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December 1, 2023 | By Stephen Jones, Hayward Tyler Inc.
Canned motor pumps (CMPs) are designed to prevent emissions by eliminating the mechanical seal. Provided here is an overview of CMPs, along with information on common uses
Emissions from process equipment not only pose safety and environmental risks, they also have a significant impact on maintenance costs and lost time of production. The increased focus on limiting emissions from conventional pumps draws attention to the biggest emissions culprit — the mechanical seal.
Sealless pumps do not use a mechanical seal, so the possibility of leaking fluids, dangerous or otherwise, from the seal into the atmosphere is eliminated. This category of pumps offers additional simplicity, by also eliminating coupling alignment, bearing-frame maintenance and auxiliary systems associated with lubricating or cooling bearings.
The two main categories of sealless pumps are canned motor pumps (CMPs) and magnetic drive (mag drive). CMPs offer increased safety through double containment, a reduced number of bearings (and inherent maintenance costs), and improved reliability when applied properly to the application. This article provides information on the operation and potential benefits of CMPs in the chemical process industries (CPI).
FIGURE 1. The diagram illustrates the difference between a mechanically sealed pump, and a CMP, which has no seal
FIGURE 1. The diagram illustrates the difference between a mechanically sealed pump, and a CMP, which has no seal
Laws of physics dictate that higher-pressure fluids want to move to lower-pressure areas. Because traditional pumps must couple to a driver (motor or turbine), the pump shaft needs to exit the pump pressure boundary (pump case). This means that the high-pressure fluid created by the rotating pump wants to leak into the atmosphere along the shaft, where it penetrates the pump case. In traditional pumps, this leakage is prevented by using gland packing or a mechanical seal.
FIGURE 2. In a fluid-fiilm bearing, the pump shaft does not come into direct contact with the bearing
FIGURE 2. In a fluid-fiilm bearing, the pump shaft does not come into direct contact with the bearing
Canned motor pumps take a different approach, using a common shaft for the pump and motor, and containing it all inside a pressure boundary. This design eliminates the pressure differential across the rotating seal and thereby eliminates the driving force for the fluid to leak. Using a common shaft for the pump and motor gives rise to other benefits also, such as eliminating coupling alignment (both hot and cold), which removes a potential failure point in the pump.
CMPs are constructed of materials compatible with most process fluids, yet the materials are not overly exotic or expensive. The typical metallurgy of the wetted components of a CMP are 304 stainless steel with the stator and rotor cans made from Hastelloy C276. Although Hastelloy C, a nickel-based alloy, is more exotic and expensive compared to 300-series stainless steel, the higher electrical resistivity significantly reduces the “eddy current” losses in the stator can, and to a smaller degree, the rotor can. Overall, using Hastelloy results in increased efficiency of the motor.
Additionally, since the stator and rotor cans are fabricated from a thin material (0.010–0.015 in.) to minimize electrical losses and reactivity, it is important to use strong and highly corrosion-resistant materials. The cans are further supported by the electrical steel lamination along the length of the lamination pack and with heavier-duty backup sleeves at the wire-coil end-turn regions. These support the thin can in dealing with the high internal pressures (up to 6,000 psi/42 MPa) within the motor.
For sealing of joints, there are typically face-to-face gasket designs and O-ring gaskets with a machined groove. For both types of joints used to seal the pump-to-motor and motor-to-end cover mating surfaces, it is important to verify the compatibility of the material with the process fluid. A standard design uses polytetrafluoroethylene (PTFE) gaskets due to their vast compatibility with various process fluids. For higher temperature and pressure applications (above 390°F and 580 psig), a spiral-wound stainless steel is typically used. The grade of stainless steel will be matched to the pump case. For special chemicals or specific applications, other gaskets may be used and the individual compatibility of those fluids with the gaskets must be assessed.
If using an O-ring joint design, a Teflon-encapsulated Viton O-ring is common, because this type of O-ring provides the corrosion resistance of Teflon (PTFE) combined with the elastomeric resilience of Viton.
An important point to understand in the context of CMP bearings is that they use fluid-film bearings. This means that during operation, the rotating surface of the shaft (or shaft sleeve) does not come into contact with the stationary bearing. Instead, as the shaft rotates, it creates a fluid film and the fluid is forced between the shaft and the bearing.
The typical design uses a hardened polished-metallic surface on the rotor, with a softer corrosion-resistant bearing material. The bearing material is designed to be easily replaceable during periods of maintenance. The bearing material used for CMPs is generally either one of two materials: graphite or silicon carbide. Graphite grades typically offer a softer, more forgiving bearing that is more tolerant of particulate matter and upset conditions. However, it will not last as long as silicon-carbide grades. Conversely, silicon carbide, being a harder material, will last longer in the right application, but is less tolerant of particulate wear and upset conditions. One material is not necessarily better than the other for CMP bearings, and material selection should be based upon the application and user requirements and preferences. Other materials, such as PEEK (polyetheretherketone) and other composites, can also be used, but are less common.
