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Old August 18th, 2004, 09:16 PM   #1
Terry Woods
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Tough Nut...

Well... let's see what you can come up with...

We have a hydraulic press. The hydraulic pressures contained in two accumulator systems close the press. The first accumulator provides low pressure (approx 500-psi) and the second accumulator provides high pressure (approx 5000-psi, approx 150F). The low-pressure accumulator provides approximately 95% of the closing. The high-pressure accumulator provides the last 5% of closing.

It is, of course, very important that the accumulators have the correct amount of water (water mixed with oil) when the accumulators charge to their "set-point pressure". This is easily determined in the low-pressure system by means of a set of float-switches.

Last week, we had a severe air leak. (Apparently, a smaller leak had been there for quite a while.) It was a few hours before the night crew identified the problem and got it isolated. In the meantime, all five bottles filled with water.

When they completed the repairs, they tried a cycle. When the high-pressure phase of the cycle began, the pressure dropped from 4500 psi to 1200 psi... in about... one second. A normal 30-second close took about 5-minutes. A normal cycle-time is five-minutes.

So... here is my problem. I need to be able to monitor the water level in the high-pressure system. At this time, the only means we have to judge the water level is by means of a set of three petcocks mounted on a tube. The tube is mounted in parallel with the airside and the waterside of the high-pressure accumulator. Of the three petcocks, the center petcock is at the required level. The other petcocks are mounted 2-inches above and below the center petcock. It is easy to determine the level of the water as long as the water is where it should be (+/- 2").

Here is a diagram of the High Pressure Accumulator System.

The problem is that once the water level moves out of that range it is impossible to know the trend. Once it is determined that the level is not where it should be, actions are taken to try to restore the water to the proper level (either add air or add water). Adding water is the easier task. Adding air, on the other hand, is a long process. While adding air, however, we do not know if we are gaining ground or losing ground. That is, once the water level is above the top petcock, it is impossible to know, on a cycle to cycle basis, whether the "at pressure" water level is increasing or decreasing.

The same issue (to a lesser degree) applies when the water level is below the bottom pet-****.

I would really like to use a 4-20mA sensor to provide continuous level information.
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Old August 18th, 2004, 09:33 PM   #2
Derek McFarland

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This link might lead you to a solution.
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Old August 18th, 2004, 10:03 PM   #3
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Can you install three float switches in the petcocks? Something like this:
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Old August 19th, 2004, 01:30 AM   #4
Eric Nelson
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Now that I've recovered from the shock of seeing a question being ASKED by Terry. (LU2T)

Any bungs at the top? If so, you might consider an ultrasonic sensor looking down to measure water level. Something along the lines like THIS from Gems Sensors.

I'm not sure if I understand, but you said an air leak allowed the water level to rise. Where did this 'make-up' water come from? Can you monitor the incoming water flow to indicate you're pi$$ing air somewhere?...


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Old August 19th, 2004, 05:08 AM   #5
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For high pressure or pressure-shock situations, skip ultrasonics (and float switches).

Two solutions that pop immediately to mind - An RF Probe level sensor would work, and provide just about any kind of output you'd like. They can also be ordered with custom-designed mounting flanges.

They are nice, as they aren't subject to heat and/or pressure disturbances, and typically require only one port on your vessel.

The second solution that pops into my mind would be to open two ports in the vessel, and use a differential pressure transducer. About the only problem with DP cells, is that you may require that the reference leg (vessel top port) be kept full of water at all times.

I've always had the best luck and response with DP cells myself, on pressure ranges from 200-3500PSI. You shouldn't have too much trouble getting to 5K or 7.5K PSI, and the resolution is great, as the DP cells only measure the water-column, not the static pressure of the system.
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Old August 19th, 2004, 08:13 AM   #6
Tom Jenkins
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A cheap solution is to use the level switches consisting of metal rods of varying lengths hung inside the chamber. If a conducive fluid is in contact with a specific rod, you close a relay contact. You can have as many lelvels as you want. I think Wilkerson is one brand, but you can see them in the Omega catalog.

