DP Transmitter Sudden Zero Reading - What’s the Cause?
The differential pressure transmitter on the separator suddenly falls to registering almost zero, even though the process level remains steady. The impulse lines are long, usually between 5 and 6 meters, and the transmitter is located below the tapping locations. What are the most probable reasons, and what checks should you do in order to find and repair the problem?
A DP transmitter that suddenly drops to zero while the real process level remains stable is a common field problem, especially when impulse lines are long or the transmitter is situated below the tapping points. Several practical elements can contribute to this behavior, and systematic troubleshooting aids in determining the actual source of the problem.
The most common cause is impulse line obstruction. In liquid service, the low-pressure (LP) leg may become clogged with debris or solidified residue, leading the transmitter to register equal pressure on both sides, resulting in a zero differential. Water or condensate buildup in gas service can produce similar problems. Another possible cause is air or gas trapped in the high-pressure (HP) leg, particularly if the transmitter is installed below the tapping point. This confined air pocket might compress and distort the actual DP signal.
Begin troubleshooting by visually evaluating both impulse lines for leaks, kinks, and moisture. Check the isolation valves to ensure they are fully open and that no three-valve manifold valves are partially closed. If the process allows it, perform a line blow-through to eliminate any probable blockages. Apply equal pressure to the manifold equalizing valve to ensure that the transmitter returns to zero correctly; if it does not, the transmitter sensor may be faulty. Also, check the zero trim and the transmitter housing for moisture ingress.
If the problem persists, isolate, vent, and refill the impulse lines with the appropriate fluid (for wet leg applications). Finally, compare the transmitter reading to a handheld calibrator to ensure sensor health before replacing.
