Sealless Design Advantages & Centrifugal Seals in Rotodynamic Pumps
Q. What are the advantages and limitations of a sealless pump design?
A. A sealless pump is used when there is a need to contain toxic, dangerous and/or valuable fluids. Application may be dictated by space, noise, environment or safety regulations. This section outlines types, nomenclatures and components of sealless rotodynamic-type pumps. Sealless pump design is founded on eliminating the dynamic shaft seal between the wetted end of a centrifugal pump and the atmosphere. This is achieved by enclosing the pump and its rotor assembly inside a pressure vessel with the pumped fluid. The pressure vessel or "primary containment" is sealed by static seals, such as gaskets or O-rings. The inner rotor assembly is driven by a rotating magnetic field that is transmitted through a containment barrier. Sealless pumps fall into two categories: magnetic driven pump (MDP) and canned motor pump (CMP), as shown in Figures 5.1.3.1 and 5.1.2.1.
Figure 5.1.3.1. Magnetic drive pump: separately coupled (closed or semi-open impeller) (Graphics courtesy of Hydraulic Institute)
Figure 5.1.2.1. Canned motor pump: close-coupled, end suction, overhung impellerThe liquid-lubricated bearing design and application considerations are essentially the same for CMPs and MDPs. Factors internal to the unit design such as pressures, temperatures, flows and heat transfer characteristics within the drive section and hydraulic performance of the pump end must be understood to properly select circulation plans and assess application questions.
Properly designed, applied and operated sealless pumps may offer the following advantages:
Improved safety when handling hazardous liquids
Eliminated leakage through primary containment to the environment during normal operation
Optional backup secondary containment
Eliminated loss of valuable liquids
Lower noise level (CMP designs)
Suction pressure usually does not affect the axial thrust
Reduced or eliminated periodic shaft seal replacement cost
Some limitations must be understood to properly apply a sealless pump:
Temperature of motor windings (CMP) or magnet components (MDP)
Control of bearing environment is required to provide clean non-flashing liquid.
Primary containment shell is relatively thin, and corrosion potential should be carefully considered.
Retraining of maintenance personnel may be required.
Drive-generated heat may affect the NPSH required with some circulation plans for volatile liquids.
Overheating of drive section may occur with loss of flow or loss of suction.
Potential for higher repair cost if bearings fail before detection
For more information about sealless pumps, see ANSI/HI 5.1-5.6 Sealless Rotodynamic Pumps for Nomenclature, Definitions, Application, Operation and Test.
Q. What is a centrifugal seal in rotodynamic centrifugal pumps?
A. A centrifugal seal is a dynamic seal that only operates when the pump shaft is rotating and has no seal effect when the shaft is stationary. It consists of an expeller or set of expellers located in a separate chamber behind the impeller, which is typically fitted with expelling vanes on the back shroud.
When the pump is running, the centrifugal seal generates pressure Pe to equalize the pressure Pb, as shown in Figure 12.3.8.3.6, so that the pump operates without leakage.
Figure 12.3.8.3.6. Centrifugal (dynamic) seal with "dry-type" packingA centrifugal (dynamic) seal needs to be combined with a backup or static seal to prevent leakage when the pump is not running. The general requirements for the backup-sealing device are that it must seal statically when the pump is shut down, and it must run dry during pump operation. This can be accomplished by dry-type packing, multiple lip seals, other proprietary devices or mechanical seals with either dry run capabilities or fitted with a separate flush.
There is a maximum allowable suction pressure Ps above which, depending on speed of rotation, a centrifugal seal will not operate properly. For this reason, centrifugal seals are not effective on the second or higher stages of multiple-pump installations, where the pumps are arranged to have the full discharge of the preceding stage applied to the suction of the following stage.
If the pumps are installed at specified intervals and elevations spread out along a slurry transport line, then it is possible to use centrifugal seals on all stages. The arrangement should be such that the suction pressures on each stage are approximately equal and do not exceed 10 to 20 percent of the discharge pressure. An analysis should be made of the centrifugal seal performance, based on actual head, flow and suction pressure, so that proper operation is ensured.