Centrifugal Pump Mystery 1

[biggles99]

Hello,

I am using a Gusher vertical sumbersion, low pressure, high volume pump to feed cutting oil into a high pressure, positive displacement pump. The pos disp pump runs at 43 gal/min (approx). The outlet of the Gusher pump reads about 10-15 psi. According to the curve for the Gusher, if it was running at 43 gal/min, it should be running with 100 ft of head or so, which I assumed was around 43 psi or so. And, the pump curve says that if it really was 10 psi on the curve, that it would be sending out over 100 gal/min.

Since it cannot physically be sending out more than 43 gal/min (an assumption), it is safe to say that the impeller of the Gusher is just sitting in its tank spinning and churning, because it wants to send out 100 gal/min but it can only send out 43 gal/min? Does that mean that it really isn't on it's curve any more? It is pretty much the churned up, aerated oil that makes the curve not apply any more?

 

[EdStainless]

The pump is on the curve, it has no choice, it just isn't where it should be. It sounds like you are running way 'back' on the curve at low flow.
Do you have a curve corrected for the density of your oil? I would guess that the catalog curve is for water.
There are a couple of ways to address this. You could slow the pump down or trim the impeller. Neither of these are practical for your case.
The easiest thing to do is install a pressure relief and return line. This will let the pump flow more oil, running nearer it BEP. By returning this oil to the tank away from the pump you will keep everything running cooler.

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[BenThayer]

as Ed said, the pump is on the curve. now you have to understand what it is telling you/us.

questions i would be asking are:

is this virgin cutting oil or recovered?
is there a inlet screen or filter if it is recovered?

you might have low outlet pressure due to pluggage on the inlet. the cutting oil likely has a low vapor pressure and you might not be noticing any cavitation.

is the pump it is supplying getting the flow it requires?
can you measure the amp pull to estimate flow?

pump curve's are rarely for specific liquids, they are in terms of feet. as Ed said, you need to multiply the feet by the density of your fluid to determine the psi you should have.

and you have really no explanation for why the 43 gpm is really a limit. if you actually have air in the system, you will see odd things in the discharge pressure but i see no real evidence that you do.

one small thing regarding returning the flow, if this puts you further out on the curve, there will be more power pulled by the pump and this could actually increase the temperature of the tank depending on its orientation, insulation, etc.

 

[zeusfaber]

I thnk the 43 gpm is determined by the downstream PD pump - which means that the Gusher is partway to running deadheaded.

EdStainless's idea of a return line should fix that.

A.

 

[biggles99]

Ed and Ben,

Thanks very much for the feedback...after thinking about it for a while and talking to our pump supplier, we are going to try two different things:


First though...there is a decent amount of air in the cutting oil, which is reclaimed cutting oil around .8 specific gravity or so.


We are going to go from a 2" inlet pipe to 3", which will get reduced to 2" right before the inlet to the horizontal centrifugal pump (the Gusher pump).

Then, we are going to put another return line right outside the pump, to get it closer to the BEP, like Ed had stated as well. We already have one on it, but it does not seem to be big enough.

The supplier's theory is still that the aerated oil is causing the cavitation, also because it also take a few minutes (5-6) before it start cavitating.

I was just thinking that there is not enough air in the oil to cause the problem, but it is definitely a possibility.

I need to work the numbers again, but it did seem like we should be in the 50 psi range on the outlet of the pump at 43 gal/min, and we're only getting around 30. I need to make sure I did the .8 SG correctly, as well.

Thanks again for taking the time, I do appreciate it and it helps me think we're trying the right things.

Thanks,

Tim