×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Power Transformer Micarta Diaphagms vs Relief Valves

Power Transformer Micarta Diaphagms vs Relief Valves

Power Transformer Micarta Diaphagms vs Relief Valves

(OP)
thread238-9659: Transformer Pressure Relief Devices

The manufacturer's manual for their 30 year old power transformer infers that the micarta diaphragm and goose neck vent protects the gas detector relay from seeing pressure transients above 5 psig, which will cause loss of calibration. It also credits the diaphragm with mitigating the severity of damage resulting from explosive pressurization from a fault.
Can a relief valve set to 5 psig with a discharge pipe routed down to the berm (to satisfy environmental concerns) also claim to provide these attributes?
Would the insurance company find a loop hole if a fault occurred?

RE: Power Transformer Micarta Diaphagms vs Relief Valves

Pressure diaphragm at the end of a goose neck protruding over the conservator (some times an additional diaphragm provided at the pipe joint where it starts from cover) is an old design that no one adopts these days. I don't know micarta- it used to be phenolic resin impregnated paper board (mica sheet) of 0.5-1.0 mm thick to break at an overpressure of 0.5 kG/cm2.PRDs are much superior and reliable with the advantage of resealing property ie after releasing excess pressure it will close automatically after giving indication at control panel. These are provided to protect the tank from rupture in case of dynamic overpressure from a severe internal arc flash from fault . So insurance company should be happy with PRD over diaphragm plate.

When you say gas detector relay is it the gas operated relay mounted on pipe connecting tank to conservator?

RE: Power Transformer Micarta Diaphagms vs Relief Valves

(OP)
prc,
A ruptured diaphragm and gradually tapered goose neck imposes very little dynamic backpressure in the transformer.
An open relief valve imposes set or blowdown pressure in the transformer plus any dynamic backpressure from the deflection shield and long discharge pipe.

The gas detector relay is mounted on the top of the transformer with a maximum of 5 feet of oil above it in the conservator. It is set to trip the transformer at 2.8 psig, which may not prevent a pressure rise past 5 psig due to thermal inertia. Loss of calibration means the pressure relay may not detect the next transient.

The ruling given B.G.E. may have been different had they substituted a relief valve for a rupture disc (attached).

RE: Power Transformer Micarta Diaphagms vs Relief Valves

Knowledge changed a lot during the past 60 years. The honorable judge is referring to the small valve in kettle. What we are now referring is 150mm (6") opening PRD that is far reliable and superior to diaphragm.
It is also to be understood that PRD or diaphragm may not work in case the fault is in a faraway place inside transformer than PRD position. Then tank will rupture. Nothing can be done about it. A survey done by CEGB,UK (CIGRE 1960) proved this beyond doubt. That is why in large transformers two or more PRDs are provided on tank cover.
The sequence of operation of relays in case of a severe fault inside the tank is - first electrical relays(over current /differential), PRD / gas operated relays.
 

RE: Power Transformer Micarta Diaphagms vs Relief Valves

(OP)
prc,

What pressure should the PRD be set to open at (ie., WHAT is it protecting)?
If it is supposed to protect the gas operated relay bellows, then shouldn't it be set at or below 5 psig minus the static head to the top of head tank?
Why wait for the head tank to go solid (5 feet static head above the PRD = 1.8 psig) before opening?


RE: Power Transformer Micarta Diaphagms vs Relief Valves

PRD's usually are to protect the structural integrity of the tank. Why wouldn't you just replace the fragile relay after a PRD opening? Even if you had the PRD set statically low enough to protect the PRD, the dynamics of a fault will cause highly unequal pressures throughout the tank.

RE: Power Transformer Micarta Diaphagms vs Relief Valves

(OP)
bacon4life,

The gas detector relay can't detect pressurization rates less than 1 psig per minute and the PRD set pressure tolerance increases to +/-2 psig on 5 psig at pressurization rates less than 2 psig/second.

Wouldn't it be prudent to have similar PRD and relay set pressures?

A goose neck on the PRD discharge also seems like a good idea.

