3D. Differential Pressure Level Measurement

1. Calculate the required differential pressure range for a given dp level measurement application.

2. Select and describe applications for wet and dry legs, chemical seals and purge or bubbler systems.

3. Explain problems that may be encountered due to change in ambient temperature and some solutions.

4. Sketch a typical installation for a dp cell and give reasons for the method used.

Liquid level can be measured by measuring the pressure it exerts due to its gravitational attraction to the earth.

P = rog h ro- density of the fluid

g - gravity

h - height of liquid column

P - pressure exerted

If the density of the fluid is constant then the pressure exerted by a column of liquid will be directly proportional to the fluids height or level. In other words, differential pressure level measurement measures the pressure due to a column of liquid and infers the level from this.

Example:

Note: The fluid level is directly related to the pressure measured by the dp cell.

The pressure measured by the dp cell needs to be related to the fluids height for proper operation. This is accomplished by determining the P that the dp cell would see when the fluid level is at the Lower Range Valve (LRV) and determining the P that the dp cell would see when the fluid level is at the Upper Range Valve (URV). This gives the necessary calibration range (DP @ the LRV to DP @ the URV) for the dp cell so that its output will be proportional to the fluids level.

Setting up a differential pressure transmitter for level measurement

There are many different methods used to measure level using a differential pressure transmitter (dp cell). Selecting the appropriate method for a given application will be covered later in this module. However, the basic steps required to properly set up the dp cell to measure level are similar and presented here.

1. The calibration range required for a particular application must be determined. The transmitters installed location must be know to be able to perform this calculation.

2. The dp cell must be properly calibrated using an appropriate pressure standard.

3. The transmitter is installed in its proper location and the connection lines are “bleed” to ensure that there is no air pockets in the line that may effect the pressure being measured. (Air pocket will cause measurement errors due to surface tension especially when measuring low pressures)

Following these steps will result in proper level measurement.

Atmospheric vessels using a standard dp cell

For vessels operated at atmospheric pressure, the high side of the dp cell is connected to the bottom of the vessel and the low side is vented to atmosphere.

Example:

Process fluid RD = .750 Assume g = 9.81 m/s2

Calculating the required calibration range:

P seen by the dp cell when fluid is at its LRV

PH = (.750)(1000 kg/m3)(9.81 m/s2)(.2m) + Patm

PH = 1.4715 kPag + Patm

PL = Patm

P = PH - PL = (1.4715 kPag + Patm) - Patm = 1.4715 kPag

Calculation (continued)

P seen by the dp cell when fluid is at its URV

PH = (.750)(1000 kg/m3)(9.81 m/s2)(1.2m) + Patm

PH = 8.829 kPag + Patm

PL = Patm

P = PH - PL = (8.829 kPag + Patm) - Patm = 8.829 kPag

Therefore, the calibration range is 1.4715 to 8.829 kPag

Calibration procedure:

This transmitter would now be calibrated for this P range by using the appropriate pressure standards.

Installation:

The dp cell is generally connected to the process with ½ inch tubing or ½ inch pipe. A block valve should be installed between the vessel and the dp cell to allow for isolation and maintenance of the dp cell without having to shut down the process and drain the vessel. The preferred location for the transmitter is below the connection point.

Differential pressure transmitters come with two connection points to each of the high side and low side. This allows the installation to be “bleed” to ensure no air pocket exist in the connection line and that the line is not plugged. The connection point not connected to the vessel is opened allowing the process fluid to “bleed” through the connecting line.

Note, this type of installation would only be practical on those applications where the connection line will not plug and the process fluid will not freeze or congeal in the lines.

Operation:

If the output of the transmitter was 12.000 mA this means that the level in the tank from the LRV is