why level tX's are calibrated from -ve to 0 in closed tanks and is reverse in open tanks?

 

>why level tX's are calibrated from -ve
>to 0 in closed tanks and is reverse in
>open tanks?

It makes no difference if the tank is open or closed, the differential pressure is the same.

You might be refering to a tank where the vapour condenses e.g. a steam drum or condensate tank, in that case the LP side gets pressureised by the condensate in the top connection so with the tank empty the LP side is under pressure, as the tank is filled the Differential comes down toward zero, when the tank is full the pressure on both sides is equal.

Note: the full differential may be effected by temperature (density differences)

Years ago we used to have transmitters that would not measure a negative pressure so for condensing liquids they were connected with the LP side connected to the bottom connection and th HP side connected to the top but I haven't had to do that in 20 years.

Roy

 

the basic equation for using differential pressure in level measurement is (rho)*g*h, where:

(rho) = density of liquid in kg/m3
g = acceleration due to gravity (9.8m/s2)
h = height of the fluid

differential pressure trx's use High and Low side pressure connections, and the measured DP which results in the level measurement is always: DP = (High Pressure) - (Low Pressure)

in a tank, where the vapor space above the liquid is at atmospheric pressure, the High side connection is attached to the process liquid, and the Low side connection is usually left open to atmosphere. If you are setting up your transmitter to simply measure from the point where the trx is located and above, your LRV would = 0, and your URV = (rho)*g*h, where "h" is the max height of the liquid that will be measured.

in a pressurized vessel, where the vapor space above the liquid is at a pressure greater than atmospheric, you have to account for that extra pressure. if you used the simple setup described in the previous example for a pressurized vessel, the extra pressure from the vapor space would be interpreted by the transmitter as additional level.

to avoid this, we still connect the High side pressure connection to the process liquid, but instead of leaving the Low side pressure connection vented to atmosphere, we connect it to the vapor space above the liquid being measured. Now that extra pressure is sensed by both High and Low side connections, and cancels out. The Low side connection is now a reference connection.

The Low side connection will often have a sealed capillary attached to it, which is filled with a liquid which will not change much in density when temperatures fluctuate. That liquid which is contained in the Low side connection exerts a (basically) constant pressure on the Low side connection as well, so if you go back to the basic DP = High - Low, imagine what happens when you vessel is empty (High = 0). Your Low side connection has a constant pressure due to the seal liquid, and your High side connection has 0.

In this case your LRV will be a negative value. Your URV will be the pressure exerted by the maximum height of liquid you are measuring minus the LRV

Example values: seal liquid exerts 10kpa, and max liquid head pressure in vessel = 9kpa

LRV = -10kPa (LRV = empty tank, ie: High side = 0, Low side = 10kPa, 0kPa-10kPa = -10kPa)

URV = -1kPa

hopefully thats helpful, this is how I always remember it