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#1

# How do you calculate the required HP/torque for cold applications

Do progressive cavity pumps require additional HP/torque in cold weather because of the increased hardness of the stator material?  How do you calculate the required HP/torque for cold applications?
01-18-2014 10:55 PM
Top #2
Yes, PC Pumps do require additional HP/Torque during cold weather. But not to forget that starting torque also depends on the fluid being pumped.

If you are pumping heavy oil and the pump is switched off at night, then in the morning, the starting torque required will be high as the oil which was inside the pump overnight will become more viscous. If this is your real concern, than there are methods to reduce the pump viscosity at inlet.
This is termed as resistant torque generated by viscosity and can be calculated quite accurately.

By correctly sizing the pump - calculating right clearance, fits (taking in account the thermal properties of both startor and rotor) and rubbing velocity, we can greatly reduce the starting torque by great extent.

Besides this, there are various methods available to reduce the pumps starting torque - i.e by use of correct joint / coupling, using adjustable strator, using the VFD drive, etc.

Operating torque can be calculated by T = 1.6 * Volume Displaced * Pressure Difference / pump efficiency.

Starting Torque for cold condition will be = Operating Torque * Factor.

Where Factor will be greater than 1.2 and will be based upon the field experience.

Hope this helps.
01-19-2014 01:39 AM
Top #3
We had a filed application and were trying to understand the contributing factors. The organic solvent being pumped required an EPDM stator (B rating). There was apparently some attack on the EPDM and the swelling along with the poor lubricating qualities of the solvent caused the pumps to lock up. I was wondering if the cold (19 deg. F) temperature could also be a problem. I really like the adjustable stator idea. Thanks for all the valuable input.
01-19-2014 04:12 AM
Top #4
The it is difficult to advise on the effect of the solvent upon the EPDM, without knowling the solvent, but there could be greater solvent interaction at lower temperatures. If an elastomer gets cooled to it Tg then it becomes a more effective damping material and so if the Tg of the EPDM you are using is around 19F then you could be putting more energy into the Elastomer and not getting it back. This would be seen as an increase in required power to start the pump.

The fact that you say the EPDM was swollen (B rated) could also mean that any plasticiser or other compounding ingredients in the material could have been leached out and this can change the physical properties of your material too.

Would be worth checking the free swell at 19F in the solvent mixture and finding the Tg of the EPDM you are working with.
01-19-2014 06:50 AM
Top #5
The solvent is ethyl acetate. What is Tg?
01-19-2014 09:11 AM
Top #6
Sorry Gary, Elastomer speak, Tg is Glass Transition and is the point where the elastomer stops being "elastomeric" and becomes more of a plastic with higher Modulus and other property changes. With most elastomers this temperature tends to be sub 0 deg C.
http://en.wikipedia.org/wiki/Glass_transition_temperature

Ethyl Acetate has a very similar Solubility Parameter to EPDM hence the ability for it to swell the elastomer.
http://en.wikipedia.org/wiki/Hildebrand_solubility_parameter

But there may be another reason, the dynamic viscosity of the EtOAc will be pretty high at this temperature so this may be where your power is going. only goes down to 32 F but you get the idea.
http://www.ddbst.com/en/online/Online_Calc_visc_Form.php

A combination of these two will most probably be the cause. Hope it helps.
01-19-2014 11:35 AM
Top #7
Where do I find the Hildebrand solubility parameter for EPDM?

I did not see it on the Wykipedia chart.

I think this is the problem. The pumps were easy to assemble and very hard to dis-assemble due to the swelling.
01-19-2014 02:18 PM
Top #8
The Hildebrand solubility parameter for EPDM will depend upon the type of Diene monomer and the ratio of the Ethylene, Propylene and Diene monomers as well as the effect of any of the additives, Carbon Black, Plasticisers etc. but typically the Base polymer for an EPDM with Ethylnorbenene as the diene with a 42:42:16% will be about 8.80 (cal/cc)^1/2. Typical levels vary from 8.12 to 8.90 with different Carbon Black's having different effects of dragging the SP up (so closer to EtOAc).

SP is a useful thing to be mindful of with polymers and swell interaction, but remember it does not cover the chemical effects purely the thermodynamic ones.

You can contact me on craigwatkinson1082@btinternet if you would like me to consult on this.
01-19-2014 04:21 PM
Top #9
Have you ever thought about the use of a Rotary Lobe Pump instead of the progressive cavity as this could possibly have a better efficiency and not have the problem of the swelling. In addition they do not need the additional power required on start up due to the frictional breakout torque of the rotor and stator?