Pressure Relief Valve

Jul. 15, 2024

Pressure Relief Valve

Pressure Relief Valves

A pressure Relief Valve is a safety device designed to protect a pressurized vessel or system during an overpressure event.
An overpressure event refers to any condition which would cause pressure in a vessel or system to increase beyond the specified design pressure or maximum allowable working pressure (MAWP).

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The primary purpose of a pressure Relief Valve is the protection of life and property by venting fluid from an overpressurized vessel.

Many electronic, pneumatic, and hydraulic systems exist today to control fluid system variables, such as pressure, temperature, and flow. Each of these systems requires a power source of some type, such as electricity or compressed air to operate. Therefore, a pressure Relief Valve must be capable of functioning at all times, especially during power failures when system controls are nonfunctional. The process fluid serves as the sole power source for the pressure Relief Valve.

When a condition arises that causes the pressure in a system or vessel to escalate to a hazardous level, the pressure Relief Valve could be the last line of defense against catastrophic failure. Given that reliability is directly linked to the device's complexity, a simple design for the pressure Relief Valve is essential.

The pressure Relief Valve should open at a predetermined set pressure, flow a rated capacity at a specified overpressure, and close once the system pressure returns to a safe level. These valves are often constructed from materials compatible with various process fluids, ranging from simple air and water to the most corrosive substances. Additionally, they must operate smoothly and reliably across different types of fluids and fluid phases.

Spring Loaded Pressure Relief Valve

The fundamental spring-loaded pressure Relief Valve was created to serve the demand for a straightforward, dependable, actuated device for overpressure protection.

The design features a valve inlet or nozzle connected to the pressurized system, a disc that seals against the nozzle under normal conditions, a spring to maintain the disc closure, and a body/bonnet that hosts the operational components. The spring load is adjustable, allowing modulation of the pressure at which the valve opens.

As the pressure Relief Valve begins to lift, the force of the spring increases. Thus, system pressure must go up if the lift is to continue. Consequently, there is an overpressure allowance for the valve to achieve full lift, generally around 10% for valves in unfired systems. This margin is slight, so some auxiliary means should facilitate the lifting process.

Most pressure Relief Valves utilize a secondary control chamber or huddling chamber to enhance lift. When the disc starts to lift, fluid flows into the control chamber, exposing a larger surface area of the disc to system pressure.

This results in an incremental change in force that compensates for the increased spring force, leading the valve to open rapidly. Concurrently, the change in fluid flow direction creates a momentum effect that further enhances lift. These mechanisms allow the valve to reach maximum lift and flow while staying within the allowable overpressure limits. Since the larger disc area exposed to system pressure after lift prevents closure until pressure falls below the set level, the design of the control chamber dictates the closing point. This difference between the set pressure and the closing pressure is termed blowdown and is usually expressed as a percentage of the set pressure.

Balanced Bellows Valves and Balanced Piston Valves

For situations with variable superimposed back pressure, using a balanced bellows or balanced piston design is advisable. A typical balanced bellows model features the bellows or piston designed with a pressure area that matches the seat area of the disc. Additionally, the bonnet is vented to ensure that the pressure area is always exposed to atmospheric pressure, serving as a telltale should the bellows or piston develop a leak. Therefore, fluctuations in back pressure do not influence set pressure, although back pressure may affect flow.

Pressure Relief Valve Bellow Type

Other Designs of Relief Valves

Safety Valve
A safety Valve is a pressure Relief Valve actuated by inlet static pressure, characterized by rapid opening or pop action (commonly used for steam and air services).

Low-Lift Safety Valve
A low-lift safety Valve has a disc that lifts automatically, determining the actual discharge area based on the position of the disc.
Full-Lift Safety Valve
A full-lift safety Valve features a disc that lifts automatically, with the actual discharge area not influenced by the disc's position.

Relief Valve
A Relief Valve is a pressure relief device actuated by inlet static pressure, exhibiting gradual lift that is generally proportional to the pressure increase over the opening pressure. It may incorporate an enclosed spring housing suitable for closed discharge system applications and is primarily employed for liquid service.

Safety Relief Valve
A safety Relief Valve merges rapid opening or pop action characteristics with a proportional response to pressure increase, applicable for liquids or compressible fluids.

