Safety & Relief Valve? Working Principle & Function | Linquip
Safety & Relief Valve

Safety & Relief Valve? Working Principle & Function

Pressure Relief Valves (sometimes called Safety and Relief Valves) are designed to open at a specific pressure and release fluid until the pressure drops to safe limits. The origins of the safety release valve are intriguing to learn about. Pressure relief valves that are properly maintained can last up to 30 years. Instead of simply replacing the valve in question, it's usually more vital to investigate the "why" behind pressure relief valve failure.

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What Is Safety Valve | Linquip

Construction of a safety valve (Reference: leser.com)

What is Safety Valve?

The safety valve functions as a fail-safe. The pressure relief valve (PRV), which automatically releases a substance when pressure or temperature exceeds preset limits, is an example of a safety valve. In the Industrial Revolution, steam boilers were the first to use safety valves. Early boilers operated without them were prone to explosions unless they were carefully operated.

Safety valves protect against pressure increases that can cause malfunctions, fire hazards, or explosions. They are used when electronic and pneumatic safety devices fail as they have only mechanical parts. By fully actuating a safety valve via the system media, it keeps it working in the event of a power failure. Safety valves open when a set pressure is exceeded; the valve first opens a little and then fully opens to remove as much unwanted pressure as possible. Safety valves open much faster than relief valves.

You can find many safety valve Companies and Manufacturers in Linquip, along with expert Service Providers.

Components of a Safety Valve

The construction of a spring-loaded safety valve is as follows:

  • The body of the valve has two branch pipes.
  • Over the valve seat are two separate valves.
  • Two pivots are used to place the lever over the valve.
  • With the help of a spring, a lever is kept tight in its position.
  • The lever is connected to one end of the spring and the valve body to the other.
  • With the help of spring force, the valve is kept in place on its seats.

Working Principle of a Safety Valve

The safety valve protects the boiler from excessive steam pressure inside the drum. A safety valve lets excess steam escape into the atmosphere if the pressure of steam in the boiler drum exceeds its working pressure. Consequently, the drum's steam pressure falls.

For example, in a spring-loaded safety valve, under normal conditions, springs exert a higher downward force than steam exerts an upward force. Due to spring force, the valve is closed. During high steam pressure, the upward force caused by steam pressure is greater than the downward force caused by spring. As a result, the valves are lifted from their seats, allowing steam to escape from the boiler.

The valve, therefore, operates in three states:

Equilibrium

A system is in equilibrium when the closing forces, i.e., the spring and any back pressure, are at an equal (balanced) level with the opening forces, i.e., the system pressure. The disc and seat are just contacting, so the flow has not yet begun.

Fully Open

At the specified overpressure, the valve is in its fully open state, allowing maximum flow. Accordingly, in order for the valves to open, the force applied by the system must exceed the equilibrium force, i.e., the spring force plus back pressure force.

Safety valves with a higher disc lift and larger discharge size can achieve a higher discharge flow rate.

Fully Closed

Having the valves fully closed results in no leakage. It follows that the closing force exceeds that of the opening force; hence the opening force is less than the equilibrium force.

Function of a Safety Valve

In general, safety valves are designed to protect lives, property, and the environment. A safety valve is used to open and release excess pressure from vessels or other equipment and to reclose and prevent the further discharge of fluid after normal conditions have been restored.

Safety valves provide a level of safety that is, in many cases, the last defense against a catastrophe. In all circumstances and at all times, the safety valve should be able to operate. It is important to remember that safety valves are not process valves or pressure regulators and should not be used as such. Only one purpose should be served by it: overpressure protection.

There are several reasons why the pressure inside a vessel or system can exceed the predetermined limit. These are the most common:

  • Blocked discharge
  • External fire exposure often called "Fire Case"
  • Thermal expansion
  • Uncontrollable exothermic chemical reaction
  • Rupture of the heat exchanger tube
  • Expansion of a fluid or vapor due to a cooling system failure.
  • Ambient temperature changes
  • Unbalance of fluid flow rate caused by inadvertently closing or opening isolation valves on a process vessel.
  • Transient pressure surges
  • Failure of electric power or compressed air to control instrumentation

Listed above are various events that can occur independently and simultaneously. There will also be a different volume or mass flow discharged depending on the cause of overpressure. For example, a thermal expansion will create a small mass flow, and a chemical reaction will create a large mass flow. Thus, an appropriate pressure safety valve must be sized and selected by taking into account a worst-case scenario.

Linquip offers a wide variety of Distributers, Experts, and Equipment for Sale of safety valves.

Usage of Safety Valves

Usage of Safety Valves | Linquip

Industrial safety valve for a steam system (Reference: spiraxsarco.com)

Safety valves are typically installed in pressure vessels such as chemical plants, electric power boilers, and gas storage tanks to protect them from exploding or damaging.

As the pressure increases on the inlet side of the valve, the Safety Valve opens the valve disc and discharges fluid (steam or gas). When the pressure lowers to the prescribed value, the valve disc closes again. The safety valve is designated as a final safety device, as it controls the pressure and releases fluids on its own without any electrical power supporting it.

In addition to ensuring the safety of the energy industry, safety valves support the safety and security of our lives.

Common Applications of Safety Valves

Typical safety valve applications include protecting against overpressure in industrial settings. There can be dangerous situations, fires, and explosions due to this overpressure. The oil, gas, and petroleum industries often use safety valves, such as downstream of pressure-reducing controls. Also, safety valves are typically used to protect boilers from overpressure in steam systems.

Below are some examples of safety valve applications:

  • Oil and Gas industry
  • Chemical industry
  • Pharmaceutical industry
  • Petrochemical industry
  • Food and beverage industry
  • Heating and air conditioning
  • Energy Industry

Oil and Gas industry

The Oil and Gas industry, including the downstream and midstream of refining and petrochemical industry, is the largest industry for pressurized applications in the world. In these applications, you will find:

  • Pressurized vessels
  • Thermal expansion in piping systems
  • Pressure-increasing stations

Chemical Industry

As far as valves in general and overpressure protection devices, in particular, are concerned, the Chemical industry has the highest demand compared to capacity. Safety valves are required in the following situations:

  • Pressurized vessels
  • Thermal expansion in piping systems
  • Pressure-increasing stations
  • Chemical reactors or autoclaves

Pharmaceutical Industry

There are a lot of special requirements for cleanability, materials, and documentation in the pharmaceutical industry. In this area, safety valves are required for the following applications:

  • Pressurized vessels
  • Piping systems
  • Reaction vessels
  • Filling stations



Features of Safety Valves

In order to protect the system against overpressure, each safety valve has some features that should be considered while selecting one:

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

FAQ about Safety Valve

  1. What is a safety valve used for?

Safety valve is usually used to protect boilers from over pressurization as well as to reduce pressure downstream. While safety valves are primarily used for safety, they are also utilized in process operations to prevent damage to products due to excess pressure.

  1. How does safety valve work?

The force Fp will decrease as the pressure in the vessel decreases. However, the flow still acts on the enlarged area of the disc at set pressure, so the valve will remain open. It is necessary to reduce the pressure further until the spring force Fs is again greater than Fp so that the safety valve can close.