Magnetically Driven Equipment for Aggressive Liquid Environments

Reprinted from the July/August 2017 Edition of World Fertilizer Magazine

Robert Mollath, P.E., Ebara International Corporation, Cryodynamics Divison USA, outlines magnetically driven equipment for aggressive liquid environments at ammonia production and processing plants.

The transportation of Liquid Ammonia (LNH3) presents unique engineering challenges for rotating equipment to operate safely and reliably in aggressive liquid environments.  In addition to being a cryogenic fluid (-30C°), ammonia sets itself apart from other cryogenic liquids for several reasons.  Most importantly, ammonia is remarkably hazardous to personnel working around the product as it is an extreme asphyxiate.  Furthermore, it is highly corrosive to copper, copper alloys, and other materials thereby requiring different material selections from traditional centrifugal pump designs.  Ebara International Corporation (EIC Cryo) tackles these challenges by utilizing a purged and isolated submerged motor design combined with magnetic coupler to transfer power between the motor (dry) side to the hydraulic (wet) side of the pump with no traditional mechanical seal necessary.   The benefit of this design is an isolated motor section for safe operation in a non-hazardous location that is surrounded by liquid ammonia.  Reference Figure 1 for arrangement of the pump assembly.

Figure 1: Pump Assembly

 

Motor Side Design

With a traditional seal-type external motor pump design, a motor shaft penetrates to the process region. Mechanical seals are necessary to seal the shaft between the motor and the hydraulic side of the equipment.  This exposes the motor to the elements and more importantly, to the hazardous area where oxygen is present, and potentially gas in an upset condition.   Applying a magnetic coupler simplifies the design by eliminating both traditional mechanical seals and any motor shaft penetration to the process side. The motor is isolated and safely operated in a non-hazardous area.  Additional benefits of this design include no alignment issues, as there is no direct mechanical connection between the motor shaft and pump shaft, and the design is more compact, allowing for a small footprint inside the plant environment.

Motor casings are fabricated in stainless steel, designed and constructed in accordance with ASME standards for Lethal Service Applications.  While not a traditional style pressure vessel, adhering to the ASME codes ensures the casing are fabricated and inspected to the highest standards for pressure containment.  Operating the motor in a GN2 environment ensures the motor windings and rotor are always dry, free of oxygen, and the casing can be kept at a higher pressure differential than the pumped fluid.  This ensures there cannot be penetration of ammonia into the motor cavity.  The outside of the motor casing is exposed to the pumped fluid so there is some available indirect cooling to the motor windings, taking heat out of the motor.  Additionally, before shipment, further inspection and testing takes place during final assembly where the motor housing undergoes a helium leakage test as an assembly to confirm the integrity of the flanged joints.  Reference Figure 2 for arrangement of the motor side of the pump.

Figure 2: Motor Side Assembly

 

Pump Side Design

By utilizing a magnetic coupler, EIC Cryo is not limited to any specific hydraulic duty as it can combine its existing expansive hydraulic combination database to meet different process requirements for ammonia fluid transportation.  This helps ensure established pump hydraulic designs with Ebara Cryo’s patented Thrust Equalizing Mechanism (TEM©) capability can be adapted to aggressive liquid applications with proven efficiency and hydraulic performance.  Reference Figure 3 for arrangement of the hydraulic side of the pump.

Due to the corrosive nature of LNH3, traditional bronze bushings and wear rings (sacrificial replaceable wear parts) cannot be used.  Rather, for the inter-stage bushings, a corrosion resistant carbon material is selected.  It has a low thermal expansion coefficient making it stable at cryogenic temperatures and has some lubrication properties in a normal wear situation. The impeller wear rings are made from Peek material which is also corrosion resistant and stable at cryogenic temperatures, making it a good substitute for replacing the bronze components.

Both the bushings and wear rings act to seal the impeller stages from one another limiting leakage losses between impeller stages. Due to the pressure gradient across the seals, Lomakin viscous and internal forces generate stiffness and damping to support the rotating shaft at seal locations.  To confirm the pump will not operate near a lateral or torsional critical speed, a rotor dynamic stability analysis is performed on both the pump side and motor side.  Where possible, EIC applies a design that is classically stiff in nature.

Figure 3: Pump Side Assembly

To increase reliability and bearing life, the hydraulic side features the Thrust Equalizing Mechanism system to balance the hydraulic forces generated by the pump impellers.  The TEM is a self-adjusting hydraulic balancing system that uses a small amount of pumped fluid routed to the balance chamber to equalize the forces generated by the impellers.  The flow to the TEM balance chamber is regulated by two components, the back wear ring annular clearance which acts as a fixed orifice and also, the TEM gap acts as a variable orifice. As the hydraulic forces increase, the TEM gap will close to increase the pressure in the balance chamber, balancing the hydraulic forces.  The opposite is also true, as hydraulic forces reduce, the TEM gap will start to open reducing pressure in the balance chamber, finding equilibrium.  The ultimate result of the functioning TEM is that the hydraulic side main bearing operates with a net zero thrust load, ensuring a long life to the bearing.   Reference Figure 4 for TEM.

