what size (kva) generator do i need to run 4 three phase pumps? 4

[jmct]

hi,
what size generator (kva) do i need to run 4 or possibly 6 three phase 6.6 kw submersible pumps 12 hours a day or possibly 24 hours a day for six months? if i boughtt a used generator what would be the maximum hours to have on it, to expect a relative trouble free six months? your addvice would be greatly appreciated.. thankyou

 

[ScottyUK]

About 60kVA or so would pretty much guarantee no problems, and in all likelihood 50kVA would be adequate. Note that kVA [≠] kW, and generators are almost always specified in kVA.

Be careful whether the set is continuously rated or is standby rated - I wouldn't go too much below 50kVA continuous rating, but you might well find that a 60kVA standby set uses the same engine and alternator as a 50kVA prime or continuous rated set.

Hours is a tough one - a few hundred hours idling would be much worse for the engine than a couple of thousand hours under load. Bill will no doubt add some more thoughts to this when he sees this thread.


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

 

[ccjersey]

If the pumps are critical to anything, you will want a backup. two sets also allows for servicing etc.

Used generators (take outs from apartment buildings and other commercial users most likely) in the sizes mentioned will be relatively cheap. On the other hand you get what you pay for, so approaching a reputable generator dealer to purchase a load tested, serviced, ready to run used generator with maintenance records might be the best plan. It will cost you more, but also should help to ensure you aren't looking for another generator after the first week or so. If you are, you should have some support/replacement unit available etc.

Engine type will also determine the relative importance of the hours on the machine. Gaseous fuel engines tend to run a long time with very little wear compared to gasoline engines. Diesels usually have a considerably longer potential life than the gas/gasoline spark ignition engines, but are also more susceptable to improper loading (too little usually) problems.

 

[waross]

6.6 kW = 8.8 Hp.
@ 600 V, 8.8 Hp = 9 amps.
9 A 3 phase at 600 V = 9.4 KVA.
The basic rule of thumb is "Largest motor at 300% plus all other simultaneous loads at 100%
For 4 pumps; 3 @ 100% + 1 @ 300% = 56.4 KVA.
For 6 pumps; 5 @ 100% + 1 @ 300% = 75 KVA.
BUT experience says that submersible pumps often run below 100% load, and your load profile allows starting the last motor at less than the rule of thumb allowance.
So, 4 pumps will probably start on a 40 KVA standby set or a 35 KVA prime rated set.
6 pumps should be a minimum of a 60 KVA standby set or a 55 KVA prime rated set.
But, this is still based on 100% loading. This is the minimum we can recommend without knowing the actual load currents of the pumps.
I have no problem with Scotty's estimate.
Hours;
Some operators of diesel powered prime sets expect a minor overhaul at 15,000 hours and a major overhaul at 30,000 hours.
I would be wary of anything over 7,000 or 8,000 hours. (If the set was at a giveaway price and oversized I may take a chance on one with up to 10,000 or 12,000 hours.
Things to be wary of:
Sets that are slow starting, a long crank period followed by one or two cylinders firing and then another, and then another, (while still cranking) until the engine finally picks up revs on its own.
White smoke when starting is normal as is a cloud of black smoke when turbo charged sets first fire, but smoke afer a set has been running for a few minutes is not a good sign.
Lots of blow-by is not a god sign.
My customer have ad mostly good luck with standby take-outs. These are typically 10 to 15 years old or older but have just a few thousand hours or less.
The sts that I see in that size range don't permit operation at an idle. Either stopped or running full speed. Any that do allow operation at an idle may be 20 years or more old. There may be exceptions that I am not aware of.
The most serious problems with full speed running of an unloaded diesel at full speed occur when the set is new, zero hours. The issue is incompletely seated rings and severe oil pumping. If this issue is addressed there should be no further issues. If not addressed the engine may be destroyed before it runs for 24 hours. So, not usually an issue with a used set.

