Plugged impulse lines
Accurate Process Variables (pressure, level & flow) using a Pressure or Differential Pressure Transmitter is measured via the impulse line. A partially or completely plugged Impulse line prevents accurate Pv measurement.
An Impulse line connects with an Instrument Tapping Point, usually behind a tapping point isolation valve.
To ensure there are no induced errors the tapping point diameter and hence cross sectional area must be greater than that of the Impulse line. This ensures the inert purge used as the barrier between the process and field instrument passes through the impulse line, into a larger diameter tapping point creating no friction or back pressure that is measured by the field instrument.
As the process begins to plug the tapping point, the opening through which the purge passes through, decreases in size and when the area becomes less than that of the impulse line, false readings will be transmitted from the field instrument.
A significant alarming side effect is that the lesser area of the tapping point, creates a restriction and allows the purge to carry back some of the process particles into the impulse line and over a period of time, these particles will grow “somewhere “ in the impulse line, leading to continual and unreliable Pv measurement.
Manually rodding out the tapping point only clears the tapping point assembly, not the blockage that is beginning to form in the impulse line.
Using ‘smart’ transmitters with statistical acoustic monitoring to help identify a blockage in the impulse line does not fix the problem, it only lets you know you have a problem (which you would know anyway once the impulse line plugged because you would also lose your process variable measurement).
With Autorodders installed correctly, the tapping point will not partially plug, meaning the purge flow is maintained and thus will not plug. The Autorodder frequently strokes a scraper through the tapping point clearing a path that is greater in area than your impulse line and therefore ensuring there are no induced errors, no purge back pressure and therefore no process particles being carried back into the impulse line.
Additionally, with Autorodder maintained tapping points, you will have increased safety and have the added benefit of constant, accurate process variable measurement.