Pump Operating Torque is higher than Output Power of the Gearbox

[bmw318be]

I have some doubt is pump initial Sizing application

50 Hz Motor with 150 HP VFD Motor 4 Poles (1450RPM) with a gearbox ratio of 5.3 with Safety Factor 2.3 is having Gearbox Shaft Torque Maximum of 3500 Nm


I am getting a motor torque from a motor and multiple by the Gearbox ratio, correct me if I am wrong because I am getting very low output torque here.

I am operating a PD internal gear Pump for various service condition:

My issue is that I could not accommodate some of service condition as it has higher running torque than the Output Gearbox Torque. Although the BHP of the pump is still within the Motor Limit of 150 HP

Some cases is running at multiple viscosity of 1500 CST, Q = 30 m3/hr at 110 Degree C , SG of 0.9
My BKW is 18 HP
Pump rated speed of 35 RPM

My calculated Operating torque or Power Required to drive at this operating condition is 3660 Nm
However, looking at VFD speed curves, I am running at approximately at 8 Hz at 35 RPM and the Output Motor torque after Gearbox is 75% of Maximum Torque which is 2625 Nm


I need to check if the pump operating Torque has to be lesser than output Gearbox Torque in order to run smoothly. I am having a VFD motor characteristic as followed as speed goes down the torque goes down too from 50 Hz to lower Hz. However, I am still operating at a speed between 5 Hz to 50 Hz.

Is there anyway to increase the Motor Torque with any coupling as my understanding Motor Torque is fixed and I am not able to upsize the motor due to range limitation. It is clear that I am having higher torque required because of lower rated speed of the pump and higher Break Horse Power .


Some terminology of Internal Gear pump that it is constant Torque.

However it seems, when we operate at various flow rate, pressure or viscosity, the BHP (Break Horse Power) and Rated RPM would various and My understanding is that we could not reduce the torque by lowering the flow, lowering the flow would increase the torque as speed is inversely proportional to the Torque and BHP or Break Horse Power is directly proportional to the Torque.

Torque (Lb ft) = ( BHP * 5252) / RPM




Lastly, I have seen that the Power for BHP of the pump can be calculated by

Pump HP = Flow (GPM) * Pressure (PSI) / (1714 * efficiency of pump )

How this formula is valid for BHP calculation for Internal Gear Pump as some liquid properties such as (Viscosity,Specific Gravity & Temperature are not part of the formula here.

Anyway, my BHP above case based on the Pump manufacturer calculation, however it far off the calculated using this formula.

 

[hydtools]

Pump torque varies with pump output pressure. See link.

http://www.airlinehyd.com/KnowledgeCenter/Hydraulic/Formulas/PumpH.asp

Ted

 

[psv1990]

First, operation of a motor at 8hz is not recomended. Motors are designed to operate at 50hz and do not provide full torque at low speeds. Here is some detailed information on motor operations. Th only way to increase the torque is to speed up the motor. You will have to put a different gear drive in to get the torque you require.

Link

The pump HP calculation you are using is only valid for clear water. You have to add correction factors based on viscosity and specific gravity. If you are pumping a slurry, correction factors for partical size and distibution are also required. These corrections will also depend on the type of pump you are working with.

 

[bmw318be]

Are you referring to centrifugal not a internal pd pump . Internal gear
Pump's is using differential pressure to love the liquid

 

[bmw318be]

Hi PsV

I am running a pump with VAriable freq drive and it is inverter duty, can be run at up to 5 Hz minimum, as you said at 7 hz the torque is not full load.

I am using a gearbox ratio of 5.3 so manage to bring up the torque by 5.3, however it is still lesser than my pump operating torque.

Option: increase pump rated speed by increase the flow, this doesnt help as it would increase the BHP of the pump.

Increase speed, would increase BHP and toque operating would exceed my gearbox output torque.

 

[MikeHalloran]

Your pump is way too big. Get a smaller one and run it faster.


Mike Halloran
Pembroke Pines, FL, USA

 

[psv1990]

You need to increase the motor speed, not the pump speed.

 

[hydtools]

Agree with Mike, need a smaller displacement pump to reduce torque demand. Then adjust speed to deliver flow required.

