Application Bulletin AB170628: Maximizing flow center pump longevity for Geothermal Systems

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1 Engineered Solutions Making Geothermal Easier Application Bulletin AB170628: Maximizing flow center pump longevity for Geothermal Systems How Can Pump Service Life Be Extended for Geothermal Systems? ASHRAE (American Society of Heating, Refrigerating, and Air Conditioning Engineers) publishes a service life estimate for geothermal heat pumps of greater than 24 years, but the pumps used in geothermal systems only have an average service life of 10 years. The key word is system, which is what the customer is purchasing. A better understanding of how pumps operate in geothermal systems, and the challenges associated with service life can help increase system longevity and reliability. Pumps Used in Geothermal Systems Almost all residential geothermal flow centers and many commercial pumping stations use wet rotor circulators to pump fluid between the ground loop and the heat pump(s). Figure 1 shows common pumps used in the industry. Wet rotor circulators use relatively low power, are small in size, and cost effective. These circulators are cooled and lubricated by the fluid flowing through the pump, as shown in the simplified fluid path diagram in Figure 2 (next page). These small passages make water quality an important consideration when using wet rotor circulators. Geothermal systems can operate over a fairly large temperature range. For example, in the Midwestern part of the U.S., ground loops may range from 30 F (-1 C) to 100 F (38 C). In winter, the colder loop temperatures can create potential for condensation. Therefore, a wet rotor circulator designed for boiler systems could be problematic when used in geothermal applications. Figure 3 (next page) shows a crucial design feature for wet rotor pumps used in geothermal applications, condensate drain holes. Coated motor windings are usually part of the pump design for lower temperature operation. Similarly, terminal box orientation, as shown in Figure 4 (next page), is important for systems with colder loop temperatures. Common Causes of Premature Pump Failure Constant Speed Wet Rotor Circulator (PSC Motor) Variable Speed Wet Rotor Circulator (ECM Motor) Large Dual Head Variable Speed Wet Rotor Circulator (ECM Motors) Figure 1: Typical Pumps Used in Geothermal Systems In a recent warranty analysis jointly conducted by Grundfos and Geo-Flo, where pumps were disassembled to determine root cause of failure, about 50% of warranty returns were water quality related, 10% were failures related to improper installation (e.g. air in the system, terminal box orientation, pump shaft orientation, etc.), and 30% were no fault found, leaving only 10% with mechanical failures. The most recent analysis echoes previous findings over the years, which would indicate that the ASHRAE service life of 10 years has the potential for a significant increase if the operation of wet rotor circulators is considered when designing and installing the system. The fact that a wet rotor circulator is cooled and lubricated by the fluid flowing through it, suggests that water quality is a major factor in pump longevity. Figure 5 (next page) includes two tables that affect wet rotor circulator operation. There are two water quality related factors to consider, chemical content, and particulate matter or dissolved solids. Generally speaking, chemical content affects the pump volute (sometimes called pump housing), which is typically cast iron. In some parts of North America, where water quality is poor, bronze or stainless steel volutes are commonly used. AB170628, rev. 28JUN2017, page 1 of 5

2 Through shaft (interior ball/check in split cone) Suction side (low pressure) Through neck ring and eye Exit vanes (high pressure) Bearing/Shaft clearance Rotor/can clearance Bearing/Shaft clearance Figure 2: Simplified fluid path inside a wet rotor circulator Figure 3: Condensate 3: Condensate drain holes (photo drain courtesy holes of Geo-Flo Products Corp.) Figure 4: Acceptable flow center pump terminal box locations (NOTE: Above positions for UP(S)26-99 and UP Consult flow flow center IOM for variable speed and other pump types.) Table 1: Water Quality Recomendations Table 2: Grain size for various soil types Figure 5: Water Quality Tables (courtesty of Geo-Flo Products Corp.) AB170628, rev. 28JUN2017, page 2 of 5

