Instrumentation Questions Part One

Pressure Safety Valves PSV's play important role in safeguarding industrial systems and equipment from over-pressurization. The valves designed to automatically release excessive pressure when it exceeds the pre-determined level.

In this article we are going to explain the PSV working principle, Parts, Types, PSV sizing steps, Applications and Differences between PSV & PRV

How Does a PSV Work?

PSVs operate based on a simple yet effective principle. They consist of a spring-loaded closure element that is held in place against the pressure in the system. 

When the pressure rises beyond the set point, the spring force is overcome, and the valve opens. This allows excess pressure to escape, relieving the system and preventing further pressure build-up.

PSV Parts and Components

Let's explore the PSV parts and components and their functions:

• Body: The valve body, usually made of cast steel or stainless steel, forms the backbone of the PSV, providing structural integrity and containment of the flowing media. It contains an internal chamber where pressure buildup is sensed.
• Inlet Connection: This connection serves as the entry point for the fluid being controlled. It's typically a flanged or threaded connection designed to match the system's pipeline.
• Outlet Connection: The outlet connection provides the discharge path for the excess fluid when the valve opens. It's also usually flanged or threaded.
• Disc (or Seat): This critical component seals the valve in its closed position, preventing fluid flow. It's typically made of durable materials like stainless steel or hardened alloys, designed to withstand high pressures and temperatures.
• Spring: The spring exerts a force against the disc, holding it closed and establishing the set pressure. The spring's compression determines the pressure at which the valve opens.
• Spring Adjuster: This external component allows for precise adjustment of the spring force, thereby changing the set pressure of the valve.
• Blowdown: The blowdown feature in a PSV ensures that the valve remains open until the pressure drops below a certain point, preventing premature closure and ensuring efficient discharge.
• Gasket: A replaceable gasket seals the connection between the valve body and the inlet and outlet connections, preventing leaks.

Types of PSV's

There are various types of PSV's available, each designed to suit specific applications and requirements:

- Conventional PSV's: These are the most common types of PSV, consisting of a direct spring-loaded closure element.

- Pilot-Operated PSV's: These valves use a pilot valve to actuate the main closure element. They are suitable for high-pressure applications where a large discharge capacity is required.

- Balanced-Bellows PSV's: These valves have a bellows assembly that balances the pressure on the closure element, allowing them to operate in both positive and negative pressure conditions.

- Relief Valves: These PSV's are designed to release excess pressure in closed systems, such as tanks or containers.

Sizing Steps of PSV's

Proper sizing of PSV's is essential for ensuring effective protection against over-pressurization. The following steps outline the general sizing procedure:

1. Identify the maximum allowable working pressure (MAWP) and maximum expected operating pressure (MEOP) for the system.
2. Establish the type of relief scenario, such as overpressure due to loss of cooling, start-up, or fire.
3. The set pressure should be slightly higher than the MEOP but below the MAWP.
4. Determine the maximum flow rate that the PSV must handle based on the relief scenario.
5. Use the flow rate to select a PSV that meets the required capacity.
6. Ensure that the selected PSV has sufficient capacity to handle the flow rate and meets the desired set pressure.

Common Applications of Pressure Safety Valves PSV's

Here are some of their prominent applications:

• Oil and Gas Industry: In oil and gas extraction and processing, PSV's are indispensable for controlling pressure in separators, tanks, pipelines and other pressure equipment.
• Chemical Processing Plants: Chemical plants frequently involve hazardous substances and high pressures. PSV's are essential for safety, ensuring safe discharge of hazardous fluids in case of overpressure.
• Pharmaceutical Industry: PSV's ensure safe processing and storage of pressure-sensitive pharmaceuticals and prevent overpressure situations that could compromise product integrity.
• Food and Beverage Industry: PSV's contribute to safety and process control in food and beverage processing, particularly in high-pressure applications involving pasteurization or sterilization.

The Differences: PSV vs. PRV

The primary difference between Pressure Safety Valves PSV's and Pressure Relief Valves PRV's  lies in their * intended function * and the "standards" they comply with. Here's a concise breakdown:

Pressure Safety Valves PSV's

Primary function: Prevent catastrophic system failures by relieving excess pressure and to ensure safe operation of equipments.

Standards: PSVs are designed and tested to strict industry standards outlined in ASME Section VIII, Div. 1, and API 520.

Performance: PSVs are designed to open quickly and fully at a predetermined set pressure, regardless of the pressure build-up rate.

Applications: They are extensively used in high-pressure applications involving flammable, toxic, or corrosive substances where safety is paramount. Examples include boilers, pressure vessels, pipelines, and chemical processing plants.

Pressure Relief Valves PRV's

Primary function: Provide pressure protection and prevent overpressure situations, but their primary role is not safety-critical.

Standards: PRV's are typically governed by less stringent standards than PSVs, often conforming to ASME B31.1 for piping systems or specific industry-specific requirements.

Performance: PRV's adjust gradually to maintain a stable pressure level. PRV's often have a slower opening mechanism compared to PSV's and might not fully open at the set pressure, especially for smaller pressure increases.

Applications: PRVs are commonly employed in lower-pressure applications where safety concerns are less critical. They can be found in water systems, hydraulic systems, and air systems.

Table of Main Key Differences between PSV's and PRV's

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