RE: Power Transformer Micarta Diaphagms vs Relief Valves

Goose neck is not a good idea. It delays the pressure transmission. We have to remember that the pressure build up after a fault is not uniform or static. The dynamic pressure moves like a wave and hit the nearest tank surface. So the limitations of PRD and gas detector relay.
PRD settings are normally between 0.42 to 0.7 kG/cm2.Too low setting is not good for PRD.

RE: Power Transformer Micarta Diaphagms vs Relief Valves

(OP)
prc,

For environmental impact and fire potential mitigation the PRD manufacturers offer a directional shield and discharge pipe that direct oil flow to the berm. My calculations indicate that the PRD flow capacity is halved at the very least.

If the encumbered PRD can only respond to slow rates of pressure rise that the gas relay cannot detect and moderate rates from decay heat following successful trips by the much quicker gas relay, then it's purpose is really to protect the gas relay and should be set-up accordingly.

The brittle micarta diaphragm is superior because it responds quicker with higher capacity.

Perhaps the only advantage of a PRD is not mitigating overpressure but avoiding the consequence of rupture; oil contamination that increases the likelihood of a successive fault.

RE: Power Transformer Micarta Diaphagms vs Relief Valves

Don't under-estimate the effect of the swan-neck on the ability of the bursting disc to relieve pressure during the fault. The directly-mounted PRD may well out-perform a similar-sized bursting disc. If you make the bursting disc and swan-neck sufficiently large then of course it will out-perform a commercially-available PRD, but experience says that bursting discs on transformers are rarely larger than 8" or so even on large transformers.

Personally I agree with prc, if the fault is big enough then the tank is going to rupture regardless of whether you install a PRD or bursting disc. The only difference is whether the rupture occurs with some of the pressure relieved to atmosphere or not. When we lost a transformer in a massive fault one of the questions we asked over and over again in the immediate aftermath was "Why didn't the PRD prevent the tank rupture?". We estimated at the time that the area of the apertures created by the tank rupture were at least 50x that of the Qualitrol 208 PRD and could well have been double that.

RE: Power Transformer Micarta Diaphagms vs Relief Valves

To my understanding gas detector relay should detect pressure rise first and isolate the transformer. Then only (or if fault is sudden and severe together) PRD will come in to play. Purpose of PRD is not to protect gas relay. Discharge pipe is not going to affect the operation setting of PRD.

RE: Power Transformer Micarta Diaphagms vs Relief Valves

Bambie,

Is the gas detector relay of this general type: https://library.e.abb.com/public/7ecd37d3c79ef688c... ? If not, do you have a datasheet you can link to. I'm assuming that you're not talking about a conventional Buchholz relay.

RE: Power Transformer Micarta Diaphagms vs Relief Valves

(OP)
ScottyUK,

And yes, the old and new gas detector relays are BUCHHOLZ designs.

RE: Power Transformer Micarta Diaphagms vs Relief Valves

Bambie, There are three type of relays- (1)Buchholz (gas operated- used with conservator with breathers) (2) Gas operated relay - Mounted over the tank (used with nitrogen sealed units) operated by collected fault gases, (3) SPR (shown by Scotty) mainly used in sealed units, operated by rate of rise of pressure inside tank, mounted on side on tank or in the nitrogen gas space

SPR is expected to do the same function as Buchholz. The characteristic curves put up by you seems of SPR type relay. So please mention correctly the type & make of relay to understand the issue.

The calculation is simply basic. Unfortunately actual PRD working is not based on the volume of gas generated. As I mentioned earlier, dynamic forces travel at lightning speed and hit the nearest side and then spread. Engineers have developed much better calculations and analysis for fault gas generated pressures, rupturing strength etc. Still I will take these with lot of doubt. Please refer IEEE draft standard PC57.156. D4.1-2015 Guide for tank rupture mitigation of liquid filled transformers. This draft is prepared based on a number papers and studies published during last 30 years.

RE: Power Transformer Micarta Diaphagms vs Relief Valves

(OP)
prc,

Thanks for the IEEE reference.

The characteristic curves are for the original Model 12 Westinghouse Buchholtz design and the newer ABB Model 12 replacement.

Both manuals state that pressure excursions beyond 5 psig cause loss of calibration and damage to the bellows.

