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Decompression Valve for an Emergency Diesel Generator

07/17/2017 1:55 AM

Dear Forum users,

I came across a requirement of Solenoid operatred Decompression valve in a specification for an emergency diesel generator.
I wish to understand what is the purpose of this valve?
Is it the same purpose as in an Internal combustion engine wherein the decompression valve reduces the cylinder compression at low rpm for easy starting?
Please guide me further.

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#1

Re: Decompression Valve for a emergency Diesel Generator

07/17/2017 3:16 AM

Assuming the generator is using a diesel engine then yes I would say this is correct.

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#2
In reply to #1

Re: Decompression Valve for a emergency Diesel Generator

07/17/2017 4:16 AM

Yes, the generator is using HSD(high speed diesel).
Further, I wish to know, whether this decompression valve is necessary for the engine always? or it depends on certain parameters (like rating of DG or type of starting or type of fuel etc.) when the valve is required to be provided.

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#4
In reply to #2

Re: Decompression Valve for a emergency Diesel Generator

07/17/2017 10:15 AM

Try starting the engine without using the decompression valve, if you can start it you don't need the valve. I have no idea what your starter motor though and you might want to think what will happen to that.

I was fortunate enough to have a small centrifugal pump with a diesel engine. It was hand cranked to start and we'd have one guy on the handle and the other ready to throw the decompression valve lever when we were up to speed. More than a few times when the valve was thrown the hand crank would basically stop and kick back with such force you risked broken arms. It was a bit of a beast. I suspect the reason the decompression valve is there is cos it's needed.

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#3

Re: Decompression Valve for an Emergency Diesel Generator

07/17/2017 9:26 AM

yes, otherwise they would need a auxiliary pony engine to start the diesel.

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#5

Re: Decompression Valve for an Emergency Diesel Generator

07/17/2017 11:04 AM

Decompression valve allows one to cycle the large diesel engine up to minimal starting rpm using the starting mechanism, that can be a small electric starting motor, air from a storage tank, or other means.

Once rpm is reached, and all other system parameters are right, the valve closes, and the self-ignition (dieseling) of fuel-air begins to take place in the compressed mixture.

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#6

Re: Decompression Valve for an Emergency Diesel Generator

07/17/2017 11:23 AM

It is for starting and stopping the thing. Particularly stopping.

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#7

Re: Decompression Valve for an Emergency Diesel Generator

07/18/2017 9:10 AM

For critical applications NFPA required that emergency generators be fully up, operational once a drop in frequency of 4 cycles is sensed. By then the generator will be almost loaded in less than 10 seconds....

Having said that, "Decompression valve is a must to have, it is a critical component that help ensure code compliance!

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#8
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Re: Decompression Valve for an Emergency Diesel Generator

07/18/2017 10:06 AM

Please elaborate a bit more on the code compliance part. Is that the fast start capability aspect being within code?

I read about a pumped hydro plant in Wales just yesterday (the plant itself dates back to the late seventies?) and it is a multi-GW plant, as i recall with a spin-up to full load of about 10 seconds. Go figure.

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#9
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Re: Decompression Valve for an Emergency Diesel Generator

07/18/2017 4:06 PM

Yes I believe it is one of the critical requirements in a patient care setting.... to ensure that within the first ten seconds, the power generators should be already delivering power to at least 80% of its full load rated capacity...

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#10
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Re: Decompression Valve for an Emergency Diesel Generator

07/18/2017 7:05 PM

"I read about a pumped hydro plant in Wales just yesterday (the plant itself dates back to the late seventies?) and it is a multi-GW plant, as i recall with a spin-up to full load of about 10 seconds. Go figure."

Was that Dinorwig? Takes a little longer from standing start: around 75s to open the sluices and spin up the turbines. If they are expecting a sudden demand then they keep the turbines at synchronous speed and just have to open the sluices which can be done in around 16s.

