Friday, January 9, 2009

Flow Control - Mislabeled???

How does a flow control control flow since all the oil in equals all the oil out?

23 comments:

Anonymous said...

Aslong as that is the correct flow, you Can't get much better control of flow than that.

JDK said...

I agree but that doesn't say much about how this is actually accomplished.

Anonymous said...

"How does a flow control control flow since all the oil in equals all the oil out?"

Is that a Trick Question JDK?

If all the Oil In equals all the Oil Out a Flow Control is an unnecessary added expense. Could be a Salesman on commission who wants extra money in his Pay Envelope.

OTOH, If Oil Flow at the Inlet is capable of more Flow than required at the Outlet, 10 GPM Pump supplying the Inlet, 3 GPM Maximum Flow required at the Outlet to a small actuator, then the restriction to flow appplied by a Flow Control is a common way to supply the reduced flow.

Not the best way since a Flow Control usually generates a lot of HEAT from the Wasted Energy it produces.

There must be a point to a question like this but it has escaped my muddled mind so far.

JDK said...

"If all the Oil In equals all the Oil Out a Flow Control is an unnecessary added expense"

Not sure I'm buying that. Because of the heat issue which you point out (I agree with that)it's certainly a component we would like to eliminate if possible.

This, I think is the reason for my question. Flow controls are sometimes necessary, do create heat, and flow in = flow out. If you agree with these then how does adjusting them in and out control flow across them.

Not supposed to be a trick question and hopefully I'm not making it one.

Anonymous said...

Go to your kitchen sink. Notice that there is no water comming out of the faucet(flow control valve). That means, we have 0 flow into the flow control valve and 0 flow out of the flow control valve. Now, get a 1 gallon jug, put it under the faucet. Now just crack the faucet(flow control valve)open. Lets say it takes 1 minute to fill the jug. We would call that. 1 GPM. At this point we have 1 gpm flowing into the flow control valve and we have 1 gpm flowing out of the flow control valve. Now, if you open the faucet(flow control valve) all the way. Lets say it fills the jug in 10 secounds. You now have 6 gpm going into the flow control valve and 6 gpm comming out of the flow control valve. The only thing you changed is, how much you opened the flow control valve. Thats how a flow control valve works. So, if you want somthing to move faster, you open the flow control valve. If you want it to move slower, you close the flow control valve.

JDK said...

"So, if you want somthing to move faster, you open the flow control valve. If you want it to move slower, you close the flow control valve."

Let's say I have a 10 GPM positive displacement pump with a relief valve to tank off of the pump.The pump feeds a cylinder. In between the pump and cylinder is a wide open flow control. I extend the cylinder and it moves at a 10 GPM speed. I turn the flow control in 1 turn and extend again. What would you say happens to the speed of the cylinder?

pnachtwey said...

The flow goes down because of increased slip in the motor due to the increased load and leakage across the seals due the pressure drop across the spool. Unless one can measure very accurately I don't think most would notice the difference in speed. The pump curve on a fixed displacement pump is almost vertical until power limited.

Anonymous is assuming he has a constant pressure source in which case he is right for his assumption.

There are two curves that must be consider. One is the pressure drop as a function of flow from the valve and the other is the pressure as a function of flow generated by the pump. The operating point, pressure and flow, is where the two curves intersect.

JDK said...

pnachtwey:
I agree with everything you said but I don't think it answers the posted question. Please consider my 10 GPM pump example.

Thank you for your input.

Anonymous said...

If you have 10 gpm going into the flow control. You will have 10 gpm coming out of the flow control. Will 1 turn of the flow control change that? I don't know. Maybe, maybe not.

JDK said...

Anonymous: I agree - that is the point of this post
"How does a flow control control flow?"

pnachtwey said...

JDK, you haven't made it clear at what level you are seeking an answer. 1. There is the physics and mechanical part the restriction where fewer atoms can go through the hole with the same pressure drop. 2. How the flow constant is computed. 3. At the big picture my solution is the correct one. You have pointed out that a flow control doesn't do that much in some cases whey you have a fixed displacement pump. That is because the fixed displacement pump curve is almost vertical. The flow remains constant or nearly so and the pressure rises until the pump doesn't have enough power or the relief valve flows.

I have been saying that one needs to know the system curve ( the resistance to flow ) and the pump curve and the operating point is where the two points intersect

The flow control does not control the pressure or flow by itself. The pump or fluid source affects the answer.

Beyond that I have know idea what you are asking for.

JDK said...

A flow control controls flow by adding restriction to the oil flow to or from the work. This added restriction will only slow the work down when it is enough restriction to raise the pressure at the relief above the cracking pressure thus allowing oil to flow across the relief. There are technically 2 paths that oil can take as the pressure rises.

