Contact US

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!

*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

Pump Sizing Calculations versus Datasheet

Pump Sizing Calculations versus Datasheet

Pump Sizing Calculations versus Datasheet

Hello all. I've recently been tasked with the requirement to size a pump for a water pumping system. I've had some trouble getting my calculations to show agreement with the datasheet for a particular (example) pump. Attached to this thread are screenshots of my calculations and the datasheet. I'm trying to use the first law of thermodynamics to compare the flow conditions on the datasheet to the rated pump power (1.5 hp shown here). For example, the datasheet shows that an operating point on the pump is 79 gpm with 52 ft of head. However, the the mechanical power required to create this pressurization is not equal to the rated power of the pump, even when an efficiency factor is accounted for in the calculations (see attached calculations). Similar issues apply for other operating points. I'd like to understand how the datasheet is created before I begin sizing the pump so I can have confidence in my results -- is there something I am misunderstanding about pump sizing that I should know? Thanks in advance for the help.

RE: Pump Sizing Calculations versus Datasheet

If its a pump problem, do not begin the calculations with the electrical stuff. We don't care about Amps, or Volts. That changes with whatever motor voltage, frequency, phase, etc. is and it does not affect pump sizing.
Where did the 18 amps and 120V come from?
What is P mechanical? The pump rated power?

Your P mechanical / electrical efficiency is not correct.
Efficiency must be hydraulic efficiency.
That is obtained from the pump curve, provided by the pump mfgr. From the "pump curve." It is for a particular flow rate. You need the pump efficiency at 79gpm.
You must use a pump that can deliver 50ft of head at your 79gpm flow rate. I don't see that in your pump data, but it is very hard to read.

Your P equation P = 1.0 HP looks correct. Your P_reqd looks correct, but that is not electric power. It is called Pump's Brake Power
Use the pump efficiency value for 79gpm to calculate that.
Electric power, or Driver power = Brake Power/ motor efficiency. It looks like you do not know the pump's efficiency and are trying to use motor efficiency instead of pump efficiency.

A pump's rated power can be anything. It must be higher than the Brake Power

Not too sure, as I cannot read the pump data in your low resolution picture, but I do not see 79gpm at 50ft of head. I think I see 75gpm at 0 ft of head. None of these pumps work for 79gpm at 50ft., no matter how you wire them. That means that the hydraulic efficiency of these pumps for 79gpm at 50ft of head is 0. And 1.036 hp/ 0 eff = EXTREMELY high Brake power requirement.

End of story for now.
Next question?

A black swan to a turkey is a white swan to the butcher ... and to Boeing.

RE: Pump Sizing Calculations versus Datasheet

You're not using the pump performance data correctly. The pump does not make 52 feet at 79 GPM. It makes 52 feet at 0 GPM or 79 GPM at 0 feet. At those extremes your pump efficiency will be zero. You need a pump curve to estimate the pump performance. The flow values at the various head pressures in your image are taken from the pump curve.

RE: Pump Sizing Calculations versus Datasheet

Thanks all for the response. Either way, I am looking to reproduce the data provided in the pump datasheet using hand calculations. If I look at each operating point (flow rate and head), I can estimate the required power at that data point. It seems that the pump performance curve is not derived using the equation P = density x gravity x flow x head (rearranged for head as a function of flow). In this case, the head (H) is inversely proportional to the flow (Q), but the performance curves do not indicate this (i.e. head does not vary according to 1/Q on the performance chart). Can anyone elaborate on this? I would have expected the pump performance curve to be a line of rated power across varying flow conditions. In other words, I'm curious now how the pump performance curves are actually generated during the pump design phase - before testing is feasible, and what the theoretical basis for them is.

RE: Pump Sizing Calculations versus Datasheet

Find a real pump curve.
It shows pump efficiency at each flow rate.
If you don't understand pump curves, none of your calculations will ever make any sense.

A black swan to a turkey is a white swan to the butcher ... and to Boeing.

RE: Pump Sizing Calculations versus Datasheet

"I am looking to reproduce the data provided in the pump datasheet using hand calculations." WHY??

Just use the data provided.

Your missing factor for power is that the shaft power needs to include the efficiency of the pump. for small pumps like these this can be as low as 50-60%.

These little circulation pumps operate in a different way to much large units where the pump head at max flow is often only 20-30% below its max head.

Pump design is well understood, but unless you're going to design your own pump is not worth bothering about - just build it and test it.

Especially for these simple water circulation pumps, static friction and constant losses have a big impact and also the efficiency of the small motors is horrendous compared to larger units.

To size a water pump first you need to know its required duty. that is normally what flow do you want and then calculation of the differential pressure/head required. Then find a suitable pump. you seem to be wildly overthinking this. IMHO.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Pump Sizing Calculations versus Datasheet

I recommend that you ask the pump supplier to size the pump as it appears you do not have the background to size a pump. Just tell the supplier the pumping conditions and the application. Once you have the pump information, then you can input that information onto the data sheet.

The pump supplier should be able to provide you with the pump hydraulic efficiency as well as the motor electrical efficiency.

There are a number of pump calculators that you may use to check the pump suppliers pump recommendation:


RE: Pump Sizing Calculations versus Datasheet

bimr is vorrectt- simply give the flow and head plus any other operating conditions to a reputable pump engineer or company, it should take 1 or 2 minutes to size and select a pump for this duty.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

RE: Pump Sizing Calculations versus Datasheet

Hi JMP1098

Have a look at this link it might give you an insight in how to size the pump.


“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

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


Low-Volume Rapid Injection Molding With 3D Printed Molds
Learn methods and guidelines for using stereolithography (SLA) 3D printed molds in the injection molding process to lower costs and lead time. Discover how this hybrid manufacturing process enables on-demand mold fabrication to quickly produce small batches of thermoplastic parts. Download Now
Design for Additive Manufacturing (DfAM)
Examine how the principles of DfAM upend many of the long-standing rules around manufacturability - allowing engineers and designers to place a part’s function at the center of their design considerations. Download Now
Taking Control of Engineering Documents
This ebook covers tips for creating and managing workflows, security best practices and protection of intellectual property, Cloud vs. on-premise software solutions, CAD file management, compliance, and more. 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