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PRESSURE RELIEF DEVICES FOR POWER ... - Cedaspe Powered By Docstoc
					                                   PRESSURE RELIEF DEVICES
                                  FOR POWER TRANSFORMERS
                                     (with electric contacts)


When it is required to limit the pressure rise inside a tank, in order to prevent an excessive mechanical stress of the walls,
it is necessary to use a safety valve set at a precise overpressure value.
The tank of oil-immersed transformers is usually fit with this kind of protecting device; as matter of fact, in case of short-
circuit due to an insulation failure, the dielectric arc between alive parts vaporises the surrounding insulating fluid which
generates a quick rise of the pressure inside the tank, with the risk of permanent deformations, or, even, of the failure of
the tank walls with the consequent flow-out of hot oil.
Due to the rapidity of this event, it is necessary to mount on the transformer an adequate protecting device which relieves
quite suddenly the excess of pressure generated inside the tank by the above mentioned failure.
Our Pressure Relief Devices “ VP “ have this protective function.


The way of working of above device is very simple: each time the pressure inside the transformer tank reach the operating
pressure of the PRD, the shutter lift slowly from its rest position disjoining from the main tightening gasket (pos 9 of the
attached sketch); in this condition the excess of pressure can’t be released in the environment, being the shutter still in
contact with the external gasket (pos 10 of the attached sketch).
Now the internal overpressure operates on the whole internal surface of the shutter, which is bigger than the initial surface:
consequently, the strength acting on the shutter is much higher than the spring-load on the other side; this brings to a
quick and high lifting of the shutter followed by an equally fast reclosure as the excess of pressure is released in a very
short time through the big opening created by the shutter lifting.

GENERAL FEATURES of our “ VP “ devices

-   Body, connecting box and shutter in cast aluminium, paint protected.
-   Screws, nuts, fasteners, pins in stainless steel or nickelplated brass.
-   NBR rubber gaskets (other kind of gasket material available upon request for use at extremely low temperatures).
-   Oil spray protection (upon request) in stainless steel.
-   Springs in high quality steel, finish shotblast plus epoxy paint (corrosion protection).
-   Electric contacts (upon request), one or two, changeover, inside the connecting box with IP67 protection degree.
-   All the components of our PRD are positioned outside the fixing flange, with two big advantages:
    a) Impossible to loose inside the tank any components like small nuts or pins.
    b) Without anything protruding inside the tank it is possible to locate the PRD in those points where there are more
    risk of generation of the overpressure, reducing the possibility of deformation of the tank.
-   Optic signal: a big red semaphore protrude outside the cap and indicates the PRD has operated; the semaphore need
    to be manually reset; the semaphore is a standard accessory on all our “VP” valves, either with or without oil spray
    protection or contacts.


Three sizes:
-   VP 50, pag5-31, up to 3000 dm3 of oil inside the tank.
-   VP 80, pag5-33, up to 9000 dm3 of oil inside the tank.
-   VP 150, pag5-35, up to 25000 dm3 of oil inside the tank, or more.
For bigger volumes of oil it is common practice to use more valves.


Our Pressure relief devices “VP” can be fit with the following accessories, all independently each other:
-   Oil spray protection: useful to give an orientation to the oil flow towards draining systems in order to avoid damages or
    injuries to persons due to hot oil.
-   One or two independent electric contacts ( microswitches, changeover-wiring diagram, inside the connecting box);
    contacts are operated through a lever by the shutter and need to be manually reset in case of operation.

File : POWER PRD ing                             UNCONTROLLED COPY                                       Rev. 4 dtd 27/10/08

Protection degree: IP67
Contacts in Silver/Nickel plated
Breaking capacity at 250 V (ac 50/60 Hz): 5A
Breaking capacity at 125 V (dc): 1 A
Current (min/max): 0,1 / 10 A


-   Sinusoidal vibrations with frequency < 120 Hz and amplitude < 250 µm
-   Dynamic stress with the following accelerations:
    Max 3g in any direction, sinusoidal, amplitude < 20 mm.
    Shock, max 10 g, in any direction.


The choice of the size of the valve depend form the space available and from the quantity of oil inside the tank.
Looking at the safety and at the operating rapidity it may be preferable to use more than one valve, eventually of smaller
Each valve can be mounted either vertically either horizontally on the transformer tank, the nearest possible to the point
where a failure may occur, or on barycentric position if there are more than one critical point, usually nest to the
transformer columns.


