Engineering designers and owners may use the PFD as a first step in designing
the measurement and control system. Important process monitoring and control
requirements are then captured as they become known during design
development. In this situation, the process design team may indicate where various
process variables are to be measured. For example, a circle with a single
letter P inside might be used to signify that the pressure at this point is important
to the process and should be measured. Likewise, the use of F for flow, L
for level, or T for temperature in a circle would indicate where these variables
should be measured. Everyone involved should understand that the process
measurement and control points shown on a PFD are by no means the only
points, they are just notable points identified during project scope development.
Many more process measurement points and control points will be
defined later and shown on a P&ID.