When considering the bearing lubricity and motor cooling, it is necessary to understand the requirements and characteristics of the canned motors as they relate to the fluid properties. Typically, the pumped fluid lubricates the bearings and cools the motor. In some instances where the pumped fluid is not compatible with the motor (because of high levels of particulate matter, viscosity issues and so on), then a barrier fluid can be used in the motor.
The motor fluid must have appropriate viscosity to provide adequate lubrication of the hydrodynamic bearings, yet also have sufficient flow through the motor to cool the parts effectively while minimizing fluid-friction drag losses. As a general rule, fluids with an absolute viscosity greater than 200 centipoise and less than 0.07 centipoise are not desired.
An alternative solution when the pumped fluid cannot be used inside the motor involves a barrier fluid. A labyrinth seal is used to isolate the pump, and the motor is filled with a compatible fluid that is different than the pumped fluid. It should be noted that, in some cases, an internal mechanical seal is used. This seal does not leak to the atmosphere, but instead, leaks a small amount of motor fluid into the pump end.
With either sealing option, it is essential to ensure the compatibility of the motor (barrier) fluid with what is being pumped. As previously mentioned, the fluid inside the motor is used to cool the motor, as well as to lubricate the bearings. The fluid will therefore increase in temperature as it circulates through the motor. As the fluid temperature increases, so does the fluid vapor pressure (and typically, the viscosity decreases), for these reasons, it is important to evaluate the thermodynamic properties of the motor fluid and ensure it will not vaporize (“flash”) in the motor. This might cause vapor lock or insufficient bearing lubrication.
The temperature rise of the motor fluid is not only a function of the amount of waste heat generated by the motor’s inefficiency, but also of the specific gravity, specific heat (heat capacity) and the flowrate of the fluid through the motor. In addition, the thermal conductivity of the fluid will affect the heat-transfer rate from the motor and ultimately, will affect the temperature rise of the motor windings. Motor winding wire has an insulation class determining the maximum temperature to which the wire can be exposed. Typically, CMPs use Class H (180°C) or Class N (200°C) wire, although other ratings are available. Note that class designations for those above Class H are typically specified in International Electrotechnical Commission (IEC; Geneva, Switzerland; www.iec.ch) standard IEC 60085.
Taking this into consideration, there are six main fluid properties of the motor fluid that are to be considered for a given application. These are the following: 1) specific gravity; 2) specific heat; 3) viscosity; 4) vapor pressure; 5) freezing point; and 6) thermal conductivity. Because the viscosity and vapor pressure of some fluids can vary significantly with temperature, it is a best practice to understand these properties at various temperatures. This would generally be at normal-operating, minimum-operating and maximum-operating temperatures. It is worth noting that most manufacturers have a wealth of knowledge on various commonly pumped fluids, so they will likely be able to offer support in defining these parameters if they are unknown at the specification stage. The viscosity and vapor-pressure properties do not vary linearly with temperature, so the three points mentioned can be used to generate an approximate curve, allowing these values to be calculated for other temperatures.
There are a variety of fluid flow paths for circulating fluid through the motor. Most manufacturers adopt those specified in the American Petroleum Institute (API; Washington, D.C.; www.api.org) standard 685 (Sealless Centrifugal Pumps for Petroleum, Petrochemical, and Gas Industry Process Service-Annex D). This standard outlines 14 different flow plans. Those familiar with mechanical seal plans will be accustomed to this concept. All these plans will not be reviewed in this article, however, the most widely used circulation plans are discussed. It is not an expectation that the user would identify the model type and circulation plan for their application. This would be done by the manufacturer’s application engineers. However, there are considerations that users should take into account when specifying a CMP, for example, the availability of cooling water (when pumping a hot liquid) or the availability of pipework to route back to a tank (for low-vapor pressure fluids).
Plan 1-S Internal circulation (hollow shaft). This is the most common circulation plan and involves a small amount of the pump discharge circulating through orifices in the pump-end (PE) bearing housing and into the motor (Figure 3). It travels through the pump-end bearing, across the rotor and then through the cover-end (CE) bearing before returning to the suction via the hollow rotor shaft.
FIGURE 3. The Plan 1-S circulation, shown here, is the most common type of circulation path for a CMP
FIGURE 3. The Plan 1-S circulation, shown here, is the most common type of circulation path for a CMP
Plan 1-SD Pressurized circulation. This circulation plan (Figure 4) is used for handling volatile fluids that have a low boiling point. The premise is that the auxiliary impeller adds pressure to the motor cooling fluid to prevent it from vaporizing inside the motor. The flow path starts from the pump discharge and travels through ports to a hole in the hollow shaft. It travels through the hollow shaft to the eye of the auxiliary impeller. The flow splits and some goes through the CE bearing and back to the auxiliary impeller. The rest travels past the rotor, through the PE bearings and back to the discharge side.