Generally the limitation to the use of this kind of valve stays in the gasket material.
Here below we give you few data regarding three type of different gasket material, suitable for use in different ambient
conditions, in mineral oil or silicon fluid, with humidity: 95% at 20°C; 80% at 40°C; 50% at 50°C.
a) Standard execution (N)
    Gasket material: NBR rubber.
    Ambient temperature (min/max): -20°C/50°C
    Oil Temperature (min/max): -20°C/110°C
b) Low temperature execution (F)
    Gasket material: special kind of NBR rubber suitable for low temperature.
    Ambient temperature (min/max): -40°C/50°C.
    Oil Temperature (min/max): -40°C/110°C
c) High temperature execution (V)
    Gasket material: Fluor-rubber.
    Ambient temperature (min/max): -15°C/50°C
    Oil Temperature (min/max): -14°C/150°C


All the part of the valve made in aluminium, corrosion protected with a coating (a oil resistant primer coat, approx 50 µm
thick, on all the surfaces, plus a polyuretahanic coat, RAL7030 colour, only on the surfaces exposed to the external
atmosphere, with a total coating thickness of approx 100 µm.
Oil spray protection and screws: in stainless steel.

Please contact our design department every time the specifications require a special valve, giving us full details.

File : POWER PRD ing                             UNCONTROLLED COPY                                       Rev. 4 dtd 27/10/08

Our Pressure Relief Devices “ VP “ can be calibrated at an operating pressure value between 30 kPa and 150 kPa.
The value indicated on the identification label is the nominal value ( Pn ) of the operating pressure.
It is necessary to define other value of the operating pressure:
   - Pe: working pressure, max value of the internal pressure at which there are no leaks of gas or of oil
   - Pt : tightening pressure, which is the max value of the internal pressure at which the valve is reclosing during testing
   - Pmin & Pmax: Min & Max operating pressure, which are the Min and Max value of the internal pressure at which the
   valve is operating (the difference between the two values is equal to the tolerance of the nominal value)

                                         Pn       Pe      P min     P max      Pt
                                         30       25        30       37,5      27
                                         40       33        38        46       35
                                         50      42,5      47,5      57,5     44,5
                                         60       52        57        66       54
                                         70      61,5      66,5       77      63,5
                                         80       71        78       86,4      73
                                         90      80,5      85,5      97,2     82,5
                                         110     105       110      118,8     107
                                         130     125       130      136,5     127
                                         150     145       150      157,5     147

Vacuum withstand test: 10 torr
Pressure withstand test: 4 bar


When issuing an order or asking a quotation, please specify the following:
-   Size of valve.
-   If oil spray protection is required.
-   Operating conditions (material of gasket: standard NBR, low temperature, high temperature).
-   Wiring diagram.
-   Nominal value of the operating pressure, Pn.
See table in next page.
Nr 3 PRD type VP080P0NH050; means that are required nr 3 valve VP080 with protection, climatic standard operating
conditions, wiring diagram H (1 changeover contact) and setted at 50 Kpa.


It is possible to supply special valves, which are in accordance with special requirements.

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     VP080 P0 N H 0 5 0


                                                             VALUE kPa

         AND SIZE


                                                           025           OPERATING VALUE 25kPa
                                                           030           OPERATING VALUE 30kPa
                                                           035           OPERATING VALUE 35kPa
                                                           040           OPERATING VALUE 40kPa
                                                           045           OPERATING VALUE 45kPa
                                                           050           OPERATING VALUE 50kPa
                                                           055           OPERATING VALUE 55kPa
                                                           060           OPERATING VALUE 60kPa
                                                           070           OPERATING VALUE 70kPa
                                                           080           OPERATING VALUE 80kPa
                                                           085           OPERATING VALUE 85kPa
                                                           090           OPERATING VALUE 90kPa
                                                           110           OPERATING VALUE 110kPa
                                                           130           OPERATING VALUE 130kPa
                                                           150           OPERATING VALUE 150kPa

                                                  F                      W/OUT CONTACTS
                                                  H                      1 SPDT CONTACTS
                                                  W                      2 SPDT CONTACTS

                                      N                                  STANDARD CONDITIONS NBR GASKETS
                                      L                                  LOW TEMPERATURE GASKETS
                                      V                                  HIGH TEMPERATURE VITON GASKETS

                       F0                                                WITHOUT PROTECTION
                       P0                                                WITH PROTECTION
                                                                         WITH OIL DUCT (ALWAYS AND ONLY FOR

       VP050                                                             PRD VP50
       VP080                                                             PRD VP80
       VP150                                                             PRD VP150 &VP150-QT

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