DP- Pad cell Transmitter with Interface Level Configuration

Differential pressure (DP) transmitters are used to measure the interface of two fluids with different specific gravities (S1 & S2). To make an interface measurement, the overall level must be at or above the low pressure tap at all times. It is important that the level be large enough to create a reasonable DP between the two specific gravity extremes. This measurement can be done with or without remote seals. However, from a maintenance point of view, it may be easier to use a remote seal assembly; keeping the wet leg at a constant height can be difficult in some applications.

To determine the calibration range of transmitter, need consider four assumptions, all described below:

1. At the lower calibrated value, 4 mA point, the tank is filled with the lighter fluid
2. At the upper calibrated value, 20 mA point, the tank is filled with the heavier fluid.
3. The taps leading to the transmitter are flooded at all times. The overall level should be equal to or higher than the upper (low pressure) tap.
4. There is always a reference level seen by the low pressure side. This can be accomplished with either a remote seal system or a wet leg. The reference level must have a constant height and density. The calculation is modified slightly for wet leg systems when the density of the high and low pressure wet legs differ.

The measured level is composed of a combination of the two fluids:

L = L1S1 + L2S2

When the tank is filled with the lighter fluid, the transmitter will be at 4 mA (or 0% of span) and L2 = L:

HP = L2S2 + dSf

LP = dSf + hSf

At 4 mA, DP = HP – LP = L2S2 – hSf

When the tank is filled with the heavier fluid, the transmitter will be at 20 mA (or 100% of span) and L1 = L:

HP = L1S1 + dSf

LP = dSf + hSf

At 20 mA, DP = HP – LP = LS1 – hSf

The tools below is used to calculate the DP transmitter ranges for interface measurement application

For calibration range of DP use the formula below:

The Transmitter 4ma or LRV formula

∆Pmin or 4ma = LS2 – HSf

The Transmitter 20ma or URV formula

∆Pmax or 20ma = LS1 – HSf

The automatic calculation (DP) above can be used to found the internface level in the second automatic calculation below (second step)

In the second step below there are four paramiters to add which are:

1. Total Measured Level (L) = Total height to be measured
2. LRV/4mA= how many mbar the transmitter is zero ( see table above)
3. Transmitter span = Span transmitter  (see table above)
4. Mesure DP=  Put the transmitter online and check how many DP reads the transmitter. Put the parameter four that you read on the transmitter.

The Interface Level (I) is the value in mm that are now in the vessel.

  Interface Calculation

 The formula used for calculation the interface in mm is:

below all the abraviation

H = Distance between taps

L = Total Measured Level

S1 = Specific gravity of lighter fluid

S2 = Specific gravity of heavier fluid

Sf = Specific gravity of reference leg Measured

DP= After transmitter calibration, present DP value

Span = Difference between absolute of maximum and minimum range

LRV = Lower range value of transmitter in DP scale I = Interface level

Advantages 

1. Low cost
2. Simple to install
3. No additional components required

Limitations

1. Span must be sufficiently large enough to measure; either the distance between taps (flange connection) or the specific gravity difference must be large.
2. Upper tap must be covered at all times.
3. Small spans are subject to errors when the temperature increase or decrease.

Example calculation:

A vessel requires an interface measurement where:

capacity of vessel 3810mm

fluid S1 the specific gravity is 1.0

fluid S2 the specific gravity is 1.08 below the figure with all dettail:

At 4mA, DP = L2S2 – hSf

DP=  (3810 * 0.834  / 1.02 / 10 ) - (3810 * 0.934 / 1.02 / 10 )

So at 4 mA = 37.35 mbar DP

At 20mA, DP = LS1 – hSf

DP= (3810 * 1.02  / 1.02 / 10 ) - (3810 * 0.934 / 1.02 / 10 )

So at 20 mA = 54.6 mbar DP

Conclusion

The calibrated span is 24.7 to 54.6mbar. When the transmitter reads 24.7mbar, the tank is filled with the lighter fluid. When the transmitter reads 54.6mbar, the tank is filled with the heavier fluid.

To determine where the interface is located (mixture of fluids), reading the DP from Transmitter (% of span). For example: If the out transmitter is 18.4ma or DP is 51.55mbar use formula below