How to Connect a DP (Differential Pressure) Flow Sensor to a DP Transmitter

The DP Transmitter should be mounted with a manifold block as illustrated in the figure 1(a) below to allow for maintenance.

Valves B and C are the isolation valves while valve A is an equalizing valve. Valve A is used along with valves B and C to zero the transmitter.

In normal operation valve A is closed and valves B and C are open. Valve A should always be opened before valves B and C are closed prior to the removal of the transducer/transmitter to avoid high pressure being locked into one leg. Similarly on replacement, valves B and C should be opened before valves A is closed to prevent damage from static pressure.

Gas measurements are prone to condensate in pipes and liquid measurements are prone to gas bubbles. To prevent these effects, a gas differential transducer/transmitter should be mounted above the pipe and a liquid differential transducer/transmitter should be mounted below the pipe with tap off points in the quadrants as illustrated in figure 1(b) and 1(c) below. Ideally the piping should be taken from the top of the pipe and rise to the transducer/transmitter for gas systems, and be taken from the lower half of the pipe and fall to the transducer/transmitter for liquid pressure measurement.  If this is not possible, vent/drain cocks must be fitted. Under no circumstances should the piping form traps where gas bubbles or liquid sumps form.

Steam pressure transmitters are a special case. Since steam can damage the pressure transmitter diaphragm, the piping leg to the pressure transmitter is normally arranged to naturally fill with water by taking the tapping of the top of the pipe and mounting the transmitter with (unlagged pipes) below the pipe as shown in figure 1(d). If this is not possible a steam trap is placed in the piping as illustrated in figure 1(e). In both cases there will be an offset from the liquid head, a similar effect occurs when a liquid pressure transmitter is situated below the pipe. In both of these cases, the offset pressure is normally removed in the transmitter set up and configuration.

Pressure transmitters require maintenance or may need to be replaced altogether, so isolation valves should always be fitted. These should include an automatic vent on closure so pressure cannot be locked into the pipe between the transmitter and the isolation valve.

You can also read: P&IDs and Loop Diagrams

While the accuracy (around 4%) and turndown (4:1) of differential pressure flowmeters is poor, their robustness, low cost and ease of installation makes them a frequent choice for industrial flow applications.