Eastman’s Therminol heat transfer fluids are widely used in industrial thermal systems for their stability, performance, and longevity. A critical component in any such system is the expansion tank, which plays a vital role in managing fluid expansion, system venting, and inert gas blanketing to prevent oxidation. This article provides a detailed overview of Eastman expansion tank installation practices, design considerations, and system integration based on the manufacturer’s technical resources.
Overview of Expansion Tank Functionality
An expansion tank is typically installed at the highest point of a thermal system and connected to the suction side of the pump. It serves as the primary venting point and accommodates fluid expansion, which can range from 25% to 30% of the system’s total volume. The actual expansion volume is influenced by the specific properties of the chosen fluid and the operating temperature range.
In systems using Therminol fluids, the expansion tank must be designed to handle both thermal expansion and the release of noncondensable gases. This is achieved through proper venting and, in many cases, the use of a double-leg expansion tank. The double-leg design provides greater operational flexibility compared to single-leg configurations, especially in systems that include degassing tanks and temperature buffer tanks.
Key Installation and Design Guidelines
1. Placement and Connection
The expansion tank should be installed at the system’s highest elevation to facilitate proper fluid and gas flow. It is typically connected to the main circulating loop at the lowest pressure point, ensuring that any gases or moisture can be effectively vented.
For systems with potential moisture ingress or high thermal cycling, it is recommended to connect the expansion tank to the pump suction side. This placement helps prevent entrained gases from reaching the pump, which can lead to cavitation and reduced efficiency.
2. Venting System Design
All expansion tank vent lines must be routed externally, ideally through a cooled condenser, to prevent vapor and noncondensable gases from entering working areas. This is particularly important in systems where the vapor space is not blanketed with inert gas.
Vent condensers help recover low-boiling components and moisture, which should be collected in a vent condensate trap or cold-seal trap. These traps must be periodically drained to prevent the accumulation of contaminants that can affect system performance and fluid life.
3. Inert Gas Blanketing
To minimize fluid oxidation, it is recommended to blanket the system with an inert gas such as nitrogen. This is especially critical in systems where the vapor space is not fully occupied by the fluid. In smaller systems, a cold-seal trap or expansion leg filled with system fluid at a low temperature can serve as an alternative to continuous inert gas supply.
Inert blanketing prevents the formation of acidic byproducts that result from the oxidation of thermal fluids. These byproducts can corrode system components, reduce heat transfer efficiency, and shorten the fluid’s useful life. Eastman highlights that high acidity levels in the fluid may indicate contamination or severe oxidation, necessitating system maintenance or fluid replacement.
4. Material and Component Selection
Expansion tank construction and component selection should be based on the operating temperature range and fluid type. For systems operating above 200°C (390°F), pump manufacturers typically recommend either water-cooled ring seals or fluid-cooled stuffing and air-cooled, extended-shaft seals. These designs help maintain seal integrity under high-temperature conditions.
For flange gasketing in high-temperature applications, spiral-wound or graphite gaskets that conform to API 601 and DIN 4754 standards are advised. These gaskets are typically made of 304 stainless steel and pure graphite. To ensure a leak-free seal, it is important to use raised-face flanges, steel bolting, and even compression of the gasket during tightening. Graphite gaskets are a viable alternative in many applications and are often preferred for their thermal stability and resistance to chemical degradation.
5. System Start-Up and Commissioning
Before starting a new thermal system, it is important to install a wire mesh strainer in the pump section to capture any debris that may have entered during installation. These strainer baskets can typically be removed after the initial start-up and debris removal.
In systems where solids or contaminants are likely to enter or be generated during operation, a high-temperature filter bypass line should be installed. This line should be equipped with valves that allow for positive isolation, enabling periodic cleaning or replacement of filter elements. Common filter types include glass fiber string-wound cartridges and sintered metal filters with micron ratings between 5 and 20. These filters require a significant pressure drop to function effectively.
When commissioning a system with potential moisture content, a gradual start-up is recommended. The expansion tank should be purged with inert gas to remove moisture from the vapor space. If the system contains a large amount of water, it may be necessary to remove and dry the fluid externally before reintroducing it to the system.
Fluid Selection and System Performance
The selection of the thermal fluid is a critical factor in system performance and longevity. Eastman offers a range of Therminol fluids designed for different temperature ranges and applications. Fluids are typically selected based on the system’s maximum operating temperature, required thermal stability, and low-temperature performance.
Low-boiling components and moisture must be managed through proper venting and trap systems, as they can lead to reduced heat transfer efficiency and increased risk of corrosion. High-molecular-weight degradation products, which are the result of thermal stress and oxidation, can also impact system performance by increasing viscosity and forming deposits.
Eastman recommends regular system sampling and analysis to monitor fluid condition and detect early signs of degradation. This allows for timely maintenance and fluid replacement, preventing unexpected performance losses and equipment damage.
Safety and Maintenance Considerations
Safety is a primary concern in the design and operation of thermal systems. The flash point of a fluid is an important safety parameter, as it indicates the lowest temperature at which the fluid gives off sufficient vapor to ignite. Flash point is typically measured using either the Cleveland open-cup (COC) method or the Pensky-Martens closed-cup method. The closed-cup method generally yields values that are 20°–30°C (30°–50°F) lower than the open-cup method, especially for fresh fluids. In-service fluids may show an even greater difference due to changes in composition from thermal and oxidative degradation.
Proper maintenance of the expansion tank and associated components is essential to ensure long-term system reliability. This includes periodic inspection and cleaning of vent condensers, traps, and filters, as well as monitoring for signs of corrosion or fluid degradation. Contaminants such as leaked process materials or incorrect fluid additions can significantly affect system performance and must be addressed promptly.
Conclusion
Eastman’s expansion tank installation guidelines emphasize the importance of proper design, component selection, and system commissioning to ensure optimal performance and longevity of thermal fluid systems. By following these recommendations, system operators can minimize the risk of fluid degradation, corrosion, and operational inefficiencies. Regular maintenance and fluid analysis further support system reliability and help extend the life of the heat transfer fluid.
Eastman’s resources, including technical manuals, system design guidelines, and testing services, provide valuable support for engineers and operators throughout the lifecycle of the thermal system. These resources are designed to help users make informed decisions and maintain safe, efficient, and environmentally responsible operations.