Although an easy-to-forget component in a thermal fluid heating system, the expansion tank is vitally important to reliable system operation.
The expansion tank is a very important and oft-ignored component of the thermal fluid system.
Most owners pay attention to the other components in the thermal fluid system. They maintain the combustion system on the heater, maintain alignment and mechanical seals on pumps, maintain instrumentation and control valves, and inspect heat exchangers. They may even have their thermal fluid tested regularly, but because it is a static piece of equipment, these same people often neglect the expansion tank.
In my consulting practice, I often visit sites where there are complaints centered on startup difficulties and fluid degradation problems. On one visit, when I asked to take a look at the expansion tank, it took about 15 minutes to find it! When we did find the expansion tank, it was obvious that no one had paid attention to this piece of equipment for a long time. It was no wonder that this facility was having problems.
Far from being a forgotten component in the system, the expansion tank is of vital importance to the reliable operation of the system. While the expansion tank is a static piece of equipment, it still needs to be monitored and operated.
Purpose of the Expansion Tank
The expansion tank serves four distinct and equally important roles in the thermal fluid system:
- Ensuring that the system is completely flooded.
- Serving as the reservoir for the heat transfer fluid to expand into as the fluid heats up.
- Separating water and other non-condensable species from the fluid on startup.
- Controlling the environment between the heat transfer fluid and the atmosphere.
I’ll take a closer look at each function.
Ensuring That the System Is Completely Flooded. It is important that the thermal fluid system be completely flooded with fluid. Any bubbles will affect the performance of the pump, can cause overheating in the heater coil, and can negatively affect heat transfer in the heat users. To this end, the expansion tank should be located at the highest point in the system.
If design constraints prevent the proper location of the expansion tank, it can be located lower, even at grade, but the tank must be pressurized with nitrogen (or other inert gas) to provide enough pressure to force the fluid to the high point. It should be noted that systems with low-mounted expansion tanks may not be as easy to fill or de-gas as systems with properly located tanks.
Expansion Reservoir (Sizing the Expansion Tank). In the part one of this series (“Choosing the Heat Transfer Fluid,” January 2007, p. 32; see Related Sites at the bottom of the page for online link to article), I stated that choosing the heat transfer fluid is the single most important decision made in specifying a new system. Here is one reason why: Depending on the temperature of the system and the heat transfer fluid chosen, the fluid can expand in volume from as little as 20 percent to more than 35 percent. The owner needs to have the following information to properly estimate the required size of the expansion tank.
- The thermal fluid to be used and its coefficient of thermal expansion.
- Minimum (startup) temperature.
- Maximum (design) temperature.
- Total volume of the thermal fluid system.
With this information, the total expansion of the thermal fluid, in gallons, can be calculated. Ideally, one would want the expansion tank to be about one-quarter full with the system ready to start up and about three-quarters full with the system at full design temperature.
Figure 1 shows a simple diagram of an expansion tank with two pipe “legs” for expansion. Note the existence of a valve on the upstream leg and a block valve between the two legs. This arrangement allows the flow of heat transfer fluid in the system to be diverted through the expansion tank when desired. During startup, or if the presence of noncondensable contamination in the fluid is suspected, fluid is diverted to circulate through the expansion tank. Air bubbles, water (as steam), and any low-boilers will separate from the fluid, where they can be released from the system.
Note that the downstream leg of the expansion tank does not have an isolation valve. It is extremely important that the fluid always have a path to travel to the expansion tank and that no part of the system be isolated from that path while the system is operational. The high coefficient of expansion of heat transfer fluids can result in extremely high pressures with only a few degrees of temperature change.
Some manufacturers offer alternate designs to the double-leg expansion tank, and these designs work with varying degrees of success. The intent of these alternatives is to accomplish the operational requirements outlined herein and, in some cases, to differentiate that manufacturer’s product from others in the marketplace. These designs will not be evaluated in this article, but they should be evaluated by experienced personnel when decisions are being made regarding new thermal fluid systems.
Placement of the tank is important. It should be positioned on the main circulation loop on the suction side of the pump. In this location, it helps ensure that the pump has flooded-suction conditions, which makes system filling and de-gassing much more efficient.
Controlling the Environment at the Fluid-to-Atmosphere Interface. The owner, or his outside resource, must evaluate the thermal fluid chosen to determine what environmental controls need to be installed on the expansion tank. The evaluation should consider the following questions regarding fluid properties (as well as others, perhaps) to make a proper determination:
If the fluid has a high vapor pressure, low flashpoint, an odor, has listed components or is prone to oxidation, the owner will be well served to place the fluid under an inert gas blanket. In most cases, the inert gas is nitrogen, although I have seen carbon dioxide, argon, and combustion inert gas. The inert blanket provides the following benefits:
- The pressure can be adjusted to keep fluids with high vapor pressures from flashing.
