The Importance of Expansion and Drain Structure in Hot Oil Systems
Preplanning during design can save time and money during planned or unplanned shutdowns.
The provisions made for thermal expansion and draining of the thermal oil are key factors to consider for the successful installation of any thermal oil heating system. Important system design principles like this often are overlooked; instead, the focus is placed on specific components like pumps, valves or the heater itself.
Because most thermal fluid heating systems are closed-loop systems, and because thermal oil expands with increasing temperature, accommodations must be made for that expansion. It also is inevitable that some part of the system will require service in the future. At that point in time, the system will need to be partially or fully drained, depending upon the nature of the service required. A little thought and preplanning can make draining a system relatively easy, potentially saving time and money during planned or unplanned shutdowns.
There are many ways to design expansion and drain systems, and all have their advantages and disadvantages. Despite the differences, most designs incorporate standard parts and related theories. Though thermal fluid heating manufacturers work with these systems daily and understand their key terms intimately, process engineers charged with thermal processing operations may not be. As a refresher, the most common components and concepts used in expansion and drain system designs are defined (see sidebar on page 25).
Next, consider the design principles for some system components and analyze how they are employed during commissioning and the everyday operation of a thermal oil system.
This multi-heater system is shown with expansion tanks, nitrogen blanket manifolds and double-leg drop piping structure.
Because the fluid within a closed-looped system is heated during liquid phase, it will expand. Your system must be designed to accommodate the liquid expansion to avoid the overflow of hot oil into the operating facility or overpressurization of your system devices and associated equipment damage.
First, let us look at how to calculate the amount a fluid will expand from its cold-fill volume to its max-operating level. Because the fluid volume is changing but the fluid mass is not, this is a simple conservation of mass equation.
The mass of fluid in the system depends upon the volume of liquid (when the system is first filled) and the ambient temperature.
In this example, we will use Therminol 55 while filling at 60°F. The equation shows the density of Therminol 55 at 60°F.
Assume total system volume — all of the users, piping, cold-fill level in the expansion tank, etc. — and calculate the total system volume at initial fill (cold).
Next, calculate the density of Therminol 55 at 550°F.
Mass is conserved. Therefore, solve for the expanded volume:
The difference in volume is the amount of fluid expansion that we must accommodate.
Now that we know how much the fluid will expand, we must select the right expansion tank size to accommodate the expansion. Generally, you will need some excess volume (commonly 20 to 25 percent) to account for the possibility of calculation errors (impossible!) or in case the system has temperature excursions above the anticipated operating temperature. Keep in mind that having some volume of vapor space (vapor-disengagement area) at the top of the tank is crucial for deaeration and vapor removal during startup or when new fluid is added. In addition, all systems should include a level switch to maintain a minimum liquid level in the expansion tank. Such factors will reduce the available volume in the tank; therefore, they must be accounted for in the final tank size selection.
For instance, continuing the previous example (calculating fluid expansion), we will presume that the next and largest standard tank size is 400 gal. Also assume the minimum fill in the tank (to satisfy the level switch) is 35 gal (typically specified by the manufacturer). See equation in box on this page.
As a percentage, this leaves you with about 21 percent volume remaining in the tank.
Selecting the Location of the Tank
Now that we have sized the tank, we must think about where we want to place it. Several factors govern the location of the expansion tank. These factors are important and should be thoroughly discussed when laying out the system. Generally, many of these factors are installation specific (e.g., location of available safe containment, maintenance access). The safety of operators and plant personnel also must be well thought out. Expansion tanks are occasionally full of hot thermal oil — making them both burn and fire hazards.
The ideal height for the expansion tank is above the highest liquid level in the system. This can be system piping, the heater or sometimes the thermal fluid space within a user (for instance, the heat exchanger, calendar roll or press platen). The bottom of the tank should be several feet above the highest point (5’ is a good rule of thumb). This provides several benefits, including:
- Creating static head above the pump inlet to prevent pump cavitation.
- Aiding in degassing of the system.
What happens if the tank cannot be placed at the highest point? Do not abandon all hope. While not ideal, there are ways to accommodate this, including:
- Using an inert-gas blanket system (usually nitrogen).
- Using a bladder-style expansion tank (only possible on lower temperature systems).
