This year, I have been discussing the components that make up a thermal fluid system. Past articles in this series have discussed the selection of the thermal fluid -- still the most important decision -- as well as specification of heaters, specification of pumps and the expansion tank. In this last installment, I will “tie the system together” with a brief discussion on piping materials, valves and insulation.
In my practice, I often have the opportunity to evaluate and troubleshoot thermal fluid systems that are not performing to the owner’s expectations. When system does not deliver heat and the heater and circulation pump are adequately sized, I generally start looking at the fluid-delivery components, including the piping and valves, to determine if they are impeding the efficient delivery of fluid to the process.
Piping DesignToo many thermal fluid piping systems are “designed” by the piping contractor, who may use a pipe diameter that he feels is appropriate and route the pipe in the most convenient (not the most efficient) manner. This often results in a poorly performing piping system. It is important to remember that proper pipe sizing is determined not only by the flow rate, but also by the length of the piping runs. Fittings and valves add “equivalent length” to piping systems, which adds to the fluid pressure drop in the pipe. Longer runs of pipe may require a larger pipe diameter to keep pressure drop from becoming a problem.
There are two principal reasons to design the piping system on paper:
- Laying the piping out allows the owner to accurately
determine the length of pipe and the number of fittings. This allows the
engineer to choose the proper pipe diameter with known pressure drop, allowing
proper pump sizing.
- Pipe grows significantly as it gets hotter, and thermal fluid systems
exhibit significant thermal expansion because of their higher operating
temperatures. Thermal expansion can generate significant forces that can have a
negative impact on pumps, process equipment and even structural steel.
Piping MaterialsSelection of the proper piping materials is important for control of leaks and for fluid life. The owner should develop (or have developed) a piping materials specification for thermal fluid service. A piping materials specification helps ensure that the thermal fluid piping will be installed with proper materials and that any future additions or modifications to the system will be consistent with the original construction materials and techniques.
A piping materials specification also allows the owner to demonstrate that recognized and generally accepted good practice was followed in the construction of the thermal fluid piping system.
As a minimum, the specification should have detailed information on:
- The proper schedule and material of the pipe.
- The proper methods of joining pipe.
- What fittings to use (including a branch connection table for
constructing branches of smaller pipe size).
- What valves to use and their construction.
Items and practices to avoid include (but are not limited to):
- Do not use copper, brass or copper-containing alloy
tubing, pipe, valves or fittings. Copper is an active metal and can catalyze
the degradation of the thermal fluid.
- Avoid threaded
connections. The properties of most thermal fluids cause them to be prone to
leakage. The threads are just a way out in many cases.
- Avoid the use of
composition (i.e. sheet type) gaskets.
Consideration for valves for thermal fluid service should include specifying the proper materials of construction, specifying the proper end connections, specifying gaskets and packing that mitigate the tendency of the thermal fluid to leak, and choosing the appropriate valve design for the particular point in the piping system and the intended use.
Materials. Valve bodies should be forged steel for smaller valves and cast steel for larger valves.
End Connections. Welded end connections are preferable as they eliminate flanges, which can leak.
Packing and Gasket Materials. Here again, carbon-based packing materials and gaskets are preferable over other materials. Consideration to live loaded packing should be given to larger valves.
Valve Design. This subject could be its own article. In general, straight-through valves, including gate valves and certain ball and butterfly designs, are used in on-off isolation service. Globe valves are used where the flow is throttled. The choice of valve design should be evaluated for each service. Most systems use ANSI spec gate valves for on-off service and ANSI spec globe valves for throttling service or where tighter shut-off is required.
Bellows-Sealed Valves. Bellows-sealed valves do not leak through the stem. Bellows-sealed valves are useful choices where:
- Personnel safety is a concern.
- The fluid may contain components that are listed under OSHA.
- Odor is a concern.
Safe Practices. In systems where a component may have to be isolated for service while the rest of the system is operating, the owner may wish to give consideration to the use of double-block and bleed valve arrangements. Such an arrangement places two valves between service personnel and the hot thermal fluid. The bleed valve directs any leaks from the primary block valve to a safe (or safer) location.
Items and practices to avoid include (but are not limited to):
- Do not use copper, brass or copper-containing alloy valves.
Copper is an active metal and can catalyze thermal fluid degradation.
- Avoid threaded connections. The properties of most thermal fluids
cause them to be prone to leakage. The threads are just a way out in many
- Avoid the use of cast-iron valves, which can crack if thermally
- Avoid the use of 150# cast valves or valves with 150# flanges.
InsulationInsulation material should be chosen with several factors in mind.
First, the insulation material should have a rated temperature that is higher than the maximum temperature attainable by the thermal fluid. While this seems like it should go without saying, I have seen systems installed where lower temperature insulation was installed. This usually occurs through poor communication with the contractor and the lack of a formal thermal fluid insulation specification. Suitable materials for insulating thermal fluid equipment are calcium silicate or glass-foam materials.
Fiberglass insulation should never be used to insulate thermal fluid systems. Fiberglass insulation has a very high surface area and can reduce the flashpoint, firepoint and autoignition temperature of the fluid if they contact one another. If thermal fluid leaks onto fiberglass and is in contact with air, a spontaneous fire can result.
Second, in sections of the piping system containing flanges, valves, pumps or other components that can leak, the insulation should be a material that will not soak up the insulation and swell. The choice here is to use a glass-foam material.
Third, the insulation material needs to be covered with a metal jacket. Do not use plastic or other low temperature jacket materials.
Finally, in order to protect personnel, overhead flanges and valves, which have the possibility of leaking, should be enclosed in a flange shield or other protective cover that will collect any drips and direct them to a safe location.
When connecting the thermal fluid system components, the effort spent in properly designing the piping system, checking for pipe stress, choosing the appropriate piping valve and insulation materials and employing safety concerns to reduce the possibility of fluid contact with personnel will result in a system that meets the process requirements, operates with less maintenance and is safe for operators and maintenance personnel.
SidebarUse the links at the bottom of the page to
continue reading this six-part series on specifying a thermal fluid heating
system. You've just finished:
Specifying a Thermal Fluid System: A 6-Part Series
Part 6: Piping Materials, Valves and Insulation
Other parts in this series include:
Part 1: Choosing the Thermal Fluid
Part 2: Fired Thermal Fluid Heaters
Part 3: Electric Thermal Fluids Heaters
Part 4: Thermal Fluid Pumps
Part 5: The Expansion Tank