LRV or lower range value and URV or upper range value are the two important values we should care, while measuring level using differential pressure transmitter
Case 1:
Here in the above diagram the differential level transmitter is fixed at the same level of the tank base.
Level controllers are cared only about the level of the tank & the DP transmitter is concerned about the pressure
For Level controller LRV & URV values only associated with level of the tank.
For level controller:
LRV = 0 ft URV = H ft ( height of the tank)
For Pressure transmitter the LRV & URV associated with the pressure at the HP (high pressure) line.
For pressure transmitter :
LRV = 0 ft URV = H ft
Then the differential pressure DP = Ph - Pl (Ph - pressure at HP side, Pl - pressure at LP side)
Here Ph is zero because the pressure transmitter is with the base of the tank.
therefore using the above formula ;
For LRV:
LRV = 0 - ( H * W * SPw ) ;
SPw is the given specific density of liquid in wet leg W is width of wet leg side LP.
Again applying the DP equation.
For URV;
URV = ( H * W * SPt) - LRV
SPt is the specific gravity pressure of liquid in tank
Case 2:
Here the situation is little different, but common. We can’t able fix the DP transmitter at the same level to the base of tank always.
Consider the DP is fixed at X ft below the tank
For a Level controller, LRV & URV values only associated with the level of the tank.
For level controller:
LRV : 0 ft URV : H ft ( height of the tank)
For Pressure transmitter the LRV & URV associated with the pressure at the HP (high pressure) line.
For pressure transmitter :
LRV = X ft URV = (H + X ) ft ; (H + X is the total height from the DP to the top of the tank).
Using DP = Ph - Pl
Here pressure at HP is not zero as the there is pressure of X ft of liquid.
therefore for LRV ; LRV = ( X * W * SPt ) - ( ( H + X) * W * SPw )
H + X is the total height from the DP to the top of the tank.
For URV: URV = ( ( X +H ) * W * SPt) - LRV