Electrical

Well, he'll get that question correct on his test.

So, to carry it a step further, closing the pump output valve completely will reduce the motor load to its lowest, but turn the pump into a closed loop water heater.

I remember there was a post about that with an exploded pump . . .

Do I need to dig that up?

Not for me. I don't think the OP will care either.

Let's wait to see if they ask about SG effect on AMP draw.

I wonder if the smart guy who measured the power consumption is running all pumps with closed inlet now in order to save power.

All pumps are imaginary.

Let's hope he tries running with no fluid next to see that effect. The terrible squealing will be the seals burning up. (made that mistake just one time)

You are not wrong there.........it can and has happened on more than one occasion!!

All that occurs is the water increases in temperature and continues to do so and then flashes off into steam.........its volume increasing 1 700 times........sudden and tremendous increase in pressure............ and there you have it..........one slightly stuffed pump, and you would not want to be anywhere near it when it exploded.........otherwise you will probably become a statistic

Had a customer once who demanded 7.5 HP pumps in their wet spray systems.

A 3 HP pump would have sufficed.

We used Serfilco PVC pumps due to the chemistry.

They kept failing. We replaced the first few under warranty, but I went to visit. This was a $100,000.00 USD line and a good customer.

They were throttling the pumps back way too much, to keep from blowing product off the conveyors and were destroying pumps in the bargain.

They relented and we installed correctly sized pumps. No more failures.

Looking at the OP's avatar I just hope he doesn't work for them. It was a well respected company but it looks like its now gone down the pan.

when we close the delivery...

do you mean the fluid entering the pump or the fluid leaving the pump, commonly called the discharge?

both will have the same effect, but it sounds to me like you are in a training course for pumps!!

So here's your explanation....

Centrifugal pumps and if its a multistage C/P lift fluid at a certain rate to a certain height depending on the RPM of the motor, which is governed by the Hz of the motor.

If you shut off the discharge, the pump will continue to lift against that closed valve, and MAINTAIN that column of fluid. As it takes less work to maintain the column than to flow the fluid the Running Current (RC) will drop. Yes the pressure will increase at the discharge side, but once it has reached its maximum it will not increase further.

The above curve shows you three things.. efficiency, POWER and Head or lift.

As the flow rate moves to the left of the curve you see efficiency drop, head increase and the HP also drop.

The curve is for a pump that at its BEP, best efficiency point produces 3500 barrel of fluid per day, not quite in the middle of the OR, operating rage and it requires slightly less than 1.5 HP to produce that amount of fluid. As the flow rate is reduced and look at the zero flow the pump will still take power about 1.1 HP.

hope that explains it for you

Now if you require a multistage pump, you would find out the lift or head required, and the stages you need based on this curve to give you that lift, then take the number of stages required and multiply that by the HP for one stage to give you the required motor HP (± 10%)

You forgot to say that the inlet of stage2 is the outlet of stage one and he has to decide which inlet he wants to close to save the most power.

You said....

You forgot to say that the inlet of stage2 is the outlet of stage one and he has to decide which inlet he wants to close to save the most power.

First.... the OP did not mention saving power, he wanted to know WHY the power dropped if you shut off the discharge valve, and I explained it to him. Please read to OP question and my answer again.

Second I did not forget anything.. In fact, I'm not sure what you mean by "which inlet he wants to close".... If you can explain, and I will listen.

However the multistage pumps I deal with do not have the ability to shut off inlets and outlets of one stage to another as my diagrams will show.

As you can see each stage is placed and secured on top of the one below it. Any "shutting off" of any stage will result in NO fluid flowing through the pump.

As you can see from the diagram above, each stage (consisting of a impeller and defuser) sits on top of each other and there could be as many as 100 in a pump section.

Following on from the diagram above, this diagram below shows the route that the fluid would take as the motor rotates the impellers at the correct RPM.

So my question to you is how do you shut off the outlet and why would you?

How does it save power? Please remember that the OP did NOT ask about saving power, only why does the power demand drop.

On several ships that I served on, many years ago, vertical dual impeller fire and bilge pumps had a valve fitted between the two stages. This valve allowed you to operate the two stages in either series or parallel, dependent on pump requirements, i.e. parallel to pump out bilges -large volume of water, or series for firemain- water at higher pressure. The pumps where Worthington Simpson pumps

Many thanks for the explanation

I am sure you would not measure power consumption for nothing. If anything it was for power saving.

But of course we can put a lot more interpretation in there until OP comes back to explain.

Nice multistage pumps you have there. I am sure OP has none. So please take my post with a grain of salt.

It was to complement your post because I saw it fit with the power scheme I was thinking about.

Never mind.

very true, you would not measure the power for nothing. In many operations you need to know the running current to be able to set the underload trips.

As for understanding what the OP said, I think it is clear.. why does the power drop.

I answered his question.

I fail to see how it could be interpreted as anything else.

As for the pumps the OP has, again assumptions will and in this case has, sent you in a new direction.

Why did I discuss multi-stage centrifugal pumps? Because I have them. The theory of centrifugal pumps is the same for one stage or multi stage pumps. Therefore it fitted the question, and I could illustrate with pump curves.

You win!

I don't consider it a win, more of, IMHO an answer that fitted the question.

But thanks for the compliment.... not many folk would have taken the time to post!

if you turn it off it will consume even less power... for exactly the same reason!

It's doing less work.

Same as running up a hill takes more energy than running on the flat.

I could go on, but even I'm bored now... can I go sleepies please?

Del

You'll find the same thing happens when you plug the vacuum cleaner hose... the motor speeds up because there is less load on it.

Really?

I always thought that the motor was just blowing harder to try to clear the blockage.

Er, because it's doing less work? You've only got to look at the manufacturer's performance curves, Boss!

Goodness me. What do they teach on Engineering courses these days?

Certainly, they don't teach them how to Google!

Dear Mr.NIKHIL VISWAM,

I presume you are referring to the Centrifugal Pump.

The power consumed by the pump (hence the Motor) is linked to the Discharge of the fluid and Head through which the fluid is to be pumped.

The formula is (W x H)/75 where W is the Weight of fluid in Kgs./Sec., (pumped per Second) and H is the Height in Metres through which the Fluid is pumped. This is known as SHAFT H.P.

Then this SHAFT H.P is to be devided by the PUMP EFFICIENCY for getting the motor H.P., and hence the Equation will be (W x H)/75 x 100/Pump. Eff.in Percentage.

Now when the Delivery Valve is closed, the W comes down and the power consumed also will come down.

DHAYANANDHAN.S

p.s. Are you still studying in college.?

But that doesn't explain why the power decreases with increasing head. If the Q/H curve slopes down from left to right (as it should for most and preferably all the range), throttling the delivery increases head and reduces flow. But the power Q*H only decreases if the fractional rise in H is greater than the fall in Q. Also the efficiency might change a little.

That is the case for most centrifugal pumps over most of the flow range, but not always. Some mixed-flow and axial-flow pumps have the opposite characteristic, with a sharp rise in power close to shutoff, to well above normal operating power. So unlike most centrifs, should not be started against a closed delivery.

I should have said "......the power Q*H only decreases if the fractional rise in H is greater lower than the fall in Q."