Valves

McDonnell & Miller diaphragm-operated cast-iron and diaphragm-assist-operated bronze ASME Safety Relief Valves are designed to protect fired and unfired hot-water pressure vessels from overpressure. The diaphragm's "oversized" effective area generates greater operating force, helping overcome the effects of fouling. In addition, these valves feature a unique fail-safe disc with sufficient area to maintain their safety relief function in the event of a diaphragm rupture. These valves are designed, manufactured, tested, and labeled in accordance with the requirements of Section IV of the ASME Boiler and Pressure Vessel Code. They are offered in various capacities to permit a close match with the boiler output rating.

Shop McDonnell & Miller Pressure Relief Valves

Webstone Pressure Relief Valves

Webstone PRVs are designed for use with water, oil, and glycol mixtures in residential and commercial plumbing and heating systems.

Shop Webstone Pressure Relief Valves

Armstrong Pressure Relief Valves

Armstrong PRVs are built to high standards and offer many outstanding advantages. Their extra-large diaphragm assures positive action.

Shop Armstrong Pressure Relief Valves

Pressure Relief Valves Available in Liberty Supply

Check The PRV Ranges Available

Liberty Supply is a customer-centric organization. We focus on hydronic and steam systems for commercial and industrial facilities. As a B2B distributor within the "needs business," our foundational pillars are knowledge, communication, and speed. Relationships are built through hard work and consistent delivery of value. No exceptions. Our team maintains that with each transaction, we start back at zero.

Contact Liberty Supply

Request a PRV Quote

Talk to an Expert on PRVs

For any further questions or support, don't hesitate to contact us:

• Phone: 844-448-0880
• Fax: 844-448-0881
• Email: [email protected]