All You Need to Know About Pressure Relief Valves

Asset Management, Compliance & Regulations, Plant Inspection September 3, 2024

If you work in a plant or operate within the mining, chemical, petrochemical, oil and gas, or energy sectors, you understand that safety is paramount. A critical component of your safety infrastructure is the pressure relief valve (PRV).

Pressure relief valves are essential safety devices designed to protect systems, equipment, people, and the environment from overpressure conditions. By automatically releasing pressure when it exceeds a pre-set limit, PRVs prevent potential damage, explosions, and safety hazards.

PRVs play a vital role in preserving the integrity and lifespan of your equipment, ensuring smooth operations while mitigating the risk of catastrophic failures. In industries where high-pressure systems are the norm, PRVs are indispensable, safeguarding both your machinery and personnel.

The Basic Functionality of PRVs

PRVs operate on a simple yet effective principle: They open at a set threshold to relieve excess pressure from a system. Once the pressure within the system returns to a safe level, the valve closes, so the system can continue to operate normally.

This automatic pressure regulation prevents the system from exceeding safe operating limits, thereby protecting critical components from stress and potential failure. The precise design and calibration of PRVs ensure that they respond quickly and reliably to provide pressure relief, making them vital for the safe operation of pressurised systems.

Applications of PRVs

Pressure relief valves are used across a wide range of industries, with their application being particularly critical in sectors where pressure control is a key aspect of daily operations.

In the mining sector, for example, PRVs are employed in a range of systems, such as slurry pumps, crushing systems, ventilation systems, and hydraulic circuits to prevent overpressure situations that could lead to costly downtime or hazardous conditions. In addition to mining, PRVs are also commonly found in chemical processing plants, oil and gas pipelines, power generation facilities, and even domestic heating systems. Their versatility and reliability make them a fundamental component in any environment where pressure management is required.

Types of Pressure Relief Valves

As multiple industries use PRVs, there is more than one type of pressure relief valve. Different types of pressure relief valves exist because each one is designed to handle specific conditions and requirements depending on the industry and operating conditions.

The diversity in PRV types allows for tailored solutions that ensure optimal safety, performance, and regulatory compliance in a wide range of scenarios.

Conventional Pressure Relief Valves

Conventional pressure relief valves are the most commonly used type of PRV. They rely on a spring mechanism to open and relieve pressure when it exceeds a pre-set limit. Valve performance can be affected by backpressure (the pressure on the outlet side of the valve), which can reduce the valve’s effectiveness by altering the set pressure.

These valves are typically used in applications where backpressure is either minimal or consistent and can be accounted for in the system design, such as in pressure vessels, pipelines, and general process systems.

Balanced Bellows Relief Valves

Balanced bellows relief valves are designed to mitigate the effects of backpressure that can influence the operation of conventional relief valves. They incorporate a bellows mechanism that isolates the valve spring from backpressure, maintaining consistent performance regardless of variations in outlet pressure.

This makes them ideal for applications where fluctuating or high backpressure is expected, such as in high-pressure steam systems, chemical processing plants, and oil refineries. Additionally, the bellows provide protection against corrosive gases or fluids, enhancing valve longevity in harsh environments.

Pilot-Operated Relief Valves

Pilot-operated relief valves are sophisticated devices that use an auxiliary pressure pilot to control the opening and closing of the main valve. This design allows for precise control over the pressure relief process, so these valves are ideal for high-pressure applications where accuracy and reliability are paramount.

They are often used in critical systems, such as those found in refineries, chemical processing plants, and offshore drilling rigs. Their ability to handle high pressures with minimal pressure loss during operation makes them a preferred choice for applications requiring tight pressure control.

Temperature and Pressure Safety Relief Valves

Temperature and pressure safety relief valves (TPRVs) are specialised valves that offer dual protection by responding to both pressure and temperature conditions. These valves are commonly used in systems where both pressure and temperature must be closely monitored, such as in water heaters, steam boilers, and solar heating systems.

