Could somebody cross check my motor-pump calculation. 2

[redfun]

We are having a motor-pump set up in our lab, that over heats after pumping water for 2 hours.( Simply because it is was built as a pneumatic set up initially).We want to run the set up over months non stop to check a polymer heart valve. So a new motor was ordered. Now I just need to cross check if the order was done correctly by my predecessor.

Calculation of shaft power of present pump ( single cylinder positive displacement pump)

Input
Present motor power rating: 1/15 HP=0.0667 HP= 49.6915 W
Power of new motor( brush less DC motor whose speed can be varied between 8-400rpm ): 0.1667 HP= 124.1915 W
Suction head, Hs= 0.44m
Discharge head, Hd= 0.44m (Hs=Hd as all that pump does is recirculation)
Density of glycerin, ?= 1261 kg/m^3 ( planning to shift to glycerin as fluid so calculations are done using glycerin density)
Gravity= 9.81 m/s^2

Requirement
Discharge, Q= 5 L/min= 0.00008333 m^3/s (Heart discharge rate)

Equations
Pump shaft power, Ps= Hydraulic power, Hp / Pump efficiency, ?

Hydraulic power, Hp (W) = Q (m^3/s) x (Hd + Hs in m) x ? (kg/m^3) x g (m/s^2)

Assumptions
I have assumed positive suction head as the height above pump center line to water source.
And the discharge head as the height from pump center line to water destination.

Calculations
Hydraulic power= 0.00008333x0.88x1261x9.81= 0.9071W

Observation
According to this calculation the power requirement of the pump is lesser than the power provided by the motor- new and old, even if a reasonable loss factor is considered. But we know that the heating could not be avoided in the present motor unless a heat removal source is provided ( well we ran the set up overnight after we kept a table fan in front of the motor –so in and around the valve endured 25 hours of testing before failing). So the questions that may be raised are would the new motor have the necessary torque-my guess is it should as it has a higher power rating than the present motor. And the second question is would it endure months of operation without overheating. I have presently not found a way of predicting if this would happen.



Presently the pump is horizontally oriented, but can I orient it vertically to avoid air trapping problem ?Now how could I do calculations that would show the difference in power requirements of vertical and horizontal positioning of pump? ( May be the effect not considering the leakage would be inconsequential because of the fact that the pump is very small, and hence the work that needs to be done against gravity is low)

I hope to achieve the 5L/min discharge by running the motor at 120 rpm, (this is the same number of reciprocations I want from the pump piston per minute.) And the discharge per stroke to be adjusted would be 42 cc.

"Please correct me if I am wrong in application of formulas or assumptions .Any observations or suggestions are welcome"

 

[micalbrch]

Can you install the pump vertically? It has suction and discharge valves, probably ball checks. Will they work in a vertical installation? I guess not because the balls need the gravity to close.

Another question: Is your pump single or double acting? You wrote that it is a one cylinder pump. O.k., but are both piston sides pumping or only the front side? That is important for the power calculation.

And please refrain from double posting. If you do not get an answer within 2-3 days in one forum, it will be o.k. to do so.

 

[cholopa]

The pressure in the equation should be the difference between suction and discharge pressure in Pa units.

Though your system is recirculating your system will have some friction pressure loss, that the pump has got to give.

 

[redfun]

@micalbrch , Sorry for the double post, I did try to delete the first one, but didn't know as to how to do that.
Now the PD pump is a single acting single cylinder one.We want it it mimic the heart operation as much as possible. As far as the valves are concerned , there is no check valve on discharge side. But there is one at suction side, it seems to be having some sort of diaphragm.So is a check valve necessary, some sort of unwanted pressure spike was noticed when when a valve was previously fixed.

@cholopa, I did see the eqn as difference between suction and discharge head in a text, but then was not sure about how it would translate in my problem.Both Hs=Hd=0.44m that means Hd-Hs=0. So I added both, with the only result that I would get a more conservative answer.
As far as friction loss is concerned. The tubes are very short. The whole set up sits on a table. So can't they be neglected?

 

[micalbrch]

You have to multiply your result with 2 as your pump is a single acting simplex piston pump. 2 because a discharge stroke is followed by a suction stroke which creates now flow out of the pump. That means that the number of strokes to provide the required flow is twice the stroke no. of a double acting pump. Therefore you need to add a x 2 in your equation.

 

[redfun]

@micalbrch, I am taking your word on multiplying the end power by 2 ( even though I cross checked and couldn't find the formula to support the same in any text.) It would simply increase the power by a factor of two and even taking any loss factors into consideration, the power would still be less that 10 W. Which is really nowhere near the power of the new 120 W motor we have ordered. So I am guessing the overheating problem should be taken care of as we are running for low power on low speed ( 120 rpm---same number of strokes are expected--120, pumping 42 ml/sec.) Now I am trying to take the valve off today to see what it is !( It looked real gross with lots of slime when I partially took of the valve lid yesterday) Our presumption is a horizontally aligned cylinder when worked over many cycle should be able to pump out the trapped air.

 

[micalbrch]

redfun: You can take my word, beleive me. But yes, it seems the installed motor is way oversized. As far as I understand your post you ordered the same motor size again. What does the pump's nameplate say about max. flow and head/pressure? Is perhaps also the pump oversized (which would not matter)? What I want to say is that the pump can be rated for a much higher pressure and it can therefore have a much bigger motor size that you need for your application. For me it seems you bought a pump from a catalogue. So, it is not customized. That is perhaps the reason for the big motor.

Yes, the pump should be able to press out any air. But is seems that it cannot. Do you have any cross-section drawing which you can post here or any photo to find out why the pump gets warm?

 

[redfun]

@micalbrch, the initial motor we had was of 50W rating, the new one we have received is of 120 W.( I am assuming ,that when you say, that it is over sized,you are talking about the rating). So our argument has been , if the 50 W motor over heated after two hours of working( and to some extent this problem was solved, when we kept a big table fan in front of it.), then a 120 W motor should work with out over heating for months. Since we got the original set up from another lab, we received no specification papers along with it. So I went online and checked to see if I could get the same. Unfortunately the medical instruments company that made the unit does not exist anymore.The pump was custom made by them.

I am uploading a photo of the set up

http://files.engineering.com/getfil...1-a19c-daa68c36cc21&file=IMG_2542_800x600.jpg