PRESSURE RELIEF VALVE OF A TRANSFORMERS or PRV
A pressure relief valve of a transformer or PRV is a mechanical safety device used for the protection of transformers, on-load tap changers OLTC and other similar equipment which can release the excess pressure inside them because of various reasons. The Pressure relief valve of a transformer mitigates heavy damage to the external shell of any equipment due to a sudden rise in internal pressure beyond a pre-determined safe limit.
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The ideal sealed transformer tank’s pressure is between 2-5 psi. But as the oil inside the tank experiences breakdown due to internal faults like short circuits in the windings etc. Heavy arcing is developed inside the tank which causes the formation of gas rapidly inside the tank as the oil decomposes or reaches the vaporization stage. The formation of gas gives a sudden rise in the tank’s pressure which can damage the fabrication of the tank if the internal pressure precedes the designed pressure that the tank can withstand.
Therefore, the sudden rise in pressure must be normalized and this is done using the pressure relief valve of a transformer. The operating times of PRVs are instantaneous. It shall be noted that prv or prd’s are not control valves. They are spring-loaded fast-acting safety valves.
CONSTRUCTION:
The pressure relief valve of a transformer is basically a spring-loaded safety valve. It has a means of providing instantaneous amplification of actuation force. It consists of a pressure die-cast aluminium flange which seats on the mounting gasket. The stainless steel diaphragm seats on the wiper gasket which is pressed by two heavy-duty reversed wound springs. It seals the port against the wiper gasket and top gasket.
The pressure die-cast aluminium cover retains and compresses the spring and is held in place by compression screws. The spring retainer prevents the dislocation of the spring during operation. It has a flag which is operated by switching the operating rod. The switch assembly for the alarm houses the NO & NC contacts for wiring.
WORKING:
These valves are designed to release the excess pressure in a very short time. When the internal pressure rises in the area defined by the top gasket beyond the predetermined safe limit which is set upon the springs of the valve, the diaphragm gets lifted from its seat exposing the area of the wiper gasket to the transformer’s internal pressure. This results in greatly exposed force causing extreme and rapid opening of the diaphragm corresponding to the closed height of the springs. As the transformer’s internal pressure is rapidly reduced to normal value the springs return the diaphragm to a sealed position.
The lift of the diaphragm operates a brightly coloured flag and the limit switch with the help of the switch operating rod. This flag is clearly visible from a great distance. It indicates that the valve has operated. The flag and the switch remain in operated condition until they are manually reset by tilting the flag down.
The operating pressure for the pressure relief valve of a transformer is a factory preset and cannot be changed at the site. The operating pressure ranges from 6-10 psi.
PRVs are suggested to be mounted on a horizontal plane. However, it can be mounted vertically.
MAINTENANCE OF PRESSURE RELIEF VALVE OF A TRANSFORMER:
The pressure relief valve of a transformer is almost maintenance-free as it is a rugged device. It is recommended that the compression screws on the cover plate should never be removed without extreme caution. The operation of the switch may periodically be checked by manually lifting the switch operating rod. It should be reset before putting the instrument back in service. The pressure relief valve of a transformer or pressure relief device shall never be painted because any exposure to paints can alter the working pressure range of the device and may cause it to malfunction.
WIRING DIAGRAM
In normal working conditions, the valve is normally seated and the flag/semaphore is also not lifted, the NO contact is closed and it gives a normal working annunciation. When the PRV operates and releases the excess pressure, the NC contact closes and generates a tripping signal for the circuit breakers along with an alarm.
It is recommended to check the operation of the flag and switch by lifting the switch operating rod manually and matching the same with the wiring diagram confirming both operations work as required before the commissioning of the device.
FAQ’s
Are the PSV and PRV same?
Yes, the pressure safety valve and the pressure relief valve of a transformer are the same.
How to release pressure from the transformer?
To release the sudden rise in pressure in the transformer tank, we use the Pressure relief valve. It is the last protection device for the transformer which operates to save the transformer from explosion by normalizing the pressure.
What is the use of a pressure relief valve in a transformer?
It helps to release the excess pressure developed inside the oil-type transformer because of serious faults inside by the opening of the diaphrgm of the valve when the pressure inside reaches beyond a set limit.
What is the principle of PSV?
When the fluid force inside the transformer is more than the set force of the valve, it releases the spring, which holds the diaphragm of the valve in place, and the PSV or pressure relief valve operates.
What is the semaphore in the pressure relief valve?
It is a deep coloured flag mostly red or yellow, it indicates the operation of valve when is vertical, actuated by the switch operating rod of the pressure relief valve or device.
When does the PRV trip occur in a transformer?
When the transformer is overloaded for long hours, the heat dissipation is too much to be contained which leads to gas formation and high pressure inside the tank.
When there is a serious fault in the winding of the transformer, which generates high heat leading to vaporization of oil and increase of pressure inside the tank
