In “Thermal Fluid Pumps for 650 to 750oF Service” (September 2006), desirable design features of thermal fluid pumps were discussed in some detail. They include:
- Hydraulic performance, including flow, head, and NPSHR.
- Casing design.
- Impeller design.
- Materials of construction.
- Baseplate to maintain proper alignment.
- Mechanical seal selection and mechanical seal environmental control.
To summarize the discussion from that article, desirable features for thermal fluid pumps include:
- Centerline mounting of the pump casing.
- Cast steel construction (or other alloys not sensitive to thermal shock).
- 300 lb ANSI flanges.
- Totally enclosed impeller.
- Equipped for adequate cooling.
- Robust baseplate to absorb piping stresses.
Not all thermal fluid pumps offer all of these features, but there are benefits to be gained from including as many of these features as possible in the pumps chosen for service. In this article, I will compare the design features in conventional sealed pumps and sealless pumps, including both magnetically coupled (mag-drive) and canned motor pumps as they apply to thermal fluid service.
Simply stated, sealed pumps have a mechanical connection from the impeller to the motor. The pumps shaft exits the pump through a seal chamber to a bearing housing located away from the liquid being pumped. The fluid is kept inside the pump by means of a mechanical seal, housed in the seal chamber.
Sealless pumps do not have a mechanical connection from the impeller to the motor. The pumpage is contained in a sealed metal “can” and cannot leak unless the can is damaged. The impeller is supported by bearings that reside in the pumpage. The pump is motivated (driven) by magnetic forces created by rotating magnets (mag-drive), or by incorporating the electric motor into the pump as an integral unit (canned motor pumps).
Sealed PumpsSealed pumps (figure 1) are more numerous by far than sealless pumps in the United States. Purpose-built sealed thermal fluid pumps are generally robust and reliable, and cost less than their sealless cousins. Sealed purpose-built thermal fluid pumps can often survive brief periods of dry running or light cavitation without serious damage to the pump.
Sealed pumps include oil-lubricated bearings that reside outside the pumpage. These bearings can wear out prematurely if they are allowed to run too hot and if the quality of the lubricating oil is not maintained. (Cooling is discussed below.) Efforts should be made to ensure that oil levels are adequately maintained. Remember that the oil level in the side-mounted oiler only indicates how much oil is in the oiler! The oil level in the bearing housing should be double checked with a bull’s-eye indicator or a level gauge. The oil should be analyzed periodically for chemical purity and for water, particularly if a pump is operated intermittently. Multiple studies have shown that tiny amounts of water in bearing oil can have marked deleterious effects on bearing life. Worn bearings can introduce excessive vibration in the pump, which often results in damage to the mechanical seal.
Sealed pumps for thermal fluid service should have features that allow for adequate cooling of the mechanical seal and the ball bearings. While some of these plans are not listed in the most current standards (i.e. API 610), pump manufacturers can still supply these plans in purpose-built thermal fluid pumps (table 1).
As stated in earlier articles, if you are going to use cooling water to cool thermal fluid pumps, it is highly advisable that the cooling water be clean. Dirty water can quickly leave deposits in the narrow passages in the cooling jackets, which can significantly compromise proper cooling.
Manufacturers offer purpose-built sealed thermal fluid pumps that use unique designs that allow air cooling of the bearings and the seal. These pumps often carry temperature limitations but can be cost effective and reliable when placed in an appropriate service.
Mechanical SealsMechanical seals for thermal fluid service are generally metal bellows seals with carbon graphite gaskets and sealing rings. It is important to remember that materials for your mechanical seals should all have maximum temperature ratings that are higher than the temperature of the thermal fluid. This means that O-ring type seals, particularly those with dynamic O-rings, should never be used in sealing of high temperature pumps.
One of the most significant factors in reducing mechanical seal life in thermal fluid pumps is oxidation and coking of the thermal fluid on the seal faces. Two strategies are generally employed to combat coking:
- Employ a cooling plan (table 1) to reduce the pumpage temperature at the seal faces.