TPRVs are designed to open when either the pressure or temperature exceeds safe operating limits, providing comprehensive protection against potential hazards. Their dual-function capability ensures that systems remain safe and efficient, even under fluctuating temperature and pressure conditions.

Power Actuated Pressure Relief Valves

Power actuated pressure relief valves are a more advanced type of PRV, controlled by external power sources such as electric, pneumatic, or hydraulic actuators. These valves are typically used in systems where precise control over the relief operation is required. The actuator can adjust the valve opening in response to system conditions, providing more flexibility than conventional or balanced valves.

These PRVs are often found in high-stakes industrial settings like nuclear power plants, aerospace applications, and large-scale chemical processing, where safety and precision are paramount.

Vacuum Pressure Relief Valves

Vacuum relief valves serve a different purpose than traditional pressure relief valves. Instead of preventing overpressure, these valves protect equipment from vacuum or negative pressure conditions that could cause implosion or structural damage. They open to allow air or fluid into the system when the internal pressure drops below the set point. Vacuum relief valves are commonly used in storage tanks, pipelines, and systems that handle volatile liquids, where negative pressure could lead to equipment failure or product contamination.

Pressure Safety Valves vs Pressure Relief Valves

Pressure safety valves and relief valves are both critical components in pressure management systems, designed to protect equipment and personnel from the dangers of excessive pressure. While they share the common goal of preventing overpressure, they differ in their design, operation, and typical applications.

Here’s a comparison between safety valves and relief valves:

Purpose and Function

Safety Valves

•  Purpose: Safety valves are primarily designed to protect people and equipment by quickly releasing pressure from a system when it exceeds a predetermined level. They are often considered the last line of defence in preventing catastrophic failures.
• Function: Safety valves operate on a ‘pop-off’ principle, meaning they open fully and quickly when the system reaches the set pressure. Once the pressure drops to a safe level, the valve closes just as rapidly.

Relief Valves

• Purpose: Relief valves are intended to control pressure by gradually releasing it when it exceeds a set point, maintaining system stability. Their primary function is to protect equipment and processes rather than being purely a safety mechanism.
• Function: Relief valves open proportionally to the increase in pressure, allowing for a controlled release of excess pressure. The valve will open gradually as the pressure increases and close gradually as the pressure decreases.

Design and Operation

Safety Valves

• Design: Safety valves are designed with a spring-loaded mechanism that keeps the valve closed under normal operating conditions. When the pressure exceeds the pre-set limit, the force overcomes the spring tension, causing the valve to ‘pop’ open.
• Operation: The valve opens rapidly and completely at the pre-set pressure, and it typically has a higher lift, meaning it allows for a large discharge area to quickly release pressure. This rapid action is essential in systems where sudden pressure spikes can occur.

Relief Valves

• Design: Relief valves are also spring-loaded but are designed to open in proportion to the increase in pressure. The spring and valve seat are engineered to allow the valve to open gradually.
• Operation: Relief valves open slowly and adjust the opening size based on the pressure differential. This proportional opening is ideal for systems where maintaining a steady pressure is more critical than quickly relieving a sudden spike.

Applications

• Safety Valves: These are commonly used with compressible fluids (like gas) and in applications where a sudden pressure surge can lead to a catastrophic event, such as in steam boilers, chemical processing plants, and gas storage tanks. Their quick response makes them ideal for systems where human safety is a primary concern.
• Relief Valves: These are typically used with incompressible fluids (like water and certain oils) and in systems where maintaining a constant pressure is essential, such as in liquid pipelines, hydraulic systems, and pump systems. They are often used where the goal is to protect equipment from gradual overpressure rather than sudden spikes.

Closing Characteristics

• Safety Valves: Safety valves usually have a ‘snap’ action, meaning they close quickly once the pressure drops below the set point. This ensures that the system pressure does not fall too low, as this could affect operations or safety.
• Relief Valves: Relief valves close gradually as the pressure decreases, ensuring that the system does not experience significant fluctuations in pressure. This gradual closing helps maintain system stability and prevents the potential for oscillations in pressure.

