Preventive Maintenance of Flow Elements


Thread Starter


Do we need to perform periodic inspection for flow elements including Orifice plates that are being used for the measurement/control/shutdown of critical process control instruments?Is there any statutary standard governing inspection requirements?

Andy Robinson

Absolutely! The only real regulations that might govern this are general statements in pubs such as ISA S84 that say you must check out your interlocks and make sure they function properly on a schedule, which is set by a number of factors. Depending on the service you can definitely chew out the hole in your orifice. It doesn't take much to really screw up your flow. Also, if you are protecting against a high-flow situation then you definitely need to check them out. The reason being that if the orifice gets enlarged over time the indicated flowrate will be lower than your actual flowrate. (Larger orifice means smaller dp, thus smaller indicated flow) You can also get cracks but those wouldn't be as frequent as run of the mill enlargement. When you take the orifice out you should send them out and get them mic'd by a qualified shop. This is a specific technique for checking the bore size on the orifice. When they are done they should stamp the size on the tab of the orifice so you know exactly what you have. Also, make sure you have the correct orientation if the orifice is beveled.

Hope this answers your questions.

Andy Robinson
Avid Solutions
Cary, NC

Standard Plan

I am not aware of any overriding legal requirements for preventative maintenance on orifice plates. I have seen intercompany (and intracompany) regulations on custody transfer applications.

Many feel that the maintenance required for orifice plates is just periodically measuring the bore size and replacing when the dimension grows by some amount. Unfortunately this is only part of the problem. According to ASME, ANCI, AGA, ISO, etc. a "sharp edged" orifice needs to have sharp edges especially on the inlet side of the plate. The discharge coefficient (Cd) is based on this requirement. The sharpness specification is to be sharp enough NOT to break a beam of light (show a reflection) on the edge. If the edge is rounded (a normal case of wear) the Cd will dramatically increase, thus giving lower DP values for a
given flow. So for custody transfer, more fluid passes than is recorded. Even minor rounding can cause major Cd changes. Consequently, it is equally important, if not more so to verify edge condition during maintenance checks.

John Catch