HOW TO CALLCULATE FLOW RATE OF PUMP WITHOUT USING ANY APPARATUS?
HOW TO CALLCULATE FLOW RATE OF PUMP WITHOUT USING ANY APPARATUS?
(OP)
I have a centrifugal pump working on suction lift for cold water at 20 C. its specs are as follows.
Suction dia = 5 inches
Discharge dia = 4 inches
Motor RPM = 1460
Motor Power = 20 kw
Impeller dia = 14 inches
from these specs I calculated head and pressure of the pump as follows:
1- Peripheral velocity of vane =
=( RPM * Dia of impeller in inches) / 229
= (1460 * 14) / 229 = 89.26 ft/sec
2- Now the head at shutoff = (Peripheral velocity)2 / 2g
= (89.26)2 / 2*32.2
= 123.7 foot of head.
[Ignored all the friction losses and suction lift
that is not too high]
3- The discharge Pressure is then calculated as = Head * 0.433
=123.7 * 0.433
=53.57 Psig
So from such values, how I can calculate Flow Rate of pump just with calculations without using any apparatus
Suction dia = 5 inches
Discharge dia = 4 inches
Motor RPM = 1460
Motor Power = 20 kw
Impeller dia = 14 inches
from these specs I calculated head and pressure of the pump as follows:
1- Peripheral velocity of vane =
=( RPM * Dia of impeller in inches) / 229
= (1460 * 14) / 229 = 89.26 ft/sec
2- Now the head at shutoff = (Peripheral velocity)2 / 2g
= (89.26)2 / 2*32.2
= 123.7 foot of head.
[Ignored all the friction losses and suction lift
that is not too high]
3- The discharge Pressure is then calculated as = Head * 0.433
=123.7 * 0.433
=53.57 Psig
So from such values, how I can calculate Flow Rate of pump just with calculations without using any apparatus





RE: HOW TO CALLCULATE FLOW RATE OF PUMP WITHOUT USING ANY APPARATUS?
At least, not easily.
My first instincts would be as follows:
1. Assume your head-capacity curve does not cross over and above your theoretical hydraulic power curve.
2. Assume a % rise in head to shut-off from BEP. Some folks like to use 25% but I think it's optimistic. My gut feel would suggest 10% to 15%.
3. Assume a maximum pump efficiency. For centrifugals, in the absence of data to the contrary, I would personally assume 75%.
4. Subtract 15% from the shut-off head you appear to have already calculated and apply 75% efficiency and full power (bhp) at that point. Apply the equation P = Q*H*SG/(3960*n), algebraically rearranged to solve for Q. You now have two points on your pump curve defined: shut-off and BEP.
5. Assume a concave down parabola with vertex at BEP and crossing the "Q" axis at (0,0) for the pump efficiency curve left of BEP. Derive it's corresponding equation.
6. Assume a concave-down parabola with vertex at BEP and crossing the "Q" axis (n=0) at a point ~5*(Q@BEP) for the pump efficiency curve right of BEP. Derive its corresponding equation.
7. Take the efficiency corresponding to ~3*(Q@BEP) and choose this as your run-out condition. Apply the equation P = Q*H*SG/(3960*n), algebraically rearranged to solve for H.
8. Find an equation in the form of Q=k*H^2 for a simple concave down parabola with vertex at the pump shut-off point that passes through the three points defined above.
Now, this is in no way founded in theory or textbook. It's just what I personally would do to "rough-in" something that might come approximately close to modelling a typical centrifugal pump if I was given nothing more than what you have to work with. I might be close; I might be out to lunch. In any case, you would have to then go back and check several other points along the locus of your hypothetical curve to validate the rather bold assumptions made, particularly at Steps 1 and 2. With a spreadsheet, you could tweak and tune things to get a fit.
The only true way to figure things out is to get the as-tested pump curve.
Regards,
SNORGY.
RE: HOW TO CALLCULATE FLOW RATE OF PUMP WITHOUT USING ANY APPARATUS?
So double checking my last post with some representative pump curves (maps) by typical vendors, showing the "eyes" (BEP) of the curve...
6. Maybe ~4*(Q@BEP) would be a better assumption for n=0 right of BEP.
7. Maybe ~2*(Q@BEP) would be a better "run-out" point to assume.
Like I said...it's something that would need some fiddling and experienced judgement to arrive at something make sense.
Regards,
SNORGY.
RE: HOW TO CALLCULATE FLOW RATE OF PUMP WITHOUT USING ANY APPARATUS?
If you failed to contact the maker, the best you can do is get more operating data.
You can at least eliminate one uncertainty by putting suction vacuum gauge and discharge pressure gauge on the pump nozzles to find out the actual differential head.
You will also need to measure the motor current and make an estimate of the power out put by comparing with the motor full load current.
If it more than 80% of the of the full load, the accuracy is much higher.
Then use the basic formula Kw = Q(m3/h)x H (M)/367/ pump effcy.
Here come the difficult part in guessing the pump efficiency.
For the size of the impeller and nozzle size, it would be some where between 60 to 70%.