If it wasn't Dinorwig, please tell me as I don't know of it.

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#13
In reply to #10

Re: Decompression Valve for an Emergency Diesel Generator

07/19/2017 9:16 AM

You are correct, and thanks for the editorial correction. Sometimes I am in a real hurry to finish typing and get back to work, I hope everyone will forgive my faux pas.

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#11
In reply to #7

Re: Decompression Valve for an Emergency Diesel Generator

07/18/2017 9:01 PM

Please let us know the NFPA code which you are referring to.

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#12
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Re: Decompression Valve for an Emergency Diesel Generator

07/18/2017 9:24 PM

It has been awhile but I believe it is NFPA 110...

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#14
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Re: Decompression Valve for an Emergency Diesel Generator

07/19/2017 9:18 AM

NFPA Standard for Emergency and Standby Power Systems

yes it is in NFPA 110, good job sir!

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#15

Re: Decompression Valve for an Emergency Diesel Generator

07/25/2017 1:23 AM

Thanks everybody for your guidance and help.

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#16

Re: Decompression Valve for an Emergency Diesel Generator

07/25/2017 8:40 AM

You don't give an idea of the size of the engine, or whether it's manual or electric start (or otherwise assisted).

The only ones I'm familiar with that use decompression to assist start were on small units - dump trucks, concrete mixers, small generators etc with swing start. These had a flywheel, big for the size of the engine. You would decompress and swing it, release the decompression, and the flywheel gives a couple of compression strokes, and with luck off she goes. I think the decompression worked by simply holding the exhaust valve open, not by a specific valve.

I could be wrong, but I've never heard of a decompression valve on vehicle, or larger stationary, diesels, for starting or stopping. Stopping is by cutting off the fuel supply at the injection pump, direct mechanically at one time, nowadays via the electrics, but I believe the principle is the same.

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#17
In reply to #16

Re: Decompression Valve for an Emergency Diesel Generator

09/12/2017 7:47 AM

The engine is electric start.

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#18
In reply to #17

Re: Decompression Valve for an Emergency Diesel Generator

09/12/2017 11:02 AM

OK, I suppose it gives the starter motor an easier time by allowing it to get up to full cranking speed while decompressed. Clearly the decompression has to be cancelled for the engine to fire and run. I've never seen an engine with that system, but that doesn't mean much!

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#19
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Re: Decompression Valve for an Emergency Diesel Generator

09/12/2017 2:59 PM

With a much lighter cranking load for the electric motor, the resultant increased in the RPM will provide a corresponding starting torque... A faster engine turn-over...

Lower electrical draw, lesser mechanical wear and tear on motor, faster engine start, faster availability of emergency power..

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#20
In reply to #19

Re: Decompression Valve for an Emergency Diesel Generator

09/12/2017 3:37 PM

That's right, much as I was saying. But the starter still has to provide the umph to continue cranking the engine when the decompression valve closes.

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#21
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Re: Decompression Valve for an Emergency Diesel Generator

09/12/2017 5:22 PM

The closure of the decompression valve I believed is timed with injection of fuel into the combustion chamber...The starter motor has already developed the momentum & torque by the time the valve closure takes place!

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#22
In reply to #21

Re: Decompression Valve for an Emergency Diesel Generator

09/13/2017 4:07 AM

Agreed, but the starter still sees a sudden increase in load, like MACA's arm in #4!

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#23
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Re: Decompression Valve for an Emergency Diesel Generator

09/13/2017 8:57 AM

The starter motor may have generated enough momentum when valve was open, and disengaged almost instantaneously when valve closed... As valved closes a burst of fuel simultaneously delivered and soon ignition commenced as fuel was already compressed by then thus engine turning over is continued..

The Increased RPM that resulted from the valve opening, and a timely sequenced closing presented a brief almost no load condition for the starter that enabled a very short engine cycling condition which provided an almost instantaneous engine ignition in the process.

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