In the process industry, both terms refer to safety devices, which generally come in the form of valves, cylinders, and other cylinders that protect people, property, and the environment. Safety valves and relief valves are integral components of process safety. However, they are used for almost identical purposes. Their main difference lies in their operating mechanisms.

Relief Valve and safety valve prevent equipment damage by relieving accidental over-pressurization of fluid systems.

In this article, we will compare Relief valves and safety valves and know the following:

• •  Relief valve
• •  Safety Valve
• •  Relief valves versus safety valves
• •  the function of relief and safety valves?
• •  What is a setpoint
• •  Pilot-Operated Relief Valves
• •  The main difference between a relief valve and a safety valve is the extent of opening at the setpoint pressure.

Relief Valve

What is the purpose of relief valves?

In the event of an overpressure, a safety valve or pressure relief valve (PRV) protects pressure-sensitive equipment. It is recommended to strip down relief valves regularly and prevent serious damage due to backpressure. Pressure relief valves are a crucial part of any pressurized system. In order to prevent system failures, you can set the pressure to open at predetermined levels. A setpoint, also known as a predetermined design limit, is set for all pressure systems. When the setpoint is exceeded, an overpressure valve opens.

A relief valve, illustrated in Below Figure, gradually opens as the inlet pressure increases above the setpoint. A relief valve opens only as necessary to relieve the over-pressure condition.

 

Figure 1: Relief Valve

A safety valve, illustrated in Below Figure, rapidly pops fully open as soon as the pressure setting is reached. A safety valve will stay fully open until the pressure drops below a reset pressure.The reset pressure is lower than the actuating pressure setpoint. The difference between the actuating pressure setpoint and the pressure at which the safety valve resets is called blowdown.Blowdown is expressed as a percentage of the actuating pressure setpoint.

Relief valves are typically used for incompressible fluids such as water or oil. Safety valves are typically used for compressible fluids such as steam or other gases.

Safety Valve

There are various types of safety valves used in several types of industries, including power plants, petrochemical plants, boilers, oil and gas, pharmaceuticals, and more. Using safety valves helps to prevent accidents and injuries that can harm people, property, and processes. Pressure builds up in vessels and systems automatically when the device is activated above a preset level. Safety valves must be configured so that their prescribed pressure is exceeded in order for them to function (i.e., relieve pressure). Ideally, excess pressure should be released either to the atmosphere or back into the pneumatic system to prevent damage to the vessel. In addition, excess pressure should be released to keep pressure within a certain range. As soon as a slight increase in pressure above the desired limit has lifted the safety valve, it opens.

Safety valves can often be distinguished by the presence of an external lever at the top of the valve body, which is used as an operational check.

As indicated in Below Figure, system pressure provides a force that is attempting to push the disk of the safety valve off its seat. Spring pressure on the stem is forcing the disk onto the seat.

At the pressure determined by spring compression, system pressure overcomes spring pressure and the relief valve opens. As system pressure is relieved, the valve closes when spring pressure again overcomes system pressure.

Most relief and safety valves open against the force of a compression spring. The pressure setpoint is adjusted by turning the adjusting nuts on top of the yoke to increase or decrease the spring compression.

 

Figure: Safety Valve

Relief valves versus safety valves

Valve relief removes excessive pressure from a system by limiting its pressure level to a safe level. Often referred to as pressure relief valves (PRVs) or safety relief valves, these valves provide relief from pressure. The purpose of a relief valve is, for example, to adjust the pressure within a vessel or a system so that a specific level is maintained. The goal of a relief valve, unlike a safety valve, is not to prevent damage to the vessel; rather, it is to control the pressure limit of a system dynamically depending on the requirements. Conversely, safety valves have a maximum allowable pressure set at a certain level, which allows escaping liquid or gas whenever the pressure exceeds it, eliminating damage to the system. It is imperative that safety valves are installed in a control system to prevent the development of pressure fluctuations that can cause property damage, life loss, and environmental pollution. 

What is the function of relief and safety valves?

The hydraulic system relies on a pressure relief system in order to regulate the running pressure. By allowing excess pressure to escape from the pressurized zone, pressure relief valves and safety valves prevent overpressure when the pressure in the system reaches a predefined limit. By venting excess pressure through a relief port, or returning it through a return line, a pneumatic system can enable the excess pressure to escape into the atmosphere. Pump-driven pressure generators and control media that cannot be vented into the atmosphere are typical examples of this type of application.

A comparison of the functions of safety valves and relief valves

Excess pressure may be relieved from the system using relief valves and safety valves. The valve opening increases proportionally as the vessel pressure increases with the relief valve. Gradually opening the valve rather than abruptly releasing only a prescribed amount of fluid. As pressure is reduced, the release proceeds at this rate until the pressure drops. By contrast, an emergency safety valve operates automatically when a predetermined pressure is reached in the system, preventing a catastrophic system failure. When the system is under excessive stress, the safety valve regulates the pressure within the system and prevents overpressure.

What is a setpoint?

Defining a “setpoint” is the process of defining a pressure level that triggers the device to vent excess pressure. Setpoint is different from pressure. Overpressure is prevented by setting these devices lower than the highest pressure the system can handle before overpressure occurs. Setting the device below this pressure prevents overpressure. The valve opens when pressure rises above the setpoint. A setpoint also known as the maximum allowable working pressure (MAWP) cannot be exceeded when deciding the pressure in pounds per square inch (PSIG). The adjustment points for safety valves are generally 3 percent above working pressures, while adjustment points for relief valves are 10% above working pressures.

Choosing between a safety valve and a relief valve.

Pressure in an auxiliary passage can be controlled by a safety valve as well as a relief valve by releasing excess pressure. Safety valves of this type are pressure-sensitive and reliable. Safety valves can be categorized according to their capacity and setpoint, although both terms often refer to safety valves. Self-opening devices open automatically when maximum allowable pressure has been reached rather than being manually activated to prevent over-pressurizing. Contrary to relief valves, safety valves are typically used for venting steam or vapor into the atmosphere. Relief valves regulate fluid flow and compressed air pressure and gases, whereas safety valves typically regulate steam and vapor venting. Put simply, relief valves are used for more gradual pressure control requiring accurate, dynamic systems, whereas safety valves are used for one set to prevent damage to a system.

Pilot-Operated Relief Valves

Pilot-operated relief valves are designed to maintain pressure through the use of a small passage to the top of a piston that is connected to the stem such that system pressure closes the main relief valve.

When the small pilot valve opens, pressure is relieved from the piston, and system pressure under the disk opens the main relief valve.

Such pilot valves are typically solenoid operated, with the energizing signal originating from pressure measuring systems.

• (This "Fire Safety Blogs" Published in  April 2024  Edition)