What is pressure relief valve ? How does pressure relief valve work

Components of pressure relief valve

The main components of a pressure relief valve include:

1. Valve Body: The main structure of the valve that contains the internal components and provides the inlet and outlet connections to the piping or equipment. It is designed to withstand the full system pressure.
2. Bonnet: The cover for the opening in the body of the valve. It can be bolted or threaded to the valve body and may house the adjusting mechanism for the set pressure.
3. Spring: A crucial component that applies force to the valve’s closing mechanism (usually a disk or piston). The spring force is adjustable and determines the set pressure at which the valve will open.
4. Disk or Piston: The component that seals the valve under normal operating conditions. It moves away from the seat to open the valve and allow fluid to escape when the system pressure exceeds the set pressure.
5. Seat: The surface against which the disk or piston seals. The seat is usually made from a material that is compatible with the fluid and can provide a tight seal.
6. Set Screw or Adjusting Nut: Used to adjust the spring tension, which sets the opening pressure of the valve. By tightening or loosening the adjuster, the set pressure can be increased or decreased.
7. Lift Lever: A manual lever found on some pressure relief valves, used to manually open the valve for testing or to relieve pressure.
8. Blowdown Ring: An adjustable feature that allows for the adjustment of the blowdown, which is the difference between the pressure at which the valve opens (set pressure) and the pressure at which it reseats (reseating pressure) after discharging fluid.
9. Sealing Gasket: Ensures a tight seal between the valve body and the bonnet to prevent leaks.
10. Guide: Aligns and directs the movement of the disk or piston within the valve body to ensure it seats correctly.
11. Nozzle or Orifice: The passage through which the relieved fluid exits the valve. Its size and shape are critical for the valve’s capacity and performance.
12. Overpressure Protection Device: Some valves may include additional mechanisms to protect against excessive overpressure in case the primary relieving device fails.

These components work together to ensure that the valve operates correctly, opening to relieve excess pressure and then reseating to maintain system integrity once normal conditions are restored.

A pressure relief valve (PRV) works by automatically releasing pressure from a pressure vessel or system when the pressure exceeds a predetermined level, ensuring the safety and integrity of the system. Here’s a step-by-step explanation of how it operates:

1. Normal Operation: Under normal operating conditions, the pressure within the system is below the set pressure of the relief valve. The force exerted by the spring (or other biasing mechanism) keeps the valve’s disk or piston seated against the valve seat, preventing the flow of fluid through the valve.
2. Pressure Increase: When the pressure in the system starts to increase and approaches the set pressure of the relief valve, the force due to the system pressure acting on the disk or piston begins to counteract the force of the spring.
3. Reaching Set Pressure: Once the system pressure exceeds the set pressure of the valve, the force exerted by the system pressure on the disk or piston overcomes the force of the spring. This causes the disk or piston to lift off the seat, opening the valve.
4. Discharge of Excess Pressure: With the valve open, the excess pressure is vented through the valve’s outlet, reducing the system pressure. The discharged fluid is directed to a safe location, typically through a discharge piping system designed to handle the fluid safely.
5. Re-seating of the Valve: As the pressure in the system decreases due to the release of excess pressure, the force exerted by the system pressure on the disk or piston diminishes. When the system pressure falls below the set pressure (minus the blowdown margin), the spring force pushes the disk or piston back onto the seat, closing the valve and preventing further discharge of fluid.
6. Blowdown: The “blowdown” is the pressure difference between the set pressure (the pressure at which the valve opens) and the reseating pressure (the pressure at which the valve closes after having opened). It prevents the valve from chattering—a rapid, repeated opening and closing that can damage the valve. The blowdown is typically adjusted by a blowdown ring or similar mechanism in the valve.
7. Reset for Next Cycle: Once reseated, the valve is ready for another cycle. It remains closed, maintaining a seal until the system pressure once again exceeds the set pressure.

The operation of a pressure relief valve is automatic and based on the balance of forces acting on the valve’s moving parts. It does not require any external control or intervention to function, making it a critical safety device in many pressure systems.

