How to realize an extreme low flow pressure compensated flow control valve 1L/24hr - 1L/72hr? 2

[karl lambrechts]

In a lab system, the pressure in the accumulator (500 ml) has to be relieved (automatically) in 24-72 hours.
Pressure at the input is nom. 380 bar (and has to drop to 0 bar). The output side is an open container (ie with a lid and diaphragm), so at 0 bar. The high pressure is used for an hydraulic braking system. The aim is to go for a passive system, so without controller or electric valve, and the idea is to put a (very) low flow control valve parallel to the braking system. This principle will result in a small continuous, but acceptable, leak during operation, because that will be easily compensated by the hydraulic pump (ca 1L/min), and this loss is negligible in comparison with the consumption of the brake system. We tried using an ordinary flow control valve, but due to small impurities in the fluid, the valve gets clogged sometimes, witch is unacceptable. A valve which opens wider when pressure drops, would give a more reliable operation.( Even 144 hrs is acceptable).
Open to ideas.

At the end it will have to be integrated in a hydraulic block with 2 input ports and 2 outputs.
It would be very nice if we could find this somewhere as a standard part (total quantity is ca 150 pieces)

Thanks

Karl
AKLA

 

[TugboatEng]

Does the flow need to be constant? You could use a solenoid valve with a wider opening and pulse it to give an average flow of 1 LPD. Increase the pulse duration as pressure drops for more constant flow.

 

[LittleInch]

Orifice with a filter cage in front?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[73lafuite]

Hello,
The smallest pressure compensated fixed flow regulator Pmax 500bar that we find go down to 0.045l / min.
In very small flow rates like this, we see that it is not impurities which block the passage of the fluid but large molecules (polymetacrylate additive type) which block the clearance especially when the pressure drops.
cordially

 

[karl lambrechts]

Thanks for the answers!
73lafuite: when you say "very small rates like this", do you mean the rate i'm asking, or the 45ml/min you mentioned?

Tugboat; no, the flow does not need to be constant, but i'm looking for a passive system, so without electrical controlled valve. But it is still an escape route.
Littleinch: I first considered, but it would ask for maintenance, and seen the answer of 73lafuite, it would be very close to impossible.

 

[LittleInch]

What maintains the pressure in the accumulator?

Can't you just relieve this pressure instead if it's a gas?

You don't actually say what the flow is that you want. That would help.

But if you're talking flowrates where you're worried about large molecules then I think a re design is required.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[karl lambrechts]

Hi LittleInch
By a pump 1 liter/min,
Relieving the gas is not possible because it is an diaphragm accumulator 300 ml (completely closed, no afterfill possible).
Flowrate is as in title 500lm/24hr up to 500ml/144 hr, that is 0,045 l/hr or 0,0007 l/min. Less would be also ok

 

[BrianPetersen]

Why why why why why?

Why do you need to do this?

If this is for an "emergency braking system", what's wrong with it holding pressure indefinitely?

If the pressure has to be released after X time for who knows what reason (and WHAT IS that reason/excuse), what's wrong with just dumping pressure at that time all at once?

What's the reason for dumping pressure in your "emergency braking system" accumulator at all?

If it's to remove a stored-pressure hazard for someone working on it, do you really want them to wait three days for it to bleed off pressure? Give the maintenance chap a pressure-release valve and let him dump the pressure at once, the moment he has to work on it!

You are never going to fulfill your requirements "as asked". So go back and ask why the system is being specified in this manner.

The last vehicle I had which developed a leaky master cylinder (due to it being almost 30 years old) ... got a new master cylinder so that it didn't do that any more.

 

[karl lambrechts]

It is not an emergency braking system, it is a security for a lab braking system (for the moment).

Point is that it must be a passive and automatic system, so that is without electrical valve and without manual push button. In this case the system will be safe after a few days: even without electricity and without human intervention.
The spec is made with care, there is no reason to question the spec, the aim is to solve the spec.

Don't only think car : that is merely 1 of the 1000's of applications with hydraulics.

 

[BrianPetersen]

What are you "braking"?

Is retained pressure causing the brake to "apply", or to "release"?

Why is it a good thing for this to apply or release after two or three days and not immediately and as opposed to being able to release upon demand - and if this achieves the "safe" condition then why is it okay for it to be "unsafe" in the interim two or three days?

