I've been working with a hydraulics test system for a little over a year. I have no formal hydraulics training, I'm an electronics engineer. Much I know about how our system works is through observation, discussing with other electronics engineers about the system and how they think certain things are working, and with a little bit of reading. I got into a battle today with another coworker, an ISE, who is trying to equate what he knows about electronic (power supplies and resistors) to hydraulics and I know empirically what he thinks is happening isn't just based on what I've observed.

Hopefully I can describe what we have without drawing a picture.

We have a piston pump that can deliver 110cc per stroke and we spin it at 1800 rpm. This pump will do roughly 40gpm at 5000psi. We have 2 valves very close to the output of this pump. One is a relief valve we set at 5800 psi, the other is a variable valve we control pneumatically to set the system pressure from about 500-5000psi. My coworker and I agreed that with no flow being required from the pump into any sort of device (actuator, valve, etc) all of the flow being delivered from the pump will be passing through the variable valve whether it is set at 500 or at 5000. My coworker got in his head that if the pump can do 5000 psi at 40gpm and any more flow demanded of it will cause the pressure to go down that when we add a load to the system that could require up to 5gpm (we'll say a solenoid controlled valve that limits flow to 5 gpm max) that the 40 gpm flow through the variable valve won't go down to 35 with 5 now going through the device but that the same 40 will keep flowing through the variable valve, 5 more will now be going through the device and the overall pressure will drop.

Even though I observe this is not the case, I couldn't explain the physics behind why he is wrong because I don't know the equations.

What is the basic math?

9/22/2011 11:05:01 PM

It's a little fuzzy as to whether you are saying your variable valve and theoretical solenoid valve are in series or in parallel downstream of the pump outlet but I'm guessing you theoretically have them in parallel.

I believe he would be right if it was a centrifugal pump, which follows a slowly depreciating pump curve. If you were to reduce the system head by opening or widening the flow path you would be dropping the system head curve, meaning the intersection of the system head curve and pump capacity would be shifted to the right -> more flow, less output pressure.

Since you have a piston pump, and here's where it gets a little fuzzy for me, it follows the PD pump curve, which is nearly an instant drop in output pressure when you exceed the flow rate required (see charts in the link below). One thing to note for you while I was googling for a graph was this article on discharge throttling which makes sense but I never knew before now. Read about halfway down.
http://www.driedger.ca/ce2_pdp/CE2_PDP.html
IMHO between your coworker having the incorrect (but not illogical) mindset and the discharge throttling variable valve setup, this may be explaining your observations not quite lining up with his theory although you haven't really mentioned what you are observing at the moment.

In all seriousness, are there no mechanical or chemical engineers where you work? This is right up their alley.

Also, did you ever figure out what was going on with your pressure leakage issues in one of your systems a while back?

[Edited on September 23, 2011 at 12:12 AM. Reason : .]

9/23/2011 12:09:43 AM

vague question written with run-on sentences

9/23/2011 12:23:34 AM

yeah, i need an illustration.

9/23/2011 12:32:00 AM

Hi Chief,

Yeah, you can think of everything in parallel with the pump with all paths making their way back to the tank. One is essentially set to relieve at 5400, one at 5800, and one is variable sets the system from ~500-5000. Most things we will have hooked up will have their on flow restrictors so that typical flows are in the 2-10gpm range and usually then only for brief periods of time anyway.


As far as the thread I made back in December asking about air in the lines. I honestly can't remember what the outcome of that was. I had only begun to get a decent understanding of not just hydraulics back them but specifically how are test stand was implemented and I wanna say my coworkers and I didn't understand some aspect of it and what the "smart guy" (at the time) in the group was theorizing was an air in the line issue was actually doing something wrong with the valving.

Btw, here is a link to a 3.4mb PDF of our pump series if you're curious
http://tinyurl.com/3l3326l

We have the p07 turning at 1800rpm.

[Edited on September 23, 2011 at 5:13 PM. Reason : ^ If what I put is still clear as mud I'll try and sketch something up]

[Edited on September 23, 2011 at 5:26 PM. Reason : .]

9/23/2011 5:13:16 PM

Well, I've had some brief experience in hydraulics, so I'll do my best to contribute something. For starters, pumps don't directly generate pressure per-say, but rather they only generate flow, and the resistance in the system generates the pressure. Therefore, if you were pumping 40gpm through a pressure controlling valve at 5000psi, and you opened an additional orifice, the flow would be divided betwen both valves, and thus, to maintain 5000psi, the controlling valve would need to close and reduce flow through that segment such that the total system flow would come back down to 40gpm, and back within the pump limitations.

If your pressure-controlling valve was not pneumatically-controlled to maintain pressure, then yes, the additional flow would cause the system pressure to drop. However, since the valve (in my interpretation at least) is self-adjusting to maintain pressure, the valve would automatically close slightly to compensate for the drop in flow, increasing the resistance in that segment and bringing the pressure back to the prescribed setting.

Hope I clarified things rather than making it worse, but if I'm mistaken anywhere, feel free to correct me.

tl;dr Whether or not one valve is pressure-controlling is key here. If so, that one will close completely if necessary to compensate for any other orifice in an attempt to maintain pressure. If fully-closed isn't enough, then would would see pressure bottom out to nothing out the extra orifice.

[Edited on September 23, 2011 at 6:06 PM. Reason : tl;dr]

9/23/2011 5:52:32 PM

^ I think this is exactly the argument my coworker is making. Again, there could be something fundamental about this variable valve that I am not understanding but I have certainly observed a flowmeter well down stream of this variable valve indicating

1) A flow rate of ~48gpm but only at ~1000psi, and
2) A flow rate of ~40gpm while maintaining 5000psi

My intuitive thought (that could be way off) is the restriction from the variable valve is generating a certain velocity of flow and also causing heat to be generated into the system. When the valve well down stream is opened such that a certain amount of flow is diverted through it instead of the variable valve, the system adjust it's flow velocity at various points such that the pump is still delivering it's rated flow it's just not all now going through this variable valve. The heat generating by the restrictions would be changed such that the variable valve isn't generating as much and any not be generated there is now being picked up by the down stream valve.

9/23/2011 5:59:25 PM

The PDF you linked to shows that the P07 pump, at 1800rpm, is capable of about 49gpm@5000psi and about 53gpm@1000psi. Therefore, flow and pressure requirements are still within limits according to your above post.

When you say that the flow rate is 48gpm at a pressure of 1000psi, is this where your system is struggling to meet the 5000psi setting and this is the best it can produce? With these pumps, your pressure is directly proportional to your delivered torque to the pump, and your flow rate is directly proportional to the RPM of the pump. What's driving this pump, and what is it (Electric motor, engine of some sort) rated to in power output? If your pump is struggling to meet 48gpm/5000psi, you may be short on power supplied - the PDF indicates you need in the ballpark of 24hp to push this pump to it's limit. Can you manage 45gpm and 5000psi?

9/23/2011 6:26:25 PM

It's a 200hp electric motor.

Keep in mind, we have a few other bleeder paths ahead of the flow meter that want be capturing the flow. I think that is the point you may be missing. I know that I can get 40gpm at 5000psi well down from the pump (actually about 50ft of piping later) but I have no idea how much may be making its way through the variable valve.

Furthermore, as I read this and think a bit more about the schematic, I believe what I was calling a 5400psi relieving valve (as it appears on the schematic) is actually the stroker control determine how much fluid per stroke we command it to deliver. I don't know where the 5400psi number comes from or if it is left over from a time before they included the additional variable valve (the "5400" psi control is managed over RS-232 that talks to a PLC, the variable valve is via a direct voltage control which I suspect is MUCH MUCH faster than trying to talk over the PLC, we typically set the stroker to 1 value at start up and leave it alone, controlling system pressure via the variable valve)

9/23/2011 6:39:08 PM

this pic summed it up well:


the pump always operates on the same curve. when you go adding stuff to the system, either in parallel or series, you change the system curve. the system is always gonna operate at the intersection. since you have a positive disp pump, you have a steep curve.

[Edited on September 23, 2011 at 6:41 PM. Reason : and for god's sake, call it a "regulator", not a "variable valve" ]

[Edited on September 23, 2011 at 6:42 PM. Reason : or "throttling valve"]

9/23/2011 6:40:17 PM

Quote :

"we typically set the stroker to 1 value at start up and leave it alone, controlling system pressure via the variable valve)"

This right here is what might be our missing piece. The PDF lists all of the maximum values for this pump - i.e. max displacement/rev. Therefore, if you have it destroked to a certain degree (by picking an arbitrary setting rather than the max setting), you're limiting your maximum flow, and therefore your maximum pressure/flow combo is limited to a lower degree than what these charts indicate.

If you want full performance of this pump, you need to have it fully stroked before you use any regulators to control pressure settings, and if this is the plan for this pump, you could probably save quite a bit of money if you ever have to replace it by going with a fixed-displacement rather than this variable-displacement pump.

You said you were an electronics engineer working off experience with this system - any chance you're familiar with the internal workings of a variable displacement axial piston pump like this?

9/23/2011 6:55:36 PM

Quote :

"the system is always gonna operate at the intersection. "

Hmmm...ok, well maybe we both (my coworker and I) are right for slightly different reasons.

