Purging the Sensing Lines of a Differential Pressure Transmitter

What if the process 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 significant 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 the sensing line eliminates stagnant fluid, 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

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 flow rate. 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 flow rate in the sensing lines should be equal. Assuming that the tubing lines are the same length, equal flow rates 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.

Instrumentation Highlights

• High performance mechanical flow controller improves the accuracy of differential pressure flow measurement
• Integral mounting with Sho-Rate™ glass tube variable area flow meter saves space and improves installation

Process Solution

A Not So Typical Solution.

Using a flow meter with a needle valve to set the flow rate would be a typical solution. However, if the process pressure varies, the set flow rate will also vary and cause an error. The Sho-Rate™ glass tube variable area flow meter coupled with the high-performance FCA8900 manual flow controller, both from Brooks Instrument, provides a reliable solution for flow measurement and control of the sensing line flow rates.

Flow Scheme

Flow can be set to equal flow rates as indicated on the flow meter, and the flow controller will maintain this preset flow rate 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 Sho-Rate™ variable area rotameter and FCA8900 flow controller, which can easily be accomplished with a pressure regulator.