In this column and the next, I will deal with items of good standard practice in process instrumentation, and some notable tricks and traps. While looking and poking around your physical plant and product, you will be using two levels of insight into the process:
- The installed instrumentation such as the meters, indicators, controllers, lights, recorders and screen displays that came with the process. These are used for day-to-day operation, so they should be within sight of the process.
- Test, investigation and trouble-shooting equipment such as calibrators, multimeters, clamp-on ammeters, oscilloscopes and electronic thermometers.
Operating InstrumentsA look at a few specific types of instrumentation will provide clarification.
Temperature Indicators and Controllers. Insist on plant-wide agreement on usage of °F or °C. Totally purge the plant of one or the other, or pay some day for scrap product or accidents. Most instruments now are field convertible. The scale range and resolution usually determine accuracy, so do not use a range greatly in excess of your working range. To avoid a dancing last digit, only use resolutions of 0.1°F/°C or tighter if the process really needs it and is stable.
Liquid- or gas-filled temperature indicators usually are too slow for prompt sight of process changes. A hazard to accurate readings is a leaky bulb or capillary, which will default to low indication.
Output Indication. If your final control device is a valve or actuator, make sure it feeds back a position signal, not a received control signal that you hope will be obeyed by the final control device. Also, note that even valve position knowledge does not necessarily imply a corresponding flow - or any flow at all.
Heater Ammeters. Moving iron ammeters are cheap and dependable. They also indicate the rms (heating effect) value even for severely phase-restricted SCR controlled loads. On such loads, average responding meters (this includes most clamp meters) can indicate low by 50% or more, though they are good on sinusoidal waveforms.
Analog ammeters can reveal erratic loads and trends better than digital meters and displays. They also are more sensitive indicators of control loop oscillation than is the temperature indication. They do, however, lack the data-recording and heater malfunction alarm and action potential that distributed control systems (DCS) can provide.
For three-phase, three-wire Y-connected heaters, I recommend an ammeter in each line. If you lose a heater or line fuse, one meter goes to zero and the other two will fall by about 13%.
For a delta connection, put the ammeters inside the delta loop, one in series with each heater. Loss of any heater takes its meter indication to zero while the other two do not change. Loss of a line fuse gives the same result as the Y-connection. Here, I assume balanced and reasonably constant resistance loads. When you know the load resistance you can readily work out the power as current squared times heater resistance (I2R). Readers of a previous column on heater materials (Process Heating, November 1999, p. 86) will recall how the severe variation of resistance of some materials can upset your calculation.
Some controlling SCRs can put out a signal representing power regardless of load resistance. These signals, along with those from current transformers and from position feedback potentiometers, can be fed into temperature controllers or DCS systems for indication and alarm action or process monitoring and recordkeeping.
You will sometimes see illogical combinations of readings such as:
- Temperature high and power stays on.
- Temperature high and power never on.
- Temperature low and power fails to increase.
If you are there and alert, you can diagnose and intervene, but it is better to arrange for the measurement and control system logic to act for you.
Convoluted Key-Handling. There will be times when you have to make a quick adjustment to some control or alarm parameter and are defeated by the tricky button-pushing sequence - just like when you could not change the alarm time on your watch without reference to the manual. Make sure that operators take the time to become familiar with critical actions, and refresh their training periodically. Be prepared to reject offers from equipment suppliers who neglect this aspect of operating the process.
In my next column, I will continue my look at instrumentation on your process by addressing test instruments.