External Heating and Cooling of Lined Pipe and Fittings
It is possible to provide a source of heating or cooling to a lined piping system by using jackets around the equipment. Here the core pipe is completely surrounded by a jacket pipe. The process material travels through the lined pipe as normal, whilst the heating or cooling medium travels through the outer jacket.
Jackets typically contain steam, hot water or oil. These can be operated at temperatures of up to the maximum operating temperature of the lining - 200oC or the required process temperature if lower. Jackets can also work successfully at cryogenic temperatures, as long as the steelwork is specified appropriately.
On a straight pipe spool the jacket usually extends almost to the end of the pipe, leaving some room to introduce the necessary vent between the steel tube and the liner. If the line is to be subsequently insulated such vents require extensions to take them outside the lagging. Connections to the jacket can be flanged, threaded or whatever the customer specification requires.
Fittings and Valves can also be jacketed, but it may be that their short length allows the process to remain heated, simply by insulating them.
The advantages of jacketing are that heat can be applied evenly to the pipe without the danger of localised hot spots, which can lead to localised liner damage or liner expansion, both of which can lead to liner collapse. Also jacketing allows for a smooth ramp up of temperatures – a useful attribute given the poor heat conducting characteristics of PTFE and PFA.
Jacketed Pipe Spool
Jacketed Equal Tee
Jacketed 90 Degree Elbow
Direct steam tracing can be used by running a steam pipe alongside the steel tube or fitting outside the liner, securing it with steel strapping and insulating the line. It is important to limit the temperature to avoid damaging the liner and avoid creating localised hot spots where the trace is in contact with the pipe.
As with steam tracing this can be used, more often for closer temperature control or for cooling control.
Electrical tracing can be undertaken with heating tapes or cables, often used in conjunction with a heat conducting cement to aid dissipation. Again this can provide good temperature control, but often a low rate of temperature rise and of course is a risk in certain ATEX zones.