What is a Safety Valve?

Figure 1: Safety valve

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A safety valve safeguards a system from overpressure, which occurs when the system's pressure surpasses its Maximum Allowable Working Pressure (MWAP). The valve opens at a predetermined pressure, initially slightly and then fully, to quickly release excess pressure. This prevents pressure-related malfunctions, fire hazards, or explosions. Safety valves are mechanically operated, ensuring functionality even during power failures, and they act as a reliable backup when electronic or pneumatic safety devices fail.

Table of contents

• Important terminology
• Spring mechanism safety valve
• Selection criteria
• Applications
• Safety valve symbol
• Safety valve certifications
• Pressure safety valve vs relief valve
• FAQs

Important terminology

• Overpressure: Excess pressure over the set pressure of the safety valve.
• Operating pressure: The pressure at which the system works under normal operating conditions.
• Set pressure: The pressure at which the safety valve's disc begins to lift and open.
• Lift: The distance the disc moves from the closed position to the position required for discharge.
• Backpressure: The pressure built upon the safety valve outlet during flow. Backpressure = Built-up backpressure + Superimposed backpressure.
• Built-up backpressure: The pressure at the outlet when the safety valve opens.
• Superimposed backpressure: The pressure at the outlet of a closed safety valve.
• Maximum allowable working pressure (MAWP): The maximum allowable pressure at a designated temperature under normal operating conditions. MAWP is the maximum pressure that the system's weakest component can handle.
• Blowdown: The difference between the pressure at which the disc lifts and the pressure at which the valve closes. Blowdown is generally expressed as a percentage.
• Blow-off capacity: The rate at which the safety valve can release excess pressure.

Spring mechanism safety valve

The most common safety valve type is a spring-loaded or direct-acting safety valve. An advantage of this type is that it is available for pressure ranges from approximately 1 to bar. The mechanism consists of the following components:

• Expansion chamber: The expansion chamber (Figure 2 labeled A) increases the surface area that the system's media pushes against to open the safety valve, allowing it to open rapidly.
• Spring: The spring's (Figure 2 labeled B) stiffness determines at which pressure the system's media can begin to open the valve.
• Disc: The disc (Figure 2 labeled C) sits on the nozzle and moves up and down to allow or prevent flow through the safety valve.
• Nozzle ring: The nozzle ring (Figure 2 labeled D) affects the pressure at which the disc reseats. A high setting can cause the disc to reseat too late, while a low setting can lead to the disc randomly opening and closing when it should not.
• Nozzle: The nozzle (Figure 2 labeled E) controls the disc surface area that the media interacts with before the valve opens. This causes the media to work against a larger surface area when the valve opens, increasing the force acting on the disc and opening the disc rapidly.

Figure 2: Safety valve with spring mechanism: expansion chamber (A), spring (B), disc (C), nozzle ring (D), and nozzle (E).

The balance between a safety valve's spring force and the input force controls the valve's opening and closing. Inlet pressure and the disc's surface area with which the media interacts determine the input force. According to Pascal's Law, force equals the product of pressure and area. Therefore, as the area of the disc that the media interacts with increases, so does the force.

The most important characteristic of safety valves is that they quickly open entirely to reach maximum blow-off capacity in minimal time. This is possible because the valve's disc has a larger diameter than the nozzle. As soon as the inlet pressure is high enough, the disc lifts. At this moment, the disc surface at which the medium can reach becomes larger. This results in an input force much greater than the spring force, and the valve completely opens.

Special safety valve versions exist for incompressible and compressible media and gasses/vapors. Safety valves for gasses and vapors often open before the set pressure is reached and open to at least 50% lift at the response pressure (see Figure 3). Safety valves of this type have a significant disadvantage: they are very susceptible to back pressure, which can negatively affect the valve's safety.

