By Mr. Phil Lawrence A. M. Inst Mech E. Sales Manager (Oil and Gas Industry)
Current design of multiphase separators allow overall uncertainty (on all three phases) of about 15% - 20% according to recent API discussions, this can be due to operator control, time delay in stabilization of the vessel, incorrect design involving future fluid levels, position of vessel in respect to the pressure head requirement (on liquid side) and also the main metering.
In particular where orifice meters are used it is necessary to perform, plate changes to facilitate turndown otherwise the performance of the system could be compromised, and to use upstream flow conditioners or velocity profile devices which add cost to a system. (Gas Outlet).
Long-term vulnerability using orifice plates in production separators can be demonstrated by examining public documents in the measurement field. (Examples cited in this document from Phillips Petroleum Embla)