Early editions of ASME MFC-8M (Connections for Pressure Signal Transmissions Between Primary and Secondary Devices) showed various differential pressure impulse tubing arrangements including taps at various angles and the use of seal pots. Most instrumentation users have little use for these tapping arrangements because they seem to offer no advantages and are more expensive to install. Most of these arrangements were eliminated in later editions of the standard. In steam service (specifically), users typically locate the taps on the side of the pipe and run the impulse tubing down to the transmitter below. In operation, condensate fills the impulse tubing to form a seal between the hot (live) steam and the transmitter that cannot withstand the temperature of live steam.
The above installation seemingly creates condensate legs of the same height in both impulse tubes. The pressures associated with these legs are canceled by the differential pressure transmitter measurement. This installation may be acceptable in most industrial applications but it can be deficient in custody transfer installations where accuracy is of prime importance. For example, what if the pressure taps are not at the same elevation? What if the transmitter is not level? In other words, what if the fills in the condensate legs are not exactly the same height?
In addition, pressure compensation is required in typical steam measurement applications. The pressure transmitter measurement will include the pressure generated by the condensate leg. The pressure associated with the condensate leg should be subtracted from the pressure measurement to calculate the pressure at the flow element. The pressure to be subtracted is typically determined by calculation using elevation information in conjunction with the estimated temperature of the condensate leg. But what if the difference in elevations between the flow element (inaccessibly located 10 meters above grade) and the transmitter (mounted on a pipe near grade) cannot be measured accurately?
In many industrial applications, the pressure associated with the condensate leg can be small in relation to the steam pressure such as when the condensate leg height is short or when the steam pressure is relatively high. In these applications, compensation for the condensate leg and/or its temperature is often not performed. However, while this may be acceptable in most industrial applications, it can introduce significant error in custody transfer installations.
One solution to these problems is to install a seal pot arrangement at each of the taps in differential pressure steam flow elements. This allows the elevations and leveling to be checked prior to putting the measurement system in service because the transmitter can be checked to measure zero differential pressure with its condensate legs full and its block valves open. In addition, the pressure associated with the condensate leg can be measured by removing the flow element from service (after the condensate leg is full and at operating temperature) and opening condensate leg to atmosphere at the flow element.
Standards such as ASME-8M may show seal pot arrangements, but they do not (at this time) explain why they are important. Standards are generally a good thing --- but they should be tempered with some understanding of the application at hand.
This article originally appeared in P. I. Process Instrumentation magazine.