Application Note

Purging the Sensing Lines of a Differential Pressure Transmitter

By Steve Brown – Brooks Instrument 04.04.17

One of the common applications for differential pressure transmitters is flow measurement by measuring the differential pressure across an orifice plate or venturi tube, or other differential pressure device. However, what if the fluid has contamination that can clog the sensing line, or if the fluid can crystalize or jell in the sensing line? In both cases, a large error can result in the differential pressure measurement.

One solution to avoid these issues is to introduce a flow of fluid down the sensing lines back into the process flow. Purging of the sensing line eliminates stagnant fluid, which is a common source of clogging. The fluid can be a gas or liquid that is inert to the process, or can even be a small amount of the actual process fluid.

Application Challenge

Introducing flow into the sensing lines creates a pressure drop in the sensing line, causing an error in the differential pressure measurement. Also, process pressures can vary, creating variations in the sensing line flowrate. This variation results in unreliable differential pressure measurement, yielding inaccurate measurement of the main process flow measurement.

To improve the accuracy of the process flow measurement, the flowrate in the sensing lines should be equal. Assuming that the tubing lines are the same length, equal flowrates will result in identical pressure drops in each sensing line, yielding an equal offset on each side of the differential pressure measurement which will null each other out.

A Not So Typical Solution

Using a flowmeter with a needle valve to set the flowrate would be a typical solution. However, if the process pressure varies, the set flowrate will also vary and cause an error.

An example of a reliable solution for flow measurement and control of the sensing line flowrates is a flow indicator like the Sho-Rate™ Series 1350G coupled with the FCA8900 flow controller. Flow can be set to equal flowrates as indicated on the flowmeter, and the flow controller will maintain this preset flowrate even with variations in the downstream pressure in the sensing line (main process pressures). The supply pressure must be maintained at the inlet of the 1350G/FCA8900, which can easily be accomplished with a pressure regulator.

Taken from: Brooks Instruments