Flush before filling

~ The importance of a system flush ~

Steve Martin is famous for stating that “A day without sunshine is like, you know, night.” Steve is not wrong in quoting the obvious. Dr Chris Wright, head of research and development at Global Heat Transfer (GHT) explains how stating the obvious could help maintain the health of heat transfer fluid in the foods,pharmaceuticals and chemicals manufacturing.

An empty system needs flushing and cleaning prior to being filled with fresh heat transfer fluid. There is a range of heat transfer fluids on the market that can be used at high and low temperatures. Some of them offer fast, dual acting flushing and cleaning that works by clearing a heat transfer system of contaminants, be they old or oxidised residual fluids, carbon deposits, loose debris, water or volatile light ends.

At some point in the life of a thermal fluid, manufacturers will need to clean out the content of their system and allow for maintenance activities to take place. In such circumstances, the ‘cleanout’ procedure is clearly defined by the manufacturer or distributor of the flushing fluid.

Most cleanout procedures entail draining the system and then refilling with a cleaning or flushing fluid. This fluid is then circulated in the system at 108-122 degrees Celsius (225-250 degrees Fahrenheit). This temperature range enables an existing fluid to be mixed with suspended loose particles which can then be circulated and ultimately drained out.

Once the cleaning or flushing fluid has been drained, the system will be refilled with a virgin heat transfer fluid or even a second fill of the cleaning or flushing fluid, depending on the condition of the fluid drained from the system. This is ultimately determined by laboratory analysis which is used to assess the levels of carbon and particles in the heat transfer fluid. This procedure is common practice for experts working in the area of heat transfer.

What is immediately obvious is that this procedure assumes the system is filled with a heat transfer fluid and ready to be drained. But what happens if the system is already empty as is the case with newly commissioned systems? The answer is simple; all systems need to be flushed and cleaned before fresh heat transfer fluid is introduced. This is extremely important as any welding slag, ferrous material or yellow metals which have not been removed during manufacture could effectively catalyse the oxidation of the heat transfer fluid and in turn, accelerate the ageing of the fluid.

The net effect is a potential reduction in the overall working life of the heat transfer fluid. At GHT we consider this to be stating the obvious, but others may not support this position. To support GHT’s position we will investigate the evidence from a recent project where GHT was commissioned to fill a new thermal system with a fluid capacity of 2,000 litres. The engineering company that built the system was commissioned by Babcock Wanson UK.

The new system was filled with Globaltherm C1, the pump was started and the temperature rose to between 108 and 122 degrees Celsius. The heat transfer fluid was then collected to visually inspect its colour and clarity. At the same time, the system strainers were examined and checked for fabrication debris.

This inspection revealed that fabrication debris had collected in one of the system’s strainers measured roughly 10 x 8 x 1 cm (length x breadth x height) in size. This represents a volume of 0.08 litres and equates to 0.004% of the system’s total volume. More worrying was the size of individual pieces of sediment, which measured around 1cm in diameter.

Heat transfer fluids and systems are built to transfer heat from one part of a system to another. However, this is not their only function as they work to drive away contaminants and wear particles from critical components of a system, such as pumps and heaters.

Thermal fluids provide a physical function and help to protect against system wear. The current inspection revealed that fabrication debris was circulated through the system and was caught in the system strainers. It is important to remember that not all debris will collect in the system strainers and some will pass through the mesh and remain in the system’s circulation. A further factor to consider is that the strainers used in this particular type of system build were only temporary and are removed once the system is operational.

The circulating debris in any system at any stage of its lifecycle is a cause for concern. High levels of debris in a heat transfer fluid can lead to the erosion of pipework as well as accelerating the wear of mechanical pump seals and the oxidation of the heat transfer fluid.

Circulating debris can also have other consequences if not managed properly. For instance, at low circulating speeds, larger particles settle out from the circulating thermal fluid. This leads to the formation of a sediment layer and increases in thermal resistance and reductions in heat transfer capacity and effectiveness. Large particles also lead to an apparent increase in fluid viscosity and this can cause pressure drops across system elements such as heat exchangers. The effect being a decrease in thermal efficiency and an increase in the energy required to pump the heat transfer fluid. In more extreme cases may be even clogging-up of the system. At high circulating speeds, large particles carry more momentum and kinetic energy and this can cause damage to the internal pipe work and accelerate erosion.

If the suspended material in a heat transfer fluid reaches over 5% of the total system volume, the fluid is no longer suitable for use. This emphasises the need to assess debris as part of a planned, preventative monitoring programme. Revealing the levels of contaminants and loose debris can help minimise unnecessary energy losses, production losses and whole system cleanouts, as well as potentially preserving the lifetime of the fluid.

Flushing prior to filling or refills is an effective way of removing particulates and contaminates. The effectiveness of a single flush in the removal of particles and contaminants has been proven time and again. Comparison of flushed values with non-flushed values shows a significant reduction, with values after the flush being between 0.1 and 0.2% of the initial starting values, thus representing 490 to 1000-fold reduction in particles per millilitre.

Fluid cleanliness is critical in establishing the reliability of equipment. Indeed, the manufacturers of patch testing kits have reported a direct correlation between oil cleanliness and component life.

The current case emphasises the importance of flushing any system before filling it with fresh heat transfer fluid. Flushing liquids such as Globaltherm C1 are effective in the removal of fabrication debris and other contaminants. This is essential in sustaining the longevity of thermal fluids and systems.