Calculating the flow through a pressure relief valve involves understanding several key parameters and using specific equations tailored to the fluid type (gas, liquid, or steam) and the valve’s characteristics. The primary objective is to ensure the valve can relieve enough fluid to prevent a system’s pressure from exceeding its maximum allowable working pressure (MAWP). Here’s a simplified overview of the process:

1. Determine Fluid Properties

• Type of Fluid: Gas, liquid, or steam, as the calculations differ for each.
• Operating Conditions: Temperature and pressure of the fluid.

2. Valve Specifications

• Valve Size and Coefficient: Obtain the valve’s effective area (usually given in square inches) and the discharge coefficient from the manufacturer’s data.

3. Set Pressure and Back Pressure

• Set Pressure: The pressure at which the valve is designed to open.
• Back Pressure: The pressure in the discharge system that can affect valve performance, especially for balanced bellows or pilot-operated valves.

4. Calculate Flow Capacity

The general formula for flow capacity is:

​

Where:

• $Q$ = Flow rate
• $K$ = Discharge coefficient (depends on the valve design and fluid type)
• $A$ = Effective area of the valve’s orifice
• $\Delta P$ = Pressure differential across the valve (set pressure minus back pressure)
• $\rho$ = Fluid density at relieving conditions

For Gases and Vapors (Including Steam)

The flow of gases and vapors through relief valves is often calculated using the ideal gas law and factors to account for real gas behavior under certain conditions. The American Petroleum Institute’s API 520 and API 521 standards provide detailed methodologies for these calculations, including factors for adjusting for superheated steam or compressible fluids.

For Liquids

The calculation for liquids is more straightforward since liquids are incompressible. The formula simplifies to:

5. Adjustments for Real Conditions

• Superheated Steam: Use specific formulas or correction factors for superheated steam, as it behaves differently from saturated steam.
• Viscosity Correction: For liquids with high viscosity, corrections may be needed to account for the resistance to flow.

6. Safety Factors

Incorporate safety factors as recommended by standards and regulations, ensuring the valve can handle peak flows under emergency conditions.

Industry Standards and Guidelines

Refer to industry standards like API 520 (for sizing and selection of pressure-relieving devices) and ASME (American Society of Mechanical Engineers) codes for detailed methodologies and guidelines tailored to specific conditions and fluids.

For precise and accurate calculations, especially in critical applications, it’s essential to consult these standards and, if necessary, seek the assistance of a qualified engineer. The calculations can become quite complex, especially when dealing with mixed phases, reactions, or non-ideal behaviors.

Selecting the right pressure relief valve (PRV) involves considering a range of specifications and factors to ensure the valve can adequately protect the system from overpressure conditions while meeting operational and regulatory requirements. Here are key specifications and considerations:

1. Set Pressure

• The pressure at which the valve starts to open to relieve excess pressure. It is typically set at or just above the maximum allowable working pressure (MAWP) of the system it protects.

2. Capacity

• The valve must be capable of discharging enough fluid to prevent the system pressure from exceeding the allowable limits. The required capacity depends on the system’s maximum flow rate under overpressure conditions.

3. Fluid Type

• The nature of the fluid (gas, liquid, steam, or mixed phase) significantly influences valve design and material selection. The properties of the fluid, such as temperature, corrosiveness, and cleanliness, are crucial.

4. Size and Orifice Area

• The size of the valve and the orifice area determine the flow capacity. The orifice size is selected based on the required flow rate to relieve excess pressure without exceeding the valve’s maximum flow capacity.

5. Temperature Range

• The valve materials and design must withstand the minimum and maximum operating temperatures of the system.

6. Back Pressure

• The pressure in the discharge system can affect valve performance. Valves are classified as conventional, balanced bellows, or pilot operated based on how they handle back pressure.

7. End Connections

• The type (flanged, threaded, welded) and size of the valve’s inlet and outlet connections must be compatible with the piping system.

8. Materials of Construction

• Material selection for the valve body, trim, seals, and other components must be compatible with the fluid and the operating environment to ensure durability and prevent corrosion.

9. Overpressure Allowance

• The maximum overpressure the valve can handle while still operating safely, often expressed as a percentage of the set pressure.

10. Blowdown

• The difference between the set pressure and the reseating pressure. Adjustable blowdown helps prevent valve chatter and ensures stable operation.

11. Operational Type

• Whether the valve is spring-operated, pilot-operated, or uses another mechanism, which affects its responsiveness and suitability for various applications.