- The inert gas blanket keeps oxygen away from the fluid and reduces the danger for fire.
- If the fluid is listed under OSHA or SARA, the fluid vent can be piped away to an appropriate collection point.
- If the fluid has an odor, the gas blanket can reduce the amount of fluid that may escape and cause an odor problem.
- If the fluid has a low resistance to oxidation, the inert gas keeps oxygen away from the fluid and extends fluid life.
Figure 2 shows the simplest arrangement for applying an inert gas blanket to an expansion tank. The supply regulator is set at the lowest pressure that will allow the system to run effectively, but generally at least 3 to 5 psig. The backpressure regulator is set for a higher pressure, generally about 10 psig higher than the supply regulator. This allows the expansion tank to build or lose pressure over a predetermined range (as the level in the tank goes up and down with expansion) and reduces inert gas consumption.
The small bypass valve shown on the backpressure regulator in figure 2 is for startup. This valve generally is a very small globe valve or a needle valve and is partly opened to vent any water and low boilers present out of the system in a controlled manner.
One problem that can occur with a dual regulator arrangement is that the backpressure regulator can occasionally experience high temperatures if hot fluid is in the tank and heats the inert gas to a very high temperature. One way to address this reliability issue is to install a pressure controller and replace the two regulators with small control valves. A single pressure transmitter can be used and either dual-pressure controllers or a single “differential gap” pressure controller with dual outputs can be used. The small control valves have reduced flow coefficients (CV) and admit or vent gas at the same rate as a discrete pressure regulator. While this arrangement is more expensive than regulators, it is more reliable, particularly when the ability of the control valves to “ride through” a high temperature event is taken into account. Figure 3 shows an inert gas control loop arrangement on an expansion tank.
Construction Standards for Thermal Fluid Heating Expansion Tanks
It is good practice to construct expansion tanks to a recognized code. These tanks endure temperature cycling and often contain liquid at a temperature above its flashpoint. For these and other reasons, it is in the owner’s interest to show that the expansion tank was built to a generally accepted code or standard.
In order to have traceability of the materials and procedures used to construct the tank, I specify that expansion tanks be constructed in accordance with ASME Section VIII, Division 1 of the pressure vessel code on projects where I have input. This requirement results in a tank constructed with materials and weld materials traceable to a national standard, and with welds made by a certified welder. A tank that will never be operated above one atmosphere positive pressure (14.7 psig) is not required to have any code stamps in most states; however, a tank that is manufactured to ASME Section VIII can be inspected and have a stamp applied for only a few hundred additional dollars.
It is important for the owner to determine for himself what local, state or other codes or standards apply at the point of use of the equipment when specifying any equipment for thermal fluid service.
Instrumentation and Appurtenances
The expansion tank is a component of the thermal fluid system and, as such, needs to be operated. As a minimum, the fluid level in the expansion tank should be recorded daily to monitor the fluid inventory. The level device can be as simple as an armored gauge glass or as sophisticated as an electronic level transmitter, but the fluid level should be monitored. Decreases can indicate loss of fluid due to leaks or volatility. Increases can indicate contamination from leaking heat exchangers, the process or other sources.
A low level switch (shown in the figures as LSLL) is installed on the tank and is connected to the heater shutdown circuit. This safety reduces the possibility of running the expansion tank out of fluid and allowing the heater run without fluid.
If the tank has an inert gas blanket, the pressure of the gas blanket should be monitored. If the inert gas blanket has a discrete source such as a high pressure gas bottle or a dewar of liquid gas, the inventory of the inert gas should be recorded as well.
Calibration of the pressure-relief valve (PRV) should be included in the facility’s mechanical-integrity program. The pressure-relief valve should be removed and calibrated periodically.
In addition, a simple pressure gauge (mounted on a connection on the top of the tank) and temperature indicator (mounted in a thermowell located low on the tank to ensure that the liquid temperature -- rather than the gas -- is being measured) are a good idea. In addition, more sophisticated users will install extra instrumentation and will monitor expansion tank conditions remotely in a control room or at a process control panel.
Figure 4 shows a typical expansion tank with minimum instrumentation and appurtenances. Symbols are labeled according to ISA standards. By contrast, figure 5 shows a fully instrumented expansion tank. (Remember that few expansion tanks have all of the devices shown.) This figure is meant to demonstrate what a fully instrumented tank may look like.
A short article can only give the basics of the requirements for a properly specified expansion tank. The reader should be aware that the expansion tank and all of its components have to be matched to the system and fluid properties, and the expansion tank should be properly sized and specified. Once properly specified, close attention to the expansion tank after startup will result in a thermal fluid system that runs more reliably over time.
Specifying a Thermal Fluid System: A 6-Part Series
Use 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:
- Part 5: The Expansion Tank
Other parts in this series include: