Mis Use of Insulation

A common observation is that lines are often insulated which should be left bare and are left bare when they should be insulated. Some of the more obvious offenders are listed below.

• Compressor discharge lines: These lines mostly lead to inter-coolers or after-coolers. During process design, it is common for engineers to mark them on the P&I Diagrams for 'personnel protection insulation'. But, when the plant is built, the majority of the hot lines are well out of reach and so should be left bare to reduce load on the cooling water system or air coolers. The lower resulting process temperature saves horsepower.

• Lines to air coolers: These often have the same problem. Headers and drop lines are insulated, yet the cooler header boxes (closest to walkways) have to be left bare to allow visual checks for leaks.

• Refrigeration circuits: It was common in the past to rely on ice buildup to provide insulation. It is far better to fit properly sealed insulation to reduce 'heat in-leak' which increases refrigeration compressor power demand.

• Corrosion under insulation: This is a fairly recent concern. There have been cases where poor sealing of insulation cladding coupled with metal temperature below 100 °C have resulted in serious corrosion due to rain penetration. Some operators have therefore chosen to leave metal bare when the surface temperature is less than 100 °C. This is false economy and insulation is still recommended - with special care taken on sealing the cladding.

There is a widespread belief that the flanged connection of the heads of heat exchangers should not be insulated since this risks leakage when the bolts expand. Provided the bolts are correctly specified and tightened, this risk is minimal. The key is correct tightening. Experiment has shown that bolt tension can vary up to 50% for identical torque settings since this is affected by the degree of friction between nut, studbolt and washer. Ideally, tension should be determined by bolt stretch (measured with a micrometer: ~0.1% of the distance between nut faces) or by simultaneous hydraulic tightening of the bolt circle. Flanges which should be left bare include high pressure hydrogen lines since any leakage will auto-ignite.

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