Protect process instruments to ensure performance

07/01/1999


Too frequently, instrumentation protection is not considered until the snow is flying and then a problem is discovered. The summer months are the best time to examine instrumentation winterization problems and take measures to ensure the instrumentation performs accurately and repeatedly when the mercury falls.

When evaluating protection options, passive protection methods are preferred over active methods.

In order of preference, six popular ways of ensuring instruments perform well in harsh winter elements are:

  • Locate instruments indoors;

  • Use instruments immune to cold weather;

  • Install nonfreezing liquid in impulse lines and meters to form a liquid seal;

  • Apply insulation and/or heat trace to impulse lines and meters to keep contents above freezing;

  • Purge impulse lines with a dry gas to keep liquids or vapors out; or

  • Purge impulse lines with a nonfreezing liquid to keep process liquids out.

If outside, enclose
Every effort should be made to locate instruments indoors or in instrument housings. Even if the enclosure is not heated, instrument reliability and life will be extended if the enclosure takes the beating from snow, rain, hail, and falling ice.

Manufacturers constantly improve product performance and broaden environmental limits that affect instrument performance. Paying a premium for an instrument that does not require winter protection may actually be a bargain when all factors are considered.

Liquid seals come in two forms, open and closed. Open liquid seals most often use seal pots to form an interface between the process media and the nonfreezing liquid in the impulse lines and meter bodies.

Closed, or integral, seals use a diaphragm and flexible liquid-filled capillary system. Process pressure changes cause slight deflections on the diaphragm. The capillary system hydraulically transmits the change to the instrument.

When liquid seals won't work, the use of insulation and/or heat trace is required.

If the process media is heated, mounting the instrument very close to the process piping or vessel and insulating everything may provide adequate protection.

Heat trace may be electric, steam, salt solution, glycol, or other media. Any of these choices increase operating cost, and require periodic maintenance. Unless a temperature controller is used, it's important to remember to turn continuous heating heat tracing off when the weather warms and on again when it gets cold.

A variety of reasons make purges the least desirable protection. Adding hardware, piping, and proximity of the purge media creates a miniature process. Also, unless purge flow rates are tightly maintained, accuracy and repeatability suffer. Purges have been successful in measuring liquid level changes in open vessels, but opportunities for this application are limited.

Avoiding instrument failures
For all the ways to protect instrumentation, there are an equal number of ways to improperly apply and/or install protection. Keep reading to learn three of the most common mistakes made while designing and/or installing instrument protection.
First, many flow meters have two impulse lines. When heat tracing is used, both lines must be equally protected. Avoid splitting a single steam trace line into two lines, one for each impulse line, and then rejoining the lines ahead of the steam trap. Steam follows the path of least resistance. The line offering the most resistance will stop flowing, the steam will condense, freeze, and possibly rupture the tracer line. It's okay to split the lines, but provide each line its own steam trap.
Secondly, when using an instrument housing, especially "home built housings," avoid temptations to mount the instrument on the same metal pedestal as is used to mount the instrument housing. Thermodynamics applied to the pedestal will cause heat, supplied to protect the instrument, to migrate toward the cold end of the pedestal. The instrument housing should be mounted on top of a pedestal and a separate instrument-mounting stand should be located inside the housing.
Finally, when using finned heaters to warm instrument housings, small buildings, etc., the fins should be in a vertical position. Finned heaters rely on convection to move heat away from the heater. When fins are horizontal, air cannot flow between the fins, and little or no heat is transferred


Dave Harrold, senior editor
dharrold@cahners.com





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