Regulatory Considerations

• Safety Valve Regulation: Safety valves are often governed by stricter regulations and standards because they are critical to human safety. For instance, in boiler applications, safety valves must comply with specific codes such as the ASME Boiler and Pressure Vessel Code.
• Relief Valves Regulation: While relief valves are also subject to industry standards, they are often governed by regulations focused on equipment protection rather than direct human safety.

Regulations are critical to maintaining compliance, particularly in industries like oil and gas, mining or chemical processing.

PRV Components and Working Principles

Being familiar with the components and working principles of pressure relief valves is crucial for ensuring their functionality and effectiveness in protecting systems from overpressure.

PRVs are precisely engineered to respond to specific pressure conditions, making their construction and operation integral to their performance. By delving into the key components and the mechanics of how PRVs operate, we can better appreciate the role they play in maintaining safety and preventing system failures.

Key Components

• Valve Body: The outer casing that contains and supports all other components. It must be robust and durable to withstand high pressures.
• Bonnet: The bonnet is the upper part of the valve that houses and protects the internal components, such as the spring and spindle. It provides a sealed enclosure that helps maintain valve structural integrity and prevents leakage. The bonnet also serves as a mounting point for the spring, ensuring that the valve operates correctly under pressure.
• Spring: A calibrated component that exerts force on the valve disc, keeping it in place until the system pressure exceeds the spring’s pre-set limit.
• Spindle: The spindle, also known as the stem, is a crucial moving part within the PRV that connects the disc (which seals the valve seat) to the spring mechanism. When pressure in the system exceeds the set limit, the force on the spindle causes the disc to lift, opening the valve and allowing excess pressure to be relieved.
• Disc: The movable component that seals against the seat under normal conditions and lifts to release pressure when necessary. The disc must be made from materials that can withstand the system fluid and temperature conditions.
• Seat: The part of the valve against which the disc seals to prevent fluid from escaping. The seat surface must be smooth and free from defects to ensure a tight seal.
• Nozzle: Directs the flow of the released fluid when the valve opens, ensuring that pressure is safely and efficiently relieved.

Each of these components is carefully designed and tested to ensure that the PRV operates reliably under the specific conditions for which it is intended.

Operation Mechanism

The operation of a pressure relief valve is straightforward but highly effective. Here’s how it works:

1. Normal Operation: Under standard operating conditions, the system pressure is below the PRV’s pre-set pressure. The spring exerts force on the disc, keeping it tightly sealed against the seat, preventing any fluid or gas from escaping.
2. Pressure Increase: As system pressure rises and approaches the set pressure, the force exerted by the fluid on the disc overcomes the spring tension.
3. Valve Opening: When the system pressure exceeds the set pressure, the disc lifts off the seat, allowing fluid to escape through the nozzle. This release of fluid reduces system pressure.
4. Pressure Relief: The valve remains open until the system pressure drops back below the set pressure. Once this occurs, the spring pushes the disc back into the seat, closing the valve and stopping the flow of fluid.

This mechanism allows PRVs to respond quickly and accurately to changes in system pressure, ensuring that overpressure conditions are safely managed.

Flow Dynamics

Flow dynamics play a critical role in the performance of pressure relief valves. Fluid velocity, density, and viscosity all affect how the valve operates:

• Velocity: As fluid velocity increases, the forces acting on the disc change, which can affect the valve’s ability to open and close effectively.
• Density: Denser fluids exert more force on the disc, which may require adjustments to the spring tension or pre-set pressure.
• Viscosity: Highly viscous fluids can cause drag on the moving parts of the valve, potentially leading to slower response times.

Engineers must carefully consider these factors when selecting and setting up PRVs to ensure they function correctly in the intended application.