Good luck..
RE: HOW TO CALLCULATE FLOW RATE OF PUMP WITHOUT USING ANY APPARATUS?
RE: HOW TO CALLCULATE FLOW RATE OF PUMP WITHOUT USING ANY APPARATUS?
Further, how this process will help me to estimate actual bhp?
I myself is not very good at pumping system.But my General Manager has assigned me to calculate the flow rate on the behalf of some basic data which i have already mentioned in the question :)
Profound Regards!
RE: HOW TO CALLCULATE FLOW RATE OF PUMP WITHOUT USING ANY APPARATUS?
Can you rent a Doppler meter to measure pipe flow? I used to do that years ago (Polysonics Doppler Meter; it came in a foam case with the pickups, clamp, grease and chargeable battery for about $100.00 per day). If you are measuring current draw anyway with an ammeter...
Regards,
SNORGY.
RE: HOW TO CALLCULATE FLOW RATE OF PUMP WITHOUT USING ANY APPARATUS?
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Regards,
SNORGY.
RE: HOW TO CALLCULATE FLOW RATE OF PUMP WITHOUT USING ANY APPARATUS?
You have proposed a good start based on limited data, but the one most important piece is missing: specific speed of impeller.
A five gallon bucket and a stopwatch will yield much, much more accurate results.
RE: HOW TO CALLCULATE FLOW RATE OF PUMP WITHOUT USING ANY APPARATUS?
I did that in the field once. It is indeed effective.
The other engineers involved got mad at me. But...it was what it was.
Star to you.
Regards,
SNORGY.
RE: HOW TO CALLCULATE FLOW RATE OF PUMP WITHOUT USING ANY APPARATUS?
Yes! unless I have data from manufacturer, i cant determine the specific speed of the Impeller. you know! in my parallel pumping system, there are two pumps with different impeller dia and rpm of motor but same dia of suction and discharge ports.
So I liked to make the head same by using the following calculations
Pump: 1
Dia: 14 inch
RPM: 1460
Solution:
1- Peripheral velocity of vane =
=( RPM * Dia of impeller in inches) / 229
= (1460 * 14) / 229 = 89.26 ft/sec
2- Now the Diff. head at shutoff=(Peripheral velocity)2 / 2g
= (89.26)2 / 2*32.2
= 123.7 foot of head.
[Ignored all the friction losses and suction lift
that is not too high]
Pump: 2
Impeller Dia: 14 Inches
RPM: 1400
Solution:
1- Peripheral velocity of vane =
=( RPM * Dia of impeller in inches) / 229
= (1460 * 10) / 229 = 61.135 ft/sec
2- Now the diff head at shutoff =(Peripheral velocity)2 / 2g
= (61.135)2 / 2*32.2
= 58.03 foot of head.
[Ignored all the friction losses and suction lift
that is not too high]
In order to make the heads of Pump: and Pump : 2 similar, i will have to increase the RPM for Pump: 2
Therefore
Pump : 1 = Pump: 2
RPM(1) * Imp. dia(1) /229 = RPM(2) * Imp.dia(2)/229
1460 * 14 = RPM(2) * 10
RPM(2) = (1460 * 14)/ 10 = 2044
Now: if we run the pump : 2 with 2044 RPM the calculations will be as:
1- Peripheral velocity of vane =
=( RPM * Dia of impeller in inches) / 229
= (2044 * 10) / 229 = 89.26 ft/sec
2- Now the Diff. head at shutoff=(Peripheral velocity)2 / 2g
= (89.26)2 / 2*32.2
= 123.7 foot of head. (equal to pump: 1)
[Ignored all the friction losses and suction lift
that is not too high]
But here is the problem of SUCTION SPECIFIC SPEED. Now i need manufacturer's data or i will have to take risk to purchase such motor with 2044 RPM or VFD :)
regards!
RE: HOW TO CALLCULATE FLOW RATE OF PUMP WITHOUT USING ANY APPARATUS?
regards
RE: HOW TO CALLCULATE FLOW RATE OF PUMP WITHOUT USING ANY APPARATUS?
It seems you are more interested to do an academic exercise as suggested by DubMac rather than trying to find out the flow rate.
RE: HOW TO CALLCULATE FLOW RATE OF PUMP WITHOUT USING ANY APPARATUS?
Thanks to all of you!
Regards!
RE: HOW TO CALLCULATE FLOW RATE OF PUMP WITHOUT USING ANY APPARATUS?
I have 22 bar (discharge pressue) Multistage HOT WATER pump ( Temp. of water is 165 C) in the factory. But now i want to upgrade pressure up to 30 Bars. Please tell me the efficient and the most economical way to enhance the pressure?
Regards!
RE: HOW TO CALLCULATE FLOW RATE OF PUMP WITHOUT USING ANY APPARATUS?
a.) approach the pump vendor for a re-rating
b.) Put a VFD on the pump and speed up the pump and drive the TDH higher - this needs vendor verification as it will alter the pump curve, increase flow, power and impeller speed. Other bad things may come out of this.