Q. We have purchased and plan to install several sealless pumps in our facility and would like to know what procedures and cautions should be followed during installation and commissioning.
A. Sealless pumps are 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. The driven shaft is completely contained in a pressurized vessel holding the pumped fluid. The pressurized vessel or primary containment device (chamber) is sealed by static seals, such as gaskets or O-rings.
The power required by the driven shaft is transmitted through a containment barrier. Methods include:
- The canned motor pump, with a liner (can) placed between an induction motor’s armature and stator.
- The magnetic drive pump, with a shell(s) located between an outer rotating element with a permanent magnet(s) and a mating inner ring that follows the driving magnets. The inner rotor may have permanent magnets, or it may be an induction device.
Canned motors are normally cooled by circulation of pumped liquid or by use of external coolant fluid to remove heat generated by containment shell eddy current losses, rotor windage loss and motor electrical losses. Stator winding temperatures should be maintained at or below the values established for the grade of insulation used.
Magnet drive pumps (MDP) are constructed with permanent magnets that have high attraction forces to ferrous metals and high attraction or repulsion forces to companion magnets, depending on the position of the poles. Extreme care shall be exercised when disassembling or assembling the drive unit to prevent damage to components and injury to personnel. Methods of piloting and separating flanges are typically provided in the design to allow for safe disassembly and reassembly of the magnet drive section.
For MDPs, the magnets in magnetically driven pumps create strong magnetic fields. Precautions to prevent harm or damage shall be taken, among which the following are the most important:
- Pacemakers—magnets from these pumps can upset the timing of pacemakers and make them malfunction. This caution cannot be overstated due to the health risks involved.
- Hands and fingers—magnets can cause parts and tools to slam together, injuring hands and fingers.
- Credit cards—credit cards or information on the credit card’s magnetic tape can be scrambled and should be kept away from all magnets.
- Computers, computer tapes, computer disks—or any computer memory device should be kept away from magnets to prevent damage.
- Watches—all watches should be removed when handling magnets. Magnets have affected the workings of mechanical spring-driven watches, as well as chip and electronically-controlled watches.
When the pump is installed and the piping system completed, the system should be backflushed to remove debris and loose scale. The pump, motor and other sensitive equipment should be protected with startup strainers, which should be removed on completion of the flushing. The viscosity of the flushing liquid should be compatible with the drive. The pump should not be run unless it is completely filled with liquid, as damage might occur. Typically, case and impeller rings depend on liquid for their lubrication and may seize if the pump is run dry. When required, priming may be done by using an ejector or a vacuum pump.
Preventing the liquid-lubricated bearings in the magnetic drive and canned motor from being operated dry is extremely important because they depend on the pumped liquid for lubrication. Lack of bearing lubrication results in heat buildup, which is the most common cause of sealless pump failure. Silicon carbide bearings fail quickly when run dry, while carbon graphite composites may handle short periods of dry run. Liquids with entrained air or gas may cause vapor pockets to form in the containment shell, leading to lack of lubrication, heat buildup and bearing failure. In addition to proper venting, a special flush of clean, vapor-free liquid shall be applied to the bearings when the pumped liquid contains entrained air or gas.
When handling cold liquids, slowly cooling down both the unit and the piping to near pumping temperature prior to startup is necessary. This can be done either with a bypass line on the discharge side or a separate, adequately sized vent line back to the supply vessel. Shocking the pump with cold liquid may cause seizure and/or vapor locking.
During initial startup the pump should be started with the discharge valve partially opened. Once the pump has reached full operating speed, open the discharge valve completely. With a variable frequency drive (VFD) or soft start control, the pump may be started with a fully open discharge valve. Prolonged operation with a closed discharge valve will damage the pump. The pump should never be operated with a closed or partially-closed suction valve.
Additional information regarding sealless pumps is available in Sealless Rotodynamic Pumps for Nomenclature, Definitions, Application, Operation, and Test, ANSI/HI 5.1-5.6.
Q. What is slip and how does it affect pump performance?
A. Slip is a characteristic of positive displacement pumps and is defined as the quantity of fluid that leaks through internal clearances of a pump per unit of time. It is dependent upon the internal clearances; the differential pressure; the characteristics of the fluid handled; and, in some cases, the speed.
The pump slip (S), as shown in Figure 1, decreases with increasing viscosity.
Slip is determined using the following equation:
S = (Dn/16.7 x 10^3 ) - Q (Metric units)
S = ( Dn/231) - Q (U.S. customary units)
When handling low-viscosity liquids, the rate of flow varies with speed but may be affected by pressure because of the effect of pressure on slip, the internal circulation of pumped liquid.
Certain pump types, including progressing cavity, timed screw pumps and timed rotor lobe pumps present specific advantages in their application on multiphase pumping (MPP). A distinguishing constructional feature of the timed screw MPP is that it is axially balanced, having double-section rotors. The internal leakage or slip through the resulting clearances profoundly affects the performance of this machine.
When handling incompressible fluids, the slip is the same through each location of trapped volume (lock), defined by the meshing screws, and the pressure rise is linear across the entire length of the screw. In MPPs, this slip is not the same in every lock, and the pressure rise across the screws is nonlinear. The pressure loading produces deflection of the rotors, which can be successfully limited by the proper choices of the rotor geometry.
Learn more about slip and how it affects positive displacement performance from the Rotary Pump Standard for Nomenclature, Definitions, Application, and Operation, (ANSI/HI 3.1-3.5).
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