Preventive Maintenance for Instrumentation

IDCON often gets the question, “What kind of Preventive maintenance should be in place for instrumentation?” from maintenance managers with mechanical backgrounds.

The exact answer will vary from plant to plant depending on equipment and level of automation, but in general on can divide the preventive maintenance tasks for instrumentation as follows:

1) Calibration

Instrumentation devices needs to be calibrated from time to time to make sure they show the right value.

The interval is different from different devices, brands and styles. Some suppliers give up to 10 years warranty on the device they sell to stay true, others are less.  But, if you are a non-instrumentation manager, look for calibrations in your instrument PM’s.

2) Control Loop Checks

Part of an instrumentation preventive maintenance program is to check your loop response.  For example, if you set a flow to 22 gpm, the flow should become 22 gpm.  In tests by control loop experts such as Fisher and others, on average, the typical plant has about 80% of their control loops respond incorrectly.

Part of the loop response problem can be design issues.  It is not uncommon to see plants select a valve that is larger than needed, which results in errors.

If a maximum flow is supposed to be 30 gpm and a larger than necessary valve is chosen, the maximum flow may be achieved at 75% open and the range will therefore be 0-75% for the corresponding 0-30 gpm, instead of 0-100%.

3) Physical Instrumentation Checks

All instrumentation should be checked physically in our opinion.  Most instrumentation people I’ve met resist doing these checks. I’m not sure exactly why.

While a packing leak on a valve, a damaged transmitter, or a damaged membrane on a regulator eventually will show up on a computer screen somewhere, it is not always a problem that can be pinpointed to physical damage, or to what device(s) that have the problem.

A physical inspection route for instrumentation equipment will pinpoint physical problems and identify problems earlier.  It is a must for a world-class instrumentation preventive maintenance program.

An idea may be to have your operators do physical inspections of instrumentation and mechanical equipment?