LEL measurement within a process tank
LEL measurement within a process tank
(OP)
Does anyone have experience installing LEL and O2 sensors such that they are measuring the environment within a tank? The tank is blanketed with Nitrogen (Fisher ACE controller on the inlet side, Protectoseal PVSV on the outlet) and we require the sensors to confirm the blanket.
The further twist is that the tank is filled with agitated polyester resin and there is a chance of splashing and contamination. Most of the vendors I have spoken to are being pretty cagey about supporting this application -- I can't be the first person to need to instrument a tank in such a fashion?!
The further twist is that the tank is filled with agitated polyester resin and there is a chance of splashing and contamination. Most of the vendors I have spoken to are being pretty cagey about supporting this application -- I can't be the first person to need to instrument a tank in such a fashion?!





RE: LEL measurement within a process tank
RE: LEL measurement within a process tank
You might not be the first person, but you may be within the first ten...
It is my understanding that a plant operator purges the tank, establishes an atmosphere that is non-explosive, then bleeds N2 into a tank as necessary. The environment inside the tank is controlled and there should be never be a source of ignition present.
The requirement for continuous monitoring of LEL and UEL assumes (in my understanding) a changing environment where a source of ignition might be present. ( Such as outside of the tank)
Remarks anyone ?
A "chapter and verse" quote from some NFPA standard probably exists that addresses this matter....
My opinion only....
-MJC
RE: LEL measurement within a process tank
MJC, it's the changing environment that's the issue, plus failure detection of the blanket gas. The tank inventory is constantly changing due to different batching formulations etc., sometimes there is a solvent rich environment, sometimes not. The risk is especially present during spraydown operations with e.g. styrene. If area nitrogen goes down, or the local regulator fails, it is intended that the LEL sensor will provide a warning.
RE: LEL measurement within a process tank
We are applying extractive analysis and knocking moisture out in the sample system for some tanks. The analysis includes oxygen and including IR hydrocarbon detection for component groups. This is a few orders of magnitude more costly than an LEL detector. The tanks are nitrogen purged.
RE: LEL measurement within a process tank
You are probably too far down the line to do this.. but
could you do the following.
1. Operate at slightly positive pressure say 0.1 barg and use pressure control to ensure the atm.
and
2. Have a high MAWP of the vessel, therefore reduce the severity of an explosion significantly. (you should do this anyway if operating in such a manner above flash point).
Other comments.
> Could you do the same as the o2 monitor by flow and level. My experience with 02 monitors on process duty is variable. in one case they were horribly expensive and we used for 1 month fought poor reliability and eventually put in another engineering solution. in another case no problems..
RE: LEL measurement within a process tank
We have a slipstream off of the positive side of a blower that measures for oxygen and returns the stream to a lower pressure area. I'm sure you could use a small diaphragm pump or something of the sort to pull off of the tank (if not pressurized). We have had some trouble with the existing unit, mainly the reactant/catalyst inside has spilled a couple of times making it necessary to replace the unit.
RE: LEL measurement within a process tank
RE: LEL measurement within a process tank
RE: LEL measurement within a process tank
Thanks.