Rear view of an internal gear pump on liquid asphalt service. Note the steam exhausting from the steam-quenched mechanical seal, hot oil lines leading to and from the seal gland and the insulated covers on the pump body and piping. Pump it hot A guide to pumping elevated temperature fluids John Hall, Marketing Communications Manager, and John Petersen, Vice President, Technical Customer Service Viking Pump, Inc., a Unit of IDEX Corporation Cedar Falls, Iowa M any pump applications require pumping fluids at higher temperatures and pressures, providing unique challenges to process reliability and operator safety. Proper pump selection and operation is at the heart of any hot fluid process, and it is good to know about pumping principles, materials of construction and design issues for common high temperature applications. Temperature references for specific fluids and materials are taken from various pump manufacturers’ literature, and should not be generalized as accepted practices, because each manufacturer has its own rating method. Always obtain specific recommendations from your pump manufacturer when selecting, installing and operating any hightemperature pump. Reprinted from June 2001 Fluid Handling Systems How hot is hot? We define a hot fluid pumping application as one processing material with a temperature between 225º F (107º C) and 800º F (425º C), which is the range typically found in chemical process industries. Certainly, there are higher temperature applications—such as pumping molten metals—but those are beyond the scope of this article. Why pump hot? Process materials and fluids are heated for one of three reasons: • The fluid is solid or semi-solid at ambient temperature and heating permits it to be moved, applied or processed more easily. Examples include asphalts, sulfur and phthalic anhydride at 300º to 350º F (150º to 177º C) to lead at 690º F (350º C). • The fluid is involved in a chemical reaction that uses heat as a catalyst, such as solvent extraction in soybean processing, cracking in refineries, hot acids for etching and hot caustics for sterilization. • The fluid is in a closed system transferring heat from a heater or heat exchanger to a process vessel. The bulk of these applications tend to be in the 450º to 650º F (288º to 343º C) range. Hot fluids are different Physical characteristics of most fluids change when they are heated. There are exceptions, but in general, as fluids are heated: • They become more corrosive. Materials of construction for pumps that work well at ambient temperatures may not be suitable at 600º F (315º C). • Vapor pressure increases. This increases the fluid’s susceptibility to cavitation, requiring greater net positive suction head ratings on the pump, or increased suction head in the system. Since required net positive suction head is a function of pump speed, oversizing a pump and running it slower than normal may be recommended. • Fluid viscosity decreases. This tends to reduce head loss but also increases the potential for slip on rotary positive displacement pumps. Increased slip reduces pump output. • Fugitive emissions of volatile organic compounds may increase. This could require a higher level of sealing for pumps and valves. • Flammable fluids are closer to their flash point. Auto-ignition becomes easier, which increases the risk of fire or explosion. which the fluid is heated to make it flow, both kinetic (centrifugal) and positive displacement pumps can be used successfully. If pressures are low or the fluid is a slurry, centrifugal pumps may be the best choice. If the fluid is viscous or shearsensitive, if flow cannot vary over changing discharge pressures or if the pressures are moderate to high, a positive displacement pump is usually best. Gear-type pumps withstand high-temperature transfer applications and are often used when the application dictates the need for a positive displacement pump. Injecting or metering a fluid into a batch or continuous process requires a positive displacement pump to provide constant, repeatable flow, whether at low or high pressures. For Process materials and fluids are heated for one of three reasons. Selecting the best pumping principle When you understand the physical characteristics of both the fluid and pump materials at high temperatures, selecting the best pumping principle is much the same as it is for ambient temperature applications. The desired flow rate, pressure, system design and fluid characteristics determine, in part, the pumping principle best for the application. Some principles can be immediately eliminated from consideration for reasons related to materials of construction. The temperature limitations surrounding elastomers—about 350º F (180º C)—limit the use of diaphragm, peristaltic and progressing cavity pumps. For transfer applications in extreme precision or repeatability, a reciprocating, controlled-volume pump is preferred. Where extreme repeatability is not required, a rotary positive displacement pump is most cost-effective. Recirculating heat transfer fluid usually requires the constant flow of a positive displacement pump to ensure even heat transfer. Some manufacturers recommend running a pump at reduced speed to counteract the effects of these liquid’s low lubricity. Running the pump slower extends pump life. Whatever the application, specifics of each system installation will help narrow the choices among pumping principles. Always work with manufacturers whose products are specifically rated for the intended operating temperatures. There is a potential hazard using an off-the-shelf pump for hightemperature applications. Pump materials and design Each pump manufacturer bases its products’ temperature ratings on experience with a particular design and material of construction. Metals are by far the most common increased corrosion potential of most fluids at elevated temperatures, ensure that the pump materials have an acceptable corrosion rate at the fluid’s operating temperature. If there are abrasives in the fluid, such as the limestone filler added to roofing asphalt, use hardened materials in critical wear areas of the pump. When solid or semi-solid materials are heated to make them flow more easily, consider