I still maintain that what you have described so far is not practical to achieve in the real world, which means you need to go back to whoever wrote that specification and find out (A) what they really mean, (B) how others have achieved compliance in the past, (C) what the fundamental underlying requirements are.

Ask WHY. There's a reason I asked "why" five times. Frequently that is about the right number of times it takes to ask "why" in succession, to actually arrive at the underlying reason why a certain thing happened a certain way.

I explained my example. It's a car. You explain your example. The rest of us have no clue. We can't read your mind.

 

[IRstuff]

So, you're basically asking for a passive leak, which could be done with a small orifice; for a flow rate of 12E-6 scfm

https://lenoxlaser.com/resources/calculators/orifice-calculator/

says about 2 microns at the initial pressure. Their equations are here:

https://lenoxlaser.com/publications/fluid-flow-through-calibrated-orifices/

Since the flow rate is dependent on pressure, you'll need to iterate the equations

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

https://www.youtube.com/watch?v=BKorP55Aqvg

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[73lafuite]

Hello,
Strongly agree with BrianPeterson.

You can always go on saying that your specification is made with care, and that you should not question it.

However, your leakage rate is "unachievable". To explain it I would have to explain to you the phenomena of drawer sticking, filterability, and leakage which decreases over time in an electro-distributor whose drawer does not move relative to the body. I have 45 years of experience in the safety brakes and we delivered equipment that maintains the pressure for months without the pump ever recharging the accumulator. Reminder: a drop of oil is 0.06cm3 and you want 7 to 2.3 drops per hour !!!!

Please tell us when you have the solution.

 

[karl lambrechts]

Dear gentlemen,
It is in a lab environment. It is a brake system, more important it is a hydraulic circuit, in which a passive relieve (safety) system needs to be build in. The hydraulics are small (in the world of hydraulics). This setup was to test electric motors, but it will eventually used in all our systems.

Why, why, why, why, why...???

Lab systems are used, but suddenly are moved to a storage room ("project is in frige") where they sit for months,years,... Then another person, student, newbee, cleaning personnel, project manager, engineer, (in this case a person with many -25..26?- years in the company)... takes it up, not knowing that there is still high pressure in the system... Time, oil, made stickers fade and fall of... Deterioration, corrosion did the rest... There are regulations ("do not store under pressure", "check pressure first", "2e person check",...) everywhere, but we are humans, so mistakes are made. On the door, the same "DANGER" symbols, and still...
There is never enough attention for danger: Planes crash, rockets explode, everyday there are accidents everywhere, on the job, on the way to,...

So passive safety must be integrated! So a technical solution to relieve pressure, without action, without electricity is a must. "Unachievable" things do only exist in the mind of people who cannot find a solution. Going to Mars was once unachievable, and going to the moon is still never done, according certain persons.

Therefor that specification.

We did the brainstorm sessions (engineers, managers, safety/quality personnel...) and this is what we came up with. If you have another solution, or if you have a complete other approach to solve this, or if you see a way to change the specification, we will be very happy to hear about.

Thanks

 

[zeusfaber]

You seem to have manoeuvred yourself into a position where loss of pressure is unsafe and unacceptable - but so is not losing pressure - and are now trying to negotiate a way between these two incompatible requirements by exploiting a difference in timescale between the two types of hazard. That prompts a few questions:

I don't know the detail of your testing process, but it sounds a bit like you are converting your braking system into one where the brakes release themselves without warning after a few hours. Are you absolutely certain that, when this happens, it can never lead to danger? Every year, I hear at least one new story of some unfortunate individual chasing an artic across the yard, or a locomotive down the hill, after they've started the engine without checking the parking brake is set, then gone off to have a brew while the cab is warming up and the brake reservoirs are charging.

You mention a window of 24-72 (or maybe 144) Hr for discharge of the accumulator. What governs the 24 Hr end of that window? Even allowing for a nonlinear pressure decay and the brake needing a respectable pressure to work with, that feels like a very long time to have to keep pressure available after the pump has died. Is it possible to adjust the process to put it into a safe state much sooner - which would allow you to depressurise the accumulator more quickly?

