×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

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!
  • Students Click Here

*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

At what pressure do I get full flow?
5

At what pressure do I get full flow?

At what pressure do I get full flow?

(OP)
Backgound,
I have a small eaton-vickers pump that looks almost like this only mine is green.
It has a 5 HP motor driving a 5 gpm pump at full flow.
https://www.youtube.com/watch?v=81nzCdIX1aw
We no problems with setting the system pressure at about 1500 psi when there is no flow. We can't adjust it much higher because the 5HP motor is limited.
The pump should provide no flow at 1500 psi.
The question I have is at what pressure do I get full flow?
It seems to me this is a function of the spring. Adjust the spring in and out adjust a proportional band where the flow is proportional to the pressure drop from 1500 psi. However, there is no way to know how far the pressure must drop to get the full 5 gpm from the pump. It seems to me that this would be a function of the spring constant. A stiffer spring would more cause the port to the swash plate to open up more with a smaller pressure drop. This essentially is changing the proportional band of the pump.

Are there specifications for this?
Are there specifications for how fast the swash plate moves?
Is it possible to replace the spring in the compensator?
As a controls person this seems simple but crude.
BTW, know the flow because I can measure the speed of the piston and know the areas of the piston but I am looking for a general case.





Peter Nachtwey
Delta Computer Systems
http://www.deltamotion.com
http://forum.deltamotion.com/

RE: At what pressure do I get full flow?

I agree with hydtools, as long there is enough power in the prime mover the pump should deliver max flow up to pressure cut off. (the diagram shows a rounded corner though)
Here is a Vickers pump with specs that look more like the one on the video, with only a max pressure compensator for max pressure stand-by. Hydtools pump also had a low-pressure stand-by compensator, but the principle will be the same.
So this pump thrives to maintain compensator pressure setting till swashplate has reached its max angle. If you ask for more flow at that position it will result in an instant pressure drop. So the flow has to be controlled with a directional valve variable orifice and of course the load pressure downstream the orifice. On the detailed schematic, it seems like the bias spring is "adjustable" or replaceable for other pressure ranges(??).

RE: At what pressure do I get full flow?

(OP)

Quote:


According to the catalog specs the pump delivers full flow at all pressures less than the compensator setting. That has been my experience, too.
This can't be so. The pump can't go instantly full stroke when it drops one psi below the pressure set point. This would cause an unstable response.
I am looking at the details.
My pump is probably the PVM018 on page 12. We bought the HPU used off of e-bay and didn't get documentation. Now I have some I think.

It looks to me like the pressure change A is 40 psi so if I have the compensator set for 1500 psi then I get no flow at 1500 psi but at 1500-40 psi I get close to 100% flow minus the droop of up to 1.2 gpm.
It looks like the pump flow vs pressure goes up the right side of that pump curve and it looks linear. In my case it looks like if the pressure drops 20 psi to 1480 I will get the rated flow minus a little for the droop and and divided by 2. Basically the pump performance is going up and down the right side of the pump curve.

I don't think akkamaan's graph has the detail I need.

What is interesting is that some pumps will have a pressure drop of 40 psi and other have a drop of 150 psi before they are on full stroke but the line look linear going up to the left.

Thanks hydtools for the document. I am not quite sure which pump I have because I am self isolated at home but I think I can find out tomorrow.

What is also interesting to me is the standard response times at the bottom of page 8. Some are very long and others like the PVM018 are fairly decent. It would have been nice to see the swash plate vs time curves. I need to stick that into my simulator. I have always assumed the pump responses were more like the 150 ms.

Peter Nachtwey
Delta Computer Systems
http://www.deltamotion.com
http://forum.deltamotion.com/

RE: At what pressure do I get full flow?

2
Their 8.5 GPM for the PVM018 (chart at bottom of page 12) is based on 1800 RPM. Your motor will be operating at less than that, ~1750 RPM depending on your actual motor.

Q absolute max = RPM x cc x 100% / 3780. At 1750 RPM, Q absolute max = 8.33 GPM.

