Correct calibration range

[cmcnaught]

I need to know if there is a engineering standard or what is good engineering practice for ranging and calibrating of a level transmitter on a tank.

I have a tank which is 5000L (100%) capacity and it has a extra 500L (10%) to overflow. When ranging the level transmitter to monitor the level in the tank, would a person range the transmitter to the 100% (5000L) capcity or to the overflow?

Thanks
Colin

 

[hacksaw]

Lots of answers to that one.

I prefer to span the transmitter above any overflow, but below any structural limit, and then alarm/interlock appropriately.

Not every one shares that approach especially in some of the older refineries. That gives the operator a clearer picture of what is happening. If the span is below the overflow, then once the meter is max'ed out, the operator has no clue as to by how much.

The bigger question is setting the lower range limit. Some prefer to span for "true empty" even though the transmitter is not displaying correctly at that limit. I prefer to span for the lowest level that can be properly indicated, with the heel of the tank being dealt with in the operating manual.

Whatever you do, make it so that the operator has a one-to-one correspondence between the remote indication and what is actually happening at the tank.

 

[jsummerfield]

There are different engineering standards based upon the industry. What is in the tank?

Do not install the measurement so that 100% is at the overflow level.

Use API RP 14C for the level control basis for offshore hydrocarbon business.


John

 

[shobi]

Other things to consider are,
is it an open tank application or closed tank,
the liquid is volatile or non volatile.

 

[cmcnaught]

Thanks for the reply, it does all make sense.

The storage tank is a vented tank and the capacity is 100% with an extra 10% to overflow and a further 10% to vent. The tank is manufatured from GRP and stores a corrosive liquid.

I believe the solution to be:

As the storage tank is designed for a 5000L (100%) working capacity and an extra 500L (10%) to overflow. The level transmitter would be calibrated to 110% giving the full working capacity of the tank. I.e. 4mA = 0L (0%) and 20mA = 5500L (110%).

The storage is only designed for a 5000L working capacity this would mean that what the operator sees in relation to the tank when the transmitter reads 18.54mA the storage tank would be 100% full (5000L). This would still allow the operator to monitor by how much the tank has been overfilled.

Trust all the above makes sense, as it did at the time of writing.

Colin