Is it possible to do a more fundamental redesign so that the brake is held off by the hydraulics working against a spring, and applied by spring force? This would give you a passive solution to the braking requirement allowing you to reduce the stored energy in your hydraulic system (note, though, the potential for becoming another unfortunate individual when you restart your brake pump).

A.

 

[zeusfaber]

There is another parameter you can tinker with:

Simply make the accumulator much bigger. Once it's larger than 65 litres, 73lafuite's valve will sort you out very nicely indeed. (Please don't try doing this).

When an idea as bad as that is the most obvious fix, it's a strong hint that you've set off down the wrong path.

A.

 

[3DDave]

No one is questioning the need to relieve the pressure. I, at least, don't understand why it is required for device to remain in an unsafe condition for several days. Let it leak down over an hour. Or 10 minutes.

 

[BrianPetersen]

I deal with risk management in industrial environments all the time.

Stored fluid pressure is indeed a line item on the risk assessment.

One of the things you absolutely must come to terms with, is that risk can not absolutely be eliminated. You just can't. At a certain point you have to rely upon someone to actually apply the padlock to the electrical disconnect before going inside the machine - and trust that the electrical disconnect is correctly marked, shuts off the correct equipment, etc. YES people fail to lock equipment out, it happens all the time. And then you have to rely upon someone to not flip the "on" switch and press the "start" button when there is someone inside the machine. YES this strategy can fail, and it does fail from time to time. But you just cannot completely eliminate the risk.

You can "reduce" the risk.

If you're worried about the equipment being stored with pressure inside it, give people a valve that they can easily open to release that pressure and instructions on how to use it. If you're worried about someone opening a fitting on a pressurised hose because they forgot to properly release the pressure, use a threaded fitting that will start to leak slowly the moment someone puts a wrench to it before it goes kaboom. If you're worried about a printed sign fading or disappearing, engrave it (or the most important bits) on the tank itself. If you're worried about it rusting, make the tank (or specifically the engraving) out of something corrosion-resistant. If you're worried about someone not being able to read English, use the appropriate ISO warning symbols. If you're worried about someone not understanding those signs ... you can't fix stupid.

Hand in hand with any CE-marked piece of equipment, is the instruction manual - "information for use". How to install and connect the machine. How to stop the machine. How to prepare the machine for storage. How to prepare the machine for servicing. What procedures to follow when doing so. And, information on risks associated with all of these procedures, and how to mitigate those risks. Invariably, people will be called upon to follow the correct procedures for doing things in order to avoid causing a problem. It's unavoidable.

Your responsibility as an equipment manufacturer/integrator is to prepare that information for use.

Your responsibility as a user is to follow that information for use. And keep that instruction manual so that people have access to it. And provide relevant people with training for that equipment.

Once again - you can't fix stupid. But sometimes you can organise it so that they just get soaked with sprayed hydraulic fluid instead of being exploded.

And yes, we've had that happen ...

 

[BrianPetersen]

And one other thing. Any pressure vessel ought to have a readily-visible pressure gauge.

Yes, a pressure gauge can fail, too. But if something is stored with (let's say) 100 bar pressure inside it, and then is left to sit for years, the most likely outcome is that the gauge would seize with the needle showing 100 bar even if the accumulator loses pressure over the years. A pressure gauge with its needle at 100 bar would give me pause before fiddling with whatever the mechanism is.

If someone chooses to ignore that ... you can't fix stupid.

 

[karl lambrechts]

Hi 3D Dave,
Yes that was a consideration. In this setup a larger leak is not a problem, but we also have smaller systems ( we are a R&D company, prototype builder) , where the "leak" would influence operation. We want to do this development only once, we try to go for 1 standard solution in all our experimental set ups. Several different solutions can lead again to wrong choices. We consider 300ml in 24 hr can serve as a general solution. We never use accumulators larger than 1L.

(Some systems do require a pressurized system all the time :for example emergency brakes needs to ne ready all the time.)

 

[karl lambrechts]

Zeusfaber,
No, we are looking for a device that reliefs the pressure very slow. It is a lab test system, so it operates under supervision, and only short. This time it is a motor test system (with brake), but this is not relevant. We are looking for a standard passive system, that relief the pressure slowly.So, after x hrs or x days, any prototype system is safe, even for people who does not know anything about pressure and hydraulics.