FIGURE 4. This diagram shows Plan 1-SD circulation, which is used for handling volatile fluids
FIGURE 4. This diagram shows Plan 1-SD circulation, which is used for handling volatile fluids
Plan 13-SE Reverse circulation. This circulation plan is used when pumping fluids with low vapor pressure, such as refrigerants, liquefied gases, ammonia and so on. The fluid is circulated only once through the motor to avoid excessive heat pick up in the fluid (which could cause cavitation). As discussed previously, any vapor inside the CMP can do significant damage to the bearings and other internal parts.
The flow path (Figure 5) is from the pump discharge through holes in the PE bearing housing, through the PE bearing, across the rotor, and through the CE bearing before exiting the motor at the cover end. The fluid would typically be returned to a suction tank or collection manifold.
FIGURE 5. Plan 13-SE reverse circulation, shown here, is used when pumping fluids with low vapor pressure
FIGURE 5. Plan 13-SE reverse circulation, shown here, is used when pumping fluids with low vapor pressure
Plan 23-S Externally cooled motor. This circulation plan is used when pumping high-temperature liquids (for example, heat-transfer oils). This circulation uses a heat exchanger to cool the motor fluid. The same fluid being pumped is used inside the motor, just at a lower temperature. The design uses a thermal barrier to thermally isolate the hot pump end from the cooler motor. This circulation requires site-supplied cooling water, or it can use an air heat exchanger, although this is less common. Generally, a small amount of fluid circulates from the impeller discharge, through the thermal barrier, and toward the heat exchanger inlet. The motor fluid circulates through the heat exchanger at the PE and then enters the motor at the CE (Figure 6). It passes through the CE bearings, across the rotor, and through the PE bearings before repeating this circuit. There is sometimes an auxiliary impeller keyed to the shaft to help circulate the motor fluid, which can be located at either of the bearing-housing ends.
FIGURE 6. This diagram shows Plan 23-S circulation, which is used for high-temperature fluids, such as heat-transfer fluids
FIGURE 6. This diagram shows Plan 23-S circulation, which is used for high-temperature fluids, such as heat-transfer fluids
Plan 1-S High temperature, no cooling. Although this flow plan uses the same circulation path as mentioned first, it is worth mentioning as an option for pumping high-temperature fluids when there is no site cooling available. This motor design uses an advanced grade of insulation (Class 400) for handling higher temperatures. This motor configuration is typically only available up to 120 hp and is often not rewindable (Figure 7).
FIGURE 7. The circulation plan shown here is the same as that shown in Figure 3, but with added insulation for handling high-temperature fluids
FIGURE 7. The circulation plan shown here is the same as that shown in Figure 3, but with added insulation for handling high-temperature fluids
Plan 53-S and 54-S slurry handling. This circulation plan is suitable when pumping slurries or abrasive fluids that do not provide good lubrication and cooling to the motor. This plan (Figure 8) uses a barrier fluid inside the motor. The barrier fluid is circulated through the motor, either originating from a seal pot (53-S) or an external source (54-S). A labyrinth seal (or internal mechanical seal) separates the pump end from the motor. An external metering pump is commonly used to ensure that the motor pressure stays above the pump discharge pressure. The compatibility of the motor fluid and the pumped fluid must be checked, because a small amount will leak from the motor to the pump across the internal labyrinth seal/mechanical seal.
FIGURE 8. The circulation plan shown here is designed for pumping slurries and abrasive fluids
FIGURE 8. The circulation plan shown here is designed for pumping slurries and abrasive fluids
When investigating the use and benefits of a CMP, whether a new installation or a retrofit of an existing pump, it is essential to know the fluid properties and operating conditions. All analyses must include expected abnormal or possible upset conditions, which can affect the performance and reliability of the pump. This requires a mutual understanding of the application considerations by the system designer, end user and by the CMP manufacturer. With this approach, a cost-effective and reliable installation will be realized.
CMPs offer the safest and most environmentally friendly pumping option when handling fluids that can harm the environment. CMP use is rising with environmental concerns, combined with features that eliminate the most common pump failure modes. Eliminating the mechanical seal, removing the requirement for hot and cold alignment, and reducing the number of bearings plus lubrication systems offer a reliable and maintenance-free pumping solution when correctly applied to the application.
Editor’s note: All images courtesy of Hayward Tyler
Stephen Jones is a senior technical marketing specialist at Hayward Tyler Inc. (P.O. Box 680, Colchester, VT 05446; Phone: 802-655-4444; Email: [email protected] ), a leading designer and manufacturer of canned motor pumps. He has worked as a mechanical engineer in various roles for over 15 years. Jones holds a master’s in mechanical engineering (MEng) and is a chartered engineer (CEng) through the Institute of Mechanical Engineers (IMechE).
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