12. Certifications and Standards

• Compliance with relevant industry standards (e.g., ASME, API) and certifications is essential for ensuring safety and reliability.

13. Reliability and Maintenance Requirements

• Consider the valve’s reliability, ease of maintenance, and availability of spare parts.

14. Environmental and Safety Considerations

• The valve should safely discharge the relieved fluid without harming the environment or personnel, requiring adequate discharge piping and, if necessary, treatment systems.

Selecting a pressure relief valve requires a comprehensive understanding of the system’s operational parameters, the characteristics of the fluid being handled, and the regulatory environment. It’s often advisable to work with experienced engineers or specialists and refer to detailed sizing and selection guidelines provided by standards and valve manufacturers.

Pressure relief valves (PRVs) are critical safety devices in various industries, designed to protect equipment, systems, and personnel from the dangers of overpressure conditions. The selection, sizing, and installation of these valves are governed by a comprehensive framework of codes and standards to ensure their reliability and effectiveness in emergency situations.

Key codes and standards that regulate the design, manufacture, and application of PRVs include:

• ASME (American Society of Mechanical Engineers): ASME Boiler and Pressure Vessel Code, particularly Section VIII, which covers pressure vessels, including PRVs for such systems.
• API (American Petroleum Institute): API 520 and 521 provide guidelines for the design and sizing of pressure-relieving devices in refineries, chemical plants, and related industries.
• ISO (International Organization for Standardization): Various ISO standards apply to PRVs, including ISO 4126 for safety devices for protection against excessive pressure.
• EN (European Norms): EN standards such as EN ISO 4126 also apply within European countries, harmonizing with international standards.

These standards ensure that PRVs are designed and tested to perform reliably under specified conditions, providing a critical safety function by venting excess pressure to prevent equipment failure, explosions, and other hazardous events. Compliance with these standards is essential not only for safety but also for regulatory approval and operational integrity of pressure systems.

In conclusion, pressure relief valves are indispensable in maintaining safe pressure levels in industrial systems. Understanding their operation, key components, and the principles behind their sizing and selection is crucial. Adhering to established codes and standards ensures that PRVs function as intended, safeguarding assets and lives against the risks associated with overpressure situations.

There are numerous manufacturers around the world that specialize in the design, production, and supply of pressure relief valves (PRVs) for various industrial applications, including oil and gas, chemicals, power generation, and water treatment, among others. Some of the most well-known and respected manufacturers in the industry include:

1. Emerson (Pentair Valves & Controls): Emerson’s range of PRVs, including those from its acquisition of Pentair, are widely used in various industries for their reliability and performance.
2. Farris Engineering (Curtiss-Wright): Farris provides a wide array of pressure relief solutions and is known for its engineering expertise and custom valve solutions.
3. Leser GmbH & Co. KG: A leading European manufacturer of safety valves, Leser offers a broad range of products for industrial applications.
4. Consolidated (Baker Hughes): Consolidated safety valves are well-regarded for their quality and are used in multiple industries, including oil and gas and power generation.
5. Crosby (Emerson): Crosby is another renowned brand under Emerson, known for its high-quality pressure relief valves and engineering support.
6. Anderson Greenwood (Emerson): Anderson Greenwood offers a variety of pressure relief devices and is known for its innovative valve solutions.
7. Kunkle Valve (Circor): Kunkle Valve provides safety and relief valve products for industrial and commercial applications, known for their durability and safety.
8. Tyco Valves & Controls (now part of Pentair): Tyco offers a variety of valve products, including pressure relief valves, for various applications.
9. Watts Water Technologies: Watts is known for a range of water safety and flow control products, including pressure relief valves for plumbing and heating applications.
10. VELAN: A leading manufacturer of steel gate, globe, check, ball, butterfly, knife gate, and engineered severe service valves for critical applications.
11. Weir Group: Weir provides a range of pressure control and relief valve solutions, especially for the mining and oil and gas sectors.

When selecting a PRV manufacturer, it’s essential to consider the specific requirements of your application, including the type of fluid, operating pressure and temperature, required certifications, and any industry-specific requirements. It’s also beneficial to assess the manufacturer’s reputation for quality, reliability, and after-sales support.