in Sealless Pump Operation (Part 1)
Understanding the common requirements and differences among these pumps is critical to achieving a highly reliable installation.
The chemical processing industry is focused on managing the total life cycle cost of plant capital equipment, including sealless pumps. Equipment downtime must be regarded as a lost profit opportunity. When added to repair parts and maintenance personnel costs, the cost of downtime can easily dwarf the initial purchase price of a pump.
Although sealless pumps are used more and more in a broad range of applications, how to select the right one remains widely misunderstood, as do their operational requirements. There are three major categories of centrifugal sealless pumps: canned motor, metallic magnetic drive and non-metallic, lined magnetic drive. Where is each type best utilized? What are the major advantages and disadvantages of each? Understanding the differences and common requirements among these pumps is critical to achieving a highly reliable installation.
Canned Motor Pumps
The single biggest advantage of canned motor pump technology is double containment. If the primary fluid containment boundary ruptures, the external motor stator shell is a true pressure vessel. It is often rated to the pump’s full working pressure, tested to assure integrity and resistant to mechanical contact or "rub-through." This feature makes canned motor pumps the choice for lethal or extremely hazardous applications. They are also well suited for high pressure operations.
Canned motor pumps are usually available in a range of configurations that are designed to provide a controlled environment for the process lubricated bearings. The various arrangements can increase the pressure in the bearing area when volatile fluids are being pumped, prevent process solids from entering the bearing area, provide a cool source of lubricant on high temperature services and so forth. These measures mean more control over the critical bearing environment, resulting in higher reliability potential.
Canned motor pumps also benefit from a wide range of monitoring options that facilitate preventive maintenance. The principal disadvantage of canned motor pumps is the relatively special nature of the motor. The same metallic liner, or "can", that provides the primary fluid boundary under normal operation adds to the difficulty of decontaminating and repairing a breached motor. While it is technically feasible to repair these motors, it is often not economical to do so.
Metallic Magnetic Drive Pumps
Metallic mag drive pumps are well suited for a wide range of pumped materials, including:
- mild acids
- liquids that are sensitive to heat input
- fluids containing moderate solids
- fluids that present difficult sealing challenges, such as liquids with dissolved solids that tend to precipitate out of solution
- high melting point liquids
- high temperature services including heat transfer oils
Synchronous mag drive pumps add less waste heat (from eddy current losses) to volatile fluids than canned motor pumps. Metallic mag drive pumps are available in sizes as large as 520 hp (400Kw) and utilize standard NEMA and IEC motors. The primary disadvantage of these pumps is that they have no secondary containment capability, making them less than ideal for lethal or extremely hazardous applications. Various secondary control devices are available from suppliers to prevent massive spills if the primary fluid containment shell ruptures.
Non-metallic, Lined Magnetic Drive Pumps
The best of these designs feature a metallic outer pump case housing lined with thick, bonded fluoroplastic that combines excellent corrosion resistance with the strength of a metallic casing. Such pumps are ideal for acetic acid, hydrofluoric acid, hydrochloric and sulfuric acids as well as ferric chloride, hydrogen peroxide, sodium hypochlorite and other dangerous liquids. These pumps tend to have bearings made exclusively from silicon carbide (SiC), due to its high resistance to chemicals.
The disadvantages of lined pumps stem from the physical limits inherent in today’s fluoropolymers. Temperatures must be limited to about 120°C, and the pumps are generally limited to smaller power levels and lower developed head due to limits in bonding strength of the liners on large diameter, high tip speed impellers.
Major Causes of Failures in Sealless Pumps
While the specific applications for sealless pumps vary, the primary causes of their failures are similar. In all cases, they result from the failure to control the operating environment of the process fluid lubricated bearings.
The leading cause of failure in sealless pumps is probably dry running. Operating a pump for even a few seconds without liquid at the bearings will shorten pump life. Much has been written and presented at various chemical industry technical meetings on the subject of dry running bearings for sealless pumps. New materials, including hybrid carbon/SiC designs, "diamond like" coatings on SiC and a wide range of carbon graphite blends, have been tried and tested. But, no material offers a panacea for long term tolerance of dry running under loaded conditions.
Bearings for sealless pumps can be designed for greater tolerance to dry running, but only at the expense of load carrying capacity, wear resistance and run life under normal operation. In general, SiC provides the greatest load carrying capacity of any available material. It also offers excellent corrosion resistance. It can’t tolerate even brief dry runs, however. Carbon graphite, on the other hand, provides a degree of self lubrication under dry run conditions, and it is more tolerant of this abuse. Yet it cannot handle unit loads as high as SiC. Nor can it withstand abrasive particles. It will therefore have a shorter useful life under even ideal conditions.
