In operational thermal fluid systems the fluid degrades over time, primarily as a result of the elevated temperatures that the fluid ‘sees’. These elevated temperatures cause the breakdown of some of the more complex hydrocarbons in to simpler, often more volatile and flammable compounds. They also facilitate the formation of larger molecules which result in less desirable fluid properties, such as increased viscosity. As with many chemical reactions, the rate of break down is a function of both the temperature and the time over which the temperature persists.
It is important to recognise that, although fluid data sheets routinely specify quite high acceptable film temperatures, usually in excess of 300oC, such temperatures must be accompanied by significant turbulent flow to minimise the time that a given unit of fluid ‘sees’ the elevated temperature.
Thermal Fluid Solutions has an on-going fluid characterisation programme which allows us to thermally degrade any fluid sample at relatively low (less than 300oC) film temperatures. Flashpoint reductions of between 10% and 60% are routinely seen. The reason for this is that flow in low volume laboratory test cells is relatively low and laminar.
From experience, we know it is important to ensure that flow rates through heating elements of the system, such as boilers, biomass heaters and Thermal Oxidisers, are consistent. A combination of regular and effective maintenance, along with good control regimes, will go a long way to ensuring this.
Good maintenance routines include inspection of the heating elements to pick up on refractory failures that cause local hot spots, regular cleaning to eliminate excessive build-up of carbon based sludge which inhibits flow, and inspection and redundancy in circulation elements such as pumps.
Control elements around the primary heat sources are the most critical. There should be an interlocked flow monitor which usually consists of a differential pressure gauge across input and output. Differential pressure is the most tangible monitor of flow and maintaining it is vital. Fluid temperature on the outlet side of the heater should be made as close as possible to the heating elements and also interlocked. System shutdown for any reason should result in the heat source being removed well before circulation is stopped.
For more information or to discuss any specific issues you may have please contact Thermal Fluid Solutions on +44 (0)1298 815862.