Pnachtwey in his comment points out one of them:
"The flow goes down because of increased slip in the motor due to the increased load and leakage across the seals due the pressure drop across the spool. Unless one can measure very accurately I don't think most would notice the difference in speed.

The 2nd path is across the relief.

In a fixed displacement pump system the flow control is used to raise the pressure at the "SYSTEM RELIEF" above it's cracking point.

In a variable displacement pump system the flow control is used to raise the pump pressure enough so that the pump starts to compensate (destroke).

The point of the question was to possibly infer that the flow control could be called simply a "Variable Restrictor" for example.

Why even think of all this? A Flow Control is only one aspect of controlling flow in a hydraulic system.

If you care the folowing link discusses this topic further.

http://www.fluidpowerjournal.com/07Covers%20and%20Editorial/07%20Editorial/FPJOH07_KummerFeature.pdf

huvva said...

dArn good Kuestion, KAn it be so the "puMp" pumps less when less is needed, or by pass flow...isn't it a wAter tower(akkumulator) providiNg All water at a coNstant presSure out of the sink fauCet....I think a kuestion like this need more inFo about the system, like pump type, Pressure control, lOad pressure etc

JDK said...

Huvva:

Please read all of the comments.

Anonymous said...

A flow control controls flow by allowing the user to choose which direction the flow goes, not the amount of input flow. If a pump flows 10GPM, then a flow control can only control how much will go to the actuator, and how much eventually makes its way back to tank.

-Josh Cosford

JDK said...

"A flow control controls flow by allowing the user to choose which direction the flow goes"

I've never seen this happen. I'm assuming you mean meter in one direction and block in the other.

Anonymous said...

Imagine you have 10 GPM of flow available from a piston pump. Imagine you have 1 relief valve, and 1 needle valve. Also imagine you have conduits and valves capable of 1000GPM, so flow losses through all components are marginal. With the needle valve fully open, you're getting 9.9 or more GPM going through the needle valve. Imagine you close the needle valve to allow 5 GPM through it. You just chose to allow 5 GPM go through the needle valve, 4.5 GPM to go through the relief valve, and maybe .5 GPM for pump leakage.

That is why flow controls allow you to choose where you want the fluid to go.

Josh Cosford

JDK said...

Josh:

"A flow control controls flow by allowing the user to choose which direction the flow goes"

I assumed you meant direction across the flow control.

My bad; thank you for the clarification.

valve actuator said...

To familiarize ourselves the flow control in industrial application , knowing how a certain system automation works will keep our business on top.Lets always be updated.Thanks for sharing this information.Good Day!

manoj said...

i have a diffrent quistion-

what if i put two similer displacement motors one gets full flow and one getsa a very small flow with flow controler assume 195 lpm and 5 lpm.

both moters are in parllel.

what if motor witch has flow controler does not have any work to do (rotating freely). is it going to get more flow and increase speed as the other motor moving the load and working hard.

JDK said...

I am not totally sure what your question is but here are a few points in a parallel circuit:
1. From the inlet point where both flows originate from to the outlet point where both flows combine the total pressure drops in each leg are equal.
2. If the flow source is constant(operating below relief or compensator settings) (to simplify - exclude internal leakages), the smaller flow motor will speed up as the load on the larger flow motor increases. This makes sense because the delte P on both legs are always equal.
3. If the flow source is variable (operating above relief or compensator setting), as the load on the larger flow motor increases, the speed of the smaller flow motor will increase but increase less than in #2 because more flow will go across the relief or the pump will further compensate.

See

http://www.fluidpowerjournal.com/07Covers%20and%20Editorial/07%20Editorial/FPJOH07_KummerFeature.pdf

for a more indepth discussion.

punter said...

got a problem -

there are two hydraulic motors .
both 1000cc radialpiston ones.

there is a fixed dis. pump for motor 01 creating a 250 lpm flow .

there is another fix pump for motor 02 creating 2o lpm flow.

motor 02 need just 5 lpm and rest of 15 lpm creating heat from pressure relief.

now i want to give this 15 lpm flow to motor 01.

there is a flow control throttle and a monoblock power beyond dcv in curcit 02.the in built pressure relief is 1000 psi.

the system pressure in curcit 01
varies as the load from 0 to 1000 psi.

i think i can reduce heat by that.
but what is the riht way to go about this.

what is your take on that.

JDK said...

Punter:

Thanks for checking in.
I think the only way that I know that this can be done is if the pressure required in circuit #1 is always maybe 150 PSI less than that in circuit #2. You could install a pressure compensated flow control between the power beyond in 2 and P in 1 set at 15 LPM. It seems as long as 1 is say 150 Psi lower you will get 15 to 1 and 5 to 2. If the gap between load pressures approaches the same, all bets are off.