- Employ a quench on the gland; generally API Plan 62, which is an external fluid quench applied to the gland on the atmospheric side of the seal. In thermal fluid service, the quench medium can be either steam or an inert gas such as nitrogen.
Sealless Mag-Drive PumpsMag-drive thermal fluid pumps resemble sealed pumps in appearance, in that they are generally horizontal end suction pumps with a separate driver. Mag-drive pumps differ from sealed pumps in that the rear of the pump is completely sealed by an alloy “can.” The impeller is driven by a set of magnets that rotate just outside of the can and magnetically couple with magnets (or a copper ring) on the rear of the impeller shaft. The impeller shaft is supported by ceramic bearings that reside in the pumpage and are lubricated by the pumpage. Because the fluid is sealed with a static seal (the can) rather than a dynamic seal (a mechanical seal), mag-drive pumps do not leak unless they are damaged.
Mag-drive pumps first came on the scene in the 1950s but were heavy and inefficient because of the magnets available at the time. As ceramic, “rare earth” magnets became available, mag-drive pumps became more efficient and more affordable. The principal magnet material specified for mag-drive thermal fluid pumps is samarium-cobalt, which is very strong and has a high Currie temperature. The Currie temperature is that temperature at which a magnetic material ceases to be magnetic. While samarium-cobalt magnets are tolerant of heat, they still have to be cooled in thermal fluid service. Cooling generally is accomplished by placing a small impeller in the rear of the pump with the magnets. This impeller circulates heat transfer fluid from the rear magnet chamber, through an external cooler, and back to the rear magnet chamber, and keeps the magnets cool (figure 2).
If properly installed and operated, mag-drive pumps can run for extended periods of time without having to be rebuilt (figure 3). The internal ceramic bearings are hard and wear very well, but they are brittle and cannot tolerate mechanical shock. Mag-drive pumps are generally intolerant of cavitation, slugging or dry running. It is advisable to monitor the pump for dry running conditions by monitoring the motor amperage and immediately shutting the pump down if a drop in amperage occurs.
Sealless Canned Motor PumpsCanned motor pumps represent a unique design because the pump and motor have been combined into a single unit (figure 4). As with mag-drive pumps, canned motor pumps contain the impeller and the driving element inside a sealed canister, or “can.” The driving component in these pumps is the rotating element of a three-phase motor, which also has been sealed to prevent the pumped liquid from attacking the copper components. The rotating element is supported by ceramic bearings that are lubricated and cooled by the pumped fluid (figure 5). A three-phase winding is wound directly on the containment shell and the impeller is driven by the windings (figure 6). In canned motor pumps, there is no separate motor, so no mechanical coupling is required. Canned motor pumps are compact and do not require the heavy baseplate of frame-mounted pumps with separate motors.
As with mag-drive pumps, canned motor pumps are intolerant of cavitation, slugging or dry running. Here also, it is advisable to monitor the pump for dry running conditions by monitoring the motor amperage and immediately shutting the pump down if a drop in amperage occurs.
Sealless Pump ComparisonsDesign differences between canned motor pumps and mag-drive pumps include:
- The containment shell in mag-drive pumps is generally thicker and clearances are greater, making the mag-drive slightly more tolerant of particulates in the pumpage.
- Bearing wear is easier to monitor in canned motor pumps.
- The external motor housing of canned motor pumps serves as a “double containment.”
Is there an ideal pump design for thermal fluid service? No. If there were, that would be the only design available.
Different designs perform better in some applications than others. The owner needs to consider the fluid being pumped, the temperature, the flow and head, where the pump is to be located, and how the system is to be operated in choosing a certain design. Cost is also a large consideration. Sealless pumps can have high upfront costs; however, if they are properly installed and operated in an appropriate service, they can be highly reliable. As with all equipment decisions, the owner will benefit from drawing on all available internal and external resources when considering a new system or the upgrade of an existing system.
Author’s Note: Pictures included herein are reprinted with permission of the manufacturers. They are included as illustrations but do not represent an endorsement or recommendation by the author. The owner should consider his application carefully and employ available experienced resources (internal or external) when selecting pumps for each application.