pumps and variable speed drives 3

[tomato5]

I would like to put a scenario and ask you to agree, disagree, correct, comment, etc. one request don't comment just for the sake of making a comment:

a decision is needed to buy either two pumps for two different flow,head conditions (all other properties/parameters the same) or one pump and a VFD

Say the larger requirement 600 gpm x 585 ft head needs 1200 bhp, at 1500 rpm, using an impeller diameter of 24 inches.

The smaller condition is 200 gpm x 570 ft head. Since the impeller diameter needs to remain the same, otherwise there is no advantage of a VFD, the smaller condition is 200 gpm x 570 ft say 400 bhp

Since head is about the same, the speed needed for the smaller case would be 1500 x 200/600 = 500 rpm

Torque needed at 600 gpm = 63000 x 1200/1500 = 50,400 lb.inches torque needed at 200 gpm = 63000 x 400/500 = 50,400 lb.inches

Select a drive which covers this working range

Select a motor which will deliver the required torque value at both speed conditions. The one remaining thing is that amperage of the drive should be greater than the full load amps of the motor, stamped on the motor nameplate. For now disregard the motor SF.

Would a drive selected in this manner prove acceptable?

thank you

 

[waross]

I think that I disagree but I won't know for sure until you post the pump operating curves.
By the way, isn't the answer in the back of the text book?

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

 

[TD2K]

That's either a really high SG fluid or a horrible pump efficiency if you need 1200 BHP for 600 gpm @ 585 ft head.

 

[jraef]

Textbook writers don't need to be concerned with reality...

This is not a homework cheat site.


"Will work for salami"

 

[tomato5]

WAROSS THERE IS NO TEXT BOOK THIS IS A REAL LIFE SITUATION DONT WORRY ABOUT CURVES OR SPECIFIC GRAVITIES THE POINT OF ASKING WAS IF YOU AGREED WITH THE PRINCIPLE THT THE SOLUTION FOLLOWED

TD2K SO WHAT IS YOUR COMMENT IN THE METHODOLOGY LEAVE NUMERICAL VALUES OF SP GR ASIDE

JRAEF AS BEFORE THIS IS NOT ANY CHEAT QUESTION

BUT WHAT IT DOES TELL ME SEVERAL WISE-CRACKS NOT ONE PIECE OF WORTHWHILE ENGINEERING RESPONSE

 

[hooverdale]

600GPM x 585FT @ 1200BHP and 200GPM x 570FT @400BHP will not going to be a real life situation unless we have to consider the curves, specific gravities and any details we can get.

But to answer your question : Would a drive selected in this manner prove acceptable? In a manner that it covers both working range and its amperage should be greater than what is stamped on the motor nameplate?

Yes it is acceptable. But it is not feasible. Well again, if we have to leave the feasibility aside then it is not a real life situation.

 

[TD2K]

I think pointing out that 1200 BHP for a pump handling 600 gpm @ 585' head is a perfectly valid engineering response. If I had a client that wanted a pump like this but by mistaken added an extra zero to the motor (as an example) and I just let the requisition sale through without commenting, they'd be right to question why I didn't catch the mistake.

You can't just apply the speed correction factor for the two flows and ignore the head because that is more or less constant. When you slow down a pump, the head drops even quicker than the flow (flow varies by the change in speed, head varies by the change in speed squared). Now, there is some rise in head at lower flows but not the amount you are hoping for unless you have one very strange pump curve. Taking a pump that originally is selected running at 1500 rpm to give you your design point of 600 gpm @ 585' head won't come close to the that head at 500 rpm, I think you'd be lucky even with the pump head rise at lower flows to get 100' of head.

Go to Goulds or another pump site and pick some pumps that fall in your 600 gpm @ 585' head and then redraw the curve at different speeds, you'll see what I mean.

 

[burnt2x]

And one needs to shout to show his knowledge? Buy a 100hp pump instead.

 

[waross]

BUT WHAT IT DOES TELL ME SEVERAL WISE-CRACKS NOT ONE PIECE OF WORTHWHILE ENGINEERING RESPONSE

I definitely disagree with your solution, even without the pump curves.
You just kissed off a couple of cheap simple solutions to your problem.
No pump curves and you don't understand the pump curves.
Let me guess what you won't tell us;
You have a very high head and when you start looking for a pump motor combination, you find that the smallest pumps that will develop the head you need are about 12 times too big.
Been there, done that, got the tee shirt.
I have successfully and cheaply boosted the head of pumps two different ways when a plant was changed and the existing pumps would not generate sufficient head for the new installation.
I don't often withhold information but I am willing to make an exception for you.


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

 

[LionelHutz]

Would a drive selected in this manner prove acceptable?

The answer is yes if the question is "should I select the drive to match the motor?".

However, jsut reading through what you posted, some parts of it appears wrong and other parts of it are wrong.