what happens to the The seal can be destroyed quickly if the material pumped is still in a solid or semi-solid condition when the pump is energized. in high-temperature applications because of their superior physical properties at elevated temperatures. One manufacturer rates maximum practical temperature for cast iron and ductile iron at 650º F (343º C), with steel, stainless steel and heat-treated ductile iron rated to 800º F (425º C). Extreme care should be used when applying cast iron at 650º F (343º C), because thermal shock can cause it to crack. That is the reason many companies will not allow use of cast iron above 400º F. Plastic or composite pump housings are sometimes used at lower temperatures. Most of these materials can only be used below 300º F (150º C), although some fluoroelastomer and lined pumps are rated to about 350º F (180º C), usually at de-rated pressures. Given a variety of metals capable of handling the temperatures, next you need to look at the chemical compatibility with the fluid pumped. With pump when the process is shut down and the liquid cools. Most pumps for these services have jacketing options that can be used to heat the pump before use and maintain its temperature when in operation. It is especially important that the pump’s stuffing box be heated if a mechanical seal is used. The seal can be destroyed quickly if the material pumped is still in a solid or semi-solid condition when the pump is energized. Electric heat tracing is another alternative for “thawing” pumps or keeping them at temperature during operation. Whatever heating method is used, it’s important to verify that the remaining material in the pump is liquified before starting it. Some manufacturers recommend overload protection on the motor or a V-belt drive to protect the pump. V-belts slip if the pump cannot be turned over. Hot pump issues Besides the materials of construction, what makes a pump suitable for hot service? Some issues that pump manufacturers contend with include: • Press-fit parts with different materials of construction. Pins and bushings with different coefficients of thermal expansion can cause loosening or high stresses in these parts. • As components expand with temperature, extra clearances are required, particularly in positive displacement pumps, which rely on tight internal clearances to act as a seal between the suction and discharge side of the pump. • Material selected for gaskets and O-rings must maintain its integrity above the operating temperature. • Springs used for pressure relief valves, seals and other functions must be rated to the maximum operating temperature. Spring rate can drop significantly when they are overheated, which will cause the assembly to malfunction or not operate at all. • Product-lubricated bearings present special challenges to the pump manufacturer. Bushing material must be compatible with the temperature at which the liquid is being pumped and running clearances must be properly selected to avoid clearance problems with temperature expansion. • Ball and roller bearings are limited to a maximum operating temperature of about 225º F (107º C). They should be located far enough away from the pumping chamber so this temperature is not exceeded. Use of lubrication suitable for this temperature must also be used. • Explosion-proof drives and controls may be required to compensate for the increased chance of fluid ignition. Sealing Sealing a rotating shaft that extends through the pump housing at ambient temperature is simple. Actually, thanks to advances in seal materials and technologies, it’s relatively easy at elevated temperatures, too. Below about 450º F (232º C), standard graphite-impregnated fluoropolymer packing may be suitable. In fact, packed pumps with water-cooled packing glands and special hightemperature packing can be used up to about 800º F (425º C). Frequently, wedge-type Teflon seals without cooling are used to about 500º F (260º C), or with a cooling quench collar up to about 750º F (400º C). New proprietary cartridge seals with outboard metal bellows are available from several manufacturers. They allow non-cooled operation to 750º F (400º C). Flushed or quenched double seals can virtually eliminate fugitive emissions, but if true sealless pumps are required, magnetic drive pumps are suitable up to about 500º F (260º C). Some canned motor pumps are rated in excess of 800º F (425º C). Operational safety Pumping hot fluids requires extra safety precautions to prevent burns and loss of life. Some safety steps include: • Segregating hot processes in separate, walled off areas, away from normally occupied areas. • Insulating hot surfaces, including pumps and piping, for both energy conservation and operator safety. • Operating the pump to avoid thermal shock that can occur when hot liquid is introduced into a cold pump or cold liquid comes in contact with a hot pump. Both situations can be hazardous, as thermal shock will Form No. 987 Front view of a large internal gear pump for moving hot resin. Note the hot oil lines leading to and from the front of the jacketed head. crack some materials. • Providing additional high temperature emergency training and providing suitable protective gear to employees in hot process areas. • Not attempting to work on a hot piece of equipment without observing proper safety precautions. • Following safety precautions recommended by equipment vendors. Common sense Usually, there is not much choice in how hot a liquid must be pumped because a specific need dictates the temperature. Here are a few commonsense rules that can help deal with high-temperature applications: • Make sure the temperature is no higher than necessary to achieve the needed purpose and to save energy costs. • Carefully consider the extra safety hazards that exist because of high-temperature operation and ensure everything possible has been done to minimize these hazards. • Seek help when applying a pump to high-temperature service. Pump construction will have to be adapted to the particular service and manufacturers will have basic recommendations to make the application successful. f Figures courtesy of the authors For more information, visit www.vikingpump.com or www.pumpschool.com
