Min. Tank Thickness of Pad Mounted Transformer

[daviandgo]

Does any IEEE/ANSI transformer standard state the minimum thickness of a pad mounted transformer tank? Or is any thickness acceptable AS LONG AS the transformer tank meets the pressure withstand requirements?

 

[dpc]

There is a specific ANSI C57 standard for pad-mount transformers, but I can't answer your question without spending a lot of time trying to find it.

I would **guess** that the pressure (and vacuum) withstand requirements govern the tank thickness.

 

[daviandgo]

Thank you DPC. The ANSI C57 pad-mounted transformer standard does not refer to any thickness requirement. There are several other transformer related ANSI standards. These standards are not available to me at this time. Since the standards are relatively expensive I am hoping that someone will point me to the relevant standard so that I can acquire it rather than the entire transformer standards collection.

 

[RRaghunath]

I have known different transformer standards IEC, IEEE as well as Indian. It is only pressure and vacuum withstand requirements that are stipulated (which indirectly dictate the tank thickness).
There is no direct stipulation of tank thickness we come across.
The view may be to better leave it to the transformer designer taking in to account the transformer dimensions etc. in addition to the vacuum and pressure withstand requirements and I agree with it.

 

[busbar]

 
A TOC for the probable standards is

http://standards.ieee.org/reading/ieee/std_public/description/dtransformers/C57.12.26-1992_desc.html

§7.6 Tanks is probably more of a performance-based description [like pressure/vacuum capability] so may be disappointing for specific material thickness.

 

[mc5w]

Tank wall thickness would be determined using the American Society of Mechanical Engineers Code for pressure vessels. In some states this is mandated by law.

There are several sections of the ASME Code for pressure vessels. These cover different size and pressure ratings. There is a special section for vessels that are subject to external pressure such as vacuum tanks and the tubes in fire tube boilers.

Padmount transformers operate at theoretically low pressures just as long as deliberate overloading is reasonable.

One thing that I have never seen in a padmount transformer is an automatic pressure relief valve but next time I see one open I will look for one.

 

[ScottyUK]

mc5w,

Virtually all transformers have a pressure relief device or PRD fitted. In Europe they are known aften referred to by the brand name 'Qualitrol'. They are normally set to lift at a few PSI above ambient pressure.

The size of the transformer would appear to govern the tank wall thickness: most tanks are slab-sided square designs, not really ideal from the perspective of pressure vessel design. The larger the tank, the thicker it has to be to avoid implosion under vacuum.

An aside on PRDs:

When one of these devices operates, oil is ejected and breaks up into fairly fine droplets. Has anyone ever considered whether this poses a fire or explosion hazard in the presence of an arcing fault? Transformer oil isn't especially flammable as a bulk liquid, but from first-hand experience it burns vigorously when atomised.



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If we learn from our mistakes,
I'm getting a great education!

 

[busbar]

 
Tank rupture without burning oil can still make a mess.

 

[RRaghunath]

ScottyUK,

NTPC the largest utility in India specify diversion of oil spill from PRD to the Transformer oil pit below using large dia pipe. This has come from their own experience with a burning transformer in 1980s.

 

[jnims]

Busbar, there has to be an interesting story to go with that picture. Can you provide any details.

 

[electricpete]

I was under the impression that ASME code applied only above a certain pressure and temperature far above what we see in transformers.


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[jghrist]

The slab-sided square design may not be ideal for a pressure vessel, but it is best for padmount transformers. For a high current internal fault that generates pressure faster than can be relieved by the pressure relief valve, the sides will deform instead of rupture. This is one reason not to require a thicker than necessary tank wall.

 

[ScottyUK]

rraghunath,

Thanks for that info - someone else has obviously decided that PRD operation poses a threat.

Our site suffered a massive transformer explosion which released several tonnes of oil in the form of mist, vented through the PRDs of the tapchanger and main tank and through the gaping hole created when the 1[½]" top plate buckled upward over 6" and the end wall deformed a similar distance. The fireball was terrifying even from the other side of plant.

jghrist,

That is an interesting idea, although it probably applies more to welded tanks than those with bolted lids. The transformer in the event noted above burst the bolted seam like tearing cotton, firing M24 bolts like mortar bombs over a significant area. A welded tank might have faired better (or maybe not?).



----------------------------------

If we learn from our mistakes,
I'm getting a great education!

 

[busbar]

 
jnims, unfortunately I received the image "without caption," so I don't know its story.

 

[stevenal]

When one of these devices operates, oil is ejected and breaks up into fairly fine droplets. Has anyone ever considered whether this poses a fire or explosion hazard in the presence of an arcing fault? Transformer oil isn't especially flammable as a bulk liquid, but from first-hand experience it burns vigorously when atomised.

Like this?

http://www.cooperpower.com/FR3/Tranvid.mpeg

 

[busbar]

Steve, you know that is a proven flying-saucer crash event near Drain, OR. The Blue Book boys continue to vehemently deny it.

 

[ScottyUK]

Scary isn't it.

Did anyone notice how the water from the deluge flashed into steam when it touched the hot oil, causing the oil to explode with even more aggression?



----------------------------------

If we learn from our mistakes,
I'm getting a great education!

 

[stevenal]

I don't believe any water is involved. (it's rare in outer space The spraying fluid is oil from the pressure relief exactly as you described. The Qualitrol rep used this as an argument for piping the discharge as rraghunath described.

 

[mc5w]

The ASME Code applies to air compressors, gas storage tanks, cryogenic tanks, and so forth. All of these operate at temperatures well below transformers.

In Massachusetts, New Jersey, and the other states that require ASME pressure vessels the requirement applies to forced hot water and low pressure steam systems that use a 15 psi relief valve.

If you want to get insurance then in almost ALL cases you do need an ASME inspected and stamped pressure vessel particularly if your transformer is located indoors or crammed between two buildings.

I will download the mpeg file later.

 

[RRaghunath]

ScottyUK,

I guess you mean High velosity water sparay system (HVWS) used for fire protection of transformer installations. These include deluge valve, temperature sensors and nozzles.

The nozzles cover the transformer completely (from all sides as well as top) and are laid out in such a way that the fine water sprays forms a blanket over the transformer with burning oil thus cutting off the oxygen supply as well as cooling the transfomer.

The fine spray when it comes in to contact with oil is supposed to form emulsion ("Emulsifier systems" is another name for the HVWS spray systems).

I have not heard of explosion due to water deluge coming in contact with burning oil and I don't think there is any such possibility in a well engineered HVWS fire protection system.

Do you have more info!