Installation and Maintenance

Proper installation and maintenance are critical to the performance and longevity of pressure relief valves. Incorrect installation can lead to operational issues, while inadequate maintenance can result in valve failure, potentially leading to dangerous overpressure conditions.

Installation Guidelines

When installing pressure relief valves, following best practices is essential to ensure they function as intended:

• Location: Install PRVs at points in the system where pressure is likely to peak. They should be easily accessible for inspection and maintenance.
• Orientation: Most PRVs are designed to be installed vertically, with the valve body upright. Installing them in the correct orientation ensures proper operation.
• Connection: Ensure that the inlet and outlet connections are free from obstructions and that the piping is correctly sized to avoid pressure drop or turbulence.
• Replacement Timing: PRVs should be replaced according to regulatory and original equipment manufacturer or Original Equipment Manufacturer (OEM) guidelines. Regular replacement is crucial because valve components, particularly springs and seals, degrade over time due to exposure to pressure, temperature fluctuations, and system fluid.

Maintenance Procedures

Regular maintenance is vital to ensure that pressure relief valves remain in good working condition:

• Cleaning: Periodically clean the valve components, especially the seat and disc, to remove any build-up of debris or corrosion that could affect sealing.
• Lubrication: Lubricate moving parts according to the OEM’s specifications to reduce wear and ensure smooth operation.
• Testing: Perform regular operational tests to verify that the valve opens and closes at the correct set pressure. Testing should be conducted at least annually or as specified by the OEM.

These maintenance tasks help extend the life of the valve and prevent unexpected failures that could lead to system downtime or safety hazards.

Inspection Requirements

Routine inspections are essential for detecting potential issues before they lead to valve failure:

• Visual Inspection: Check for signs of corrosion, wear, or damage to the valve body, connections, and components, and that the PRV is free from debris or contaminants.
• Leak Detection: Inspect for any leaks around the valve seat or connections, which could indicate a compromised seal.
• Performance Check: Verify that the valve operates correctly during system pressure fluctuations. Record any deviations from expected performance.

Regular inspections are a key part of any asset management strategy.

If you’re looking to enhance the reliability and safety of your systems, partnering with Asset Management Engineers (AME) ensures you will receive expert guidance and support in maintaining or replacing PRVs and other critical components.

Troubleshooting Common Issues

Even with proper installation and maintenance, pressure relief valves can experience issues that need to be addressed promptly:

• Valve Leakage: If the valve leaks when it should be sealed, check for debris on the seat or a weakened spring that may require replacement.
• Valve Sticking: A valve that does not open or close smoothly may have corrosion or buildup that needs to be cleaned, or it may require lubrication of moving parts.
• Inconsistent Operation: If the valve opens at pressures lower or higher than its set point, recalibration or spring replacement may be necessary.

Industry Standards and Regulations

Being critical safety devices, PRVs must meet stringent industry standards and regulations to ensure their reliability and effectiveness. Compliance with these standards is a legal requirement in many industries, including oil and gas, mining, chemical processing, and power generation.

Overview of Relevant Standards

Several key standards govern the design, testing, and operation of pressure relief valves and each one is established by respected organisations in the field:

• American Society of Mechanical Engineers (ASME): ASME’s Boiler and Pressure Vessel Code (BPVC) Section VIII is one of the most widely recognized standards for pressure relief devices. It outlines the requirements for the construction and testing of PRVs to ensure they perform reliably under pressure.
• American Petroleum Institute (API): API Standard 520 and 521 provide guidelines for the sizing, selection, and installation of pressure-relieving systems, including PRVs, in the oil and gas industry. These standards are crucial for ensuring that PRVs are appropriately chosen and maintained in high-risk environments.
• International Organization for Standardization (ISO): ISO 4126 specifies safety devices for the protection of pressurised systems, including the design and testing of PRVs. ISO standards are internationally recognised, making them essential for global operations.
• Australian Standards: In Australia, AS 1271 is the primary standard governing PRVs. It specifies the requirements for valves used in pressure vessels and boilers, ensuring that they are designed, tested, and maintained to the highest safety standards.
• European Standards: The Pressure Equipment Directive (PED 97/23/EC) is a European Union regulation that sets the standards for the design, manufacture, and conformity assessment of pressure equipment and assemblies. The directive applies to a wide range of equipment, including pressure relief valves (PRVs), and aims to ensure safety and reliability across the EU.