- Sealing the tank with nitrogen in the vapor space.
We will explore each option.
Inert-Gas Blanket System. If an inert gas blanket system is used, either plant nitrogen (if available) or bottled nitrogen may be used to fill the vapor space in the expansion tank to a relatively low pressure (5 to 10 psig is typical). This will vary depending upon the net positive suction head (NSPH) requirements of the pump and the vapor pressure of the fluid. The amount of nitrogen used will depend on how often the system temperature is cycled. Remember, the change in temperature will change the system volume. If the system is cooled, the fluid will contract, and the liquid level in the expansion tank will drop. This change in level (and, thus, volume) must be made up by adding more nitrogen. Conversely, if the system is heated, the liquid level will rise and push some of the nitrogen out.
Bladder-Style Expansion Tank. Using a bladder-style expansion tank separates the liquid in the tank from the vapor space. The vapor space then is charged with air to some pressure. As the system expands, the air in the vapor space increases in pressure. As the system cools, the pressure in the vapor space will fall (but never below the original charge pressure). Also, as previously mentioned, bladder tanks can only be used at lower temperatures because they typically are made from some type of rubber compound. Commonly, they are found in water-glycol systems. They also can be used in thermal oil systems if the bladder materials are compatible with the oil and the operating temperature.
Sealing the Tank with Nitrogen in the Vapor Space. Sealing the tank with nitrogen in the vapor space is similar to using the bladder-style expansion tank, but no barrier exists between the fluid and vapor space. To keep the fluid from oxidizing, an inert gas must be used.
Though all three methods can be used, careful consideration must be used if either of the last two options is chosen to ensure the rise in liquid level does not increase the pressure in the tank above its design rating. A pressure safety valve is critical to protect the tank if it is not open to atmosphere.
Degassing the System
Each thermal fluid system, when initially filled, will contain several undesirable constituents. As the system is operated over time, or when maintenance is performed, new fluid can be added that can create conditions similar to the initial fill. If not removed, these gases can create flow stability problems and pump cavitation that will prevent the system from operating reliably. The most common constituents are:
- Low molecular weight components of the thermal fluid (also known as low boilers).
This electric thermal oil system package includes an integral expansion tank and tower.
These constituents must be removed for trouble-free system operation. The ease with which these gases can be removed is heavily dependent upon the system piping design and expansion tank elevation.
Generally speaking, piping should be designed to minimize local high points, and the expansion tank should be above the highest point of the system. Piping also should be designed to include high point vent valves. If the tank is the highest point, then removing these troublesome elements from the system can be accomplished relatively easily through the disengagement space in the expansion tank. Otherwise, high point vent valves must be used, and the system should be vented (burped) intermittently until vapor is no longer present.
Depending upon the piping design and expansion tank elevation, there are a few methodologies that can be used to effectively get the vapor to the highest point. These include a double-leg drop or heatup line piping arrangement connecting to the expansion tank or a deaerator.
A double-leg drop arrangement allows all or a portion of the fluid to flow through the tank during commissioning. A heatup line piping arrangement achieves the same purpose, but it does so with smaller piping and by using pressure differential across different parts of the system to drive flow through the expansion tank. By forcing circulation through the expansion tank while the fluid is being circulated, the vapor in the system can disengage from the liquid level in the tank. As a result, it can be vented from the top of the tank. This can be done with the system open to atmosphere or using an inert gas blanket with settings that allow it to sweep the vapor from the expansion tank through continuous purge.
A deaerator performs a similar function as the double-leg drop arrangement, but it is a part of the system-return piping. Also, a deaerator is used continuously rather than exclusively during commissioning. While deaerators are effective for continuous deaeration, it is worth noting that they are not always as effective as forcing large volumes of flow through an elevated expansion tank. Also, they do not necessarily negate the need for a double-leg drop or heatup line piping structure.
Simple centrifugal deaerators are common, but there are many types that can be used effectively. Regardless of the vessel design, the basic premise is to allow the vapor and liquid to disengage and rise up to the expansion tank instead of disengaging within the expansion tank. The vapor should flow up and out of the system, and the liquid flow down and back to the suction side of the pump.
This thermal oil heat exchanger secondary loop package offers a modular expansion tank tower mounted above the pump skid.