I have difficulty verifying this change to overpressure protection for the transformer (and gas detector relay) because the relief valve Vendor cannot provide:

1) relief capacity with a directional shield and discharge pipe attached.
2) relief capacity for pressure differentials other than 15 psig
3) relief capacity for transformer oil (or a conversion factor to apply to air scfm)
4) any test data confirming that a relief valve replacement for a micarta diaphragm with the same inlet diameter has a higher capacity and a quicker response time.

RE: Power Transformer Micarta Diaphagms vs Relief Valves

As per my understanding Buchholz relay has no bellows, only floats or buckets operated by the gas collected(alarm) and velocity of oil flow to conservator. (trip) I believe what you are referring is SPR for which calibration may get affected by pressure. But SPR is for detecting such pressure rises before PRD release and why to worry to save it when transformer itself is getting damaged.

There is no need or possible that PRD will release all the gas generated. From the disturbance records, what I have seen, is PRD is operated after several ms after electrical relays trip the transformer. I don't think any one can honestly answer the questions raised by you and there is no relevance to actual site situation during a severe fault. IEEE standard mentioned above may clear some of your doubts.

RE: Power Transformer Micarta Diaphagms vs Relief Valves

Hi prc,

Here's the ABB manual for the relay. It is definitely not a conventional Buchholz relay, although it seems to serve a similar purpose to the float chamber in a Buchholz design, i.e. a device for capturing gas evolved slowly from a relatively gentle fault rather than a gross failure which would operate the Buchholz surge trip or activate a PRD. I haven't seen one of these before.

RE: Power Transformer Micarta Diaphagms vs Relief Valves

Thank you Scotty for that manual. It is from ABB, Alamo, USA. I was also not aware of it. It is a combination of two relays (2)& (3) mentioned in my earlier post. I knew that both these relays are used in US, but first time to see a combo. I am curious to know whether any gas operated relay is additionally used in the conservator pipe in addition to this relay on tank top.
Since this is not used in the pipe line to conservator, but over the tank, 1.5 meter below conservator oil level, we need not be concerned about over pressure. The manual is also not saying anything about PRD setting matching with setting of this relay.

RE: Power Transformer Micarta Diaphagms vs Relief Valves

(OP)
prc,

How else could you provide overpressure protection for the relay?

If the PRD can provide this protection, why wait for the conservator to go solid?

RE: Power Transformer Micarta Diaphagms vs Relief Valves

(OP)
Scotty and prc,

IEEE C57.12.10 has answered some of my questions:

5.1.7 The pressure relief device must relieve 5000 CFM at 10 psig (but it doesn't specify the fluid)

5.1.9 The sudden pressure relay must withstand a pressure of +/- 15 psig without damage.

PRD protecting the SPR seems to be implied.

RE: Power Transformer Micarta Diaphagms vs Relief Valves

Bambie,

Just so you appreciate that the PRD really doesn't guarantee to prevent a tank rupture:



For a sense of scale, the top plate is 40mm thick and the cover bolts are M24. The Buchholz relay has been torn in half toward the centre right of the image.

RE: Power Transformer Micarta Diaphagms vs Relief Valves

(OP)
ScottyUK,

Exactly my point; the PRD cannot protect the transformer from a fault.

Since the PRD can only respond to slow rates of pressure rise that the SPR cannot detect and moderate expansion rates from decay heat following successful trips by the much quicker SPR, then (ipso-facto) it's purpose is really to protect the SPR and should be set-up accordingly.

RE: Power Transformer Micarta Diaphagms vs Relief Valves

Hi Bambie,

My point is that the energy release was so violent and massive that the transformer was doomed regardless of the SPR / PRD / Buchholz because the fault can't be cleared fast enough for an alternative outcome. The fastest-acting relays would be the differential schemes, but they are still limited by the breaker operating times. When the transformer is completely destroyed, protecting one minor component isn't worth worrying about.