Conventional Safety Relief Valve
This type features a spring housing vented to the discharge side of the valve, with operational characteristics affected by variations in back pressure.
Balanced Safety Relief Valve
This design minimizes the effect of back pressure on key operational factors such as opening pressure, closing pressure, and relieving capacity.

Pilot-Operated Pressure Relief Valve
Combining a major relieving device with a self-actuated auxiliary pressure Relief Valve, the pilot-operated type is designed for robust operation.

Power-Actuated Pressure Relief Valve
This variant requires an external energy source to control and operate the major relieving device.

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Temperature-Actuated Pressure Relief Valve
This valve can be triggered by external or internal temperature fluctuations or by pressure on the inlet side.

Vacuum Relief Valve
Designed to allow fluid ingress to prevent excessive internal vacuum, this valve automatically recloses after normal conditions are reestablished.

Codes, Standards, and Recommended Practices

Numerous codes and standards exist globally governing the design and application of pressure Relief Valves. The ASME Boiler and Pressure Vessel Code, commonly known as ASME Code, is the most widely recognized.

While most codes and standards are voluntary, they are crucial as manufacturers and users can include them in purchasing and construction specifications. The ASME Code is mandated by many state and provincial legislatures in the United States and Canada.

This code outlines the rules for designing and constructing pressure vessels. It covers various aspects, including fired vessels, nuclear vessels, and unfired vessels, along with subjects like welding and nondestructive examination. Vessels produced in compliance with the ASME Code must incorporate overpressure protection, with type and design detailed meticulously in the Code.

Terminology

The following definitions, sourced from DIN standards, include terminology frequently utilized across various standards. In instances where commonly used terms lack definitions in DIN, ASME PTC25.3 serves as a reference. This glossary is not exhaustive but offers guidance.

Operating pressure (working pressure)
Is the gauge pressure present under normal operating conditions within the protected system.
Set pressure
Is the gauge pressure at which direct loaded safety Valves initiate lifting under operational conditions.
Test pressure
Is the gauge pressure at which direct loaded safety Valves begin to lift under test stand conditions (atmospheric backpressure).
Opening pressure
Is the gauge pressure at which lifting achieves the capacity necessary for predetermined flow. It equals set pressure plus opening pressure difference.
Reseating pressure
Represents the gauge pressure at which the valve closes.
Built-up backpressure
Is the gauge pressure developed on the outlet side when relieving.
Superimposed backpressure
Denotes gauge pressure on the outlet side of a closed valve.
Backpressure
Is the total gauge pressure on the outlet side during relief (built-up + superimposed backpressure).
Accumulation
Denotes pressure increase over the designated maximum allowable working gauge pressure of the protected system.
Opening pressure difference
Refers to pressure rise over the set pressure necessary for adequate lift.
Reseating pressure difference
Is the difference between set and reseating pressures.
Functional pressure difference
Comprises the sum of the opening and reseating pressure differences.
Operating pressure difference
Denotes the pressure difference between set and operating pressures.
Lift
Represents the distance the disc moves away from the closed position.
Commencement of lift (opening)
Is the initial measurable movement of the disc or the detection of discharge noise.
Flow area
Denotes the cross-sectional area upstream or downstream of the body seat calculated from the minimum diameter to estimate flow capacity without deducting for obstructions.
Flow diameter
Is the smallest geometrical diameter upstream or downstream of the body seat.
Nominal size designation
Refers to the inlet's nominal size for a safety Valve.
Theoretical flowing capacity
Represents the calculated mass flow from an orifice with a cross-sectional area matching that of the safety Valve's flow area, not accounting for flow losses.
Actual flowing capacity
Is the flowing capacity measured through testing.
Certified flowing capacity
Is actual flowing capacity reduced by 10%.
Coefficient of discharge
Is the ratio of actual to theoretical discharge capacity.
Certified coefficient of discharge
Relates to the coefficient of discharge reduced by 10% (also known as derated).

