Calculating Horsepower Required for Wanner Hydra-Cell Pro^® Pumps

We undersized a motor on a Model D35EKSTNNECA. The customer used a VFD for 3.4 to 20 GPM at up to 1,200 PSIG. This article describes what we missed and how we fixed it.

The Wanner^® horsepower formula for a D35:

Maximum pressure on this job is 1,200 PSI. The required RPM for a specific GPM can be calculated using the following formula:

Our customer required their direct-coupled pump to operate across a broad turndown range (3.4–20 GPM) at a constant pressure and used a VFD to adjust the RPM/flow rate.

Rated GPM and rated RPM are the two constants. On a D35 “E” cam that is 34 GPM at 1,150 RPM.

Put in target GPM and solve for target RPM.

At 3.4 GPM the E-cam D35 needs to run at 115 RPM:

At 20 GPM the target RPM is 676.5.

Plug those targets into the horsepower formula at 1,200 PSIG:

• 3.4 GPM @ 1,200 PSIG → 2.98 HP
• 20 GPM @ 1,200 PSIG → 17.5 HP

So a 20 HP motor is required?

Wrong. The limit is shaft torque. Most motors hold constant torque across a 10:1 turndown, but that “constant” is the torque at the motor’s rated RPM. This motor will run well below rated RPM.

Calculating Torque

Maximum motor torque in inch-pounds:

20 HP at 1,150 RPM is 1,096 in-lb. That is the maximum a standard 20 HP motor can hold from 115 to 1,150 RPM.

Is 1,096 in-lb enough?

Use the lowest operating RPM in the same formula. At 115 RPM the torque requirement is 1,643 in-lb. Always check torque at the slowest shaft speed.

So hte question becomes what HP is required to maintain a constant torque of 1,643 in-lb?

Result: 30 HP to deliver 1,643 in-lb and keep it across a 10:1 turndown for a direct drive system using a VFD.

Torque vs. Horsepower

Torque is the twist needed to beat resistance. On a Wanner Hydra-Cell Pro^® pump that resistance is discharge pressure.

Wanner’s HP formula tracks rotational speed for displacement. When the motor is not at rated RPM, check torque. When the horsepower to provide the rquird torque exceeds the flow-and-pressure HP, it determines the HPr.

Problem

At high pressure and cut RPM the VFD tripped on amps. On a constant-torque load the motor still has to make torque at low speed, so current stays up (or climbs a little) while horsepower falls. That current exceeded the VFD limit.

The skid, VFD, electrical and floor space were all built around a 20 HP size motor. Increasing to 30 HP meant a new motor, base, coupling, drive and electrical system — expensive!

Solution

20 GPM at 1,200 PSIG requires 1,630 in-lb when the motor shaft is below rated RPM. What if the motor ran at 1,150 RPM and only the pump shaft was reduced?

That was the fix: slow the pump mechanically and let the motor run full speed. The practical answer here was changing to a belt driven system. A 900 RPM motor or a gearbox would have worked but cost more time and money.

Calculating sheave diameter and belt length are will be a separate article, in this example Wanner helped size those.

Parts: a belt-guard base, sliding motor mount, sheaves and belts. Footprint stayed close. Some pipe moved. Nothing electrical changed.

Automating the Math

Wanner’s Partners Portal spreadsheet (Technical Information → Calculation Tools) does flow/pressure HP and torque HP at reduced RPM. It only covers six models, and you still have to figure operating RPM by cam. After this job we posted a more inclusive online calculator:

https://innovativepumps.com/calculators/Hydra-Cell-Pro-Pump-Horsepower-Calculator.htm

It covers every Hydra-Cell Pro model, sets RPM from a cam dropdown, and also shows maximum pressure available at a chosen motor HP — useful for PRV selection when a customer can live with a little less pressure on a smaller motor.

 

Related Articles

• Motor sizing for high-temperature liquid
• Components of a high-pressure pump skid
• Sealless pressure regulating valves
• Assembling pump to motor with an adapter
• D35 Hydra-Cell Pro