Torque Speed Curve Of Pump

If your numbers are correct, the new impeller is less efficient, so everything should be fine.

The new, larger, impeller ought to give more performance in terms of flow and head than the old impeller. If the the absorbed power reduces from 3.6kw to 3.1kw, then the new impeller is more efficient, not less efficient.

You would normally expect an increase in efficiency with increased impeller diameter, but not to the extent that it surpasses the increased hydraulic power that results from the increased performance. Even a vertical system curve would not allow for this.

So either the figures are wrong, or there is something we are not being told. What else is changing, apart from the impeller diameter? Is the pump speed being changed, or the discharge system?

I use speed-torque curves for motors all the time, but I've never seen a speed-torque curve for a centrifugal pump, and as far as I know (correct me if I'm wrong), centrif pump manufacturers generally provide power curves not speed-torque curves.

If the impeller diameter is being increased, the performance curve and power curve should be different, and these should be used in conjunction with the system curve to establish absorbed power and recommended motor size.

Hello VishalB,

I'm not sure why you have marked this 'off-topic' when you are providing more info on the original question.

Your table shows what has changed to cause a drop in efficiency and absorbed power. The head has increased from 40m to 48.4m. This means the duty point has moved leftwards on the pump curve. (It doesn't tell us why the head has increased.) So although the increased impeller diameter would normally result in a corresponding increase in absorbed power, in this instance the increased head means reduced flow and power.

In fact, you already have all the info you need in this table to make the motor selection, and the figures check out ok. It is customary below 11kw to use a motor rated for 125% of absorbed power, or more. So in theory you could drop to a 4kw motor, but it is probably best to stick with 5.5kw. There is little difference in cost, it will give you more scope for further changes in the future, and the coupling and baseplate will be sized for the 132 frame motor, so the 112 frame of the 4kw motor might not fit anyway.

I am still puzzled by your reference to T-S curves for the pump. Firstly it looks like you are operating the pump at fixed speed, so what is the point of a T-S curve in the first place? Secondly, even if you could create a pump T-S curve it would not tell you where your duty point lies on the pump curve. Thirdly, for it to be meaningful it would also have to relate speed and torque with flow and head, and with duty point, and I don't think this is possible on a simple curve. I would be interested to see one of these pump T-S curves you have.

There are still some unknowns here that beg some questions:

• Why has the head changed?
• Why does the SG change so much?

Dear Holzfeller, Thanks for your inputs. - This pump is a warehouse spare of an operational pump of a process unit and the service being: Spent Solvent. - It was proposed some time back to use it for another service - Blowdown Water in another process unit. - As there is only a slight difference of duty conditions, we took up the case with pump manufacturer who suggested impeller dia change from 190mm to 199mm. There was no change in motor rating suggested by the manufacturer. - Now, as the motor is not available we have to buy a new one and for that we will have to provide inputs to motor manufacturers such as rating, T-S curve etc

OK, but that doesn't answer Holzfeller's question about T-S curve. I've never seen one either and I'm also curious. For one thing, a pump does not have a unique torque (or power) at a given speed, it depends on the system curve.

I suspect this is based on concern about torque needed as the pump accelerates from startup. For a "squared-torque" machine like a centrif pump, (torque varies as speed²) there is no problem. If the motor has enough power at rated speed, it will have enough torque at all points to get there.

Cheers.......Codey

T-S curve for your reference...it is for the subject pump only...

OK thanks. I'd just comment that the 100% torque on curve I means 100% of torque needed by the pump at full speed. You can work out what that is in N.m from the data you gave in #3. The motor rated torque (full speed, rated power) is higher than pump required torque, obvious as the motor power is > pump power, and speed is same.

If you get hold of the torque/speed curve for the motor it will show higher torque than curve I at all points. I assume the pump torque ~ 18% at zero speed is due to a stiction effect, but the motor has higher torque than that at zero speed, even with star-delta starter.

I still don't think you need worry about the T-S curve, it's all covered by the power data.

Cheers.......Codey

MANY THANKS .....

I can see, how +vely my problem was received at your end and the solution suggested only after detailed review, but in a very short time....thanks once again...