Why PD pump ( internal gear ) is constant Torque Vs Terminology of constant torque drive?

[bmw318be]

Hi,

a PD pump is a constant torque application; the torque requirement does not vary with speed, which is the definition. Regardless of the speed you run it at, the HP to Speed ratio remains the same because the energy necessary to move the fluid varies in a linear manner with speed. That means that at 50% speed, you need 50% HP, which means you still need 100% torque.

I am curious that if i am operating the pump with VFD,does the torque varies at lower speed assuming running at 300 rpm vs 600 rpm
, the BKW would increase therefore torque would varies.

Does the constant vfd torque, 110 Kw motor 4 poles: 1470 rpm mean regardless running at lower frequency from 50 Hz to 5 Hz, my torque would still be approx 740 Nm .

I am just unable to understand constant torque in pd pump why they call it constant where obviously the power required at different speed or different fluid characteristic ( flow, pressure , viscosity) varies according and torque would not be constant

 

[hydtools]

Torque require to drive the positive displacement pump varies as the system pressure against which it must pump. If system pressure is constant at all flow rates, then the pump torque is constant at all speeds.

Ted

 

[btrueblood]

What hydtools said, with one caveat: there is torque "ripple" with any pd pump, i.e. the torque varies at gear passing frequency for a gear pump, and with the plunger pulsations of a plunger pump, etc.

 

[electricpete]

One can deduce this relationship using the following logic:
Fluid Power = Volume Flow Rate * DP = Torque * Speed (neglecting losses)

Re-arrange:
Volume Flow Rate / Speed = Torque / DP [Eq 1]

For pd pump, since the volume discharged per rotation is relatively fixed, the volume flow rate tends to be directly proportional to speed. Therefore the quantity on LHS of Eq1 does not vary with respect to speed. If quantity on LHS of Eq1 doesn't change with speed, then quantity on RHS cannot vary with respect to speed. Therefore Torque ~ DP

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(2B)+(2B)' ?

 

[bmw318be]

Hi
Sorry is Volume flow rate = pump flow rate

Q1 Is the theoritical BKW ( Break Kilowatt )for pd Pump internal gear could be calculated given Q (m3/hr), viscosity (CP), SG and what is the rated speed of pump ?

https://www.dropbox.com/s/2h82yer8sav8hxk/9 cases.png

The following is Various flow from low to high,the following application : gearbox was selected based on the highest rated speed, gear ratio is 5.23 however looking to drive low flow of case 9 where rated speed is very low 34, the torque demand is higher than geared motor available output torque.

Q3. Would using a constant torque VFD motor 5 to 50 Hz is feasible ?

Q4. How do you actually select a type of motor for driving pd pump with low speed to high speed where usually my gear reducer is usually accomodate highest flow. Highest flow means higher rpm so the torque is okay for high flow. However, i see that at low flow rate the gearbox does not help to increase my output torque as a result, to drive pump for low flow has torque issue.

sizing my gearbox to accomodate low flow would bring up the torque as gear ratio would be higher than 5.23.However to bring up the torque, the speed to drive highest flow need to compromised as a result lower flow rate.

Q5.Is it recommended to upsized the motor to higher rating or I find a constant torque for this application ( seems derating vfd motor is the right term)

Understant that constant torque motor has variable HP at 10 to 50 Hz assuming 22 KW to 110 Kw. At 300 to 1500 rpm.

Q6. Is the speed characteristic proportional ?

Q7. Is the Hp characteristic proportional or linear with Frequency ?

Eg at 30 Hz or 50% speed, 50% Hp or 55 Kw

 

[hydtools]

Why are you reluctant to consider a smaller pump? It would reduce your torque demand which seems to be your concern.

Ted

 

[bmw318be]

Small pump is not able to accomodate the flow .

 

[hydtools]

Run it at higher speeds to get higher flows.

Ted

 

[bmw318be]

Are you suggesting increasing the flow rate to higher flow so the speed would match with some of the cases . However by increasing the
Speed the existing friction loss would be greater and as a result higher diff pressure .

 

[hydtools]

No. I am suggesting running a smaller pump at higher speed to create the flow rates you require. Same flow rates, same flow losses, same pressures, lower required pump torque.

Ted