Off-Design Operation & Undisclosed Fluid Properties
Because sealless pumps rely on the process liquid for bearing cooling and lubrication, they are application sensitive. When pump suppliers recommend a specific pump configuration and bearing material selection, they do so based on anticipated operating conditions identified by the buyer. Deviations from these design conditions, including variations in pumped liquid characteristics, extreme changes in flow rates, contaminants and changes in temperature or viscosity, can have detrimental effects on the reliability and bearing life of sealless pumps.
Although a common practice, applying too many "safety factors" to pump rated flows is detrimental to all types of centrifugal pumps.
Oversizing results in off-design operation of the pump. It increases radial bearing loads, disrupts the pump’s hydraulic thrust system, and may compromise the motor’s internal cooling capacity by decreasing fluid flow. With some fluids, such as polymers, this last factor can result in the formation of particulates that clog critical internal flow passages.
In addition to the secondary formation of solids mentioned above, sealless pumps are no more (or less) tolerant of entrained process solids than conventionally sealed pumps. Again, the central issue is bearing lubrication. Bearings made of hard materials, such as SiC, can tolerate the passage of solids more than soft materials like carbon graphite. Additionally, some manufacturers offer pump configurations specifically designed to exclude particulates from the bearing area. This is accomplished through screening or centrifugal separation or by supplying a clean secondary flush and a flush restriction device. It is critical that the nature of the entrained solids is discussed with the manufacturer to assure proper pump selection. Specifics such as particle size, percentage by weight or volume, abrasive hardness and tendencies to agglomerate — are all important details that the manufacturer must know.
Special Considerations for High Vapor Pressure & Volatile Liquids
When pumping high vapor pressure liquids, especially at pressures and temperatures close to the bubble point, additional information such as fluid specific heat and, if available, the actual vapor pressure curve, will be useful. Because the process fluid is used in sealless pumps to remove heat due to electrical inefficiency, containment shell eddy current losses, bearing friction and hydraulic inefficiency, the temperature of the fluid within the motor or magnetic coupling section of the pump will increase.
A potentially damaging condition exists if the line between liquid and vapor phase has been crossed when a higher temperature fluid is returned to suction pressure. Bearings may be running "dry" (i.e., in vapor rather than liquid.) Depending on the internal flow path of the specific pump design, this vapor may be routed back into the suction of the pump impeller, resulting in cavitation. The presence of vapor in the motor or magnetic coupling area reduces heat rejection and can result in overheating. Be sure to disclose as much detail as possible about the vapor pressure/temperature curve for your specific liquid to your supplier. In the case of liquid mixtures, include information on the highest vapor pressure constituent. Gas dissolved in the process liquid will naturally increase the likelihood of flashing within the pump. This fact, too, should be communicated to the manufacturer.
Venting and Start-up Procedures
This issue, important to all centrifugal pumps, is often given inadequate attention by operators. Even brief periods of operation with improper venting will cause sealless pump bearings to run dry. In the case of carbon graphite bearings, accelerated wear almost always happens under these conditions. With SiC bearings a rapid rise in bearing temperature occurs. When full venting is finally attained, the bearings are easily shocked thermally. Extremely hard, brittle materials such as SiC can shatter under these conditions. Secondary damage to pump internals, resulting from the passage of the razor sharp fragments of the shattered bearings, is also likely. The following simple procedure can be used to assure complete venting of even hard to vent systems. It assumes the pump is empty of liquid and that both suction and discharge valves are closed.
1. Open suction valve.
(Pump fills part way)
2. Close suction valve.
3. Open discharge valve.
(Once the pressure equalizes, air
will rise into the discharge piping)
4. Open suction valve.
5. Start pump.
Misapplication of Design and Materials
When selecting a sealless pump, note that the process of application engineering for this type of pump is in many ways similar to selecting a proper mechanical seal and seal support system. As mentioned, the more detail manufacturers know about the service, fluid properties and intended operation of the pump, the better they will be able to select a configuration and proper materials of construction. This helps ensure long, reliable service. Be prepared to supply information on not only design point conditions but the full range of intended flows and pressures. Be as exact and comprehensive as possible in identifying fluid properties.
Also, don’t forget to describe piping and control system details. Try to detail any upset conditions that the pump may experience. Remember that corrosion rates are accelerated by increases in temperature. A solid understanding of all these factors will help the manufacturer select a pump with the highest potential reliability for your service.