Measuring liquid level in tank full of solids

I'm trying to find a good way to measure the level of liquid in a tank full of solid particles.

In comparison, this application is very similar to a salt brine tank on a water softener. A perforated bottom level in the tank with a pump pulling off below that, and solids above the perforated level.

Currently, there is a pressure transmitter on the bottom of the tank below the perforated level.

The problem being that sometimes the pump pulls liquid out faster than it drains through the perforations and skews the level reading. I've thought of a 'stilling well' with similar screening to prevent the solids from moving into the pipe.

Any other ideas out there?

best practice is to use ultrasonic level transmitter than to convert pressure reading in to level where the density/ visocsity is always changing.

The problem is the solids remain in the tank and the liquid is drained from it, so an ultrasonic would still reflect the top level of the solids. This is not a dissolved density change, these are particles about 1/2" square, water is pumped in throught the bottom, then drained back out. Kind of a 'washing' effect.

How about the reverse, use a BUBBLE TUBE?

How about a second pressure transmitter just above the perforations?

If the original pressure transmitter reading is greater than the second one, its reading is accurate. If not, you've pulled an air gap under the perforations. The second transmitter should always be correct for level, but you may need to keep the original to keep from running the pump dry.

Good luck,

Can you get strain gages in the legs?

RussB said:

Can you get strain gages in the legs?

Click to expand...

Probably could, but wouldn't be easy. Tank is about 35' tall, 20' wide.

One concern over putting another pressure transmitter above the perforated bottom is plugging or interference with the sensor by the particles.

A bubbler may do the trick.

I'm also considering another DP above the perforations with some type of screen or other protective device to keep the particulate away.

Thanks for the replies so far.

you could place a small tube with a fine screen at the top of the perferation keeping the top open to allow the liquid to flow into the tube then take you measurements there with whichever senser you use. as the water settles above or with your solids it should indicate how much water is mixed in with your solids. the problem would be maintaining the screen itself so that water can pass through it.

cntrlfrk

Can you install a vent pipe through the perforation to the top of the tank to equalize the pressure? Unless the pump can actually drain faster than the perforations can flow then you need more perforation area.

cntrlfrk said:

One concern over putting another pressure transmitter above the perforated bottom is plugging or interference with the sensor by the particles.

A bubbler may do the trick.

I'm also considering another DP above the perforations with some type of screen or other protective device to keep the particulate away.

Click to expand...

Since there is no flow in the sensing line, plugging should not be a problem especially with the 1/2" size you mentioned. If there is smaller "dust" that tends to stick together, putting a screen on the tube will make it much more likely to plug. An occasional flush should deal with any accumulation unless it sets up like concrete.

A bubbler above the perforations would work also. A bubbler below the perforations will have the same problem the pressure transmitter has when pumping faster than the liquid can drain. After all, a bubbler is just another way of using a pressure transmitter.

Good luck,

There is a company that makes a strain gage that attaches to tank legs, not under the legs. It operates off the flex of the legs as load increases. It has to be calibrated to the tank capaicity. I don't remember who makes it, let me dig around and see if I can come up with it. That combined with an ultrasonic sensor or pressure sensor may make it possible to determine fluid level.

Use a dip tube, pump a small volume of air down this, measure the pressure (back pressure) in the tube, this will be proportional to the level. We use this on pumping stations, tube is 2" diameter and 2 meteres long, Air in and pressure signal is taken from the top of the tube.

Alaric said:

There is a company that makes a strain gage that attaches to tank legs, not under the legs. It operates off the flex of the legs as load increases. It has to be calibrated to the tank capaicity. I don't remember who makes it, let me dig around and see if I can come up with it. That combined with an ultrasonic sensor or pressure sensor may make it possible to determine fluid level.

Click to expand...

Here's one vendor of bolt on strain gage load cells.

www.kistlermorse.com

Any continuous capacitance unit (a long, typcially flexible probe the height of the tank) would see the water level,



Even with the cubes surrounding the probe, because the dielectric of water is so high.. What could "fool" a capacitance probe is water soaked cubes that press against the probe after the water level has dropped. The probe could very likely 'see' the water on the surface of the cubes and register that as a valid water level. Capacitance units always have sensitivity adjustments, and high sensitivity capacitance unit might well differentiate between all water and wet cubes. Its the kind of thing you could test in a pail of water half filled with with some cubes.

Head pressure should work very well. There are DP transmitters designed specifically for tanks with 3' or 4" (I forget which) flanged spuds on the side of the tank. They are known as flange mount level transmitters.



The transmitters have a extension tube that moves the sensing diaphragm on the end through the spud nozzle and into the tank. The sensing diaphragm on these models is usually about 2" in diameter. Installers have to careful not to bang or damage the exposed diaphragm.

A diaphragm that size provides not only for excellent measurement, but it is not a recessed cavity like a standard pressure transmitter has, which could fill with residue and clog.

Another option is sanitary mounting, using a triclamp fitting.



The sanitary diaphragm, like the flange mount level transmitter, is large diameter.

I'm curious as to whether a tank of the size you have is on legs, or whether it sits on a pad.

mellis said:

Here's one vendor of bolt on strain gage load cells.

www.kistlermorse.com

Click to expand...

Thats the one I was thinking of!



Another thought is to install a stand pipe next to the tank attached at the drain, design the stand pipe so that the solids can't enter it. Then measure the fluid level in the stand pipe.