Bill
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"Why not the best?"
Jimmy Carter

 

[itsmoked]

Don't forget sound issues. A loud set 24/7 will often cause grievous complaints depending on where it is.

Don't forget permitting. That would be illegal anywhere around here that electricity could be purchased. (air quality)

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

Good points Keith.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Kiljoy]

Speaking of noisey generators. A customer of ours was running our equipment on a generator and just by chance, a local inspector was sleeping in the hotel across the street and was woken up by the generator. He walked over and shut them down. Dough!

 

[jmct]

hi,
thanks very much for your advice, it has been very help full, i have the opportunity to purchase a generator which was removed from a factory, its probably 10 years old or more but only has around 2000 hours on it. Its 175 kva with a cat 3208 turbo diesel engine, its a little big for my needs at the moment i know but i might need the extra power in the future and i'm getting it at a good price... it was hard wired in the factory, is it a big job to fit a box to it and wire up four 32 amp power points etc. if you could give me advice it would be greatly appreciated. thanks guys

 

[waross]

I would take one feed from the generator to a power distribution point, that would be similar to a main service, and feed the pumps individually from there.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[JGudgeon]

Just as a matter of interest, I once serviced a large water pump which had run non-stop for fourteen years. It was, of course, a Lister Diesel pump lifting water from one channel to a higher channel on the East Anglian Fens.

 

[waross]

I saw quite a few one, two and three cylinder Listers in Central America. They threw enough oil out that if you just topped the oil up every day you didn't have to change it.
Incredibly rugged machines. I saw one that threw its oil into the cooling air path. Going past the hot cooling fins a small amount of oil "Coked" onto the fins. Over time enough coke built up to completely fill in the spaces between the cooling fins. Eventually, with very little cooling, the coke deposits caught fire. The engine was stopped and eventually the fire went out. The owner scraped all the coke of, replaced a few parts that were badly worn and put the engine back into service. As far as I know it was still in running order when it drowned in a hurricane some years later.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[rhatcher]

waross's approach to kva sizing seems to be a good basic rule assuming that the pumps start one at a time (sequentially) and that they are on a soft start. This is a safe assumtion in most cases except, for example, power plants (where atarting current is not an issue).

That being said, the 175kva set that you are looking at should be adequate in almost any case. If the generator has an output breaker, you should be able to connect it to an adequately sized bus to feed the 4 or 6 starters that each have their own short circuit and overload protection. Be certain to adhere to any electic codes that apply in your area with respect to installation and circuit breaker/fuse protection and good luck.

 

[ScottyUK]

Starting current is certainly an issue in modern CCGT power plants, especially in these days where EPC contractors are trying to cut every penny out of the project by making everything almost big enough to do its job. Safety margins are non-existant and the standard response from the likes of GE, Siemens and the like is "We don't care about your specification. Our standard design is what we supply; take it or leave it". That might be fair enough if the standard design wasn't also the cheapest and smallest they could get away with, but it is.


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

 

[rhatcher]

You are right ScottyUK. In general, most things now are based on low cost and the way to improve the economic efficiency is to reduce size and safety margins. Gas turbine units are, by definition, the low cost solution to adding power to a grid.

When I made the comment that starting current was not an issue for power plants, I was thinking large, coal power, plants with multiple units running to produce MWs of power. They start 5000hp motors across the line without thinking about it.

You are correct to point out that my statement was too general and did not apply to every case. I would add to your point by saying that gas turbine units, like any other 'packaged system', are notorious for sizing motors in the service factor range and requiring maximum performance, and minimum life, from every component.

 

[ScottyUK]

Absolutely. For example, GE have the most awful design of seal oil system with the DC emergency motor piggybacked on top of the AC motor and driving the pump via the AC motor's shaft extension. If the pump itself develops a fault or the AC motor has a bearing failure - either of which would require the emergency pump to run - then what use is the DC motor? This is a safety backup to keep hydrogen in the generator! One of many examples of the utter crap designed by the accountants from the gas turbine OEMs.

By the way... have you been away for a while? Welcome back!


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