Usually the expansion tank is installed at the highest point of the system and is connected to the suction side of the pump. It may also be connected to the main circulating loop at the lowest pressure point. It should serve as the main venting point of the system, as well as provide for fluid expansion, which can be 25-30% of the total system volume. Actual fluid expansion volume depends on the physical properties of the fluid selected and the operating temperature range.
All expansion tank vent lines must be routed, preferably via a cooled condenser, to a safe external location, so that vapor may not enter working areas. The normal design choice will be a double-leg expansion tank which provides higher flexibility in normal operation than a single-leg expansion tank with degassing tank and temperature buffer tank. With careful attention to design, particularly to venting systems for non-condensable and water, both single-leg and double-leg designs may be used and can provide satisfactory service.
Low boilers and moisture must be collected in a cold-seal trap, and the contents of the trap must be discarded periodically as part of routine operating procedures.
An effective way to minimize fluid oxidation is to blanket the system with an inert gas such as nitrogen. In small systems the nitrogen may be replaced by a cold-seal trap or an expansion leg filled with system fluid maintained at low temperature.
Before a new system is started, a wire mesh strainer should be installed in the pump section. These strainer baskets may be removed after debris removal from the startup.
When operating where solids or contaminants might enter or be generated in the heat transfer system, it is advisable to install a high-temperature filter bypass line that can be positively isolated with valves for periodic cleaning or replacement.
Filter elements are commonly glass fiber string-wound cartridges or sintered metal filters in the 5-20 micron range. These filters require a significant pressure drop between the inlet and outlet of the bypass.
Recommended flange gasketing for high-temperature heat transfer fluid systems is spiral-wound or graphite types conforming to API 601 and DIN 4754 specifications.
Standard materials for spiral wound flange gaskets are type 304 stainless steel and pure graphite. To avoid leaking with spiral wound gaskets, it is important to use raised-face flanges, allowing steel bolting and even compression of the gasket during bolt tightening. Graphite gaskets are an acceptable alternative for many applications.
Generally, sheet gasketing with various binders can be unacceptable for Therminol 66 and some other fluids because of incompatibility of the binders with these fluids.
The heater may be electrical, fuel-oil or gas-fired, and is the most critical component in designing a heat transfer system for use with Therminol fluids. With the proper balance of heating capacity, temperatures and fluid velocity, the service life of the heat transfer fluid is increased to an optimum level. Another important factor for good life is that systems must be protected from contamination by foreign materials.
Two basic designs of fired heaters for use with Therminol fluids are liquid tubes and fired tubes. In liquid-tube heaters the heat transfer fluid is pumped through the tubes as it is heated. The hot gases pass outside the tubes. In fired-tube heaters the fluid flows through the heater shell with hot gases passing through the tubes.
When bulk fluid temperatures higher than about 240°C (460°F) are required, a liquid-tube heater must be used unless a specific heater design is devised to force a uniformly steady turbulent flow of liquid over the fired-tube surfaces.
Most Therminol fluids are liquid when transferring heat. To avoid hot spots in the heater, the fluid should be pumped over or through the heating surfaces at sufficient velocity so that no area of fluid stagnation occurs. Since heating is not perfectly uniform in fired-tube heaters, the maximum heat stress conditions must be calculated to determine what film temperatures will be encountered.
Fluid velocities over heat transfer surfaces must be relatively high to develop turbulent flow. This helps to avoid excessive film temperatures that may be detrimental to heat transfer surfaces and to the fluid. The heater manufacturer should be consulted for the required flow velocities.
Organic heat transfer fluids, such as Therminol fluids, have a slow oxidation reaction with air in the presence of insulation materials when the fluid temperature is above 260°C (500°F). Porous insulation, such as calcium silicate, offers a larger reaction surface with poor heat dissipation, which, along with possible catalysis from the insulation material, can cause a temperature buildup. This temperature rise may result in ignition of the fluid when the saturated insulation is exposed to air, such as for repairs.
This phenomenon is not fully understood but appears not to occur with cellular glass, possibly because of its closed cell structure. Cellular glass should be used in all areas where leakage is a possibility. The principal leakage areas usually are near instrument connections, valve packing glands, flanges and other sealed surfaces. As a precaution, eliminate any source of leakage promptly. Replace leaky gaskets and oil-soaked insulation, and repack valve stems. Cover insulation where leaks might occur with metal covers. Install valves, where possible, with the stems in a horizontal position so that leaks will drip away from the insulation.
The piping layout for systems using Therminol heat transfer fluids should be sized to provide the normal required flow rate at an economical pressure drop.
Because the system will undergo temperature changes, adequate flexibility to relieve thermal expansion and contraction stresses is essential. Schedule 40 carbon steel pipe or equivalent should be used throughout the system. The tendency to leak through joints and fittings is a characteristic of most organic fluids unless these fittings are very tight.
The best way to prevent piping leakage is to weld all connections. Where access is necessary, raised-face flanges with weld neck joints are recommended.
To help insure good seating and sealing of the spiral-wound gaskets recommended for Therminol fluid piping, this procedure should be followed:
Clean flange faces free of loose rust and dirt. Remove any weld spatter. Assure that the flange faces have no gouges or grooves and that they are aligned properly, since gaskets cannot correct for these problems.
Check alloy stud bolts and nuts to assure they are clean and free of rust and thread shavings, and lubricate the threads. The bolting stress and torque are defined by the supplier of the gasket. The torque is also a function of the diameter and the thickness of the gasket.
Torqueing is made by tightening opposite studs to the required torque values using small increments. Tighten studs in the sequence 9, 3, 6 and 12 o'clock, and repeat with adjoining studs.
Pumps must have enough capacity and pressure head to circulate the fluid at the required rate through the system. Pumps are generally centrifugal, seal-canned, glandless, or magnetically driven. They must conform to appropriate standards. The pump housing may be cast steel for most systems, or may be made of other appropriate materials for very low or high temperatures.
Pump manufacturers usually specify, for temperatures higher than 200°C (390°F), either water-cooled ring seals or, preferably, fluid-cooled stuffing or air-cooled, extended shaft seal and bearing.
On pumps with a stuffing box, at least five rings of laminar graphite packing should be provided. Inert blanketing of the seal with steam or nitrogen eliminates deposit formation from oxidation, which can lead to seal leakage. A secondary seal provides additional safety in case of sudden seal failure.
Regardless of the type of pump selected, the flow rate should be checked regularly against the pump characteristic performance curve originally supplied. To prevent alignment problems and seal leakage, it is important to avoid pipe support stresses on the body of the pump. Each pump should be fitted with a control device to switch off the heat source in case of pump failure. If expansion loops are used in the pump section piping, they should be horizontal or vertically downwards. Loops should not be vertically upwards because this forms a trap which can collect air and vapor that seriously hampers pump performance.
Forged-steel valves with deep stuffing boxes are satisfactory for systems utilizing Therminol fluids. Gate and globe valves with an outside screw should be used throughout the heat transfer system. Gate valves do not always provide an absolutely tight shutoff.
Various types of packing are used to seal valve stems on high-temperature systems, and generally five rings are specified on valve stems to assure a good seal. Valve stem bellows will provide virtually leak-free operation.