Like I pointed out above, with no restrictions anywhere, I am observing 48gpm. This tells me the pump is delivering at least this amount of flow. Now this doesn't square exactly with the curves for our pump (that can be seen at the tinyurl I posted) for pressure/flow, but I'm going to go on the idea that we actually aren't commanding the pump to deliver it's full flow but some amount less. This would make sense from a "lets not wear this thing out prematurely" perspective. So, with that said, if 48gpm is likely all it is delivering, and using the throttle valve I can set 5000 psi and get about 40...my question would be, is the other 8 still being delivered through the throttle valve? That is, the energy delivered to the system went somewhere right? Did it get consumed raising the pressure such that very little flow would be going through the throttle valve?

9/23/2011 6:58:30 PM

The energy is being lost through efficiency losses in the pump because you may have it destroked slightly. Axial pumps have a relatively fixed volumetric leakage past the moving pistons during operation, and when you destroke the pump, your output volume decreases, but your leakage volume is relatively constant. Thus, your volumetric efficiency drops off, and so your missing energy is just going into heat within the pump itself through friction and fluid leakage into the case drain of the pump past the pistons. If you fully stroke the pump, you get your max efficiencies, max flows and pressures, and meet all the listed values on the PDF.

The extreme case of a fully destroked pump implies zero output flow, and thus your volumetric, and total, efficiencies both hit 0% and the power consumed is only what is required to move the rotating group against friction, as no work is produced since no fluid moves. However, most pumps cannot be fully destroked, as the efficiency losses are too great to make it worthwhile.

9/23/2011 7:04:56 PM

Quote :

"So, with that said, if 48gpm is likely all it is delivering, and using the throttle valve I can set 5000 psi and get about 40...my question would be, is the other 8 still being delivered through the throttle valve? That is, the energy delivered to the system went somewhere right? Did it get consumed raising the pressure such that very little flow would be going through the throttle valve?"

if your flowmeter is on the discharge piping of the pump and is accurate, then, no, there's not a magical 8 gpm that is going somewhere and not showing up on the flowmeter.

operation at the intersection of the pump curve and the system curve is one of the most fundamental principles of fluid dynamics.

[Edited on September 23, 2011 at 7:25 PM. Reason : adf]

9/23/2011 7:24:16 PM

Yeah, as soon as I got downstairs I thought "dur, it can only do 48 at 1000psi" but I was being scorned into doing the dishes and didn't come back to fix it.

.

9/23/2011 7:28:53 PM

haha

i think the system is behaving exactly as should be expected

9/23/2011 7:32:24 PM

Quote :

"So, with that said, if 48gpm is likely all it is delivering, and using the throttle valve I can set 5000 psi and get about 40...my question would be, is the other 8 still being delivered through the throttle valve? That is, the energy delivered to the system went somewhere right? Did it get consumed raising the pressure such that very little flow would be going through the throttle valve?"

Referencing this - you have a 200hp motor, but what sort of controller setup do you have on it? If you have it set such that you're getting 1800rpm and 48gpm@1000psi, and then you increase the load valve setting to generate 5000psi, your pump will require more input torque to increase the outlet pressure. If you haven't modified your input power to the electric motor, the rpm would drop in response to the rise in torque, and so you would see a decrease in outlet flow paired with that increase in outlet pressure. Of course, if you're still maintaining constant RPM and you haven't modified the pump stroke, you should still see the exact same value for volumetric flow, as it can't vanish into thin air.

9/23/2011 7:38:15 PM

which is why the pump curve is almost vertical for a PD pump

[Edited on September 23, 2011 at 7:46 PM. Reason : theoretically, it is a vertical line]

9/23/2011 7:42:00 PM

Quote :

"If you have it set such that you're getting 1800rpm and 48gpm@1000psi, and then you increase the load valve setting to generate 5000psi, your pump will require more input torque to increase the outlet pressure. If you haven't modified your input power to the electric motor, the rpm would drop in response to the rise in torque, and so you would see a decrease in outlet flow paired with that increase in outlet pressure. Of course, if you're still maintaining constant RPM and you haven't modified the pump stroke, you should still see the exact same value for volumetric flow, as it can't vanish into thin air."

To be certain...there are definitely some unknowns that I think I need to figure out and of course it will all likely finally all make sense.

As far as the motor, it's fixed at 1800rpm as best I can tell. We have a meter on it and when we set the system to ~500psi it pulls about 60A@480V. At 5000psi I want to say it's in the 240 ballpark, both numbers with very little flow out of what we call our pump module. How much flow is going through the throttler in this setting is sort of the missing link.

If I get some time we've been meaning to produce some plots and I'll post em up if I can.

9/23/2011 7:59:44 PM