If we are using a differential pressure transmitter with an orifice plate. The pipe on which the orifice is installed is having two valves, one at the upstream and one at the downstream of the orifice. In normal conditions, both valves are opened, liquid passes through the orifice, creates the differential pressure for the transmitter to read.

What will happen when suddenly both valves are closed and there is a possibility of trapped liquid between the two valves. Will this create a differential pressure across the orifice?

 

No, the DP across an orifice plate will diminish as the static pressure equilibrate in its closed volume when the valves are closed.

Let me venture a guess as to why you're asking - a DP transmitter indicates a flow when the valves are shut off.

Several things to consider:

- the DP transmitter needs to zeroed.
- the shutoff valves are not really shutting off, they're leaking
- maintenance issue, like there's standing liquid condensate head in impulse tubes

 

In the common three-valve manifold used for flow transmitters, there is a bleed valve connected directly between the two transmitter ports. This should always be opened before either of the two isolator valves is shut - especially on liquid service at high pressure.

If one leg is shut off and the other left connected to the process, even a very small leak in the isolated port will allow the pressure in that line to fall and possibly apply more than the safe working pressure differential to the transmitter - causing permanent deformation of the diaphragm. Note: this doesn't have to be a sudden event.