Differential Pressure type Level Transmitter

In any process industry, level measurement is very important for Process safety and control applications. 

What is DP transmitter (DPT)?

The DP Transmitter device is used to measure level or Differential pressure as an inferential measurement. In a DP Transmitter, the diaphragm senses the head pressure developed by the height of the material in the vessel. 

This quantity is multiplied by a density variable to get the correct level measurement in case of level measurement.

DP transmitter working Principle: 

The DP transmitter is exposed to equal pressure on the high and low pressure sides when the liquid level is at its maximum; therefore, the differential pressure is zero. As the tank level goes down, the pressure applied to the low pressure side goes down also, and the differential pressure increases.

Calibration of DP Transmitter:

• Ask panel man to put the controller in manual mode for control loop.
• Hook up HART Communicator and verify some parameters by refer to data sheet. Typical parameters are, tag number,  PV, LRV and URV.
• Isolate the instrument from the process.
• Release both pressure and drain low and high side liquid throughout manifold drain.
• Open both plug at seal pot
• Top up seal liquid for low side until half of seal pot.
• Remove tubing at high side of manifold
• Hook up pressure calibrator at high side and isolate the vent valve
• Hook up a multimeter in series with the signal to the DCS to measure current signal.
• Apply pressure as per calculation when level equivalent to zero
• Multimeter should show 4mA
• If not, do zero adjustment at transmitter using HART Communicator
• Apply pressure as per calculation when level equivalent to 100 % Multimeter should show 20mA
• If not, do span adjustment at transmitter using HART Communicator
• After completion of the job ask panel operator to put loops back in normal mode or normalize the MOS
• Fill the calibration form and file it for future reference.
• Note: There is many way to perform calibration for DP level but the important thing is the DP reading at HART communicator should same with the LRV and URV

Usage and Example:

A differential pressure transmitter is a common and well-understood technology for liquid level measurement. If the tank/vessel is closed or pressurized, a DP measurement must be made to compensate for the vapor pressure of the tank/vessel. DP Transmitter Level Measurement for Closed Tank In closed tank DP level measurement, LP leg is connected to the top of tank. 

There are two methods in closed tank DP Level Measurement: 

1. Dry leg method: Dry leg method is used in normal closed tank when process vapors are not corrosive and condensations at ambient temperature are not expected. 

2. Wet leg method: The wet-leg installations are used when the process vapors would otherwise condense into the compensating leg, thereby exposing the low-pressure side of the d/p cell to unpredictable hydraulic heads, or when the transmitter must be sealed from corrosive vapors. 

Wet-leg application in which the compensating leg (LP side) is prefilled with a chemically inert liquid that will not freeze or vaporize under operating temperature conditions. Diaphragm seals with capillary filled in shall be used in highly Toxic, highly corrosive, and Hazardous liquid level measurement. 

Special Cases: For Boiling Application like in steam drum, the DP range shall be reversed considering steam will be always condensed at the ambient temperature. Additional measures to be taken care of for the compensation in the Control System for proper control of the drum level. Installation and performance characteristics shall be followed as per ASME Boiler and Pressure Code. 

For Cryogenic Application, special consideration for installation must be taken care of considering Liquid will be evaporated at the ambient temperature conditions while using DP type level. Input Documents from engineering aspect: - P&Id - Equipment, Mechanical Datasheet - Process Datasheet

When LP side of the DP transmitter is filled with any gas/air/, then we call it as Dry Leg & we apply Dry Leg Method for calculations. At 0 % Level (LRV) = Pressure acting on HP leg – Pressure acting on LP leg = H2 x specific gravity – (H1+H2) x 0 (In absence of liquid) = 200 x 0.9 – 0 = 180 mmH2O At 100 % Level (URV) = Pressure acting on HP leg – Pressure acting on LP leg = (H2+H1) x specific gravity – (H1+H2) x 0 (In absence of liquid) = (200 + 1000) x 0.9 – 0 = 1080 mmH2O Calibration Range of DP Transmitter = URV to LRV = 180 to 1080 mmH2O

When LP side of the DP transmitter is filled with liquid then we call it as Wet Leg & we apply Wet Leg Method for calculations. At 0 % Level (LRV) = Pressure acting on HP leg – Pressure acting on LP leg = H2 x SG1 – (H1+H2) x SG2 = 200 x 0.9 – 1200 x 1.0 = 180 – 1200 = – 1020 mmH2O At 100 % Level (URV) = Pressure acting on HP leg – Pressure acting on LP leg = (H2+H1) x SG1 – (H1+H2) x SG2 = 1200 x 0.9 – 1200 x 1.0

= 1080 – 1200 = -120 mmH2O Calibration Range of DP Transmitter = LRV to URV = -1020 to -120 mmH2O. Suitable method based on application, we need to select and calibrate Lower Range Value (LRV) and Upper Range Value (URV).

We need to find out correct calibration range before ordering the Transmitter as long as possible. That is quite important during engineering phase. 

DP Level Transmitter installed elevation effect: The DP Level Transmitter calibration parameters will vary depending on installation & seal system also. 

Generally we can see three possibilities of installation of a transmitter in the field as per following. 

1. Transmitter installed Exactly at HP tapping point (Ideal & preferred way of installation). 

2. Transmitter installed above HP tapping point (Not preferable, Chance of bubble formation in the impulse line). Only acceptable for Diaphragm sealed Level Transmitter. 

3. Transmitter installed below HP tapping point (Error can be compensated effectively). 

During engineering phase, Elevation of transmitter shall be verified in the 3D model and must be taken into consideration during Level calculations 

Note: Use your application specific details and refer respective guideline for selection of DP type level transmitter or any other transmitters, this blog content is not intended for selection of any DP level transmitter.