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jcsignature
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Join Date: Sep 2011
Posts: 10

Starting Torque - Speed Curve Analysis

10/17/2011 1:44 AM

Can anyone give me some idea about this, I am comparing speed torque curve from vendor, there is a requirement where in case that the pumps which need a large starting torgue (torque is larger than 30% of rated torque), the Vendor shall state starting torgue in the proposal. Why it is needed to inform the large starting torque and what is the effect of this in the pump or motor selection?

I have seen one curve which has a starting torque of greater than 30% and the vendor informed this at pump data sheet but the starting torque in percent( lets say 40%) is less than the percentage of torque at shutoff (say 55%) where discharge valve is closed and also less than torque at rated capacity (say 75%) where discharge valve is open. Another curve is the starting torque (say 75 %) is greater in percentage than torque at rated (say70%) and torque at shutoff.(say 65%). What analysis/possibilities can happen or occur in this situation? Thanks..
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AYP
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#1

Re: Starting Torque - Speed Curve Analysis

10/17/2011 4:34 AM

To answer your question we have to look at the two starting conditions for the pump. (Torque will have impact during starting of the pump.) For

1. Pump running at the EOC

2. Pump start up against the pressurised header (more relavent to the parallel operation & starting of stand by pump on low pressure signal from the discharge header)

Look at the case-1 here typically one can say that when working pump is tripped stand by pump should come online automatically. In such a case pump will run at EOC (not exactly at EOC but you can say close to EOC) momentorily which require enough motor power to take care of the power at EOC.

Now consider the case -2 pumps running in parallel operation. in the start up case when one pump running the discharge header (pressurised header) will be at the rated discharge pressure. the same will act on the second pump & stand by pump (even though NRV is located at the discharge of the pump; from process safety point of view NRV is not considered as safe isolation medium).

Now when we try to start second pump it will have to overcome the back pressure & then it will take to it's rated speed. To overcome the back pressure motor has to exert higher torque at the start up. That is why we always ask for the superimposed TS curve for pump & motor. You can say the condition as starting against the load.

The same condition exist in normal running condition that when the discharge header pressure is low the stand by pump will come on line which shall have higher stating torque.

So EOC is going with the power & open discharge valve condition goes with the higher starting torque of the motor (refer attached curves which clearly specifies higher starting torque requirement for open discharge valve condition).

In all in the design stage itself designer has to specify whether the pump shall run at EOC or start against the open discharge valve or shall be suitable for both.

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PWSlack
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#2

Re: Starting Torque - Speed Curve Analysis

10/18/2011 11:20 AM

Torque is created by the load.

A centrifugal pump "sees" very little torque as it starts, however as the speed rises, the torque rises at something like the 3/2 power of the speed (other readers please correct this). If the impeller is light, no problem. However if it has a high mass then the motor has to accelerate this mass to full speed, which will result in a higher starting torque than it would otherwise. Hence the requirement for the vendor to say so as torque and current are nearly linearly related in a motor. A motor starting a high inertia load will pull a high current for a longer time than it would otherwise, and this may affect the correct selection of the overload protection device(s).

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K_Fry
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#4
In reply to #2

Re: Starting Torque - Speed Curve Analysis

10/18/2011 2:14 PM

the torque rises at something like the 3/2 power of the speed (other readers please correct this).
Well, OK, I'll correct it. I think the value it is actually the 6/4 power.
I can't say this with any real authority, but 3/2 sounds pretty darn close to me.

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Codemaster
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#5
In reply to #2

Re: Starting Torque - Speed Curve Analysis

11/05/2011 7:41 AM

Can't agree with the 3/2 power. Assuming for simplicity the system is all friction, flow varies as speed, head as speed2, power as speed3. So torque = power/speed varies as speed2.

The torque/speed curve in #1 confirms this, above about 20% speed. The higher torque at lower speed due to stiction effect.

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RAMConsult
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#3

Re: Starting Torque - Speed Curve Analysis

10/18/2011 12:11 PM

It has to do with the availability of sufficient torque from the motor. For every point on the pump curve it is essential that the motor torque be above the pump torque or there may not be sufficient (ignoring inertia) accelerating torque from the motor to overcome the resistance provided by the pumping system. The result is that the motor can no longer accelerate the pump, it stalls and trips on overcurrent.

As AYP states the load that the motor sees is highly dependent upon the conditions at the pump, similarly the ability of the motor to develop sufficient torque is highly dependent upon its terminal voltage. AYP's Graph one above shows the motor curve at 100% and 80% voltage, note that at reduced voltage the motor curve is below the pump curve and chances are the motor will not accelerate beyond that speed.

It is a system, simply bolting a motor and pump together does not guarantee that they will perform to their individual specifications.

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