Basis for level trips at 90%, why not 100%..(of working vol) 1

[jamesbanda]

I'm looking at a non-volatile tank with material in with a high flash point 90C.

Its a pressure vessel with at atomspheric vent, design pressure ~100 psig. Relief case is only fire..

I've set the trip point - a switch at 90%, ive been asked to raise this to the top tan line.. (or as near as possible to it..) to maximise the working volume.. on first looks it seems a resonable request.. as the response time before overfill is at least 30 mins a trip system will act within 30 seconds.. the fluid is low hazard..

I have an independent transmitter (density basis).. this is hard switch..

i know these are engineering judgements.. 100% seems wrong. 95% seems the max i would be comfortable with. but i cannot exactly say why..

so i'd appreciate some sound judgements on if this has been done before..

J

 

[chemebabak]

What is the service of this vessel? That is, is it a vapor liquid separator? Is it a surge drum? To where is the vapor from the vessel routed? Is it always open to atmosphere or is the vent used only in case of maintenance?

These are questions to consider to determine the set point of the high high liquid level trip. My engineering judgment states that, as long as liquid cannot overflow from the vapor outlet and damage some other piece of equipment, the high high liquid level trip can be set as high as mechanically possible. The important design consideration is that the high level alarm and high high level trip should be spaced far enough away that the operator has enough time to maintain the vessel prior to trip.

On the other hand, if the liquid could overflow and damage some other piece of equipment, then the high level trip should be set MUCH lower.

 

[jamesbanda]

thanks for your comment its a standard storage tank for 50-100 cp viscosity material, high flash point 90c or so..

 

[Latexman]

There are good reasons to stick with standard or traditional limits on level. Consistency makes it easy for everyone in the future. They don't have to remember that one tank in the whole plant that doesn't comply.

There may also be good reasons to change the setpoint. Will the system not work satisfactorily at 90%? Yes, there is extra volume to be had, but is this a real issue or just a whim?

Personally, if the system will work at 90%, I'd keep the established standard. If it appears it is going to be a problem during operation, setpoints are extremely easy to change.

Good luck,
Latexman

 

[Fzob]

I don't know how tightly run or well trained the operation is, but my experience is that 100% is not good practice.

All it takes is one person to misunderstand that 100% means no material can ever be added above the set point and you will have a spill on your hands at some point in the future.

Instruments fail or fall out of calibration, interlocks fail, alarms and interlocks get bypassed, and all sorts of other risks that are difficult to quantify (especially 5 or 10 years from now) can stymie even the best operational group.

Assuming there little or no environmental or safety issues; from my perspective, unless there is a significant financial gain to justify the risk I would go with your standard.

Regards

 

[CH5OH]

the 10% vapour volume @ the top serves 2 purposes:
1.allow liquid to expand (min and max operating temperature)
2.avoid vapour pockets @ top of tank (reinforcement beams,...),blow-off valve in contact with complete liquid surface @ all times

 

[786392]

Moreover
the tank's containment capability will be stressed with(almost)in-compressible liquid's huge force (pressure multiplied with the top surface area) instead of vapors/or compressible(gases etc.)

whenever the mechanical venting falters and

will that(tank's containment strength) withstand every now& than exertion of such force(s)

if erratic operation takes place due to inflow surging or outflow disturbance.

Please do consider these practical possibilities in line with latexman & others pointed out above.

Hopefully this helps in deciding way forward and getting a reasonable logical rationale for 10% margin removal.

Best Regards
Qalander(Chem)

 

[mpritchett]

I believe the original post sets the 100% level at top of tangent line. Depending upon vessel size there can be a significant vapor space above the tangent line.

My comfort zone is not in the relative percentage of filling, but in the accuracy of the measurement and the time required to respond to the trip. With the worst accuracy, can the transient effects from the trip be accommodated by remain surge volume with some reasonable safety factor. If it can, then the set point for the trip is acceptable, if not, the set point needs to be lowered.

 

[BlackSmoke]

I'm definitely not an expert on instrumentation, but my understanding was that 90% was used because using 100% as an interlock was risky because the measurement depends on the expected density of the solution for differential pressure meters. Thus if there is a difference between the actual fluid density and the calibrated density of the instrument, in some cases the meter never reaches 100%. I've seen an overfilled column where the level transmitter showed 99% while it was overfilled.

In my opinion don't risk it just to save a few hundred mm straight- side.