When planning routine maintenance shutdowns of thermal fluid systems, plant engineers should prepare for a smooth procedure without incident. It pays to remember that prior preparation prevents poor performance.
Routine maintenance shutdowns of thermal fluid systems involve a staged process before, during and after the operation to reduce the amount of idle time incurred in the plant. Some jobs need to be organized well in advance to get the right equipment in place as well as replacement components. This allows each task to be run more efficiently and avoids having to shut the plant down more than once.
Three months prior to shutdown, plant engineers should check pressure relief valve tags and certificates for when they are due for testing. If the test is due within a 12-month period of the next planned shutdown, arrangements should be made for testing or replacing. The system will need to be partially drained for this to be carried out. During the shutdown, the valve can be tested, tagged and the certificate retained.
Two months prior to shutdown, the thermal fluid should to be tested to allow sufficient time for reports to be returned and quotes for any thermal fluid work to be carried out. Heater, burner and flue servicing should be arranged, either through the thermal fluid manufacturer or a servicing contractor.
A full check of the thermal fluid system should be carried out. Inspectors should look for slow leaks where there are joints in the pipework and consider fitting spray guards to prevent splashes from high pressure leaks.
The areas that should be checked for leaks include valves, gaskets, pressure gauges and pumps. This will allow for correct replacements to be ordered in time. These checks will allow for repairs to be made during the routine shutdown.
One month prior to shutdown, arrangements should be made for thermal fluid to top up the system. For many companies, this is the only planned shutdown for the year —and the only time the thermal fluid is cooled to an ambient temperature. Once cooled, thermal fluid can reduce its volume by up to 30 percent (depending on fluid type) from its hot volume, taking the system off the low level and requiring topping up.
At this stage, if not before, plant engineers may want to notify their maintenance contractors that work is being carried out on the system, and that assistance will be required to drain, flush and refill it, or to supply fluid.
During the main shutdown, the header tank should be checked for cleanliness when the system is drained. Plant engineers may need to be prepared for urgent cleaning of the header tank to remove residues that may have built up. High and low level fluid sensors also need to be checked as well as pipework for correct working flow.
Dump and overflow tanks need to be checked for their contents and arrangements made for them to be drained. Plant engineers also should inspect strainers and clean filters if fitted. If no filters are fitted, they should consider refitting them after any work on the system has been carried out to protect the pumps. After running the system cold for a few hours, the filter can then be removed and the system can run as normal.
Vents and drains on the system should be checked for blockages and cleared. On header tanks that include a thermal buffer section that stops hot oil coming in direct contact with the fluid in the header tank, any water from the thermal buffer side of the header tank should be drained. This helps prevent violent thermal expansion of the oil. These systems include a drain on the thermal buffer section of the tank that can be used to drain water that has collected.
The expansion tank is critical to the operation of the system. Its main purpose is to allow a place for the heat transfer fluid to expand into when heated, and a source to draw fluid away from to keep the system full when cooled. It also has a built-in reserve tank in case there is a small leak as it keeps the system full.
It is important to monitor the level in the expansion tank. If the level drops from its normal operating position, it may mean that a leak has developed somewhere in the system. Some general rules of thumb are to fill the expansion tank up to low level when the system is cold and then top up the system between 1 quart and 130 gal (1 to 500 l), depending on system size, to give the system a “head” of oil.
The temperature of the expansion tank should be less than 140°F (60°C) to prevent oxidation by the air inside the tank. To reach this temperature, a nitrogen blanket could be installed on the head of the tank to remove any oxygen molecules and prevent oxidation.
The timescale for a drain down would depend on the system volume, and the maintenance provider should aim to complete a drain down within a half day or a full day for a large system. This would include time taken for the replacement of valves and a refill of the system.
Draining the system can be rather messy and time consuming, but by shutting down the heater and letting the pump continue to circulate the fluid through the system, the fluid can be cooled to a safe temperature for draining. A separate pump and not the system’s pumps are required to drain the fluid into drums, an IBC or a tanker.
The system will drain faster if the oil is still warm because the viscosity will be lower. This has the added benefit of removing most of the sludge and loose particles from the system. Caked-on or carbonized material will not be removed. To do this, the system should be cleaned with a proprietary cleaner and flushing fluid.
Some thermal fluid detergents allow a combination of flushing and cleaning. The dual action works to rid heat transfer systems of potentially harmful contaminants such as old or oxidized residual fluids, carbon deposits, loose debris, water and volatile light ends.
Refilling the system also requires the use of a separate pump to draw new fluid from a container, making sure that all drains and vents are closed and resetting all the valves to the same positions as previously recorded. The system pump can subsequently be started without applying heat, allowing the fluid to circulate and any air pockets to be removed.
Once the system is circulating at the proper level in the expansion tank, heat can be applied in 27°F (15°C) increments until the heat transfer fluid reaches 221°F (105°C). If the primary pump starts to cavitate, there is water in the system, and increasing the temperature to 239°F (115°C) will boil off the water. This could take anywhere between a few hours and a number of days. Alternatively, if a light ends removal kit is installed, it can be used to remove water more efficiently.
After all signs of water in the system have gone, the heat may be turned up to 257°F (125°C) and gradually increased in steps to 284°F (140°C). Having established that there is no water present, a nitrogen feed can be reinstated. Only after these measures have been taken is it safe to turn the heater up to normal operating temperature.
Following shutdown, engineers should replace any lagging or jackets that may have been disturbed and replace any used spill kits, taking care to dispose of any waste fluid, and to replace any stocks of thermal fluid used. Spill kits are used in an emergency and require the use of personal protective equipment, irrespective of the size or nature of the spill.
Absorbent socks are the most important item in a spill kit. They are used to form a temporary bund or dam to contain fluid. Pillows also are used to stem fluid flow in a confined area or as an ad-hoc drain blocker. Final absorption of a spill should be carried out using pads once containment is complete. They should be left in contact with the fluid for about two to three minutes or until saturated. They will hold about a quart (or liter) of fluid, depending on viscosity. An approved waste disposal contractor should dispose of the used absorbent material.
Ultimately, if engineers remember the five Ps of maintenance shutdowns — prior preparation prevents poor performance — this complex process will become manageable. Shutdowns may become so routine and seamless that the interruption becomes a productive part of the process. Good planning will save the costs of downtime and, in the long term, reduce maintenance bills.