Conventional pumps using external electric motors, mechanical seals and conventional lubrication oil (or grease systems) have created reliability, safety and operational challenges for downstream units, refineries, and chemical and petrochemical facilities. In some applications, they are employed in a “1+1” configuration—one operating and another on standby—because of maintenance concerns.
These challenges have caused some to consider other options for modern pumping systems in today’s plants and facilities. Modern submerged electric motor pumps can be an alternative to these conventional pumps. In a submerged electric motor pump, the pump and electric motor are mounted on a common shaft and the combined unit is submerged in the pumped liquid. At first glance, it may seem unsafe or unreliable to submerge the electric motor with the pumped liquid, which could be flammable or explosive. However, this arrangement is both safe and reliable.
Conventional centrifugal pump units couple a pump casing to an external electric motor. This conventional pump should have a shaft seal—often two seals—to allow the shaft to extend outside of the pump casing. Seals are susceptible to failure. Risks and operational problems can result from an external pump concept, the pump seals, leaking the pumped liquid to the atmosphere and conventional lubrication systems. These issues can cause unscheduled shutdowns and serious safety issues.
For many services and pumped liquids, a good method for safe pumping is to completely submerge the combined pump and electric motor in the pumped liquid. The unit is then isolated from air; motor gaps and voids are filled with the pumped liquid; and the need for seals and couplings is eliminated. The system is intrinsically safe and reliable.
Submerged electric motor pumps are used in a wide range of midstream and downstream applications.
Submerged Electric Motors
With common centrifugal pump applications, motors are usually rated around 50 to 300 kilowatts (kW). However, there are pumps for applications with large capacity and often high head (discharge pressure) requirements.
Large ratings demand electric motors rated upward of 600 kW, with many high-pressure applications exceeding 2 megawatts (MW).
In submerged electric motor pumps, generated heat is absorbed by the pumped liquid, and there might be concerns for heat dissipation and thermal performance. Electric motor temperature rise should be evaluated and considered on a case-by-case basis. For relatively hot liquids (10 to 30 C), special designs are needed.
Submerged motor designs are not suitable for very hot liquids because the heat dissipation and high temperature issues cannot be managed properly with current technologies.
For low temperatures and cryogenic services, heat dissipation and cooling is handled by the cold liquid passing through the bearings and motor windings to assure a cool, reliable operation.
Each submerged motor pump system has unique characteristics depending on the processes being applied, the pump’s size and location, and the source and details of the pumped liquid. There are challenges in designing, constructing and operating different classes of submerged motor pumps, but there are solutions.
Submerged motor pumps are usually mated with variable speed drives (VSDs) to enhance flexibility and optimize pump performance by varying the pump’s speed with complete control. By utilizing a VSD to regulate the rotating speed, greater variation in discharge pressure for a given pumping capacity can be achieved.
In addition to increased performance versatility, a VSD will allow the compact variable speed pump to be started at a reduced frequency (soft start), decreasing the starting current level and lowering the required current. This is important for large motors above 0.6 MW.
In-Tank Submerged Pumps
In-tank submerged pumps can transform traditional storage and pumping systems. In these designs, conventional side nozzles on tanks, extremal pumps and suction-feed lines from tanks to pumps—as well as various risks, safety issues and operational problems with them—can be eliminated. In-tank submerged pumps are designed to draw down the tank liquid, leaving as little as possible.
In-tank submerged pumps should be removable from the tank for maintenance. Many of these pumps are retractable, employing a specially designed system to insert the pumps into a stationary column that is almost as deep as the tank.
The weight of the pump pushes on a spring-loaded suction valve that opens to allow the column to be flooded with the liquid in the tank. The pump then generates the required capacity and head to discharge the liquid through the top of the tank and then to the downstream facilities.
For maintenance, when a submerged motor pump is lifted from above, the suction valve closes and seals, allowing the column to be purged (for example, with inert nitrogen). The submerged motor pump is then safely removed for inspection and maintenance.
Because pumps are installed and removed through the top of tanks, all peripheral tank connections (side nozzles), which would otherwise be required, are safely and reliably eliminated. This allows the integrity of the tanks to be enhanced by removing any potential for leaks from connections. This feature also allows tanks to be located below ground if required.
Vessel-Mounted Submerged Pumps
Submerged motor pumps have many configurations. One configuration that has been widely used for high-pressure pumping is the vessel-mounted submerged pump. These are used for extremely powerful and high head services, with flow ratings upwards of 2,000 to 4,000 cubic meters an hour (m3/h), and discharge pressures of 250 barg or more.
These pumps are contained in a pressure vessel (also known as a suction vessel) built to the appropriate vessel codes, most likely American Society of Mechanical Engineers (ASME) Section VIII Division 1. Usually the operation of facilities or plants depends on these pumps, which often operate 24/7 for prolonged periods. Reliability is essential for them.
By closing the suction and discharge valves before applying an inert gas purge to the suction vessel, the suction-vessel-mounted pump may be easily removed for maintenance.
Special Materials & Designs
Conventional materials, such as low alloy steels, used in traditional pumps are not suitable for the designs of submerged pumps. Special materials and designs are required for these pumps as new challenges and requirements—such as close coupling and operation of the pump and electric motor, the unit’s submerged operation, flooded liquid in the electric motor, higher speed than ordinary designs, etc.—are raised.
For example, some submerged pumps need carefully selected aluminium alloys for housings and many rotating components. These are light and suitable materials that can be cast in required complex geometries. They are particularly important for some specific applications and pumped liquids.
In some cases, the yield strength of aluminium alloys increases as the temperature decreases, while not becoming brittle. The material is commonly used for low temperatures and cryogenic services.
Aluminium alloys are corrosion resistant with thermal expansion properties that maintain exact fits and clearances in wide temperatures ranges.
As another example, advanced ceramic rolling-element bearings are needed for these pumps. Many of these new materials have not been widely used for pump applications, and they are not listed or referred in pump codes such as American Petroleum Institute (API) 610. Therefore, great care is needed to properly evaluate and verify them. Recently ceramic rolling-element bearings have been used in different pumps with successful and long-term reliable operation.