Figure 3: Safety valve mechanism for gasses and vapors (left): nozzle ring (A) and flow pattern (B). Blow-off characteristic of a safety valve for gasses and vapors (right): set pressure (1) and lift (2).

Selection criteria

To protect your system against overpressure, it is essential to understand the five selection criteria below. Please read our technical article on selecting safety valves to better understand these criteria:

• Set pressure
• Backpressure
• Discharge capacity
• Operating temperatures
• Valve and sealing material

Applications

A safety valve's purpose is primarily for industrial applications to protect against overpressure, which can cause dangerous situations such as fire or explosions. Industrial safety valves are often found in:

• Oil, gas, and petroleum industry: For example, subsurface safety valves, or downhole safety valves, are common on offshore oil wells. In the case of equipment malfunction, a safety valve can shut off rapidly to prevent oil and gas from flowing up the well in unsafe conditions.
• Energy: Safety valves in power plants are common for compressible gasses such as steam and air.
• Sanitary: Stainless steel safety valves are ideal for industries that require sanitary conditions. For example, in the food, beverage, and pharmaceutical industries.
• HVAC: Safety valves relieve pressure in the case of blocked discharge, thermal expansion, or external heat that can damage the components.

Safety valve symbol

Figure 6: Varying safety valve symbols

Safety valve certifications

Safety valves must comply with various national and international standards for safety and quality. To ensure that the product complies, please consult local standards.

TÜV

The TÜV certification assesses a product's safety. It verifies that it meets the minimum requirements under the Pressure Equipment Directive (PED) /68/EU. The PED outlines the standards for designing and manufacturing pressure equipment such as pressure relief devices, steam boilers, pipelines, and pressure vessels operating at a maximum allowable pressure greater than 0.5 bar.

ASME

The ASME (American Society of Mechanical Engineers) ensures the specification and accreditation of pressure vessels, boilers, and pressure relief devices.

ISO

The ISO standard is a general specification for pressure relief valves, regardless of the application's media.

Pressure safety valve vs relief valve

Pressure safety valves and relief valves have important similarities and differences.

• Similarities: Both valve types are safety devices designed to automatically open at a set pressure level to prevent overpressure in a system.
• Differences: Pressure safety valves are designed to open rapidly and quickly relieve pressure. Pressure relief valves open more gradually to control the system's pressure decrease. Pressure relief valves are part of normal operation.

Learn more in our comprehensive pressure safety and relief valve comparison article.

FAQs

What does a safety valve do?

A safety valve rapidly reduces a system's pressure when it rises to unsafe levels. The safety valve continues operating until system pressure returns to safe levels.

What is the difference between a relief valve and a safety valve?

A relief valve will not immediately halt the operation of downstream components, whereas a safety valve will.

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What are the types of safety valves?

Common safety valve types are direct-acting, pilot-operated, and balanced bellows.

What is an ASME safety valve?

An ASME safety valve meets the requirements of the ASME pressure vessel code's Section I. These valves must have a large constant flow rate at no more than 10% overpressure.

At its core, a pressure safety valve is a specialized device engineered to automatically release excess pressure from within a system when the pressure exceeds maximum allowable working pressure. These valves are viewed as pressure relief devices that are crucial in all industrial systems, designed to automatically responding to overpressure events and preventing the system from reaching dangerous levels that could lead to equipment damage, process disruptions, or, in extreme cases, catastrophic failures. A pressure safety valve, often referred to as a safety relief valve, PRV or PSV, is a crucial component that ensures the integrity and safety of industrial processes. Its significance lies in the pivotal task of maintaining safe operating conditions within a system, preventing catastrophic failures and safeguarding both personnel and equipment.

