Pressure / Flow in a parallel multi-pump system

[Ben_M]

I’ll preface this by saying that I have absolutely zero engineering experience in high pressure systems, so please cut me some slack. I managed to get some three phase motors cheap, and I have some various pressure washer pumps laying around, so I thought it would be fun to fabricate a parallel pump system with the goal of learning some of the basic principles of hydraulics. If I happen to end up with something that that works well for cleaning up my engine projects and shop, even better!

I’m covered on the electrical & control side – still playing with the design, but to start, I’m planning to build a rotary phase converter and control the motor speeds with variable frequency drives. For the sake of simplicity, the mechanical coupling from the motors to the pumps will be a dampened belt-drive. On the logic side, my plan is to install flow sensor and pressure sensors ahead of the check valves, and a single flow/pressure sensor on the combined line. Each of the sensors will connect to a compute resource (microcontroller, Arduino IDE, or small computer – TBD) for data logging purposes, and the input data will have the ability to control the VFDs.

Here’s a simple diagram:


The pumps have integral pressure-actuated unloader valves which will stay in place for the time being.

So to start with the basics, what equations are used to determine flow rate in a parallel coupled system like this one? Assuming that the pumps are spinning at design RPM and producing the PSI/GPM in the diagram, would we use the mean PSI & GPM to calculate the final flow and pressure? E.g – (3000+2000)/ 2 = 2500 PSI, and (3+2)/2 = 2.25 GPM? If this is correct, does it scale without regard to the number of pumps?

Are the check valves actually necessary in a system like this? My thought was that without a check valve on P2, the unloader valve on P2 would stay open, preventing the pump from doing any useful work and pressurizing it beyond its design limit of 2000 PSI. Is this a correct assumption?

Any recommended elementary reading on the subject? Also, I would more than appreciate any design guidance, advice or criticism… new subject matter for me.

Thanks!

 

[MFJewell]

Are the motors designed to run off of VFDs? If not, you will most likely ruin them.

If both pumps are running at full capacity, your output flow rate would be 5gpm. However, you may end up deadheading pump 2 if pump 1 is running at full speed. If the output of pump 1 is more than 2000 psi, you may have a differential pressure across CV2 that will keep it closed. This will depend on how freely flow can go out of the orifice. The system pressure will depend on pump output, line size, and orifice size. Use Bernoulli's equation to compute.

 

[KoachCSR]

At first glance, in using two pumps with very different pump curves, you're going to have a flow balancing nightmare. With one pump outputting 3,000 psi, you won't see any forward flow from the 2,000 psi pump...to use both at the same time, you'll need to add a control valve or orifice plate to the 3,000 psi pump discharge to knock its pressure down.

You mentioned that these pumps are pressure washer pumps, but connected to different motors (rather than the small engines normally supplied with them, I assume) - correct? If you're connecting different motors than originally supplied with the pumps, your pumps curves are likely affected and you may not see the performance you're expecting above - how does the HP and rpm of your "new" motors compare to the ones originally supplied with them?

Edit: As far as recommended reading, being unfamiliar with fluid mechanics as you say, I'd recommend grabbing an old fluid mechanics textbook (such as the one authored by Crowe, Elger, and Roberson). Once you acquire some basic knowledge, Crane Technical Paper No. 410 is an excellent reference.

 

[mk3223]

Fist, without any flow or pressure control at discharge, the pump is to be flow toward the end of the pressure-flow curve. And, the flow of each pump will be balanced hydraulically due to the pump discharge pressure and the line pressure loss.
The check valves CV1 and Cv2 are needed, so the fluid won't be flowed backwards to the pump.

 

[3DDave]

If you have two electronic boost modules in parallel, one producing 12 volts and the other producing 9, isolated by diodes, how much current will they shove into a circuit?

 

[hydtools]

The pumps are positive displacement pumps and will deliver fixed flow at fixed speed at a pressure developed by the plumbing and spray gun at that flow. Flow will be constant, 5 gpm, until the system pressure demand reaches 2000 psi when pump 2 will unload and pump 1 will continue to deliver 3 gpm. When the gun valve is closed and the pressure rises to 3000 psi, pump 1 will unload.
This assumes the motors have the capacity to drive the pumps to max pressure without losing rpm. If the motors lose rpm the pumps flows will drop proportionally.
The pumps will operate at the same pressure until pump 2 unloads at 2000 psi.

Ted