If you have the Industrial (not Mobile) PVM018 (pg 12) and your compensator setting is 1500 psi, then with the 1.2 GPM "droop", you will have 8.33 GPM - 1.2 GPM = 7.13 GPM @ 1460 psi, and 7.13 / 2 = 3.56 GPM @ 1480 psi.

You originally asked "The question I have is at what pressure do I get full flow?" If by full flow you mean the absolute max flow this pump could put out, you will only get this flow at 0 discharge psi, meaning pump is just putting out free flow, like dumping fluid out of its discharge directly into a bucket. As soon as pressures begin to climb, volumetric efficiency begins to reduce. For some pumps, it can go down below 90%. For most pumps, it's in the 90% to 96% range at the higher pressures. This is all based on no test experience, just looking at mfr performance curves.

RE: At what pressure do I get full flow?

(OP)
I know about motor slippage.
What I didn't know is how far the pressure must drop before the swash plate was fully tilted and providing full flow. I was wondering about the slope on the right side of the pump curve. Apparently there is a lot of difference depending on the model. I can see there is no standard even within the same company or series of pumps. This needs to be another parameter in my pump model.

Quote:


If you have the Industrial (not Mobile) PVM018 (pg 12) and your compensator setting is 1500 psi, then with the 1.2 GPM "droop", you will have 8.33 GPM - 1.2 GPM = 7.13 GPM @ 1460 psi, and 7.13 / 2 = 3.56 GPM @ 1480 psi.
Yes. This means that the variable displacement pump must be sized a little larger than what would be required for a fixed displacement pump.

If I have a VFD driving a fixed displacement pump I think that would be optimal. The motor and drive would be more expensive but the pump itself would be cheaper.




Peter Nachtwey
Delta Computer Systems
http://www.deltamotion.com
http://forum.deltamotion.com/

RE: At what pressure do I get full flow?

Quote:

This means that the variable displacement pump must be sized a little larger than what would be required for a fixed displacement pump.

Instead of a larger size pump, just set the compensator setting higher. If you need up to 1500 psi with max swashplate angle, just set the compensator at 1600 or 1700 psi. That sounds too obvious and I know you are aware of that. I guess I'm not fully understanding the issue at hand for you. If you're trying to program something, then yeah it varies by pump brand, series, and even by displacement size within a series. Just have to read the charts, graphs, and data, and get the calculations going. Tedious process for sure.

RE: At what pressure do I get full flow?

(OP)
Yes, I am trying to simulate a hydraulic servo system. I need to keep the pressure as constant as possible. The pump I have seems to be fully on stroke with just a 40 PSI drop. That is good. Having the pump come on stroke with 200 psi drop is not. This affects the accumulator size if I want to minimize the pressure drop. I am taking into account the motion profile so instantaneous demands can be calculated. This is the easy part since I have done this before. My pump models have been kind of crude. Now I want to simulate the curve using the A and B parameters in the document that hyd_tools provided. I also need to determine the operating point on the curve as pressure changes and also the response of the pump since the swash plate doesn't move instantly.

I am going to use this information in my next H&P article. I have a python program written that sizes cylinder diameter, rod diameter, valve size and accumulator size. It outputs a graph of the motion profile, the instantaneous flow requirements. How much the volume of oil or gas in the accumulator is changing, the system pressure and the gas volume in the accumulator. For the small cylinder size I use standard sizes but for the larger sizes there are no standards so the user must override the programs calculation with the next available size. The program will recalculate the results but not override the users input.


Peter Nachtwey
Delta Computer Systems
http://www.deltamotion.com
http://forum.deltamotion.com/

RE: At what pressure do I get full flow?

(OP)
HydraulicsGuy said

Quote:


If you have the Industrial (not Mobile) PVM018 (pg 12) and your compensator setting is 1500 psi, then with the 1.2 GPM "droop", you will have 8.33 GPM - 1.2 GPM = 7.13 GPM @ 1460 psi, and 7.13 / 2 = 3.56 GPM @ 1480 psi.
I have a simulation and equations but I get a different answer from you. The difference is due to the flow droop. You are there will be 1.2 GPM droop at 1500 psi. Doesn't the flow droop apply to the maximum pressure? I don't see when the flow drop would be a constant and that severe if I changed my pressure set point to 1200, 1500, 1800 psi.