RE: Power Transformer Micarta Diaphagms vs Relief Valves

True, PRD cannot protect transformer from fault. It is intended for protecting tank from rupture. But effectiveness depends on the point of fault with respect to PRD position on tank, intensity of fault etc, etc. Its purpose is not to protect SPR,sure. In fact SPR is used mainly in US ( it was introduced to serve the function of Buchholz relay in nitrogen sealed units),but PRD is provided universally. From the discussions that I have seen in this forum, Buchholz is many times reliable than SPR. In case of a fault inside tank, the dynamic pressure developed is many bars (see the earlier mentioned IEEE std) SPR is expected to trip unit on change of pressure rise( C57.12.10 specifies within 3 cycles) and PRD operates many cycles later.

RE: Power Transformer Micarta Diaphagms vs Relief Valves

There are 4 pressure cases to consider:
1. Full vacuum while filling the transformer. I found it odd that the relay did not explicitly state whether it could withstand full vacuum or that it needed an isolation/equalization valve.
2. Slow overpressure due to a maintenance mistake such as:
a. Forgetting to open the valve to the conservator
b. Over filling the transformer with cold oil, then having the oil warm up and expand.
c. Filling the LTC with a 1 inch oil hose and having an 1/8th inch breather hole
d. Malfunctioning nitrogen blanket system.
3. Moderate speed pressure rise from a moderate energy fault
4. Rapid pressure rise from a high energy fault

In cases 2 & 3, the PRD is provided to prevent tank failure. As a side effect, the PRD would also protect the SPR only for case 2. Also note that the SPR doesn’t prevent anything in case 2. A big advantage of the PRD over the rupture disk is that the transformer reseals itself. A bit of spilled oil is far better than emptying the entire conservator, then having to spend a week doing a vacuum refill.

In case 3, the most important aspect is prevention of a catastrophic fire and collateral damage by preventing tank rupture. If the damage is minimized enough so the transformer can be repaired, consider it a bonus.

In case 4, the tank may fail no matter what protection is provided.

RE: Power Transformer Micarta Diaphagms vs Relief Valves

(OP)
bacon4life,

For cases 2 and 3 what set pressure would you recommend for the PRD (knowing your gas relay detector bellows needs protection from 5 psig excursions)?

RE: Power Transformer Micarta Diaphagms vs Relief Valves

For case 3, even if you can salvage the transformer, you replace the gas detector relay.

RE: Power Transformer Micarta Diaphagms vs Relief Valves

(OP)
bacon4life,

What pressure would you advise prc or ScottyUK to set their PRDs at?

(Hint: you get to choose either 5 psig or 10 psig or 15 psig)

RE: Power Transformer Micarta Diaphagms vs Relief Valves

I am indebted to Scotty and PCR for all they have shared with me through eng-tips, so I doubt they need advice from me. Qualitrol makes PRDs at 2.9 PSI and 4 PSI. The setting needs also to account for the head above the PRD.

RE: Power Transformer Micarta Diaphagms vs Relief Valves

Bacon4life, we are all learning from each other.
Qualitrol and others(Commem, Messko, Sukrut ) all make PRDs up to an operating pressure of 25 psi. Manufacturer/user selects the appropriate one.
Normally in India, customers prefer a setting of 7 psi normally. If we select a too low setting chances of mal operation is more. Thumb rule for selection is as below:
Nominal operating pressure in psi = ( maximum conservator oil head over PRD + 1-2 psi tolerance in setting+ 1 psi, pressure drop for PRD)1.3 resealing factor.

RE: Power Transformer Micarta Diaphagms vs Relief Valves

Hi bacon4life,

As prc says - this is a place for all of us to learn. I learn from prc almost every time he posts something. smile


bambie,

My opinion - as an equipment operator / maintainer, not a designer - would be to keep the setting as low as was consistent with reliability. The design features of the transformer are very significant factors influencing the chosen set pressure:
  • conservator?
  • silica breather?
  • hermetically sealed?

If a SPR on one of 'my' transformers had been subjected to overpressure due to a serious fault I would likely replace it. The transformer would be going out for rebuild anyway because we don't have that capability in-house and the cost of the new relay wouldn't be a large additional cost in the overall scheme of things. If the fault was less aggressive but the PRD had still operated I'd get the SPR inspected, function tested and calibrated by the instruments group.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources