Why we connect LP port of DP transmitter to top of the tank????
Have you ever noticed a tube connected from top of the closed tank to dp level transmitter??…why DP transmitter LP connected to tank top in closed tank level measurements .
Have you ever noticed a tube connected from top of the closed tank to dp level transmitter??? You might have..And if your job is in process instrumentation field,may be you are aware that there must need a connection to the LP side of the transmitter from tank top if we are measuring level in a closed tank.
But do you know the reason behind???
Why we need tapping to Lp side from top of the tank in closed tank measurements???
And why we don’t need them in open tank measurement???
To understand the concept let us examine three cases separately .
.
Case 1: Open tank
Here high pressure side is connected to the tank bottom and low pressure side is opened to atmosphere. the transmitter measures differential pressure with respect to atmospheric pressure.In other words the HP side pressure is a measure of tank level .
Here
DP due to liquid height
=h×S.G
So measuring HP side pressure is measuring level in open tank
Case :2 closed tank LP not connected to tank top
Let’s look at another condition
we have a closed tank and suppose we are measuring level without connecting LP port of transmitter to the tank top.
Does the transmitter shows correct reading???
The answer is no…..
Why?????.
Because in closed tank there may be vapour pressure developed above the top of the liquid and because the tank is closed .These pressure developed may be due to the evaporation of the liquid or the process itself cause pressure development such as pressurised closed tanks.These pressure above the top of the liquid surface may be huge like boiler steam drum or it may be a small one like that in closed water tank . whatever it is we don’t know anything about this pressure. And Our Dp transmitter HP side is sensing this extra pressure in addition to the head pressure due to liquid level. So it will show a value which is more than the actual level.
Let us examine the mathematical proof of this condition .See above figure
In this case ,
pressure acting on HP side
HP =h×SG+ k (suppose vapour pressure=k)
Pressure acting on Lp side
LP =0
DP = HP-LP
= h×S.G +k – 0
=h×S.G+k
=DP due to liquid head
+vapour pressure
But the actual level corresponds to h×S.G
So the factor k makes error in level reading
Case: 3- LP port connected to tank top
In this case we are connecting the LP port to the tank top. What happens in this situation is that the pressure in addition to the liquid head is simultaneously acting on HP port and LP port .So in effect the created DP is the contribution of liquid heads only.see the calculations below.
In this case ,
pressure acting on HP side
HP =h×S.G+ k
Pressure acting on Lp side,
LP =k
DP = HP-LP
= h×S.G +k – k
=h×S.G
=DP due to liquid head
So in short the tapping to tank top from transmitters LP port is mandatory for closed tank level measurements using DP transmitter.And this tapping cancels out the pressure developed above the liquid surface or extra pressure created in addition to the pressure due to the liquid head .So Dp transmitter gets this Pressure in its HP side as well as in LP side .So the net effect of of differential pressure applied on DP transmitter is due to the liquid head only .Means it get a differential pressure proportional to level only .
