Pressure Relief Valve

Pressure Relief Valve

Product Range

Pressure Relief Valve - Product Range

TypeMajor DiameterRated PressurePrevailing Use
T 125 - VS 150125mm0,3 ÷ 1 barbig power transformers
VS 100100mm0,3 ÷ 1 barmedium power transformers
T 80 - VS 8080mm0,3 ÷ 1 barsmall power transformers
T 50 - VS 5050mm0,3 ÷ 1 barcable boxes - small tanks


Description and General Specifications

Aim of the safety valves on electric transformers
The transformer tank filled with cooling liquid is a container subject to internal pressure and then has to be provided with one or more safety valves suitably calibrated for the maximum allowed pressure, so that overpressure caused by internal faults can be instantaneously relieved through the valves, thus avoiding greater damages such as the deformation or even the burst of the tank and the spraying of hot oil with subsequent fire risks. It is necessary to protect the transformer tank with a suitable equipment capable of almost instantaneously discharging overpressure already at the development, before the structure integrity is damaged and the safety valves has just this function.

General features
The safety valves consist schematically of:

  • a valve base comprising the valve opening venting area with its specially profiled gasket and a seat for an O-ring gasket on the flanged end towards the transformerís tank;
  • a valve cap pressed against the profiled gasket by calibrated helical spring, thus making the valve completely tight up to the rated pressure;
  • a splash diverter to avoid damages caused by hot oil sprinkles (on request);
  • a single or a double electrical contact (on request).

Mechanical protection degree : IP 65

Insulation : 2000V 50Hz between terminals and earth for a 60 seconds time

Cable gland : PG 13,5

Microswitch breaking capacity : 10 A 250V AC 1A 125VDC

Safety valves – types
The safety valves are built with different Major Diameters and rated pressure to satisfy the requirements of the various applications.

Safety valves series VS : the advantage of safety valves serie VS consists in the total absence of projecting parts in the transformers tank making the mounting point choose easier. These valves can be mounted, with regards to their base plane, both horizontally on the cover and vertically on the transformer walls at the points where their safety action is presumed to be more necessary.

Safety valves series T : the advantage of safety valves series T is that, showing a good effectiveness and reliability of valves serie VS, their simplicity consent to obtain very competitive prices.

Operating instruction and Maintenance

Instruction for mounting safety valves

The data about the transformer point where a short circuit is most likely to occur, the preferential direction the resulting shock wave may have, the intensity this one can reach, all depend on the transformer power, its transformation ratio, its construction characteristic and the behaviour of the other installed protection equipment. Therefore, is not possible to give strict rules about safety valves application. It is the manufacturer who must decide each time and on his own experience the valve type and its position.

It is important to take these general suggestions into due consideration for a correct choice :

  • the safety valves must be mounted near the points where a failure is most likely to occur. When a single valve is mounted the barycentric position with regards to these points shall be chosen;
  • the exhaust rated diameter shall be connected with the transformer oil quantity and with the number of mounted valves;
  • the presence of gas or air-bubbles under the valve cap may negatively affect valve opening speed. Therefore, before operating the transformer, it is important to let possibly accumulated air come out through the suitable breather screw and above all to mount the valve so that the possible gas- bubbles developed during operation can not accumulate on the valve;
  • the operating pressure must be definitely lower than the maximum allowed pressure in the tank, but higher than possible pressure peak reached during operation.

Mounting and Maintenance

The safety valves mounting is carried out by means of the suitable fastening holes of the flange, after the splash diverter removal and after the insertion of the O-ring gasket supplied with the valve. After the transformer filling, the air developed under the valve must be breathed by unscrewing the suitable breathing screw. This breathing screw shall be tightened again as soon as the oil starts to come out. During operation the safety valves do not need a particular maintenance. Nevertheless, it is convenient to regularly check the electric contact good operation and to verify if there is no gas accumulation.

Instruction for ordering safety valves

The exhaust rated diameter shall be connected with the transformer oil quantity and with the number of mounted valves. When a single valve is mounted the barycentric position, with regards to the points where a failure is most likely to occur, must be chosen. To help the Technical Department to state types and number of valves to be mounted on a transformer, is possible to study the following indicative table :

In case of tanks containing larger quantities of oil must be mounted 2 or more valves of suitable dimensions 3 (i.e.:ifoilquantityis95.000dm mustbemounted3valvestypeVS150).

Finished Product Quality Control Tests

Hydraulic test for tightness for the body : the aluminium alloy case is mounted on the wall of a tank filled with oil at the temperature of 90°C and at the pressure of 300 kpa. After 8 hours no leakages must be observed.

Dimensional check.

General function test : the safety valve is mounted on a testing pressure circuit at the atmospheric pressure value. The pressure, controlled by a microprocessor based monitoring unit and recorded by a special recording instrument, slowly increases up to the Operating Pressure value – 5 kpa. The valve remains at this pressure value for 6 hours. During this period the valve must be completely sealed. Following three different gradients the pressure increases again to the Operating Pressure value. Itís verified that the effective pressure of opening of the valve is contained in the following interval : O.P. -5% < O.P. < O.P. +8%.

Check of contacts actuation : this test is performed by means of visual indicators mounted on a panel board. Commutation tolerance, commutation differential and electrical circuits are verified.

Check of electrical contact unit mechanical protection degree : IP 65.

Insulation test : carried out by means of a microprocessor controlled testing unit.