When thermal oil freezes

The consequences of thermal oil freezing can be both damaging and costly in any kind of process engineering application. The worst thing you could do is attempt to use a system in which the oil is already frozen as this can damage the pump and associated peripherals, including the motor and control equipment.

Often companies believe that the only problem is frozen fluid, only to find that in fact they have a substantial repair bill that will haunt them once the fluid is returned to a usable state. To avoid such problems, it is critical to first understand the qualities of the oil itself as well as the needs of the application.

Many of the oils designed to operate at extremely high temperatures, of between 340°C and 400°C for instance, will freeze at 0°C or higher, some as high as 15°C.

This is fine provided you have planned your application correctly in the first instance - so that the oils are never exposed to such low temperatures.

Most responsible providers offer charts that clearly delineate the extremes that their own oils can withstand

Just as certain fluids are intended to be used at ultra-high operating temperatures, others are specifically for use in low temperatures - typically between -50°C and +120°C to +220°C, with an extremely versatile fluid functioning between temperatures as extreme as -90°C and +300°C.

The easiest way find the right oil for your application is to consult a specialist, who will already be conversant with the temperature extremes at which different types of oil can operate. But failing that, most responsible providers offer charts that clearly delineate the extremes that their own oils can withstand.

System and storage design and location are key. Evidently there is less choice of system design since there may well be other factors that have greater sway over the location of each point in the process - and even over the design of the pipework.

However, it is usually possible to choose storage locations that allow oil to be kept at a reasonable ambient temperature at all times.

The problems that arise as a result of freezing oil can be avoided by leaving the circulation pump running when the boiler is not on.

The friction and heat from the pump will keep the oil at a sufficiently high temperature to prevent it from freezing.

Companies should also store stock oil that is not already in use. This is especially important for ‘top oil’ that might need to be called into use quickly. If this is impossible, or in cases where oil is stored in an unheated building such as a shed or warehouse, purpose-designed barrel warmers should be considered.

The most common problems associated with freezing oil result from winter shutdowns or periods of low production output, when ambient temperatures in the plant and storage areas are often allowed to drop.

The first precaution is to heat up the system very slowly after the shutdown in case there is condensation in the system and the oil. The next step, if the temperature has been extremely low, is to heat up the ring-main first and then introduce the heat exchangers one at a time. Both these steps will help minimise the impact on the system.

Companies should also have sufficient oil in stock as lead times for delivery may well be longer with some companies, especially in peak periods. They should also consider the fact that, if a plant is suffering from cold weather, the roads and logistic networks are probably also suffering.

Protection on the power supply should also be adequate, with appropriate standby power measures in place to ensure continuity should there be an outage. After all, without power there is no heat. Having said that, should there be a power outage in a process application you may well find that frozen thermal fluids are the least of your worries.

Given these problems it would be sensible to ask whether turning to steam as a heat transfer method would be a solution. However, in a cold climate steam systems are subject to freezing, burst pipes and damaged components.

Furthermore they require extensive maintenance on steam traps, valves, condensate return pumps, expansion joints and water analysis and treatment. In contrast, thermal fluid systems require no traps, condensate return, blowdown or water additives.

If the proper fluid is specified and the measures discussed are taken, you should be able to develop a sufficiently strong understanding of the properties and care of thermal fluids to keep both your own stocks and associated equipment in good condition.