Hot or cold - don’t forget to top up

Paul Boughton

Andy Burns explains the best practice for restarting thermal fluid systems after a shutdown.

Have you ever wondered where the hottest place on earth is? It would seem that the record belongs to the Mponeng gold mine in South Africa. Two miles below the surface, rock temperatures can reach 150˚F (65.5˚C). Workers can only survive there if slurry ice is pumped into the mine and the walls are insulated with concrete.

As impressive as this temperature is, it does not come close to the ones in which thermal fluid operates in industrial environments. For example, the Globaltherm range includes oils which are designed to operate in temperatures of up to 752˚ (400˚C). As chemically stable and secure as these oils are, they will suffer deterioration caused by heat. This means regular maintenance and topping up as often as necessary, are essential actions for any operation. 

The volume of thermal fluid can expand by 25 to 30% when the fluid is hot, compared to its volume when it’s cold. Industrial plants normally operate incessantly, which is why thermal oil is kept at a constantly high temperature. Because of this, the volume of thermal fluid is always expanded. However, periodic shutdowns for planned maintenance result in a decrease in temperature, which makes the oil volume shrink, often below the low level switch.

As a rule of thumb, the expansion tank should be one third full when the heat transfer fluid is cold, and half full, when the fluid is hot.

Volume reduction caused by problems with system venting or leaks can result in very low thermal fluid levels in the header tank. When this happens, there is a good chance the system will not restart without a top up, because it has ‘tripped out on low level’ or fallen below the low-level switch indicator.

In this situation, the best advice is not to panic. There are several ways to resolve the problem, depending on the type of heat transfer fluid your company is using, the size and capacity of your system and the amount of fluid you require.

Is volume loss inevitable?

In a nutshell, yes. Assuming the system vents adequately, if it generates more light ends than it breaths out, the volume will not change significantly. However, this will cause low flash points, fire points and auto ignition temperatures which is another, more serious issue all together. Heat transfer fluids degrade over time, because of extreme temperatures and continuous temperature fluctuation. The two main chemical processes during which thermal fluids degrade or break down are thermal cracking and thermal oxidation.

Thermal cracking is the process that takes place when heat is applied under pressure to the thermal liquid. It results in the breaking of the fluid's molecules into smaller molecules. This chemical reaction releases several types of fractions, depending on the type of oil in question. The first by-products of thermal cracking are light ends (also called light distillates or low boilers). These fractions have a low boiling point and are highly volatile.

The production of light ends is inevitable, because it's the chemical consequence of heating up thermal fluid. The quantity of light ends in the system has to be constantly monitored, because their presence lowers flashpoints and can create dangerous situations in the event of a loss of containment. DSEAR/ATEX 137 law requires flash points to be managed by taking regular and representative samples, and submitting them for expert advice. This goes hand in hand with surveys that demonstrate a safe system and knowledgeable staff or operators. 

The flash point is the minimum temperature at which the vapour released from a liquid will ignite in the presence of an external ignition source and oxygen. Flash points vary depending on the type of thermal liquid used. 

Because of their volatility, light ends have to be managed and removed, despite the fact that this causes thermal oil levels to shrink. There are several ways of managing and removing light ends. For instance, good system ventilation maintains steady flash points. Alternatively, Global Heat Transfer’s Light Ends Removal Kit (LERK) is an entirely automated system that removes light ends and manages flash points, fire points and auto ignition temperatures by a process of continuous distillation.

The only downside to managing and removing light ends is the loss of thermal fluid from the system. This also causes the existing thermal oil in the system to be worked particularly hard, which means its deterioration rate can grow exponentially. To avoid this situation, it’s important to top up thermal fluid as often as necessary.

When the thermal fluid level is very low in the system, the first priority is to investigate why it has decreased. The reasons could be anything from a high number of light ends to leaks. Finding and fixing whatever caused thermal fluid levels to drop is an essential step before topping up. After this is achieved, there are several ways of adding more thermal oil to the system, depending on the situation.

Topping up

The best time to top up thermal fluid is after a system shutdown. Topping up can also be done while the system is operational, but only after certain precautionary measures have been taken. The best practice is to contact your thermal fluid provider and get a specialist team to take samples and check the condition of the oil before topping up.

Topping up thermal fluid is an essential, yet very simple operation, which can be very useful, save costs and precious time, especially when production is resumed after a system shut-down. 

When thermal fluid levels are low, the existing fluid is worked excessively hard, which could lead to system breakdowns. To avoid this, thermal fluid top up should be facilitated as often as possible. To ensure you get the optimal support for your needs, the best thing to do is let your thermal fluid provider know at least a month in advance before total system shut-down. 

The name of the hottest place on earth, the Mponeng mine, means ‘look at me’ in the local Sotho language. Sometimes, the only way to detect system issues or weaknesses is by taking a close look at the hot and unwelcoming environment of heat transfer fluid. Regular thermal fluid testing and analysis is essential throughout the year, and becomes particularly relevant during shut downs, because it entails a comprehensive inspection of the system. This allows you to top up thermal fluid levels whenever necessary, ensuring you have a fully working and efficient system.

Andy Burns is technical business manager at Global Heat Transfer, Stone,  Staffordshire, UK. 

Fig. 1 (GHT052 - Hot or cold don't forget to top up - two - OP.jpg) The header tank on a thermal fluid systems.

Fig. 2. (GHT052 - Hot or cold don't forget to top up - three - OP.jpg) A low level switch (removed for cleaning) shows a build-up of carbon as a result of oxidisation.

Author Pic (GHT052 - Hot or cold don't forget to top up - one - OP.jpg) and Quote: “Thermal cracking is the process that takes place when heat is applied under pressure to the thermal liquid. It results in the breaking of the fluid's molecules into smaller molecules.”
Andy Burns, 
Global Heat Transfer