There are numerous types of Pressure Safety Valves (PSVs). Apart from correct sizing and installation, deciding on the appropriate choice for your specific application can be a daunting task. The main objective of this article is to provide an overview of the most prevalent types of PSVs, as well as a comprehensive understanding of their operational principles. This article will focus on the following varieties:

• Conventional Spring-loaded PSV,
• Balanced Bellows PSV,
• Pilot Operated PSV.

Conventional Spring Loaded PSV

A conventional spring-loaded pressure safety valve (PSV) is a mechanical device designed to protect equipment and process systems from excessive pressure. It is commonly used in various industries, including oil and gas, chemical, pharmaceutical, and manufacturing.

Conventional spring-loaded PSVs operate based on the principle of spring compression. The spring is pre-set to a specific pressure, known as the set pressure or set point. When the pressure in the equipment or process system exceeds the set point, the spring force is overcome, causing the valve to open and relieve the excess pressure.

Main Components of a conventional spring loaded PSV

The main components of a conventional spring-loaded PSV are:

1. Valve Body: The valve body is a housing that contains the internal parts of the PSV. It is connected to the process system or equipment that requires protection.
2. Spring: The spring is a key component of the PSV. It is responsible for controlling the opening and closing of the valve based on the set pressure. The spring is selected based on the desired set pressure and the required capacity of the PSV.
3. Disc or Disk: The disc or disk is the component that seals the valve seat when the PSV is in the closed position. When the pressure exceeds the set point, the force exerted by the spring is overcome, causing the disc or disk to lift, allowing the excess pressure to be released.
4. Nozzle: The nozzle is the inlet of the valve through which the excess pressure is discharged.
5. Bonnet: The bonnet is the cover that protects the internal parts of the PSV. It is designed to withstand the operating conditions and pressures of the process system. The bonnet has a vent hole, which is plugged (in contrast with balanced bellows PSVs).

Basic operation of a conventional spring loaded PSV

The basic operation of a conventional PSV is straight forward.

1. Opening Pressure: The set pressure is the pressure at which the PSV is fully open and discharging the excess fluid or gas to maintain the system pressure below the set value. It is typically slightly higher than the operating pressure to ensure reliable operation. The difference between the opening pressure and the set pressure is called the overpressure. The overpressure is determined by the accumulation.
2. Set Pressure: The opening pressure is the pressure at which the PSV starts to open and relieve excess pressure. It is predetermined and set based on the specific system requirements and safety regulations.
3. Simmer Point: The simmer point of a conventional spring-loaded Pressure Safety Valve (PSV) refers to the condition when the valve starts to release a small amount of fluid or gas to maintain the system pressure just below the set pressure. It is also known as the “lift-off” point. It is usually about 2% below the set pressure.
4. Reseat Pressure: The reseat pressure is the pressure at which the valve closes after relieving the excess pressure. It is typically lower than the set pressure and ensures that the valve remains closed until the system pressure rises above the desired threshold again. The difference between the set pressure and the reseat pressure is called the blowdown. It accounts for the pressure drop that occurs during the PSV operation and helps maintain the pressure within an acceptable range. It is usually around 7%, but is sometimes adjustable.
5. Shut-off Pressure: The shut-off pressure is the maximum pressure that the PSV can handle without any leakage. It is lower than the reseat pressure and ensures that the valve remains closed even during extreme pressure conditions.

A small note about the simmer point

When the pressure in a system exceeds the set pressure of the PSV, the valve begins to open gradually. Initially, the valve lifts just enough to allow a small amount of fluid or gas to escape, relieving the excess pressure. This allows the pressure inside the system to drop slightly, eventually stabilizing at a level slightly below the set pressure.

The simmer point is important because it helps prevent unnecessary and frequent cycling of the PSV. By releasing pressure in small increments, it helps to maintain the system pressure within a specified range without fully opening the valve. This avoids sudden and large pressure imbalances that could potentially harm the system or its components.

Once the simmer point is reached, the PSV remains in a partially open state until the pressure further increases and reaches the full set pressure, at which point the valve will fully open until the opening pressure is reached, to discharge the excess flow and protect the system from overpressure.

Advantages & disadvantages of a conventional spring loaded PSV

Some of the advantages of conventional spring-loaded PSVs:

1. Simplicity: Conventional spring-loaded PSVs have a straightforward design with fewer components, making them easier to manufacture, install, and maintain compared to other types of PSVs.
2. Cost-Effectiveness: They are generally more affordable than other PSV types due to their simpler design and lower manufacturing costs.
3. Wide Range of Applications: Conventional PSVs can be used in various industries and applications, including oil and gas, chemical, pharmaceutical, and manufacturing, making them versatile and widely applicable.
4. Reliable Operation: These PSVs have a proven track record of effective pressure relief in protecting equipment and process systems from excessive pressure, providing reliable operation in a range of conditions.
5. Compatibility: Conventional spring-loaded PSVs can easily be integrated into existing systems without significant modifications, making them compatible with different setups and configurations.

