Variable Displacement Pump vs. Fixed with Variable Freq. Drive for 5000 psi Hydro system

[EdHenrie]

Hi,
I'm a Mech Eng. with limited hydraulic experience and I'm specifying a hydraulic system for use as a test stand.

Application details:
Fluid: Skydrol (Phosphate Ester) = EPR seals
Max Pressure/Flow: 5000 psig at low flow (<1 gpm)
Normal Pressure/Flow: 3000 psig at 5+ gpm (intermittent)
Low Pressure/Flow: Controllable at 50 psig very low/zero flow

Initially, I wanted to use a Variable Displacement Axial Piston Pump (w/fixed RPM motor) and manually or electro-mechanically control the output pressure/flow. The vendor I contacted for quotes couldn't find a good solution for my criteria using that type of pump and suggested a fixed displacement axial pump with a variable frequency drive on the 3-phase motor to effectively give flow control with pressure regulation via pilot control valve.

My Question(s):
What are the drawbacks to using the Fixed Displacement Pump with VFD? Are there any?
One advantage I get is the built-in electronic control-ability, of which I'm a big fan, and have the knowledge to implement.
Am I asking the right question? What am I missing? Would YOU implement the fixed/VFD design in YOUR test stand, or would you lean towards the Variable Displacement Pump with fixed RPM motor?

Thanks in advance for your opinions and suggestions.

Regards,
Ed

 

[HPost]

Hi Ed

In my experience, the biggest challenge you'll see is the efficiency.

The questions you ask are fair and worth asking.

The mechanical efficiency of the pump will be chronic at low speeds and likewise, the volumetric efficiency will remain constant and as a percentage of output flow at low speeds, the losses will be very large. You may find that the pump cannot make the desired pressure. Unless that is you procure a bespoke pump with very high volumetric efficiency.

I am not sure who told you the pumps with EP seals are not available, I have seen lots of pumps that have been adapted to run on skydrol, you just need to strip the paint off. Otherwise the skydrol will do it for you and leave it in the tank.

If you can get a piston pump, you'l find it much easier in the long run.

Good luck

Adrian

 

[hydtools]

See Dynex:

http://www.dynexhydraulics.com/fluids.htm

Ted

 

[hydtools]

http://www.vickershydraulics.ru/pages/hydraulics/pumps/pdf/512702en0999a.pdf

Ted

 

[hydtools]

http://parkerdenisonpk.com/hydraulic_pumps/LT1-00059-1.pdf

Ted

 

[EdHenrie]

Adrian,
My post was not clear. The EPR seal pumps are available, but the vendor couldn't find a pump that met the 5Ksi and flow requirements through their suppliers. Since this is a test stand, efficiency is not a design criteria. We currently have a test stand that runs Skydrol that I built from parts I was provided and told to use. That concept didn't work out so well since the pump and all components were rated 3Ksi, but the test requirements went to slightly below 5Ksi. Management decision...<ugh!> I also ended up with much melted paint and an alimentary dysfunction (All's well now. Wear those gloves!). Anyway, thank you for your reply.

 

[EdHenrie]

Ted,
Thank you for the links. Amazingly on point.

Thanks,
Ed

 

[tbuelna]

For a test stand hydraulic pressure supply requiring the range of flow (0-5gpm) and relatively high max pressure (5000psi) noted, I would use a piston pump and DC motor to pressurize an accumulator. And then use an arrangement of valves to precisely regulate the fluid pressure and flow delivered to the test article.