Basics of Pressure Safety Valves

A. Definition and Purpose

At its essence, a pressure safety valve (PSV) is a One of the most critical automatic safety devices designed to prevent overpressure scenarios within industrial systems. Its fundamental purpose is to safeguard equipment, processes, and personnel during an over-pressure event in a pressurized vessel or equipment by automatically relieving excess pressure when the system's operational limits are exceeded. This fail-safe mechanism is indispensable in avoiding potential disasters that could arise from uncontrolled pressure build-up. A pressure safety valve is carefully engineered to automatically open and release surplus pressure from pressure vessels or equipment, ensuring a rapid return to normal conditions before securely re-closing to prevent any further fluid release.

B. Components and Mechanism of Action

1. Valve Disc, Seat, and Spring

The core components of a pressure safety valve include the valve disc, seat, and spring. The valve disc is a movable element that sits against the seat, forming a tight seal under normal operating conditions. The spring, calibrated to a specific tension, exerts force on the disc, counteracting the system pressure. When the pressure surpasses the inlet pressure, the spring tension is overcome, causing the valve disc to lift from the seat. This rapid action creates an opening for the excess pressure to escape, reinstating a balance within the system.

2. Set Pressure and Opening Mechanism

The set pressure, also known as the relief pressure or popping pressure, is the threshold at which the pressure safety valve activates. It is a predefined value based on the system's operational requirements and safety considerations. The opening mechanism involves the interplay between the spring force and the opposing system pressure. When the system pressure exceeds the set pressure, the valve opens, allowing the discharge of pressurized fluid. This controlled release ensures that the system returns to a safe pressure range, preventing potential hazards.

Types of Pressure Safety Valves

A. Spring-Loaded Pressure Safety Valves

Spring-loaded pressure safety valves are the most common and straightforward type. Their design includes a spring that applies force to the valve disc to keep it closed. Once the system pressure surpasses the preset limit, the spring force is overcome, allowing the valve to open and discharge excess pressure.

B. Pilot-Operated Pressure Safety Valves

Pilot-operated pressure safety valves use a pilot valve in addition to the main valve. The pilot valve senses the system pressure and controls the opening and closing of the main valve. This design offers greater accuracy and is often used in applications with variable pressure conditions.

C. Other Varieties

Beyond the primary categories, various other specialized pressure safety valves exist to serve a specific industrial requirements. These may include balanced-bellows safety valves, diaphragm safety valves, or pressure relief valves with specific features designed for distinct operational scenarios. Exploring these diverse variants ensures optimal selection based on the unique needs of different applications.

Working Principles

A pressure safety valve (PSV), often referred to as a pressure relief valve (PRV), is a type of safety valve used to control or limit the pressure in a system; it is designed to open at a predetermined set pressure to protect equipment and systems from being subjected to pressures that exceed their design limits. Here's how it typically works:

1. Set Pressure: Each PSV is set to open at a specific pressure known as the 'set pressure,' which is determined based on the maximum allowable operating pressure of the system it protects. The set pressure is usually a little higher than the system's normal operating pressure but below the maximum pressure the system can handle.
2. Spring-Loaded Mechanism: Many PSVs are spring-loaded. A spring inside the valve applies a force that keeps the valve closed under normal conditions. The force exerted by the spring is calibrated to balance against the system pressure.
3. Pressure Exceeds Set Point: When the system pressure increases and exceeds the set pressure of the valve, the force exerted by the pressurized fluid overcomes the spring force. This imbalance causes the valve to open, allowing fluid (gas, steam, or liquid) to flow out of the system through the valve.
4. Relief of Excess Pressure: By opening, the PSV relieves the excess pressure, discharging enough fluid to bring the system pressure back down to a safer level. The discharge is typically directed to a safe location where it will not pose a hazard.
5. Automatic Reclosing: Once the pressure in the system falls below the set pressure, the spring force pushes the valve back to its closed position, preventing further release of fluid. This automatic reclosing function is crucial as it allows the PSV to respond to future overpressure events without manual intervention.
6. Overpressure Protection: The primary function of a PSV is to prevent equipment damage, explosions, or failures due to overpressure. By limiting the system pressure, it ensures the safety and integrity of the equipment and the safety of personnel.
7. Testing and Maintenance: Regular testing and maintenance of PSVs are crucial. They should be periodically inspected and tested to ensure they open at the correct set pressure and are in good working condition.