Some disadvantages that should be considered:

1. Limited Back Pressure Handling: They may not handle high back pressure situations as effectively as other PSV types, potentially leading to reduced performance or valve failure under certain conditions.
2. Inaccurate Set Pressure Control: Achieving precise set pressure control can be challenging with conventional PSVs, as the compression of the spring can introduce variations and potential inaccuracies in the actual opening and closing pressure.
3. Limited Temperature Range: Conventional PSVs may have limitations in operating at extreme temperatures, as the materials and components used in their construction may degrade or malfunction under very high or low temperatures.
4. Maintenance and Adjustment Needs: These PSVs require periodic maintenance, such as spring testing and adjustment, to ensure that the valve functions properly and remains within the desired set pressure range. This maintenance needs to be performed by trained personnel.
5. Lack of Flexibility: Unlike some other PSV types, conventional spring-loaded PSVs can offer limited flexibility in terms of set pressure adjustment or adaptation to changing process conditions, potentially requiring the replacement of the entire valve for significant changes.

Balanced bellows PSV

A balanced bellows pressure safety valve is a type of pressure relief valve that utilizes a bellows element to maintain a balanced pressure on the valve disc. On the other hand, a conventional pressure safety valve operates without a bellows and relies on the system’s pressure to act directly on the valve disc.

One of the key advantages of a balanced bellows pressure safety valve is its ability to handle back pressure more effectively. The bellows element in these valves helps to isolate the valve disc from the inlet pressure, resulting in better control and stability even in situations where there is significant back pressure, up to 35 – 50%.

Main components of a balanced bellows PSV

The main components of a balanced bellows PSV:

1. Valve Body: The valve body is the main housing of the pressure safety valve. It provides a connection between the inlet and outlet of the valve, allowing the flow of fluid or gas through the system.
2. Bellows Assembly: The bellows assembly is a crucial component of a balanced bellows pressure safety valve. It consists of a flexible metal bellows that acts as a sensing element. The bellows expands and contracts in response to changes in pressure, thereby helping to control the opening and closing of the valve.
3. Spring: The spring in a balanced bellows pressure safety valve provides the necessary closing force to keep the valve in a closed position. It counteracts the pressure acting on the bellows, ensuring that the valve remains closed until the set pressure is reached.
4. Disc: The disc, also known as the valve disc or disk, is a movable component that seals the valve seat. When the pressure exceeds the set limit, the disc lifts off the seat, allowing the fluid or gas to be discharged through the valve.
5. Seat: The seat is a stationary component against which the disc closes when the valve is in the closed position. It provides a tight seal to prevent any fluid or gas from passing through the valve until the set pressure is reached.
6. Bonnet: The bonnet is the cover that encloses the internal components of the pressure safety valve. It provides protection and helps to secure the different parts in place. Important: the vent hole in the bonnet is unplugged, in contrast with conventional PSVs.
7. Nozzle and Flanges: The nozzle serves as the inlet for the fluid or gas entering the valve, while the flanges provide connections for attaching the valve to the piping system. Proper sizing and compatibility of the nozzle and flanges are essential for a secure and leak-free installation.

Advantages & disadvantages of a balanced bellows PSV

Some of the advantages of a balanced bellows pressure safety valve compared to a conventional pressure safety valve:

1. Improved Reliability: The balanced bellows design helps to minimize the effects of back pressure, ensuring reliable operation and reducing the risk of valve failure.
2. Better Set Pressure Accuracy: The bellows element helps maintain a consistent and accurate set pressure, ensuring precise control over the pressure relief process.
3. Increased Stability: The balanced bellows design provides greater stability during operation, especially in scenarios where there are fluctuations in back pressure. This helps to prevent premature valve opening or closing.
4. Enhanced Flexibility: The bellows allows for a wider range of operating conditions, making the valve suitable for applications with varying back pressure requirements.
5. Reduced Maintenance: The use of a bellows element can minimize the wear and tear on the valve disc, resulting in longer service life and reduced maintenance needs.

A balanced bellows pressure safety valve (PSV) offers several advantages, such as increased sensitivity and stability, as well as better performance in high-pressure applications. However, there are also a few disadvantages to consider when comparing it to a conventional PSV:

1. Complexity: Balanced bellows PSVs tend to be more intricate in design compared to conventional PSVs. The presence of a bellows assembly makes them more complex to manufacture, install, and maintain. This complexity can lead to higher costs for procurement, calibration, and servicing.
2. Limited temperature range: The bellows within a balanced bellows PSV is sensitive to temperature changes. While they work well within a specified temperature range, extreme heat or cold may affect their performance. Conventional PSVs, on the other hand, do not have this limitation and can operate across a wider temperature range.
3. Potential for leaks: The bellows assembly in a balanced bellows PSV could be more susceptible to leaks compared to conventional PSVs. The presence of moving parts and the need for proper sealing introduce the possibility of leaks over time. This necessitates regular inspection and maintenance to ensure proper functioning and reliability.
4. Size limitations: Due to the design requirements of a balanced bellows PSV, they may have size limitations. If a system requires a very large or very small PSV, it may be more challenging to find a balanced bellows PSV that meets the specific size requirements compared to a conventional PSV.

