×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Calculating power consumption from knowing suction & disch. pressures
3

Calculating power consumption from knowing suction & disch. pressures

Calculating power consumption from knowing suction & disch. pressures

(OP)
If the suction & discharge pressures are read-off from gauges located on a running centrifugal pump suction & discharge how is the power consumption calculated assuming the pump is pumping water & flow rate is known ?

RE: Calculating power consumption from knowing suction & disch. pressures

A couple ways:

1. Take an amp reading on the motor, get the motor performance (efficiency & power factor) to back into the pump bhp that way.

2. Get a performance curve, go to flow and match that to the horsepower.  All pump curves should have the performance and power requirements plotted.

You can compare the answers from both different approaches and come up with a good answer.

Of course, you could also put a torque coupling between the pump and motor and measure the torque directly and (since the speed is known) calculate the power fairly easily.  Problem is, torque couplings tend to be just a tad on the expensive side and not too many people have them just laying around.

RE: Calculating power consumption from knowing suction & disch. pressures

(OP)
Thanks Tstead but I made a small mistake in my post.  I said I knew the flow but that was wrong.  Actually I would like to calculate the flow knowing the suction & discharge pressures.

RE: Calculating power consumption from knowing suction & disch. pressures

In that case, you need to know the impeller diameter if you want to get to it from the performance curve point of view.

You don't need to know the flowrate - or pressure readings if you go about it from the motor power point of view - you do need, however, reliable info on the motor (e.g. power factor & motor efficiencies).

Another trick, if you don't know the impeller diameter and if you are running on 1750 RPM speeds, run the pump to shutoff head, measure the discharge pressure, convert this to TDH in feet and take the square root.  You'll get approximately the impeller diameter in inches.

RE: Calculating power consumption from knowing suction & disch. pressures

(OP)
Is it not possible to calculate the TDH knowing both suction & disch. pressures ?  Would the difference between the two not be the TDH ?

RE: Calculating power consumption from knowing suction & disch. pressures

TDH = (Disch. psi - Suct. psi) x 2.31 ft/psi

Horsepower is approximately (GPM X TDH) / 4000 (for water)

tstead is right about the impeller diameter.  If your pump runs at 3500 rpm, the head will be 4 times what it was at 1750 rpm (head is proportional to speed squared)

RE: Calculating power consumption from knowing suction & disch. pressures

You can calculate the TDH as ArtR said above, that is not a problem (you do need to know the specific gravity though - that is a very crucial part of the equation just like suction and discharge pressure).

The horsepower eq. he gave is water horsepower - not brake horsepower.  To find the brake horsepower using just flow and head, you NEED to know the efficiency of the pump at the operaitng point.  Since there is no gauge you can mount on to the pump and measure the efficiency, you need to go about it through measuring or calculating the horsepower directly.  Using that, you can back into the pump's efficiency if you really want that piece of information at that point (it would be uselss at that point).

By the way, H.I. standards say to use 3960 not 4000.  Small difference, I know.  Also, if you were to run through the derivation of that equation (it's not difficult at all), you'd see that it is actually 3956, but I digress.

Tim

RE: Calculating power consumption from knowing suction & disch. pressures

One other suggestion in addition to the excellent answers already posted:

If you use a Tach to determine the speed of the motor, then you can compute shaft horsepower from the motor's torque-speed curve.  Multiply SHP by estimated pump efficiency to get fluid power.  Divide fluid power by differential pressure to get volumetric flow rate (carry your units through).  Surprisingly, The SG of the fluid is NOT required using this approach (or when using motor input power and efficiency to estimate shp).  The units of volume flow rate times differential power convert to units of power without any dependence on SG.




Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members! Already a Member? Login


Resources

White Paper - The Criticality of the E/E Architecture
Modern vehicles are highly sophisticated systems incorporating electrical, electronic, software and mechanical components. Mechanical systems are giving way to advanced software and electronic devices, driving automakers to innovate and differentiate their vehicles via the electric and electronic (E/E) architecture. As the pace of change accelerates, automotive companies need to evolve their development processes to deliver and maximize the value of these architectures. Download Now
White Paper - Model Based Engineering for Wire Harness Manufacturing
Modern cars, trucks, and other vehicles feature an ever-increasing number of sophisticated electrical and electronic features, placing a larger burden on the wiring harness that enables these new features. As complexity rises, current harness manufacturing methods are struggling to keep pace due to manual data exchanges and the inability to capture tribal knowledge. A model-based wire harness manufacturing engineering flow automates data exchange and captures tribal knowledge through design rules to help harness manufacturers improve harness quality and boost efficiency. Download Now
White Paper - Modeling and Optimizing Wire Harness Costs for Variation Complexity
This paper will focus on the quantification of the complexity related costs in harness variations in order to model them, allowing automated algorithms to optimize for these costs. A number of real world examples will be provided as well. Since no two businesses are alike, it is the aim of this paper to provide the foundational knowledge and methodology so the reader can assess their own business to model how variation complexity costs affect their business. Download Now

Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close