What is a safety valve and what is the purpose of a safety valve

AshlinMay 22, 2020

178 8 minutes read

What is a safety valve and what is the purpose of a safety valve?

A safety valve can be considered as a pressure reducing or pressure removing device. When there is any excessive internal fluid pressure then this valve would open so that the damage in the system can be prevented. These valves are commonly used in gas and steam lines. In an unprotected pressure vessel or a system if the pressure level exceeds the safe pressure level, then there could be catastrophic effects on both plant and personnel. The major purpose of a safety relief valve is to protect any pressurized system from the effects of exceeding its design pressure limit. These valves are designed to automatically discharge gas pressure or liquid from any pressure-containing system and thus it prevents excessive pressure and protects plants and personnel. In order to fulfill all this, a safety valve should be properly sized, selected, installed, and maintained.

What are the characteristics of the safety valve?

These valves are mostly used for compressible fluids such as steam or other gases
Only if the pressure drops to the normal pressure, the safety valve would close otherwise it would stay fully open.
In a safety valve, the inlet port is smaller when compared to the discharge port
An external valve lever is used in the safety valve and it can be seen at the top of the valve body, which is used as an operational check.

What is the difference between the safety valve and the relief valve?

A relief valve would release the liquid or open the valve in a proportional manner so that it can maintain some system pressure. In the case of a safety valve, it would quickly lift and reduce the pressure instantly. The major difference is in the capacity and set point. The relief valve would relieve pressure in case of overpressure conditions, it has an operator that would give a control signal to open the valve. Safety valves operate without the help of an operator. The relief valve would open gradually as the fluid pressure increases while the safety valve opens fully when the set pressure is reached. The safety valve will open fully if the system achieves the opening pressure and there is no half or semi-open position for a safety valve like the relief valve.

How to select a safety valve

According to the media (air, gas, steam, liquid)
Set pressure, popping pressure, and relieve pressure
According to the required capacity for the application
Factors such as temperature, back pressure, and corrosive media or environment must be considered
According to the operational characteristics
Normal working pressure is the operating pressure of the system under full load
Maximum allowable working pressure, it is the maximum pressure existing normal operating conditions
Set pressure, it is the pressure at which the valve starts to lift
Overpressure, it is the pressure that exceeds the set pressure and the valve starts to lift in this pressure

What is a safety relief valve

Safety relief valves are widely used in most of the process equipment. These valves can be used as either a relief valve or a safety valve. These valves can be used in gas and vapor system as safety valves and they can also be used as a relief valve in a liquid system.

What is safety valve gagging

Gagging is the process of arresting the stem mechanically from lifting. Gagging must not be done in cold conditions.

What are the components of a pressure safety valve

The above figure shows the major parts of the conventional pressure safety valve

Cap –it is the top part of the safety valve, which covers the adjustment screw this could be threaded or bolted to the bonnet.

Bonnet – it is the middle part of the valve and it houses the stem, spring, spring buttons, and guide plate. This device could be open or closed type.

Body – it is the bottom-most portion and it encloses disc holder, disc, nozzle blow-down ring, and locking nut.

Adjustment screw – This device is used for the calibration of pressure safety valve in a specified set pressure

Stem – This device properly aligns the components and transmits the power of the spring to the disc assembly

Spring and spring buttons – Spring will help to keep the valve closed and the spring button can uniformly distribute the spring force.

Lift lever – Lift lever is used for manual pressure relief

Disc and disc holder – Disc is the device that would be affected by process pressure and they are vulnerable to galling, erosion, and pitting

Nozzle – Nozzle is the entrance by which the process fluid enters into the valve. There are many types of nozzles available they are fully threaded and removable, semi threaded and removable, semi, welded in the valve body, semi pressed and removable.

How safety valves are constructed and what are the materials used for the construction

Sometimes safety valves must operate under severe conditions like low temperatures, corrosive fluids, etc. So special materials are used for the construction. Mostly the pressure containing components of the pressure valves are constructed by using bronze, cast iron, or stainless steel. Bronze is used for small screwed valves and these valves are used for the low and medium sectors. Higher pressure valves are constructed by using cast steel.

The internal components of safety valves such as spindle or guides which is the moving part must be constructed by using materials that will not easily corrode. The seats and discs must be able to resist corrosion, so stainless steel is used for their construction. Sometimes nozzle discs are constructed by using special alloys such as Hastelloy or monel.

