HTF System

Usually, the expansion tank is installed at the highest point of the system and connected to the suction side of the pump. It can also be connected to the primary system part of the system at the point where the pressure is lowest. It must serve as the main discharge point of the system and allow the expansion of the fluid, which can represent 25-30% of the total volume of the system. The actual expansion volume of the liquid depends on the physical properties of the chosen liquid and the operating temperature of the installation. All expansion vessel outlet pipes must be installed in such a way, preferably via a cooled condenser to a safe outdoor location, that condensation vapour cannot enter the work area. The normal design choice will be the expansion tank with double connection which, usually, offers more flexibility than a single connection expansion tank with degassing tank and heat buffer tank. With particular attention to design, especially for systems for degassing non-condensable liquids and water, both systems can be used (single or double) and can operate perfectly.

Low boiling point fractions and moisture must be collected in a cold seal tank. This tank must be regularly drained and checked as part of the facility’s regular maintenance program.

An effective way to minimize oxidation or degradation of the thermal fluid is to fill the vacuum space with an inert gas such as nitrogen. In small installations, nitrogen can be replaced by a cold seal tank or by an expansion pipe filled with system fluid and maintained at low temperature.

Before using a new thermal installation, a strainer must be installed in the pump section. These coarse filter screens can be removed after removing the waste (metal particles, welding droplets, nuts, etc.) at start-up to avoid direct damage to the pump(s). After this start-up period, it is recommended to use a fine by-pass filter to collect and remove oil degradation products.

The filter elements are generally glass fibre cartridges resistant to 300°C or sintered metal filters for higher temperatures with a fineness of 5 to 20 microns. These filters involve a significant pressure drop between the inlet and outlet of the bypass.

The recommended flange seals for high temperature thermal transfer systems are spiral wound or graphite type that meet API 601 and DIN 4754 specifications.

Standard materials for spiral wound flange seals are 304 stainless steel and pure graphite. To avoid leaks with spiral joints, it is important to use well flanged flanges, which allows the use of steel bolts and proper compression of the joint material during tightening. Graphite seals are an acceptable alternative for many applications. In general, film joints containing different binders are not suitable for Fragoltherm® 660, Therminol® 66 and some other fluids due to the incompatibility of the binders that are dissolved by Fragoltherm® 660 and Therminol® 66 as well as some other fluids.

The boiler can be electric, oil or gas-fired and is the most critical component in the design of a heat transfer system for use with Therminol fluids. With a good balance between heating power, temperatures and liquid velocity, the service life of the heat transfer fluid is increased to an optimal level. Another important factor for the useful life of the liquid is that the systems must be protected against contamination by foreign matter.

The two basic boiler models for use with Therminol fluids are those with liquid tubes and those with hot tubes. In the first type, the oil is pumped through the tubes while the hot gases circulating around the tubes heat the tubes, and the other is exactly the opposite, the hot gases pass through a system of tubes that bathe in the oil in order to transfer the heat.

When the mass temperature exceeds approximately 240°C (460°F), the first system must be used, unless a particular design of the heating tubes is designed to force a turbulent and constant flow of liquid over the heat tube heat exchanger surfaces.

Most Therminol® fluids are in liquid phase during heat transfer. To avoid “hot spots” in the boiler, the liquid must be pumped on or through the exchange surfaces at a sufficient speed so that there is no liquid stagnation. Since heating is not perfectly uniform in gas systems, maximum thermal stress conditions should be calculated to determine which extreme surface temperatures will be encountered.

The speeds of the liquid on the heat transfer surfaces must be relatively high in order to develop the correct turbulent flow. This avoids excessive heat transfer surface temperatures that could be detrimental to heat transfer surfaces and/or liquids. The boiler manufacturer must be consulted for the required flow rates.

Organic heat transfer fluids, such as Fragoltherm® and  Therminol® liquids, have a slow oxidation reaction with air in the presence of insulating 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, together with the possible catalytic reaction of the insulating material, can cause an increase in temperature. This temperature increase can cause the liquid to burn when the saturated insulation is exposed to air, for example for maintenance. This phenomenon is not fully understood, but does not seem to occur with glass foam, perhaps because of the closed cell structure. Foamed glass should be used in all areas where leaks are possible. The main leakage areas are generally located near instrument connections, valve seals, flanges and other sealed surfaces. As a precautionary measure, immediately eliminate all causes of leakage. Replace leaking seals, oil-soaked insulators and recondition valve jams. Cover the insulation where leaks can occur with metal coatings. If possible, install the valves with the pins in a horizontal position so that leaking oil can flow away from the insulation.

The layout of the piping using Therminol® heat transfer fluids must be designed to achieve the required normal flow rate by maintaining a permissible pressure drop.

As the system will experience temperature variations, sufficient flexibility to expand and contract thermally is essential. Carbon steel pipes (40) or equivalent must be used throughout the system. The tendency to leak through seals and fittings is a feature of most body fluids, unless these fittings are perfectly adjusted.

The best way to prevent leaks in pipes is to weld all fittings as much as possible. When access is necessary, it is recommended to use non-welded flanges.

To ensure that the recommended spiral seals for Therminol fluid lines are properly installed and sealed, the following instructions must be followed:

Remove rust and dirt from the flange surfaces, remove welding splashes, ensure that the flange faces do not have protruding parts or grooves and are properly aligned, as the joints cannot correct these problems.

Check that bolts and nuts are clean and free of rust and debris, and lubricate the threads. The tightening torque of the bolts is determined by the joint supplier. The torque is also a function of the diameter and thickness of the joint.

The tightening torque is obtained by tightening the opposite nuts/bolts in small successive steps up to the required torque values. Fasten the nuts/bolts in order 9, 3, 6 and 12 hours and repeat the operation with the adjacent nuts/bolts.

The pumps must have sufficient capacity and pre-pressure (static system pressure) to allow the fluid to flow through the system at the required speed. The pumps are generally centrifugal pumps, whether they are sealed jammed systems or magnetic drive pumps, they must all comply with the required standard(s). The pump casing can be made of cast steel suitable for most systems or can be made of other suitable materials, designed for use at very low or very high temperatures.

Pump manufacturers generally prescribe water-cooled sealing rings or, preferably, liquid or air cooling with an elongated shaft seal and bearing for temperatures above 200 °C.

At least five lamellar graphite rings must be present on pumps with compression seal. The inert coating of the joints by steam or nitrogen process eliminates the formation of oxidation material deposits, which could lead to a leak at the joint. A second seal provides additional safety in the event of untimely sealing problems.

Regardless of the type of pump chosen, the flow rate must be checked regularly and compared with the characteristic performance curve of the pump originally supplied. To avoid alignment problems and leaks, it is important to avoid improper positioning of the piping, which can cause tension on the pump bearings. Each pump must be equipped with a temperature sensor to deactivate it in case of a problem. If expansion loops are used in the piping of the pump part, they must be oriented horizontally or vertically downwards. The loops should not be mounted vertically upwards as this forms a trap that can collect air and vapours that can seriously hinder the pump’s performance.

Forged steel valves with deep cable glands are sufficient for systems with Fragoltherm® and Therminol® liquids. Butterfly and ball valves with external screw must be used throughout the heat transfer system. Butterfly valves do not always provide an absolute seal.

Different types of blockages are used to seal the valve rods in high temperature systems and, in general, five rings are recommended on the valve rods to ensure a good seal. The bellows rods ensure virtually leak-free operation.