Hydtool's pdf file is a great help.

However, now my HPU simulator just got more complicated because there are new parameters.
Ton and Toff, are on stroke and off stroke times. I use these to simulate the pump swash plate doesn't move instantly.
Qdroop and Pdroop for the flow and pressure droops. I am assuming these droops occur over the maximum flow for the Qdroop but the Pdroop is a difference between the set point pressure at 0 flow and the set point pressure - Pdroop for the full flow.

I have a formula now
Q(p)=max(min(Qmax-Qdroop*p/Pmax, Qmax(Pset-p)/Pdroop),0)
I then run that through a low pass filter to simulate the response time of the swash plate as it changes stroke.




Peter Nachtwey
Delta Computer Systems
http://www.deltamotion.com
http://forum.deltamotion.com/

RE: At what pressure do I get full flow?

Quote:

You are there will be 1.2 GPM droop at 1500 psi. Doesn't the flow droop apply to the maximum pressure? I don't see when the flow drop would be a constant and that severe if I changed my pressure set point to 1200, 1500, 1800 psi.

I agree with you. Apologies there. I was assuming that the "A" and "B" values on that graph on pg 12 applied to all compensator settings, but after looking more closely, I believe they apply to only the 315 bar (4568 psi) compensator setting. Trying to answer questions here and do the work at home thing with family around doesn't always work out well.

The only thing is, the "B" value on pg 12 is not in-line with the "Delivery and Efficiency" graph on pg 18. According to that graph on pg 18, the volumetric efficiency (Ev) never drops below 96%, even at 4500 psi. Flows would be:

Q = RPM x cc x Ev% / 3780
Q = 1750 x 18 x 100% / 3780 = 8.33 GPM @ 100%
Q = 1750 x 18 x 96% / 3780 = 8.00 GPM @ 96%

This is a "droop" of only 0.33 GPM, not the 1.2 GPM given on pg 12. I'm not sure how to rectify this. I guess you rectify it by saying the 1.2 GPM droop (from 0 psi to 4500 psi) applies when you have the pressure compensator control on the pump. When you don't have any pump controls on the pump, then the 0.33 GPM droop (from 0 psi to 4500 psi) applies.

RE: At what pressure do I get full flow?

(OP)
Hydtools, another issue. You assume the slippage is 50 RPM. That is a good estimate for a motor like I have at full load. However, when the swash plate is neutral there is little or no load but friction. The slip will be smaller. I wonder if slippage is taken into account with the volumetric efficiency. I am look into your point about the droop.

Peter Nachtwey
Delta Computer Systems
http://www.deltamotion.com
http://forum.deltamotion.com/

RE: At what pressure do I get full flow?

Quote:

when the swash plate is neutral there is little or no load but friction.

We tested an HPU a few months ago. We measured current with the swash plate neutral, pump in pressure-compensated state. For reference, full load amps was 57 per motor datasheet. Measured amps was 25. I had found a US Dept of Energy document that had some generic curves for Efficiency and Power Factor when at much less than full load. Using approximate Efficiency and Power Factor from that document, and applying the motor equation P = V x A x Eff x PF x 1.73, I calculated that the motor was still outputting 15 HP. It was spinning at say 1750 RPM, so:

T = 15 HP x 63000 / 1750 RPM = 540 lb-in.

Full load torque per motor datasheet was 148 lb-ft = 1776 lb-in.

So in the pressure-compensated state, the motor was still needing to generate a good amount of torque, ~30% of its full load torque per my calculation.

Another way to calculate it, simpler but probably not as accurate: 25 Amps / 57 Amps = 44% of full load torque.

Quote:

The slip will be smaller.

I agree. So you can interpolate between 1750 RPM and 1800 RPM for say 30% or 44% of full load torque (if you want to use my numbers above).

Quote:

I wonder if slippage is taken into account with the volumetric efficiency.

Speculating that they probably measure the RPM of the pump shaft or the motor shaft with a tachometer or other device, measure the pump output flow and pressure, and calculate the volumetric efficiency for the different pressures. So they're basically saying, 'if this is your pump discharge pressure, this is your volumetric efficiency, regardless of what speed the motor is at'. I hope that makes sense. I don't see them trying to calculate motor slippage and then using that for their input RPMs.

RE: At what pressure do I get full flow?

I want to add that other pump manufacturers actually publish motor power required in the pressure-compensated state. Using that power figure and speed, you can calculate torque and compare to the full load torque to see where you are speed-wise. From one of the pump mfr graphs that I saw, the power was quite a bit less than the 15 HP that I calculated for the pump we had, for a comparable size pump. I want to say it was like 5 HP, so a lot less torque, therefore much closer to the synchronous speed of 1800 RPM. If you want me to point you to where I saw this in one pump mfrs catalog, let me know.

RE: At what pressure do I get full flow?

Current measurement is a poor indicator of power output of a motor. The efficiency of a motor at no load is about 0%.

RE: At what pressure do I get full flow?

How so? For discussion:


RE: At what pressure do I get full flow?

I mean why is

Quote:

Current measurement a poor indicator of power output of a motor.

RE: At what pressure do I get full flow?

(OP)
Do hydraulic people really sweat the details of the HPU motors that much? I can understand getting a motor that is powerful enough and efficient but do you really look at the torque curves etc?

Peter Nachtwey
Delta Computer Systems
http://www.deltamotion.com
http://forum.deltamotion.com/

RE: At what pressure do I get full flow?

When using engines, yes. I want the engine to approach its peak torque as the load causes the engine speed to decrease. So I am interested in the torque curve.

Ted

RE: At what pressure do I get full flow?

In mobile hydraulics, such as medium to large size excavators there is a lot of torque curve matching going on (using electronics) to get the absolute most hydraulic power out of the machine and also optimisation to get the absolute lowest fuel consumption for a certain job. CTL forestry machines is another application where both performance and efficiency is high up on the agenda. Also machines that put out almost all their power through hydrostatic driven tracks or wheels such as some dozers or snow grooming machines are highly optimised I suspect.
These are all machines that are in daily operation and where all the power from the diesel engine goes to hydraulics so it makes sense.




When it comes to building (electric) HPU's for various machines in one-off or low production numbers there is no high tech electronic optimisation going on from the little I have seen.
Some HPU's may run only one hour a day. Or maybe even a few hours a week. Or even a month. For such applications it's not worth the effort to optimise the way it's done in the mobile hydraulics industry.


I have not been in contact with industrial hydraulics running 24/7, it would be interesting to hear what the approach and priority is there.

RE: At what pressure do I get full flow?

(OP)

Quote:

Instead of a larger size pump, just set the compensator setting higher.
This doesn't really work. The goal is to reduce the pressure variation. It depends on if the compensator ( proportional ) band is greater than 10% of the supply pressure. If so then the variance of the pressure will still be more than 10% unless the system can be run at a consistent speed. If the pressure compensator band is 20% of the supply pressure then it will supply 50% of max flow at 10%. This requires a pump twice as big to keep the pressure variation at only 10%. This shouldn't be too much of a problem when the system pressure is high.

I have used the specifications for the pump I have as supplied by hydtools. The pressure variation is much less that 10% and the accumulator is smaller because the pumps compensator band is relatively small.


Peter Nachtwey
Delta Computer Systems
http://www.deltamotion.com
http://forum.deltamotion.com/

RE: At what pressure do I get full flow?

Isn't a pump compensator more like a PI than just a P?

RE: At what pressure do I get full flow?

(OP)

Quote:


Isn't a pump compensator more like a PI than just a P?
That is a good question to think about. It would be nice if it were so but...
I say no for two reasons.
1. There is only a spring. That is for the adjusting the system pressure level. The spring constant determines the proportional band. What would be controlling the integrator?
2. If the pump did have a PI controller the pressure would stay at about the system pressure and there wouldn't be the proportional droop.

Hydtools provided info on the pumps and it is easy to see there is only a proportional gain. The document shows the pressure-flow curve is a quadrilateral with the top part drooping due to efficiency and the right side sloping to the left due to the pressure droop. If there was a an integrator there wouldn't be the pressure droop. The pressure would be pretty constant around the set point until the flow rate varied. When the flow rate varied the pressure would go up or down until the integrator wound up or down compensate for the flow change.

I sell hydraulic controls. We have controlled a few pumps by monitoring the pressure and trying to keep the pressure constant. The problem is that the swash plate pumps need to have feed back on the swash plate position and few pumps have swash plate position feed back.

I would like to see VFD driven electric motor drive fixed displacement pumps. The motion controller knows exactly how much oil is being used so it can adjust the pump speed accordingly for the pressure changes. This idea would result in a more expensive motor and drive but a cheaper fixed displacement pump. The next advantage would be efficiency and the another advantage is control because the system pressure could be kept more constant since pumps are not equally efficient over their whole pressure flow curve. I have not been provided this information yet to find the optimal solution.

There are one big food processing site where we are going to be controlling 10 400 HP pumps. The idea is to better match the capacity to the requirements. Not all pumps need to run at the same time most of the time. The issue is how does one break down the load sharing.

Peter Nachtwey
Delta Computer Systems
http://www.deltamotion.com
http://forum.deltamotion.com/

RE: At what pressure do I get full flow?

Look at how the pump works inside.
The bias spring (non adjustable) always tries to swivel the pump to max displacement.
When the set pressure is reached the adjustable pressure valve opens allowing oil into the swivel cylinder which swivels the pump back against the bias spring.
This swivel cylinder is the integrator.

The schematics in the datasheet for the PVB5 are a not that clear
On at page 14 in this datasheet there is a fairly good schematic:
https://dc-us.resource.bosch.com/media/us/products...


Edit: an even simpler theoretical sketch here

RE: At what pressure do I get full flow?

The volume of the cylinder that actuates the swash plate adds up the fluid that the pressure control valve is feeding it. That conceptually sounds like integration to me.

RE: At what pressure do I get full flow?

(OP)

Quote:


The volume of the cylinder that actuates the swash plate adds up the fluid that the pressure control valve is feeding it. That conceptually sounds like integration to me.
This is why on 16 Jul I made the comment that this is a good question to think about.
I can understand why you think this but you need to understand the difference between integrating and non-integrating systems.

Look at the document hydtools posted. The graph of flow vs pressure shows the flow increases with the pressure drop. This is a simple proportional band. If there was an integrator then the flow would not drop except when the demand makes a sudden increase.

In a hydraulic cylinder the open loop model has no integrator for velocity but there is one for position. Same system so why the difference? It depends on what you are trying to control.
You need to understand the difference between integrating and non-integrating systems. A non-integrating system goes back to ambient or quiescent state when the control signal gone. In a hydraulic system system the velocity drops to zero but the position does not revert back to the original position because position is an integrating process.

Likewise the ambient position for the pump is the spring extended in the diagram Jacc posted. It will not hold the swash plate position when the pressure is gone to oppose the spring.
Trust me on this.
BTW, I got inducted into the IFPS hall of fame for my work in motion and force control and control theory for hydraulic systems and the flow vs pressure plots show that there pressure compensator is a simple proportional band.

Temperature control systems are non-integrating. If the control/power is taken away the cool down or warm up to ambient temperature. Accumulators are integrating systems. They integrate the net flow.



Peter Nachtwey
Delta Computer Systems
http://www.deltamotion.com
http://forum.deltamotion.com/

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


Resources

White Paper - Implementing a Multi-Domain System
IoT systems are multi-domain designs that often require AMS, Digital, RF, photonics and MEMS elements within the system. Tanner EDA provides an integrated, top-down design flow for IoT design that supports all these design domains. Learn more about key solutions that the Tanner design flow offers for successful IoT system design and verification. 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