How excess pressure takes place in a system and How severe condition arises in a system

Transient pressure surge
Plant fire exposure
Heat exchanger tube failure
Ambient temperature changes
Failure of the cooling system and it would cause the vapor to expand
Uncontrolled exothermic reactions in a chemical plant

How does a safety relief valve work

If the pressure rises above the set pressure in a system, the disc will begin to lift off its seat. The spring will start to compress and the spring force also increases. So the lift will happen only if the pressure is increased again and after that, the flow through the valve occurs. Overpressure is the additional pressure rise needed before the safety valve will discharge at its rated capacity. Overpressure is dependent upon the set pressure and the standard for the particular application. The disc in the safety valve is arranged in a way that a slight increase in pressure than the set pressure could cause the valve to open. When the pressure reaches a safe level in the system the valve would return to its closed position. The disc is arranged for rapid opening in order to achieve this most safety valves have a secondary chamber formed by a shroud, skirt, or hood around the outside diameter of the disc.

What are the types of safety valves

Conventional safety valve

This type of valve, it has a spring housing that is vented to the discharge side of the valve. The operational characteristic of the valve is affected by the changes in the backpressure of the valve. This is the simplest type of safety valve and they are used in the backpressure and are very small.

Balanced safety valve

Balanced safety valves are capable to remove back pressure effects with the help of bellows or other equipment. These valves are spring-loaded safety relief valves, these valves are installed when the percentage build-up back pressure in the exhaust system is allowed to exceed the percentage overpressure applicable to the safety valve. Balanced safety valves are of two types, piston type, and bellow type.

Pilot operated safety valves

A pilot-operated safety valve is a pressure relief valve, in which the major relieving device is with a self-actuated auxiliary relief valve. The relieving device is controlled by the auxiliary relief valve. Pilot operated safety valves are of two types they are diaphragm and piston type. The diaphragm type is used for low-pressure applications. In the case of the piston-type valve, it has a main valve that uses a piston-shaped closing device and an external pilot valve.

Types of safety valves on the basis of the nozzle

Full nozzle safety valve

In this type the nozzle is the raised face portion of the flange, it is used for liquid and vapor relief and also has a closed bonnet.

Semi nozzle safety valve

These valves are also used for liquid and vapor relief. Nozzle consist of insert screwed into base connection can be flanged, weld ended or screwed

Base nozzle safety valve

In this type, the nozzle is formed from the base of the valve, mostly these valves have a screwed connection but they are also available in flanged or weld-ended and these valves would fully open at twenty-five percent overpressure and they have closed bonnet.

What is an electromatic safety valve

This valve is a pressure switch initiated safety valve and it is set to lift on a particular pressure but a regular safety valve will only lift after the accumulation of pressure.

How to install the safety valve

During the plant commissioning procedure, the valve would be lifted and there would be seat damage if there is debris or dirt. So the system should be flushed out before installing a safety valve if done so foreign matter won’t pass through the valve. The valve must be installed in a place where dirt and debris must not collect. The safety valve should be mounted vertically in a pressure vessel. The pressure drop in the valve inlet piping must not exceed more than three percent of the set pressure. In steam applications, the safety valve must be installed above the steam pipe. It should not be installed below because, the steam will condense, fill the pipe, and wets the upstream side of the safety valve seat.

How to troubleshoot and maintain a safety valve

The system operating pressure must not be too close to the valve set pressure. A safety valve needs a five to ten percent difference between the operating and set pressure. Safety valves must not be set to less than 15PSI. It should be in the range of 20-25 according to the pressure that the vessels could handle. We must not install safety valves horizontally safety valves are meant to work only in a vertical position, if not it won’t work properly. Safety valves must be stored in a dry environment and protected from weather, they should not be removed from the skids or crates until immediately prior to installation. The valve must be kept in a way that the inlet is down so that damage can be prevented. Safety valves must be subjected to an annual periodic inspection. If the safety valves are installed outdoor then they would be exposed to wind, air, snow, dirt, so certain safety features must be done. The inlet neck of the safety valve and the valve body must be insulated and the exterior part of the insulation must be weatherproof. Certain spares must be needed for the safety valve they are flat lapping plate, high-temperature lubricant, a lapping compound of grit size.

What can be the cause of safety valve leakage

Leakage can happen if there is dirt or scale sitting on the seating surface. Lifting of the lever can remove the dirt. Mostly the leakage can be caused after the initial manufacture and test, this problem occurs from the damage during transit. If the installation is not proper it can cause valve leakage. Teflon coat or pipe dope can cause valve leakage. So it must be checked when the valve is installed we must ensure that it is not too low on threads if this gets into the valve it can stick on the seating surface and can cause a leak.

What are the applications of safety valves

Power plants and heat exchangers cycle
Pressure reduction and air system
Refineries and paper mills
Pressure booster system of water and air
Chemical and pharmaceutical plants
Offshore application
Steam and industrial boiler system

To get more info about the control valve check the following link

AshlinMay 22, 2020

178 8 minutes read