Best level transmitter to use

[tona2008]

Hi,

I'm looking for some help in finding a better way of measuring the level of an acid recovery tank. the tank is used to separate impurities from Hydrochloric Acid. The "contaminated" acid is heated to 235 F and the surface starts foaming and releasing vapors which affect the use of ultrasonic or radar level transmitter. We need to maintain a constant level in the tank. The controller opens an acid valve based on the level of the tank. We currently use a differential pressure type of level transmitter but we need to keep injecting N2 to maintain a constant head pressure on the vessel. This is because there is a small draft on the vessel that removes the evaporated acid and takes it to the condenser where it is condensed and recovered. We have tried ultrasonic and radar type of level transmitters which have not worked.

Any suggestions will be appreciated.

Thanks,

 

[danw2]

I can see a problem with ultrasonic - vapors and foam. Less so with radar, so what was the problem with radar?

It isn't clear why the current DP doesn't provide an accurate level. Is the solution density constantly changing as the acid boils off ? That's a classic DP head pressure/level issue.

What exactly is the manifestation of the DP level problem?

Does the DP have remote seals on capillaries?

Or is it plumbed with impulse tubing with a dead head cooling effect?

 

[sparky970]

I have used Guided Wave Radar probes with a teflon coating for level measurements in HCL.

 

[zdas04]

I am looking at a Vibrating Fork Level Switch for an application where I can't tollerate any grossly moving parts (like a float switch). I'm not sure how well it will work, but the literature looks promissing. I'm looking at the Rosemount 2120 for this application (very turbulent surface, lots of solids). It might work for your problem.

David

 

[tona2008]

Dawn2

We are currently using a bubbler type DP system to measure the level. This transmitters sends a signal to the computer which in turns controls a feed valve to add acid as required. Due to all the vapor created, the pressure on the upper portion of hte tank keeps changing constantly which affects the accuracy of the level transmitter. In addition, there is a slight draft that removes the vapors from the top of the tank. we try to compensate this pressure fluctuations by injectin nitrogen to try to maintain the pressure constant. There is a change of properties of the solution as it boils. The solid impurities usually precipitate to the lower portions of the tank changing the viscosity of the solution. This also presents a problem when using The DP bubbler type of transmitter. We have not tried the guided wave radar transmitter. I'm not sure if the investment will be worth trying.

 

[controlnovice]

Radar is usually ok with foaming, but guided wave radar would work even better.

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This is normally the space where people post something insightful.

 

[danw2]

Is your DP sensing a single bubbler tube with respect to atmosphere or the vapor pressure?

Omega provides a diagram for pressurized tank level with a bubbler:

In pressurized tanks, two sets of dip pipes are needed to measure the level (Figure 7-4B). The two back-pressures on the two dip pipes can be connected to the two sides of a u-tube manometer, a differential pressure gage or a d/p cell/transmitter. The pneumatic piping or tubing in a bubbler system

should be sloped toward the tank so that condensed process vapors will drain back into the tank if purge pressure is lost. The purge gas supply should be clean, dry, and available at a pressure at least 10 psi greater than the expected maximum total pressure required (when the tank is full and the vapor pressure is at its maximum). An alternative to a continuous bubbler is to use a hand pump (similar to a bicycle tire pump) providing purge air only when the level is being read.

Bubblers do consume inert gases, which can later accumulate and blanket processing equipment.

They also require maintenance to ensure that the purge supply is always available and that the system is properly adjusted and calibrated. When all factors are considered, d/p cells typically are preferred to bubblers in the majority of applications.

http://www.omega.com/literature/transactions/volume4/T9904-12-PRESS.html

 

[JimCasey]

THe vibrating fork is a good level switch, but not an analog quantitative measurement as would normally be considered appropriate. It'll also make a splattery aerated mess when it is in contact with the surface. If you want point level, a capacitance switch or a thermal dispersion switch would trip when the level contacts the probe.

A fluorocarbon-coated capacitance level transmitter would probably work. Drexelbrook have been the leader in this sort of instruments for many years.

Injecting a small flow of N2 into BOTH sensing lines would work, especially since the vapors are being collected and (I presume) treated. Ensure that the sensing lines are the same length and that the flowrates are the same. Brooks 1350-8900 purge flowmeter with a constant-flow relay were designed to do this. One on each sensing line.

Ultrasonic with a stilling well would work a lot better than just sticking an ultrasonic transducer into the top of the vessel.
Similarly with radar. Youd probably find that foam does not form inside the stilling well. The fluid in the stilling well would degass once, and have relatively little mixing with the contaminated predominating fluid, especially if the level is not changing much.

Load cells-weighing the vessel. Works well as long as the fluid density is constant and it's possible to isolate the vessel from any piping loads.

There are lots of ways to skin this cat.