Level switch testing

Tuning fork: any solid object in contact with its waggly bits, like a broom handle passed through a suitable flange on the vessel, perhaps?

Capacitance switches: nasty things, usually selected when nothing else will do. No simple solution here. Fill the vessel up with someting innocuous is about as good as it gets.

Gap switches: optical? Put something solid and opaque in the gap, stuck on the end of that broom handle, perhaps!

Seriously, consider that the vessel will, on unlikely occasions, overfill. Consider carefully what happens to the liquid that spills over, its capture, retention and proper disposal. Well-designed tank installations will have a bund, probably sized to accomodate 105-110% of the vessel's contents in the event of a mechanical failure near the bottom of the tank.

There is no short-cut to anywhere worth going and a formal Hazop is highly recommended.

On the end of the "broom handle", fit a cup, larger than the switch, and with a sealing edge. Also a pipe to fill it with the same liquid used in the tank.

Put in place, fill up, and check output. When done, remove and let the cupfull of liquid add to the current contents of the tank.

Broom handle is good, cost effective, BUT say we don't want to or cain't open the tank to insert said broom stick?

So Maybe---

The tuning fork switch in some of our applications are in powder applications. I am thinking of installing a small air or N2 supply tube that would blow a jet of air or N2 on to one of the forks. I am hoping that this would disrupt the vibration enough to set the switch. I have asked for a setup to test this. One problem is (there is always at least one recognizable and several not so obvious) that the supply into the vessel will be at a pressure greater than the tank is rated for, so failsafe guards will have to put in place to prevent the inadvertant opening of the unregulated supply.

A simular setup for the gap switches( ultrasonic as made by KayRay/Sensall owned by Thermo somebody and run by somebody else, I can't keep track). I plan to try to run a line from the recirculating pump that will flood the gap in the switch with what ever the content of the tank is. This would be controled by a simple ball valve from a convienent location.

In both of the previous apps the switch and pipe nozzle mounting would have to be in a flange with piping and target aligned prior to installation in the tank.

I am still thinking on the capacitance probe (Rosemount, Foxboro, Endress-Hauser, et.al.) installed in a vertical position in the top of the tank, hangs down about 6 feet and reads level by reading the chance in capacitance as the liquid assends the probe.

Whaddua think? Will it work? Any thoughts on the latter?

As you are measuring liquid, try a tube around the probe which is normally open at both ends. Fit a valve/cover at the bottom, so it can be closed for testing. Then fill the tube (known dimensions/volume) for testing/calibration. Must be default open, or the level will only be sensed once it has raised beyond the top of the tube.

Much easier to design the testing in, rather than add later - but not impossible if the tube can be within the dimensions of the sensor mounting.

I am thinking along these same line however the valve or removable cover inside a tank of 98% caustic will be difficult to operate. perhaps the tube may be enclosed on the bottom with only a few small holes that could flow the level liquid in for measuring. During testing the chamber could be flooded with enough circulated caustic so as to overcome the drain ability of the chamber, thereby rasing the level to trip point. Now for the difficulty--- caustic in solution has a tendancy to coat and solidify in a sort of soap like gel. This may plug the holes, not allow any tank product to enter the chamber, bad level reading result. D a r n.

Also, with small holes, there would be a delay in the reading, as the chamber would fill slower than the tank.

How quickly does the sensor get coated, thickly enough to stop working? - Can you use this build-up to determine the safe number of cycles before cleaning?

I am thinking of installing a small air or N2 supply tube that would blow a jet of air or N2 on to one of the forks.

Not likely to work unless the N2 were at least supersonic: these things work by detecting a change in natural frequency of vibration as a result of the density between the waggly bits changing.

capacitance

Here's a story for the learning of.

Many years ago an analog capacitance level measuring instrument was installed in a glassed vessel full of nasties. It calibrated fine at initial set-up. After about 18 months someone overfilled the vessel with the capacitance gauge only showing part-full at the moment of the event. The reason? Something other than the liquid with which it had been calibrated had been intoduced to the vessel without going round the HazOp analysis loop in the presence of a controls engineer to consider "something other than". Horrible things. Be afraid. Be very afraid....

Yes. That would do it too.

If I understand you post correctly you need an auxillary system to verify operation of your tank level sensors. Google "Freescale Semiconductor" and look at their AN1516 liquid level control. This is an updated version of the "tube, open to the bottom of the tank. Pressurize with an inert gas and read the (pressure converted to the appropiate schedule) required to vent a bubble".

This system, using the older but sensitive Wallace & Tiernan or Heise analog gauges, is still in use in many fuel farms and volatile fluids storage facilities. Usually the inert gas is Nitrogen and is precisely regulated to allow pressures in at the bottom of the sensing tube or pipe to not be greater than that which will cause a bubble of gas to eacape. This precise pressure is then calibrated in the appropiate schedule (feet, inches, meters, gallons, liters, etc). This system can possibly be made portable for use such as you need for determining the accuracy or operation of your in tank level sensors if the tanks are not pressurized.