download - TERMAN

Pressure Relief Devices
for oil filled electric Transformers
Description and general specifications
Operating instructions and maintenance
Finished product quality control tests
page
page
page
02
03
09
1

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 transformer’s 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
1 A 125V DC
Safety valves - types
The safety valves are built with different Major Diameters and rated pressure to satisfy the requirements of
the various applications.
TYPE MAJOR DIAMETER RATED PRESSURE PREVAILING USE
T 125 - VS 150 125mm. 0,3 1 bar big power transformers
VS 100 100mm. 0,3 1 bar medium power transformers
T80 - VS 80 80mm. 0,3 1 bar small power transformers
T 50 – VS 50 50mm. 0,3 1 bar cable boxes - small tanks
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.
2

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 gasbubbles
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 :
VALVE TYPE
T 50
VS 50
T 80
VS 80
VS 100
T 125
VS 150
OIL QUANTITY
up to 1.000 dm 3 (900Kgs.)
up to 1500 dm 3 (1300Kgs.)
up to 4.000 dm 3 (3.600Kgs.)
up to 6.000 dm 3 (5.400Kgs.)
up to 20.000 dm 3 (18.000Kgs.)
up to 20.000 dm 3 (18.000Kgs.)
up to 35.000 dm 3 (31.500Kgs.)
3

In case of tanks containing larger quantities of oil must be mounted 2 or more valves of suitable dimensions
(i.e.: if oil quantity is 95.000dm 3 must be mounted 3 valves type VS 150).
SAFETY VALVES type 50T – 80T – 125 T
4

SAFETY VALVE type VS 50
Safety Valve type VS 50
5

SAFETY VALVE type VS 80
Safety Valve type VS 80
equipped with 1or 2 change-over contacts
6

SAFETY VALVE type VS 100
88.0
100.0 100.0
4
158.0
DN100
Ø114.0
Ø135.0
Ø180.0
18
Ø26.0
100.0
112.0
4
102.5
102.5
25.0
170.0
15.0
Safety Valve type VS 100
equipped with 1 change-over contact
7

SAFETY VALVE type VS150
127.0
4
199.0
DN150
Ø168.0
Ø189.0
Ø240.0
142.5
160.0
Ø 18
Ø26.0
4
285.0
294.0
50.0 190.0
16.
Safety Valve type VS 150
equipped with 1 or 2 change-over contacts
8

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. It’s 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.
I EDITION AUGUST 2006
Terman '90 S.r.l., Via Ghisalba, 13-20/21, 20021 Bollate (MI) – Italy. Tel: +39 02 38 30 37 12, Fax: +39 02 38 30 37 19,
E-Mail info@terman.com, www.terman.com
C.F./P. IVA 09970270154 – C.C.I.A.A. 1332904 – Trib. Milano Reg. Soc. 302729 – Cap. Soc. € 119.000
9