Figure 4: Thrust Equalization Mechanism

The bearings for the hydraulic side are deep groove ball bearings and are product lubricated by the pumped ammonia.  Having product lubricated bearings takes advantage of the cooling capability of cryogenic liquids, removing any heat generated.  Similarly, after the fluid passes thru the bearings, it passes thru annular cavities in the magnetic coupler to remove any heat generated by eddy current losses before returning to the suction side of the pump.  Combining the TEM with a product lubrication scheme ensures the bearings and magnetic coupler can operate maintenance free for thousands of hours in lower viscosity fluids.  Since the bearings for the motor side are not product lubricated the design features similar deep groove ball bearings that are sealed in a special low temperature grease for lubrication.

The casings feature locations for mounting accelerometers allowing operators to take a condition based approach to determine maintenance intervals.  The accelerometers can be connected to a monitoring system for both trending and spectral frequency analysis, delivering further detailed vibration analysis and prediction of machinery health.  A condition based approach allows operators to extend maintenance intervals based on machinery health rather than performing maintenance based on a fixed number of hours.

Finally, pumps feature a helical axial flow inducer for operation at low liquid levels, (reduced NPSHr) ultimately reducing storage tank dead stock levels to a minimum without the harmful effects of cavitation.

Installation System Configuration

Applying a magnetic coupler allows for compact equipment designs that can be installed in both vessel mounted and retractable installations.  Retractable designs allow rotating equipment to be installed into large top entry storage tanks. When combined with an Ebara Cryo sealing suction (foot) valve, the pump can be removed or installed into a live storage tank while the column is in an inert and isolated condition, further increasing safety for maintenance personnel.  An external motor design would not be practical in a top entry storage tank as it would require a shaft of 30 to 40m in length.

In a retractable type of installation, the power cable and instrumentation cables are enclosed in flexible stainless steel pressure retaining conduits.  These conduits span the entire length of the column from the motor cavity to the headplate and are the lifeline that connects the motor casing to the outside of the tank.  This umbilical configuration provides the path for the GN2 purge gas to flow from the external side of the tank, and is the key component for enabling the use of the purged and isolated motor.  The associated headplate design to seal the top of the column are custom engineered under ASME Section VIII Division I Rules and can be configured with any nozzle orientation to meet the customer requirements.  Additionally they can be designed with extra nozzles to aid in the monitoring of the GN2 purge gas or can be configured to interface with gas detection systems to monitor for the presence of air or ammonia.

The external tank portion of the electrical system can be designed for NEC, ATEX, and IECEx hazardous area locations depending on the geographical location and desired rating by the end-user.  The designs feature a flexible multi-conductor power cable that can be connected to a cast Flameproof (Exd) or stainless steel increased safety (Exe) junction box enclosure with certified cable glands.  A flexible power cable gives many options for mounting the enclosure in the best position to optimize space in the plant environment.  Finally, the system features an IP66 rating for protection against the elements.

Factory Acceptance Testing

EIC Cryo tests all LNH3 equipment in its full-scale cryogenic test stand to prove hydraulic performance, TEM operation, as well as the magnetic coupler functionality.  The test fluid used for Factory Acceptance Testing (FAT) is liquid propane (LPG) which simulates the temperature of ammonia without the additional hazardous area concerns.  Pumps are tested from maximum allowable flow to shut-off to evaluate the hydraulic performance and efficiency across the desired operating range.  Lastly, the NPSH test is performed to confirm cavitation-free operation at low liquid levels.   The FAT is performed with the motor housing in the purged and pressurized case to simulate the site operating condition. Additionally, the motor cavity is monitored for leakage to ensure complete sealed integrity in the cryogenic condition.

Case Study

EIC was contracted to supply an intank retractable (ACR) liquid ammonia pump for an EPC building a liquid ammonia processing and send-out facility.

Design Requirements for Pump Duty:

Flow: 9.0 m3/hr (40GPM)
Differential Head: 300m (985ft)
Temperature: -32°C (-25.6°F)
Design Pressure: 27 barg (392 psig)
Low NPSHr for reduced dead stock levels in storage tank
Technical evaluation showed that in order to optimize specific speed (increase efficiency) a twelve stage configuration would be applied.  The pump would also be outfitted with a high suction specific speed inducer to help reduce the required NPSH and ultimately reduce storage tank levels to a minimum.

A two pole motor was magnetically coupled to a proven existing hydraulic combination and TEM design.  This allowed EIC to supply a known hydraulic duty with proven reliability to the Purchaser while meeting a shorter lead-time.

A support umbilical with power and instrumentation cables of thirty meters in length was designed to allow for the installation into a top entry storage tank.  The material selections for the sacrificial wear components were designed using Peek and Carbon material as discussed herein.

Factory acceptance testing proved the pump meets all the end-user acceptance criteria requirements and the equipment is ready for shipment to Purchaser facility.

Conclusion

By utilizing the design features as described herein, Ebara International Corporation offers rotating equipment solutions for tough LNH3 applications that reduces safety concerns and combines the proven Thrust Equalization Mechanism (TEM©) capability for long term reliability as well as proven hydraulic performance.  Installations to date are in operation in China, Taiwan, Thailand, Singapore and other locations around the world.