3 In some cases, stainless steel volutes can help prevent pump failures. However, depending upon chemical content of the fluid, this approach could simply transfer the problem to another metallic system component, rather than solving the issue of water quality. Particulate matter like fine sand or clay can build up in the small passageways shown in Figure 2, or can erode internal pump components. The high percentage of water quality related warranty returns are either due to chemical content of the fluid, or fluid that has not been properly filtered during installation. Figure 6 shows both corrosion and erosion of a pump volute. Figure 7 is an example of particulate matter accumulating in the pump. Because a wet rotor circulator is cooled by the fluid flowing through it, a large air bubble could enter the volute, causing the pump to lose its prime, and air lock the pump. In this condition the pump will operate at a very high temperature without lubrication. Continued operation will lead to pump failure. Poor water quality and air in the system are major contributors to premature pump failure. Figure 6: Example of pump volute erosion (left) and corrosion (right) (Photo courtesy of Geo-Flo Products Corp.) Solutions Figure 7: Example of particulate matter in the pump (Photo courtesy of Geo-Flo Products Corp.) Fortunately, there are relatively easy solutions for extending pump life and creating a longer system service life. One of the easiest solutions is the use of a filter with the flush cart when purging air from the system, as shown in Figure 8. Typically, a 100 micron filter is used during flushing/purging of air from the piping. The filter will catch pipe shavings and debris that may have been introduced during installation. However, as illustrated in Figure 5, silt and clay can be less than 75 microns. Therefore, once air is purged from the system, a finer filter (1 micron) should be used to reduce the potential for sand, silt, and clay to eventually end up in the pump. Even systems with a Y- strainer installed (e.g. 20 mesh screen = 841 microns) cannot filter dissolved solids and fine sand. Figure 8: Flush cart with filter AB170628, rev. 28JUN2017, page 3 of 5

4 Water quality is the overriding factor in how the pump performs over the life of the system. Although filtering is essential, it cannot affect the chemical content of the fluid (e.g. low ph). Municipal water systems and well water can be detrimental to system longevity. If in question, bringing treated water or a pre-mix water/antifreeze solution to the jobsite can eliminate the guesswork of working with local water. Even in areas that do not need antifreeze (i.e. the loop temperatures never get cold enough to freeze), it is sometimes beneficial to bring water to the jobsite or to use an antifreeze solution that includes inhibitors. Treatment of the fluid is another option. If water and/or an anti-freeze pre-mix solution is not hauled to the jobsite, the loop fluid should be treated with an additive, such as Fernox s F1 inhibitor. Besides water quality, installation and application of the pumping system should be considered to avoid an air-locked pump or premature failure due to the orientation of the pump. For pressurized flow centers, an expansion tank or fluid reservoir above the pump suction connection will help eliminate a flat loop (no static pressure on the loop) in the summertime when the HDPE ground loop piping expands, and loop pressure drops (see Figure 9). As the pressure drops, air bubbles increase in size, and can coalesce into larger bubbles, eventually ending up in the pump. Avoid installing the flow center at the highest point in the system. A non-pressurized flow center can also provide the additional fluid capacity, as well as air elimination (Figure 10), providing that the flow center has pressure and vacuum relief components installed. Improper pump orientation (pump shaft in a vertical position See Figure 11, next page) or incorrect terminal box position (where condensation can drip into the motor), are installation related issues that can be addressed when the system is installed. These topics are addressed in most manufacturers literature. HDPE expansion tank Fluid Reservoir Figure 9: Examples of additions to pressurized flow centers to help with seasonal loop pressure flucuations Figure 10: Non-pressurized flow center Figure 11: Improper pump orientation, pump shaft in vertical position AB170628, rev. 28JUN2017, page 4 of 5

5 Summary Consideration of installation and application of the pumping system can dramatically impact service life, as discussed above. In summary, the following items are important tips when installing or servicing a geothermal system: Remove air from the system with a good quality flush cart. Ensure that the fluid is filtered during flushing/purging of air from the piping. Additional filtering with a finer filter is recommended to eliminate sand, silt, and clay particles. Always be cautious of local water quality. If in doubt, haul water to the job site or bring in pre-mix water/antifreeze solution. Water treatment with good quality inhibitors is another option. Cast iron volutes will last a long time if the loop fluid has good water quality. Provide a means to control air in the system. For smaller applications, a fluid reservoir like the Geo-Prime tank (Figure 9) acts as an air separator. Non-pressurized flow centers (Figure 10) are also a good option, providing that pressure and vacuum relief is included. Larger pressurized systems may benefit from an air separator. Pressurized flow centers should include an expansion tank (Figure 9). NOTE: Geo-Flo has a white paper on expansion tank sizing for geothermal systems on the website ( in the literature section. Review manufacturers installation documentation for pump and terminal box orientation to help prevent premature mechanical/electrical failures. If the tips above are utilized during installation of the system, pump life expectancy can be greatly extended, lowering overall system life cycle costs, and increasing customer satisfaction. For more information, contact Geo-Flo Products Corporation or visit the website at GEO-FLO PRODUCTS CORPORATION 905 Williams Park Drive Bedford, Indiana U.S.A. Phone: Fax: Engineered Solutions Making Geothermal Easier AB170628, rev. 28JUN2017, page 5 of 5