Despite these disadvantages, a balanced bellows PSV still offers many benefits in terms of enhanced performance and sensitivity, making it a suitable choice for certain applications.

In summary, a balanced bellows pressure safety valve offers advantages in terms of back pressure handling, reliability, set pressure accuracy, stability, flexibility, and maintenance. These benefits make it a preferred choice in applications where back pressure is a significant factor to consider.

Pilot Operated PSV

Pilot operated PSVs are commonly used in systems where a high-pressure fluid or gas needs to be controlled. They consist of two main components: the pilot valve and the main valve. The pilot valve is responsible for controlling the opening and closing of the main valve.

Main elements and operation of a POSV

The pilot valve is usually connected to a sensing line, which allows it to monitor the pressure in the system. When the pressure exceeds a predetermined setpoint, the pilot valve opens, allowing the fluid or gas to flow through the main valve and be safely discharged. Once the pressure is reduced to an acceptable level, the pilot valve closes, stopping the flow.

One of the advantages of pilot operated PSVs is their ability to provide precise and reliable pressure relief. The use of a pilot valve allows for better control, ensuring that the valve opens at the desired setpoint and closes once the pressure is reduced. This makes them suitable for applications where tight pressure control is necessary.

The pilot valve handles the precise handling of the set point. The main valve takes care of the capacity. The main valve is closed by means of an unbalanced piston. The area on top of the piston (A_Dome) is higher than the area of the main valve seat (A_Seat). Until the setpoint has been reached, the pressure on top and below of the piston is the same. This results in a downward force, which increases with increasing pressure. This means that the tightness of the valve increases the more the pressure approaches the set pressure. This is in contrast with conventional spring operated valves, which start to leak when the set pressure is approached.

When the pilot reaches the set pressure, it depressurizes through the pilot vent, because of which the pressure in the dome decreases. This creates an upward force in the piston, resulting in the opening of the main valve. It is not shown on the picture below in detail, but when the pilot opens, there is no constant flow from the pressure/sensing line to the pilot vent. Pilot valves are mostly ‘non flowing’ to prevent the pilot from clogging with fluid, debris, polymers, etc. This ensures the correct and precise working of the pilot.

Another advantage of pilot operated PSVs is their ability to handle higher pressures compared to spring-operated relief valves. Pilot operated PSVs can handle larger flows and higher pressures, making them suitable for high-pressure systems where the operating pressure is close to the set pressure of the PSV.

Lastly, pilot operated PSVs can handle much bigger back pressures than conventional PSVs without the need for delicate bellows.

Advantages and disadvantages of a POSV

Advantages of pilot operated safety valves:

1. Precise Pressure Control: Pilot operated safety valves offer precise and reliable pressure relief, ensuring that the valve opens at the desired setpoint and closes when the pressure is reduced. This makes them suitable for applications where tight pressure control is necessary.
2. Higher Pressure Handling: Pilot operated safety valves can handle larger flows and higher pressures compared to spring-operated relief valves. This makes them suitable for high-pressure systems where the operating pressure is close to the set pressure of the safety valve.
3. Better Back Pressure Handling: Pilot operated safety valves can handle much bigger back pressures without the need for delicate bellows. This makes them ideal for systems with significant back pressure, ensuring efficient and reliable operation.

Disadvantages of pilot operated safety valves:

1. Increased Complexity: Pilot operated safety valves tend to be more complex in design compared to conventional spring-loaded safety valves. The presence of a pilot valve requires additional components and increased manufacturing, installation, and maintenance complexity.
2. Regular Maintenance Required: Pilot operated safety valves need periodic maintenance to ensure proper functioning and reliability. The pilot valve, in particular, may require inspection and cleaning to prevent clogging and ensure accurate pressure control.
3. Higher Costs: Pilot operated safety valves are generally more expensive than conventional spring-loaded safety valves due to their increased complexity and additional components. The procurement, calibration, and servicing costs associated with pilot operated valves can be higher as well.

Conclusion

This article gives an introduction to the biggest differences between the main types of PSVs. It should provide you some key knowledge to help you find the right PSV for you application. As always, discuss your specific application with the vendor / constructor to find the best solution and discuss its limitations.

Further reading:

Pressure/Safety/Relief valve? A clear definition

5 step PSV design

Set pressure – ASME – PED