If you want 4-20 mA check into the capacitance or resistive type units. They also have a long metal rod, and as the leel changes give you a proportional signa. I'm on th road, and I can't think of any brands off the top of my head, but again the Omega catalog has some examples.
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Old August 19th, 2004, 08:53 AM   #7
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Endress & Hauser have analog level sensing in the rod style Tom mentions. They should be good for high pressure applications.

edit: The E+H 11500 Z goes to 500 Bar (7500 PSI ?)
3 strikes and you're out

Last edited by JesperMP; August 19th, 2004 at 09:15 AM.
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Old August 19th, 2004, 09:07 AM   #8
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Have you taken a very careful survey of the equipment? If you could tap into the wet side piping and the dry (high side) you should be able to make a DP work. It sounds like there must be some ports available if you're measuring pressure already; try to 'Tee' into available piping.

If there are no fittings/nipples available, it might be possible to insert tubing in the top leg of the available (illustrated) top port (work the end up to the high end of the vessel) to give you more range on the high side for a DP. It would be kinda hokey but better than nothing and easy to prototype... if you can get past the concern of losing the tubing in there.
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Old August 19th, 2004, 09:18 AM   #9
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Old August 19th, 2004, 01:19 PM   #10

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i would use a float switch mounted on the lower pipe. with a connection to the top ****.
danfoss makes them for high pressures.
if installation is ready for a push the level should be close to the float. i presume, take care you have a float with three positions on it.
the air side is different story.
as the compressor will have to push 300 bars just sometimes i would prefer to use a gas bottle with nitrogen and when pressure is low open valve of bottle.
if you really use an aircompressor it must be a expensive one.
if you add water be sure to use degassed water in.
skype shooter paul.deelen
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Old August 19th, 2004, 01:51 PM   #11
Peter Nachtwey
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Use sound.

The pipes will oscillate at different frequencies as a function of the water height. This would have the advantage of a longer detection range and not having to modify the plumbing as the exciter/detector would be external. It may take some effort to calibrate the sounds with the different frequencies. I would use a FFT to find the frequency spectrum at different levels. This information would then need to be correlated to the different elevations of water.

Just kidding, but it would work.
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Old August 19th, 2004, 04:42 PM   #12
Greg Gauper
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Trying to think outside the box a little....

Instead of monitoring the oil level in the accumulator (at the destination) could you instead monitor the fluid level in the pump reservoir? In other words, note the level in the tank when the accumulator is charged with fluid, and then while idle continue to monitor the fluid level. If there is no air leak, the system should not have to add fluid so the level in the reservoir should remain constant. If the air leaks down while the system is idle, more fluid will be drawn from the reservoir to maintain the pressure and the reservoir level will gradually drop. So you would set up your logic to detect a fluid level drop after the accumulators are initially charged with fluid. Your logic would have to disregard the fluid level change when your system was actually using the fluid from the accumulators, but it might be possible to look for a level change while idle. You could also monitor the recovery time of the accumulator after the system used the fluid. If the time gets progressively longer, this indicates you're gradually loosing air pressure.
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Old August 19th, 2004, 04:45 PM   #13
Tom Jenkins
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As an extension of Peter's idea, I suggest you cage a canary near the top of the tank. As the level rises the canary should make more and more noise, providing the analog signal you need. You would also have a HIGH-HIGH level alarm if the water gets so high the canary drowns.

If cost or the ASPCA are a concern, you could substitute a personal injury attorney for the canary. This has added social cleansing benefits.
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Old August 19th, 2004, 07:59 PM   #14
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Being familiar with boiler systems and how they work, a DP system will work. People out there that use bublers should know what I'm talking about. Water is water, no matter what pressure, don't bubblers work at sea level and at the top of a mountain?,, the low side always measures the water column above it! Pressure does not matter as long as both sides experience the same pressure, you will measure the water column!

PS: Tom, PETA would have a problem with your answer, People Eating Tasty Animals,, don't want to eat overworked birds!
"A good scientist is a person with original ideas. A good engineer is a person who makes a design that works with as few original ideas as possible. There are no prima donnas in engineering." Freeman Dyson

Last edited by Bitmore; August 19th, 2004 at 08:07 PM.
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Old August 21st, 2004, 09:00 PM   #15
Terry Woods
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I'm still waiting on a response from the canary vendor.

I'll let you know how it goes.
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