There are many different types of instruments for measuring pressure. The functions of these instruments are to monitor and control pressure in a process stream.
The following are typical pressure instruments that will be designated on a P&ID. ISA 5.1 was used as a basis for this list. In some cases, the name of the subgroup that ISA 5.1 assigns is not part of the instrument name. Control valves and PSVs are not always included in Instrumentation but they have been included here because this is how they are called out in ISA 5.1.
Flow nozzles are used to meter the flow of gas or liquid in a pipeline from the upstream to the downstream of the nozzle.
See YouTube Video - Flow Nozzle
Flange Flow Nozzle
Designed to be inserted between two flanges.
Holding Ring Flow Nozzle
Used when you have to matching different materials. The holding ring and pins are made of the same material as the pipe the nozzle is to be inserted in.
Tapped Flange Flow Nozzle
Similar to the flange flow nozzle but with a 1/2 inch tap.
Weld-in Flow Nozzle
Welded in line.
The flow tube measures the pressure difference between the large and small tapped straight sections. These are simular to venturi tubes.
- See Orifice Plate
The pitot tube measures the flow difference between the front of the tube (facing the flow) to the back of the tube.
See YouTube Video - Pilot Tube
The venturi tube measures the pressure difference between the front and the back of a restricted section of pipe.