Introduction
Plant operators expect pumping systems to be reliable, robust and resilient, especially in high-volume, heavy-duty chemical fluid-transfer applications. In reality, however, pumping systems are often delicate ecosystems in which system failures and breakdowns may occur, causing unexpected downtime and increased repair or replacement costs.
In today’s chemical and industrial marketplace, centrifugal-style pumps account for as much as 75% of pumps used in critical fluid-transfer applications. However, this article will illustrate how recent advancements in the design and operation of another type of pump technology – positive displacement sliding vane – can be a more efficient, reliable and safer option when transferring critical liquids in both stationary and portable systems.
The Challenge
It’s an unavoidable fact of pump operation: mating shafts require alignment. Perhaps it is a direct-coupled pump with alignment between the motor and pump. Or a reduced-speed pump with alignment between the gear motor and pump. In either case, a coupler or multiple couplers transfer torque from the prime mover to the pump, possibly through an intermediate speed-reduction device. Each of these components (motor, gear reducer and pump) must be accurately leveled so they align along the shaft centerlines. Once leveled, each component must be carefully positioned to eliminate all angular and axial offsets. There are many coupling types, but each will fail if these careful alignment procedures are not accurately performed. Coupling failure leads to expensive downtime, lost reliability, increased maintenance expenses and even failure of major downstream equipment.
Shaft fatigue is an additional consideration. Initially, a misaligned shaft will absorb the induced load. That load is transferred two times per revolution, which is even more significant at motor speeds of, for example, 1,800 revolutions per minute (rpm). Several hours later, that shaft has absorbed tens of thousands of cycles. Eventually, the stress from repeated load-deflection cycles will cause the shaft to fail. When a pump shaft breaks, the downtime is problematic for plant operators because the pump unit is not moving fluid, production schedules are missed and critical downstream equipment is starved.
The ideal for any plant operator is a set-and-forget system that eliminates any chance of premature failure, but the reality is that there are a wide variety of things – both operationally and in the way the pump is handled – that will cause pumps to fall out of alignment.
The first is vibration, which occurs whenever a pump operates. It’s undeniable that some amount of vibration will take place when a pump is running; it is when the level of vibration is excessive that the shafts will come out of alignment. A second major cause of misalignment is maintenance. When system maintenance is required, pumps must be decoupled from the motor. If the reinstallation is done in haste, the pump will be out of alignment when recoupled. This can be caused by a technician that has not been trained properly, or perhaps by someone who is rushing to complete the job because he has been stretched thin. In either case, the plant operator will be surprised by an unexpected and urgent maintenance event when the shaft, mechanical seals, bearings or couplings fail.
Simply put, the shafts must be properly aligned every time the pump unit is maintained or relocated. The ultimate repercussion of a misaligned shaft is that the shaft, coupling, bearing or mechanical seal will fail, necessitating the need for another costly maintenance call to either repair or replace the pump, all while the downtime is hampering the producer’s ability to meet the requirements of strict production cycles.
Some pump manufacturers have been able to work around vibration problems or misaligned shafts with the development of close-coupled motors, where the motor shaft also operates as the pump shaft, a design that is popular with centrifugal pumps. While this design does eliminate the need for shaft alignment, the operational window of close-coupled pumps is relatively small (requires clean liquids, cannot run dry, small viscosity range, sensitive to liquids with high vapor pressures), which does not make them viable for use in most chemical-handling applications.
The Solution
Due to the functional shortcomings of centrifugal pumps, as well as the deleterious effects caused by shaft misalignment, there is great demand for an “alignment free” pump series with the ability to deliver maximum reliability and uptime while also offering functionalities like self-priming, line-stripping, product recovery, dry-run capability, solids handling, thin/thick viscosity flexibility and 70% to 90% pump efficiencies.
Such a pump series would eliminate couplings altogether and provide a true ZERO-alignment design with the following benefits:
- Elimination of premature mechanical-seal failures, frequent downtime and increased costs
- Increased uptime, reliability, productivity, and longer life for seals, shafts and bearings
- Simplified startup/installation and maintenance tasks resulting from a drop-in replacement design that, in contrast to coupled designs that require costly laser aligning and time- consuming dial indicators, eliminates the time-sensitive need for careful shimming, checking and rechecking
Blackmer®, Grand Rapids, MI, USA, a product brand of PSG®, Oakbrook Terrace, IL, USA, a Dover company has found the solution with its GNX Series Sliding Vane Pump. The GNX Series pumps are an enhancement of the legacy Blackmer GX Series models, which have been in service since 1959 and are cast-iron pumps with an internal gear reducer for use in the transfer of various chemicals.
The GNX pumps use the proven features of the GX pumps and take them to the next level with the incorporation of a commercial-grade, single-stage gearbox. This innovative gearbox fits between the motor and pump and is held in place by a permanent dowelled connection that creates a structural link between the high-speed and low-speed sides of the system.
This redesigned gearbox is constructed of rigid aluminum housings (Frame 7) and cast-iron housings (Frames 8 and 9) and has NEMA C-face or IEC D-flange motor-input connections. The gearbox output has a proprietary hollow shaft that is compatible with the pump’s keyed shaft.
This capability results in the industry’s first alignment-free pump that can produce flow rates from 20 to 500 gpm (76 to 1,893 L/min).
The alignment-free design is well suited for both fixed and portable applications with the gearbox possessing the ability to create turndown ratios ranging from 2:1 to 13:1, resulting in more precise flow rates. Typical commercial-grade gearboxes have more moving parts and a beefier housing, which can result in higher purchase costs. Many rely on two-stage or three-stage speed reduction, which means that the lowest speed ratios between the high- and low-speed sides of the system are typically 3.0 or 4.0. This creates a maximum pumping speed that is usually less than 600 rpm, which is not enough for the demands of high-volume fluid-transfer operations.
The GNX pumps are designed to fit into the same footprint as the original GX pumps. This allows the alignment-free pumps to retain a compact footprint and to be a drop-in replacement in existing installations. The GNX features a traditional 90-degree ported option; the GNXH model offers a 180-degree ported option for use whenever horizontal discharge piping or vertical pump mounts are desired.
A final benefit of the alignment-free pumps is that the permanently connected gearbox eliminates all couplings and allows all rotating components to be totally enclosed without the need of guards. This eliminates pinch points for the operator or technician, resulting in improved safety. Furthermore, seal leakage and product spillage are reduced by incorporating the Blackmer locked-rotor design into the GNX pumps (which was not available on the legacy GX pumps). These safety improvements are important to successfully pass U.S. Occupational Safety and Health Administration (OSHA) inspections and audits, as well as those regulatory tenets used by other governments globally.
Operationally, the alignment-free pumps maintain the best traits of the legacy series, namely high efficiency and dry-run capability, without damage during the self-priming or line-stripping stages. Other benefits of the alignment-free pumps include integral head-mounted drive with oil-lubricated, helical gears; sliding vane pump design that adjusts for wear to maintain flow rates; and adjustable relief valve for protection against excessive pressures. This combination of time-proven operational features and the alignment-free gearbox mean that the pumps will deliver increased reliability, uptime, seal life, bearing life, shaft life and safety, with decreased and simplified maintenance.
Conclusion
We may think of pumps as rugged, robust pieces of machinery – and, when operated in a vacuum, they are – but because pristine operating conditions are rarely achieved, it is imperative that pumping systems are designed and operated in a way that reduces the amount of harmful stress that is placed on their components. One area that has been difficult to achieve and maintain is shaft alignment, but with its alignment-free GNX Series Sliding Vane Pump design, Blackmer is confident that this worry will soon become a thing of the past.
