How do you decide what will be the HH, H. LL and L Alarm points in Tank Gauging. Let say for example I have a 2 meter open tank with a liquid of 0.8 SG. Normal operating level will be 700mm... Is there a standard guidelines for this? how about RANGE of transmitter that will be use? Any standard also?

Thanks! I am new with engineering dept. so I need you guys...

 

I do not know of a "standard" we usually go with low low 5%, low 10%, hi 90% and hihi 95% but it depends on your process. If it is an issue to have an overflow at 75% then you better warn before then..............

Dave Ferguson
Process Control Engineer

 

Before deciding the RANGE, you need to choose the HH and LL points, since the RANGE must encompass those points. For the tank described, I would typically go with a low range limit of 0 or 100 mm below the LL setting (whichever is greater) and a high range limit of 2000 mm or 100 mm above the HH setting (whichever is less).

The HH setting should be the maximum level you can withstand before tripping the supply to the tank to prevent overflow.
The LL setting should typically be the minimum level you can withstand before tripping the pump9s) draining the tank to avoid cavitation in those pump(s).

The H and L points should typically be chosen to give an operator ample time to take corrective action to avoid hitting the HH or LL points, while also minimizing nuisance alarms.
Blindly choosing alarm settings based on tank dimensions should be avoided.

This assumes the tank was sized properly to begin with.

 

When looking at alarm settings, remember that the alarm is there to wake up the operator and get him/her to take some action. So if it is going to take 10 minutes for the operator to get kitted up in PPE, get on his bike, and cycle to the tank farm to shut off a valve then you need at least 10 minutes of filling time between the alarm going off and the stuff starting to come out of the overflow. Less than 2 minutes is usually not enough - more than about 1/2 an hour and another alarm will come up and the first will be forgotten.

The capacity of the tank divided by the maximum fill/empty rate gives the holdup time. Look at the time needed for your operators to react and take corrective action, and the level range in % between alarm settings and disaster should be the same as the % of reaction time to holdup time. The HH is a backup for the H so the distance between them is not so critical - unless there is an automatic trip on the HH you want to avoid.

 

In the company where I work the process engineers mostly decide where the various level alarms are set.

They design the process, select the tank sizes so they should know the implications. For example a HH trip might need to be quite low on the tank to allow for entrained liquid to run down. One tank we have in our plants has a HH set at around 30%. If the plant trips when everything settles down the tank is almost 100% full.

For simple tanks 90 / 95% is fairly common.

Hope this helps
Roy