Electricity + Control November 2018

PLANT MAINTENANCE, TEST + MEASUREMENT

Preventive maintenance for instrumentation What kind of Preventive maintenance should be in place for instrumentation? The exact answer will vary from plant to plant depending on equipment and level of automation, but in general one can divide the preventive maintenance tasks for in- strumentation as follows:

Calibration Instrumentation devices needs to be calibrated from time to time to make sure they show the right value. The interval will differ depending on the type, brand and style of device. Some suppli- ers give up to 10 years warranty on devices they sell while others may give less. If you are not an instrumentation manager, look for calibrations in your instrument PM’s. Control loop checks Part of an instrumentation preventive maintenance programme is to check your loop response. For ex- ample, if you set a flow to 22 gpm, the flow should become 22 gpm. In tests conducted by control loop experts, on average 80% of control loops in a typical plant respond incorrectly. Part of the loop response problem can be design issues. It is for example 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 cho- sen, 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%. Physical instrumentation checks All instrumentation should be checked physically. While a packing leak on a valve, a damaged trans- mitter, or a damaged membrane on a regulator will eventually show up on a computer screen somewhere, the problem cannot always be pin- pointed to physical damage, or to which particu- lar device(s) might be experiencing the problem. A physical inspection route for instrumentation equipment will determine physical problems and identify problems earlier. Taking proper care of test and measure- ment equipment In order to avoid causing damage to costly meas-

A physical inspection route for instrumentation equipment will determine physical problems and identify concerns earlier.

urement equipment it is vital to steer clear of the following mistakes: Using the wrong jacks: At a nuclear power plant, an electrician turned his DMM into a molten mass by measuring from a 480 V bus to ground while having his test leads plugged into the cur- rent jacks. That 2 A fuse isn’t designed to protect against this sort of abuse. Using the wrong setting: Most meters have some protection against this. For example, if the DMM is set on the 20 Vdc scale, but you are meas- uring 120 Vac. This typically will not harm the me- ter, but you will just see ‘OL’ on the screen. Never try this stunt when the meter is providing power from its battery to the resistance terminals as you could seriously damage the meter. Storing it for immediate access rather than keeping it in a meter case: Most DMM cases have a pouch for the test leads which should be used. Wrapping the test leads around the meter and toss- ing it into a gang box (or shoving it into a tool pouch) is a sure way to degrade the integrity of those leads. Using the wrong meter: If your meter is rated CATIII, do not use it at the service switchgear. If it is rated up to 1 000 V, do not even take it out around medium-voltage equipment. Using the current jacks: This isn’t in itself a problem, but if you are in an industrial setting, it probably is. That is because a typical DMM can measure small current values that are greatly ex- ceeded by those you would really care about in an industrial setting. Quick, how much current is that 100 HP motor drawing? Do not use your DMM current jacks to try to read that. Instead, use a current clamp or other accessory. Typically, these will convert the current to a voltage you can read on the meter’s display as the amount of current. You can safely use the DMM’s current jacks, but be very aware of the application and environment before even considering doing so.

32 Electricity + Control

NOVEMBER 2018