Ted

 

[bmw318be]

Hi,
The selection is based on maximum flow rate and sonce it has various liquid properties is where the challange is,

I can accomodate some higher flow and torque demand is within the limit, however for sone it exceeded.

Using smaller pump, i wouldnt be able to accomodate higher flow abd would have npsh issue

 

[psv1990]

The torque on your motor is rated at full speed at 50hz. See if your VFD can operate at a constant torque mode. That should give you more power at your lower speeds.

 

[hydtools]

Start using numbers rather than continuing discussion with relative terms like larger, smaller, higher, lower to decide what will work.

Ted

 

[MikeHalloran]

It is possible and permissible to run some, not all, electric motors above their rated HP levels for limited periods of time.

Example: I ran a 10 HP CNC mill's spindle motor at 13 HP, as permitted and documented by the mfgr, for 6 seconds, the time it took to bore a 2" hole through 2" of aluminum, once a minute or so. It was fun to watch the chips bounce off the ceiling.

Gearboxes don't work that way, at least not for torque.
For power, maybe.

Consider reconsidering your calculations.







Mike Halloran
Pembroke Pines, FL, USA

 

[bmw318be]

Hi

I attaching the case on below with figures: Some cases has very low margin

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

 

[hydtools]

Looking at the worst case torque margin, I estimate you need a 22% smaller pump. Pump torque requirement will be reduced by 22%. Torque required is proportional to pump displacement for positive displacement pumps. You flow will also be less by 22%, so you need to run the pump 22% faster if the max flow rate you have in the chart is what you require. From the data in cases 1 through 4 your pump displacement changes about 20% downward from case 2 to case 4; not a very good positive displacement pump.

Ted

 

[bmw318be]

Hi , i understand there is 2 possibilities:

1. Use inverter with constant torque, how could this be achieved ?

2. I am using standard type motor which has vfd suitability derate it from 50 to Hz, my question here is it the torque characteristic goes down as the speed of motor goes down, however at the pump side the torque demand is increasing. Seems this is the issue running.

3 thinking also so upsize the gearbox ratio to bring up the torque

 

[hydtools]

Item 3: Increasing the gearbox ratio will increase output torque but also reduce the pump speed, thereby reducing pump flow. Same effect as reducing the pump displacement.

Ted

 

[Pumpsonly]

You mentioned you can not up size the motor due range limitation. What is the limitation? Is it power supply or the gear box limitation to suit the frame size of the motor?

If it is the power supply limitation, try looking into a lower speed (6 pole) motor and a lower ratio GB.
Lower speed motor will give you higher torque. You have to do your calculation again.

Talk to motor vendor for a inverter duty motor that can provide the required torque at low speed / frequency.
They usually put a high flow fan or use a fan driven by a separate motor to provide better cooling at low speed operation.

 

[bmw318be]

Pumpsonly

Yes power supply limitation, cant upsize the motor,

6 pole motor at 100 full speed has much lower torque compared to 4 pole.

I am not familiar with VFD, just want to understand for constant VFD, would lowering the frequency from 50 to 5 Hz, it would have 100% torque. My motor has confirm me that its Exproof motor would have variable torque, I am keen to know from your view if the Inverter supplier could do something on the torque of the motor.

 

[hydtools]

Why can you not change the pump?

Ted

 

[Pumpsonly]

The torque from a 6 pole motor is definitely higher than a 4 pole motor of the same kW.
But I am not sure what will be the available torque at the gear box out put shaft. Of cause you need to change the GB ratio also.
You need to do the calculation yourself.

The out put torque of the motor is direct variable on the current.
To have high torque you need high current. The current generate heat in the stator winding.
standard motor depends on the cooling effect of the air forced across the the body by the shaft driven fan. If you run a motor at low speed and drawing high current( torque)the winding will get over heated.
That is why you can only use up to a certain percentage of the rated torque at Hz other than the design. There are other factors affect the motor performance when you run the motor other than the designed HZ.

The inverter supplier would need to know your operating Hz range and the power /torque requirement.
They will also need the motor characteristic.

That is why for some high power VFD system, the end-user will specify the motor and VFD must be from same manufacturer.