The following definitions are drawn from ASME PTC25.3 and are not included in DIN:

Blowdown (reseating pressure difference)
Refers to the difference between actual popping and reseating pressure, usually expressed as a percentage of the set pressure.
Cold differential test pressure
Is the pressure measurement at which a Valve is tested at ambient temperature, with adjustments made for service conditions like backpressure.
Flow rating pressure
Indicates the inlet static pressure at which the relieving capacity of a pressure relief device is evaluated.
Leak test pressure
Is the inlet static pressure specified for conducting a quantitative seat leakage test according to established protocols.
Measured relieving capacity
Is the relieving capability of a pressure relief device assessed at the flow rating pressure.
Rated relieving capacity
Refers to the portion of the measured relieving capacity authorized by the relevant code or regulation for a pressure relief device's application.
Overpressure
Denotes an increase in pressure over the set pressure of a pressure Relief Valve, commonly expressed as a percentage of set pressure.
Popping pressure
Is the increasing static inlet pressure point at which noticeable lift occurs, or discharge transitions from intermittent to continuous.
Relieving pressure
Is the set pressure together with overpressure.
Simmer
Describes the pressure range between set and popping pressures.
Maximum operating pressure
Indicates the highest pressure anticipated during system usage.
Maximum allowable working pressure (MAWP)
Denotes the top permissible gauge pressure in the operational position of a vessel meant for a specified temperature.
Maximum allowable accumulated pressure (MAAP)
Refers to the maximum working pressure plus the accumulation, per applicable codes for normal or fire contingencies.

Storage handling and transportation of Safety Valves

Storage and handling
For optimal function and seat integrity of a safety Valve, cleanliness is paramount; precautions must be taken to prevent contamination during storage. Inlet and outlet protectors should be preserved until the valve is ready for installation. It is essential to maintain absolute cleanliness of the valve's inlet. Best practices include storing the valve indoors in its original packaging, away from dirt and other contaminants.
Safety Valves require careful handling to avoid any impact; rough treatment can alter the pressure settings, deform components, and impair seat integrity and functionality. The valve should never be manipulated using the lifting lever.
When utilizing a hoist, ensure the chain or sling encircles the valve body and bonnet uprightly to facilitate proper installation.

Installation
Improper cleaning of connections often leads to damage upon initial use. Before installation, ensure that both valve inlet and the connecting vessel/line are thoroughly cleaned to eliminate all contaminants.
Debris entering safety Valves can cause significant damage; thus, inspected and cleaned systems holding the valves for testing and final installation are crucial. Newly constructed systems, in particular, require thorough cleaning to eliminate any foreign matter that could impair seating surfaces upon valve activation. Gaskets must be correctly sized for specific flanges, fully clearing the valve's inlet and outlet to prevent flow restrictions.
When installing flanged valves, evenly tighten connection studs or bolts to avoid warping. Avoid applying wrench pressure to the valve body when dealing with threaded valves; utilize the hex flats on the inlet bushing instead.
Ensuring proper valve installation requires precise design attention for both inlet and discharge piping, with guidance from relevant international, national, and industry standards.

Inlet piping
Connect the valve as directly and closely to the protected vessel as possible. The valve should be oriented vertically in an upright position, mounted directly on a nozzle from the pressure vessel or a short connection fitting allowing unobstructed flow. Installing a safety Valve in any other configuration can negatively impact its operation.

Discharge piping
Strategically simple and direct discharge piping is preferred, with a "broken" connection near the valve outlet whenever feasible. Discharge piping should lead as directly as possible to a safe disposal point. Ensure that discharge piping is adequately drained to prevent liquid accumulation on the downstream side of the safety Valve.
Support for the discharge piping must come from a separate source to withstand reactive forces during relief. Additionally, the safety Valve must be secured against swaying or vibration disturbances.
If discharging into a pressurized system, utilize a "balanced" design valve to avoid adversely impacting performance and set pressure. Any fittings or pipes with smaller diameters than the valve outlet must not be employed.
The bonnets of balanced bellows safety Valves need constant venting to ensure proper function and to provide an indicator in case of bellows failure. It is important not to obstruct these vents. For flammable, toxic, or corrosive fluids, divert the bonnet vent to a safe location.

References and Images for this Page..

Crosby® - Pressure Relief Valve Engineering Handbook
Anderson Greenwood Crosby - Technical Seminar Manual
Spirax Sarco - Alternative Plant Protection Devices and Terminology

It is essential to recognize that a pressure Relief Valve functions as a safety device, safeguarding pressure vessels or systems from catastrophic failures. Thus, the application of pressure Relief Valves should only be entrusted to fully trained personnel, strictly adhering to regulations set forth by the governing codes and standards.