pumps and variable speed drives
pumps and variable speed drives
(OP)
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
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





RE: pumps and variable speed drives
By the way, isn't the answer in the back of the text book?
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: pumps and variable speed drives
RE: pumps and variable speed drives
This is not a homework cheat site.
"Will work for salami"
RE: pumps and variable speed drives
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
RE: pumps and variable speed drives
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.
RE: pumps and variable speed drives
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.
RE: pumps and variable speed drives
RE: pumps and variable speed drives
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
RE: pumps and variable speed drives
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.