Replacing the orifice plates was one of the recommendations made in a report auditing a number of orifice plate custody transfer installations. Manufacturing the orifice plate would seem to be straightforward because it can be fabricated using specifications in existing standards that are available from a number of sources.
Orifice plate standards have smoothness requirements that result in an orifice plate that is relatively smooth to ensure that the appropriate geometry of this physical aspect of the orifice plate is maintained. On the other hand, the user had an internal requirement that the equipment located in the piping requires relatively rough surfaces. Investigation revealed that the source of this requirement was the user's experience that equipment with relatively smooth surfaces tended to leak over time. Therefore, plant-wide specifications required rough surfaces to avoid leakage by the gasket.
The orifice plate supplier was "pulling his hair out" regarding the smoothness of the orifice plate surface because he had to deal with these conflicting requirements in the same orifice plate. In particular, standards require a smooth surface to achieve accurate measurement while the plant (his customer) required a rough surface to avoid leakage.
The seemingly conflicting requirements were resolved after some initially frantic discussions. Once the facts were determined, it became clear that the orifice plate needed to be smooth for accurate measurement. Yet it also had to be rough to properly seal the piping. But the measurement occurs in the flowing fluid whereas the sealing occurs away from the flowing fluid outside of the pipe per se.
It is important to recognize that the smooth and rough requirements apply to different parts of the orifice plate even though neither requirement explicitly states this. Manufacturing can then proceed by fabricating the entire orifice plate per the relatively smooth orifice plate specifications. Afterwards, the part of the orifice plate that is not used for measurement (the non-wetted annular area located outside of the inside diameter of the pipe) can be made rough so as to meet the sealing requirement of the gasket.
It took a little convincing... but the orifice plate was eventually fabricated to meet both seemingly conflicting requirements.
This article originally appeared in P. I. Process Instrumentation magazine.