Preventing Fluid Degradation
If properly designed and maintained, thermal fluid systems are easy to operate, and the life of the fluid within the system can be sustained for a long time. If the fluid should degrade, it will diminish the performance of your system. There are a couple of reasons why fluid degradation could occur: oxidation and contamination.
- Oxidation. Thermal fluids, when exposed to oxygen, especially at elevated temperatures, will oxidize and break down into lower molecular weight components. They also can form acidic byproducts that can damage system components.
- Contamination. The most common contaminant is water, but contamination can also be due to a leak on the process side of the system (a heat exchanger tube, reactor jacket, etc.).
Preventing oxidation is simple in concept; either remove the heat or remove the oxygen at the interface point in the expansion tank. Removing the oxygen typically is done with an inert gas blanket as previously described. If you do not have the ability to inert the expansion tank, and it is directly open to atmosphere, removing the heat is the only solution.
As a rule, the expansion tank and the piping leading to the tank should not be insulated. This allows any hot fluid that expands into the tank to cool before it hits the vapor-disengagement space at the top of the tank. Most often, expansion piping alone does not allow enough surface area to cool the expanding fluid before it gets to the tank. In those cases, the use of a thermal buffer allows the fluid to cool by creating more surface area and residence time than the expansion piping alone.
Preventing contamination can be tricky. It is not always a dramatic event that is noticeable. Impurities often creep into a system over time, and they will not necessarily create an increase in system volume or an obvious change in performance. Because there are many potential sources of contamination, and because it can be hard to detect with observation alone, the best defense is to have the fluid tested at least yearly. Most fluid manufacturers offer this service at no charge. Based on the results, the fluid manufacturer usually can pinpoint the root cause of the contamination.
Drain Tank Sizing
Inevitably, at some point in the life of any thermal fluid system, the system will need to be drained to perform maintenance or to perform a repair. There is also a benefit to routing things like pressure safety valve (PSV) discharge and emergency drain lines to a location where the thermal oil is safely contained. Routing these devices to small drums or inadequate containment vessels may not be sufficient. Many smaller systems do not have a dedicated drain tank, and this can be a problem in case an emergency repair is needed or a safety relief event is triggered. It is highly recommended to have the drain tank sized for the entire system volume (e.g., smaller systems upward to 10,000 gallons) or adopt a design that segments the system in such a way that sections can be isolated and drained independently. This is common with larger systems. Normally you want a 20 to 25 percent buffer with the drain tank size. Depending upon your insurance carrier, the drain tank typically will be placed away from any building or main process areas.
It is also noteworthy that relatively low cost reversible fill-and-drain pumps are available. They will make filling and draining of the system considerably easier. When designing the system piping, be sure to pay special attention to creating low point drains and connecting that piping back to a central drain tank. A small amount of investment in a drain tank, fill-and-drain pump and well-planned drain piping can save hours or even days during commissioning and maintenance activities.
Common Terms for Thermal Fluid Heaters
The most common components and concepts used in both expansion and drain system designs are defined.
Expansion Tank. Vessel designed to contain thermal expansion within a closed loop thermal fluid system.
Drain Tank. Vessel designed to contain all or part of a thermal fluid system’s volume.
Containment Area. An area designated as a safe zone for vents or overflow discharge from pressure safety valves and/or vents and drains of a thermal fluid system.
Inert Gas Blanket. A system that applies inert gas pressure (typically nitrogen at the expansion tank) to prevent oxidation of thermal oil and/or to create system pressure to overcome height restrictions for an expansion tank.
Thermal Buffer. A vessel that cools thermal oil before it enters an atmospheric expansion tank to help minimize oxidation of the oil.
Deaerator. A vessel created to continually assist in the degassing of a thermal fluid system.
Cold Seal Tank. Vessel constructed to create a cold fluid seal between ambient air and any hot fluid that may be present in the expansion tank, helping to minimize oxidation of the thermal oil.
Heatup Line/Valve. The line connected to the expansion tank, from a low pressure part of the system, to create flow through the expansion vessel when the heatup valve is open to assist in deaeration during thermal oil cookout.
Double-Leg Drop. A piping structure, consisting of a block and bypass valve, that forces a high volume flow rate through the expansion tank, to aid in deaeration during thermal oil cookout.