Insane Hydraulics

Site theme image

Post-Compensation For Dummies - Part Two (Flow Sharing)

Today, as I continue my talk about the post compensated flow controls, we will (finally!) be sharing some flow. If you were brought here by the "magic of an internet search engine", I suggest you read part one first - there's a cool hydraulic schematic that draws itself at the bottom of the page there!

Let us recall what we learned so far:

At this point, we already know enough to start considering multiple simultaneous functions and look into how all this post-compensated flow-sharing thing really works.

Our objective is clear. We want to control several hydraulic functions at the same time, and we want to control them "good" even when our variable displacement load sensing pump is saturated. We tried our normally open compensators, and we saw that they work OK when the pump has enough oil to go around, but when we push our levers a bit further things go haywire. We don't want that. What we want is stable and predictable control - in other words - if we pull on a lever we want to see the respective function move, saturated pump or not.

Let us think about it. How would we like our multi-consumer hydraulic system to behave when the saturation condition happens? We do understand that the pump can't physically give more flow, so to satisfy our main request - "see a function move when the respective lever is pulled" - we would have to accept the fact that the limited amount of oil will have to be shared among the functions equally - i.e. we would see all the functions slow down when new consumers are added.

Our pump has the load sensing controller, which keeps its outlet pressure delta P higher than the LS pressure that our DCV feeds it. Very simple. But what happens to the delta P when the pump is saturated - i.e. is at max. displacement? Well - the delta P, obviously, goes down. So in order to keep the controllability, we need to create a system that would allow all of our orifices to work with the same delta P. When the pump is under-saturated - the delta P should be equal to the pump's compensator's delta-P setting, when the pump is saturated - the delta P will obviously be lower, but as long as it is kept equal across all the orifices in our flow-dividing system - the flow sharing condition will be met!

Let me say this once again:

"As long as the pressure-compensating system keeps an equal pressure drop across multiple orifices - the flow sharing requirement is met".

If the pressure drop is nominal (i.e. defined by the pump delta-P setting) - the expected function speed will be nominal as well. If the pressure drop is reduced (but still equal for all orifices) - the speed of functions will drop, but it will do so equally for all functions! Here lies the beauty of this system - the relation of speeds between the functions is still defined by the orifices, so we haven't lost control over the functions even at reduced speed!

Now, have a look at this 2-function flow-sharing hydraulic system:

Let us consider three "scenarios":

In the first case our orifice + compensator flow-controlling system would be like:

- Hey, pump, I'm Function 1, here's my load pressure for you, can I get some pressurized oil?

And the pump would be like:

-How much did you say the load pressure was? 200 bar? Great - I'll give you 220, knock yourself out!

In the second scenario the dialog would be:

Function 1:

- I am still working with 200 bar, dude!

Function 2:

- Oi, lads, I need oil too! My consumer needs 80 bar, STAT!

The Pump:

- I don't care about your 80 bar, man, I have the 200 bar from the first dude already, so I'll keep trying to dish out the 220 because I am that serious about delivering 20 bar on top of the requested highest load signal, you know. But I do see that somebody (I am looking in your direction, function 2) opened a "leak" in my pressure line. Well, I guess I'll have to increase the flow now.

Function 2:

- Well... can I at least use my own 80 bar for the compensator? Ple-e-e-ease?

The Pump:

- No, dude, you crazy? We must all be working with the same delta P at all times, what part of "at all times" are you not getting? Function 1 is toiling at 200, so take the 200, which is the highest load at the moment, and shove it up your compensator, if you please! Thus you'll be getting the 200 at the outlet of your flow controlling orifice and work with my hard-earned 20 bar delta P like all normal functions do!

Function 2:

- All right, all right! I'll do that!

And in the third "scenario":

Function 1:

- I need more flow, so I'll open my orifice! My function's still at 200 bar!

Function 2:

- Me too, me too! I also need more flow, at 80 bar, remember?

The Pump:

- I am sorry, boys, you're asking too much from me, I am at my max displacement. Hey you - with the highest load pressure - Function 1, I know that I promised to be giving 20 plus bar at all times, but I am afraid I failed you, my friend. You opened the restriction so wide, that all my cubic inches are capable of keeping up now is a mere 10 bar above the 200... I can only promise you that I will take your load signal, which is the highest of all the other load signals, and will send it to all the other functions so that they all face the same reduced delta P as you...

Function 1:

- No worries! At least it is I who will be telling other functions which delta P their orifices will be working with!

Function 2:

- Now can I use my own 80 bar for the compensator? Ple-e-ease?

The Pump:

- You serious? I told you functions before that you all will be working with the same delta P! So take the 200 from number one and make sure that that's the pressure at the outlet of your orifice, or else!

Function 2:

- Jeez, dude, chill out! You're lucky my compensator is downstream, otherwise, I'd be getting all the flow in the world right now!

See the point? All the "upstream ends" of our restrictors are connected to the pump outlet, and so are subject to the same pressure, and all the "downstream ends" of our restrictors have "pressure-inducers" that are conveniently piloted by a common pilot source which comes from the LS shuttles, and thus guarantees that the pressure in all downstream ends equals the highest load pressure.

Now we can also see the biggest drawback of this system. The fact that the compensators are placed downstream the flow controlling elements means that while you can easily build such a system with logic elements for uni-directional operation, things become much more complicated if you need directional control, because if you want to control everything with a single spool (like all normal valves do), you will have to jump through a lot of hoops wrapping the flow around the flow-restricting part of the spool, then directing it through the compensator, and then back to the spool "for directional purposes". You'll be facing some heavy design challenges here.

Be it as it may - the principle is simple, knowing it is cool, and sharing it is even cooler!


See what I just did here?.. Sharing?...