pressure relief device

7/29/2019 Pressure Relief device

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

PRESSURE RELIEF DEVICESFOR POWER TRANSFORMERS

(with electric contacts)

GENERAL FEATURES

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 whichgenerates a quick rise of the pressure inside the tank, with the risk of permanent deformations, or, even, of the failure ofthe 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.

WAY OF WORKING

The way of working of above device is very simple: each time the pressure inside the transformer tank reach the operatingpressure of the PRD, the shutter lift slowly from its rest position disjoining from the main tightening gasket (pos 9 of theattached sketch); in this condition the excess of pressure cant be released in the environment, being the shutter still incontact 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 aquick and high lifting of the shutter followed by an equally fast reclosure as the excess of pressure is released in a veryshort 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 morerisk of generation of the overpressure, reducing the possibili ty of deformation of the tank.

- Optic signal: a big red semaphore protrude outside the cap and indicates the PRD has operated; the semaphore needto be manually reset; the semaphore is a standard accessory on all our VP valves, either with or without oil sprayprotection or contacts.

MANUFACTURING PROGRAM

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.

ACCESSORIES

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.


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ELECTRIC CONTACTS

Protection degree: IP67Contacts in Silver/Nickel platedBreaking capacity at 250 V (ac 50/60 Hz): 5ABreaking capacity at 125 V (dc): 1 ACurrent (min/max): 0,1 / 10 A

DYNAMIC CHARACTERISTICS (MECHANICAL STRESS WITHSTAND)

- Sinusoidal vibrations with frequency < 120 Hz and amplitude < 250 m- Dynamic stress with the following accelerations:

Max 3g in any direction, sinusoidal, amplitude < 20 mm.Shock, max 10 g, in any direction.

CHOICE OF THE VALVE AND MOUNTING INSTRUCTIONS

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 smallersize.Each valve can be mounted either vertically either horizontally on the transformer tank, the nearest possible to the pointwhere a failure may occur, or on barycentric position if there are more than one critical point, usually nest to thetransformer columns.

CLIMATIC CONDITIONS

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 ambientconditions, in mineral oil or silicon fluid, with humidity: 95% at 20C; 80% at 40C; 50% at 50C.a) Standard execution (N)

Gasket material: NBR rubber.Ambient temperature (min/max): -20C/50COil Temperature (min/max): -20C/110C

b) Low temperature execution (F)Gasket material: special kind of NBR rubber suitable for low temperature.Ambient temperature (min/max): -40C/50C.Oil Temperature (min/max): -40C/110C

c) High temperature execution (V)Gasket material: Fluor-rubber.Ambient temperature (min/max): -15C/50COil Temperature (min/max): -14C/150C

CORROSION PROTECTION

All the part of the valve made in aluminium, corrosion protected with a coating (a oil resistant primer coat, approx 50 m

thick, on all the surfaces, plus a polyuretahanic coat, RAL7030 colour, only on the surfaces exposed to the externalatmosphere, with a total coating thickness of approx 100 m.Oil spray protection and screws: in stainless steel.

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


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OPERATING PRESSURE VALUES

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 thevalve 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 2740 33 38 46 35

50 42,5 47,5 57,5 44,560 52 57 66 5470 61,5 66,5 77 63,580 71 78 86,4 7390 80,5 85,5 97,2 82,5

110 105 110 118,8 107130 125 130 136,5 127150 145 150 157,5 147

Vacuum withstand test: 10 torrPressure withstand test: 4 bar

ORDER INSTRUCTIONS

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.ExampleNr 3 PRD type VP080P0NH050; means that are required nr 3 valve VP080 with protection, climatic standard operatingconditions, wiring diagram H (1 changeover contact) and setted at 50 Kpa.

SPECIAL EXECUTIONS

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

PRODUCT

ANDSIZE

PROTECTION

OPERATING

CONDITIONS

WIRING

DIAGRAM

OPERATING

PRESSURE

VALUEkPa

LEGENDA

025 OPERATING VALUE 25kPa030 OPERATING VALUE 30kPa035 OPERATING VALUE 35kPa040 OPERATING VALUE 40kPa045 OPERATING VALUE 45kPa050 OPERATING VALUE 50kPa

055 OPERATING VALUE 55kPa060 OPERATING VALUE 60kPa070 OPERATING VALUE 70kPa080 OPERATING VALUE 80kPa085 OPERATING VALUE 85kPa090 OPERATING VALUE 90kPa110 OPERATING VALUE 110kPa130 OPERATING VALUE 130kPa150 OPERATING VALUE 150kPa

F W/OUT CONTACTSH 1 SPDT CONTACTSW 2 SPDT CONTACTS

N STANDARD CONDITIONS NBR GASKETSL LOW TEMPERATURE GASKETSV HIGH TEMPERATURE VITON GASKETS

F0 WITHOUT PROTECTIONP0 WITH PROTECTION

QTWITH OIL DUCT (ALWAYS AND ONLY FORVP150-QT)

VP050 PRD VP50VP080 PRD VP80VP150 PRD VP150 &VP150-QT


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pressure relief device

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