Regulatory Compliance

Compliance with industry regulations is not optional—it is a legal obligation for businesses operating pressurised systems. Failure to adhere to these regulations can lead to severe repercussions, including fines, legal action, and more importantly, safety risks.

Regulatory bodies such as Safe Work Australia and state-based safety regulators enforce these standards to protect workers and the public from the dangers associated with overpressure. Non-compliance can also lead to operational downtime, increased insurance premiums, and damage to a company’s reputation. Therefore, it is essential for businesses to regularly audit their compliance with relevant PRV standards and regulations.

Updates and Amendments

Industry standards and regulations are not static; they are regularly updated to reflect new technologies, materials, and safety insights. Staying informed about these changes is critical to ensure your company remains compliant and continues to operate safe pressurised systems.

Regularly reviewing updates from organisations like ASME, API, ISO, and Australian Standards bodies can help you stay ahead of the curve. In some cases, amendments to standards may require modifications to existing PRV installations or the adoption of new maintenance practices.

PRV Inspection Services by AME

Proper inspection and maintenance of pressure relief valves are critical to the safety and reliability of pressurised systems. AME provides specialised PRV inspection services that ensure compliance with industry standards and mitigate the risks of valve failure.

With nearly two decades of experience, AME is your trusted PRV supplier, delivering premium pressure relief valves from the industry’s best. Our team of qualified professionals provides comprehensive PRV inspections tailored to the mining, oil and gas, mineral processing, manufacturing, industrial, construction, rail transport, and government infrastructure sectors. We ensure that certified replacements meet the strictest safety and performance standards, so your pressure relief valves operate safely and efficiently.

AME’s PRV inspection services are designed to ensure that your pressure relief valves operate safely and efficiently in compliance with Australian Standards and WHS regulations

Our Inspection Process

AME follows a thorough and methodical inspection process to ensure the reliability and safety of your PRVs:

• Initial Assessment: A detailed evaluation/inspection of your existing PRVs on-site, including a review of their installation, condition, and compliance with relevant standards in accordance with AS 1271/AS 3788 & API 576
• Component Inspection: A close examination of PRVs for any signs of damage to the anti-tamper seal, tag , any clogging, dust, corrosion, or damage that could affect safety and performance.
• Reporting and Recommendations: A comprehensive report on the condition of your PRVs, including any issues and non-compliances found and recommendations for certified replacements, or adjustments.

Benefits of Partnering With AME

Choosing AME as your PRV inspection and supply partner offers several advantages:

• Expertise: AME’s team comprises certified professionals with extensive industrial experience, ensuring precise and reliable pressure vessel inspections.
• Advanced Technology: We use cutting-edge non-destructive testing technology to detect even the smallest flaws, so you get accurate results.
• Safety and Compliance: AME prioritises operational safety by conducting thorough inspections that ensure PRVs and pressure vessels meet regulatory standards and operate safely.
• Comprehensive Reporting: Our detailed post-inspection reports contain expert insights, recommended actions, and best practices for maintaining pressure vessel and systems integrity.
• Customer Focus: AME tailors its services to meet the specific needs and schedules of clients, emphasising a customer-centric approach.
• Commitment to Excellence: AME is dedicated to maintaining high safety standards, extending equipment lifespan, and promoting operational excellence through its inspection services.

As a trusted PRV supplier, AME delivers top-tier pressure relief valves from leading manufacturers, guaranteeing certified replacements that uphold the highest safety and performance standards.

Contact us today to discuss your PRV inspection and supply needs!