Written by: Steve McKenzie at Piping Design Central
With staturated steam, the impulse lines and the wet end of the pressure transducer are usually arranged to run full of condensate. The aim is to have both HP and LP impulse lines full of liquid to an equal height above the transducer so the transducer requires no zero offset and there is no zero drift due to condensation during operation. The impulse lines are often unlagged and sometimes a condensate pot is fitted to each line to provide a reservoir of condensate and help provide stability and a degree of adjustability to the condensate level. Top tappings can be used with condensate pots on saturated steam provided the impulse tube from tapping to pot can drain back to the steam line. The aim is to keep the lines (from pot to transducer) equally full of condensate while running and when shut down for short periods. It would be theoretically possible to arrange a top tapping setup for saturated steam, but any condensation which forms and remains in the impulse tubes or the transducer body would affect accuracy.
With superheated steam any condensate will tend to be evaporated when in contact with saturated steam so a known level of condensate in the impulse tubes cannot be maintained easily. The plan normally adopted for superheated steam is to keep everything hot to minimise condensation, and to arrange for any condensate formed to drain back into the process line. Top tapping with transducer above process line and insulated impulse tubes is an easy way to arrange this.
There's always been a lot of debate about this subject, and often the arrangements seen are a house standard as opposed to a custom arrangement.