Applications and Industries

A. Diverse Applications in Various Sectors

Pressure safety valves find widespread applications across diverse sectors due to their critical role in maintaining safe operating conditions. From petrochemical plants and manufacturing facilities to energy production and water treatment, these valves are integral to preventing overpressure events and ensuring the reliability of industrial processes.

B. Key Industries That Rely on Pressure Safety Valves

1. Chemical Industry: In chemical processing, pressure safety valves safeguard against potential hazards associated with the handling of unstable substances.
2. Oil and Gas: Petrochemical refineries heavily rely on pressure safety valves to protect equipment from overpressure situations, ensuring continuous and safe operations.
3. Power Generation: Power plants, whether thermal or nuclear, use pressure safety valves to maintain optimal pressure levels in steam systems, preventing damage to turbines and boilers.
4. Water Treatment: Pressure safety valves play a crucial role in water treatment plants, preventing excessive pressure in pipelines and ensuring the integrity of the water distribution system.

Exploring these applications highlights the versatility and indispensable nature of pressure safety valves across various industries.

Selection and Installation

A. Factors to Consider When Choosing Pressure Safety Valves

Selecting the right pressure safety valve is a critical decision that involves several key considerations:

1. System Parameters: Understanding the specific requirements of the system, including operating pressure, temperature, and flow rates, is crucial for selecting a valve that aligns with these parameters.
2. Valve Type: Choosing between spring-loaded or pilot-operated valves based on the application and industry requirements ensures optimal performance.
3. Material Compatibility: Considering the compatibility of valve materials with the processed fluids to prevent corrosion or degradation over time.

B. Proper Installation Practices

Ensuring the correct installation of pressure safety valves is essential for their effectiveness and longevity. Key practices include:

1. Location and Orientation: Placing the valve in a position where it can effectively release excess pressure and ensuring the correct orientation for optimal performance.
2. Piping and Venting Considerations: Properly sizing and designing the piping system connected to the valve to facilitate smooth pressure relief and venting. This involves accounting for potential backpressure and minimizing bends or restrictions in the piping.

Considering these factors and following proper installation practices is essential for the optimal performance of your pressure safety valve. For professional guidance and expert installation, trust THINKTANK, a company committed to ensuring the highest standards in safety valve installation for industrial systems.

Maintenance and Inspection

A. Regular Checks for Optimal Performance

Effective maintenance and regular checks are vital to ensure the pressure safety valve functions optimally over time. The following practices contribute to its longevity:

1. Periodic Schedule: Establishing a routine maintenance schedule based on industry standards and the valve manufacturer's recommendations.
2. Visual Inspection: Regularly examining the valve for visible signs of wear, corrosion, or damage that could compromise its performance.
3. Functional Testing: Conducting periodic tests to confirm the valve's response and proper operation under simulated pressure relief conditions.
4. Calibration Verification: Ensuring the valve's calibration aligns with the specified set pressure, providing accurate and reliable relief.

B. Identifying Signs of Wear and Issues

Routine inspections should include a detailed assessment for potential wear and issues, focusing on the following indicators:

1. Leaks: Detecting any fluid or gas leakage around the valve, which may signify a compromised seal.
2. Corrosion: Checking for signs of corrosion on the valve body or components, addressing potential material degradation.
3. Unusual Sounds: Listening for abnormal sounds during valve operation, as they could indicate mechanical issues.
4. Irregular Pressure Relief: Monitoring for instances where the valve activates inconsistently or fails to relieve pressure, suggesting a malfunction.

By incorporating these maintenance practices and attentively identifying signs of wear, operators can ensure the pressure safety valve remains reliable and ready to safeguard against overpressure events.

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Summary