dftpTried & Tested

Reprinted from the July 2016 Edition of Hydrocarbon Engineering

Tyler Brower, Ebara International Corporation USA, discusses the evolution of the submersed motor pump for use in the hydrocarbon and petrochemical industries.

Intro

One of the worst phrases we can often hear in the business environment is “because that’s the way we’ve always done it”.  Any company or industry that fails to look for better alternatives or improvements to current technology or processes is doomed to fall behind their competitors, and possibly fail as a business as a whole.  No industry or business is immune to this truth, and the petrochemical industry is no exception. Given the sometimes dangerous nature of the products handled, it is very easy to continue using the same piece of equipment over and over because it has been proven to work, or work good enough. Seal-type external motor pumps have always been the work-horses of the petrochemical pumping industries, but what if there was a better way?

In this article we will present how the submersed motor pump (SMP) is a modern solution for the needs of the petrochemical and hydrocarbon industries as an alternative upgrade for seal type external motor pumps. For decades, SMPs have been widely accepted in the LNG industry for a wide range of applications that include both land based and vessel mounted installations. SMPs are also used in various natural gas liquid (NGL) applications related to LNG production including ethane, propane, butane, etc. SMPs are manufactured in a multitude of installation configurations including vessel (pot) mounted, in-tank retractable, and in-tank permanent mount, allowing them to be installed in any application currently using external motor style pumps.

History of Submerged Motor Pumps

The first patent for the transportation of liquefied gas was obtained by Godfrey Cabot in 1915 for the “Means of Handling and Transporting Liquid Gas”. It wasn’t until the 1950’s that the technology was actually realized with work taking place simultaneously both in the United States (US) and Europe to design and build the first liquefied gas carrier vessel. In 1958 the US was the first to market with the commissioning of the Methane Pioneer, which delivered its first cargo to the United Kingdom in January, 1959.  The Methane Pioneer utilized the currently available technology with a 1000 gpm deepwell pump, serving as the cargo pump in each tank.

The deepwell pumps operating aboard the Methane Pioneer had difficulties in operation due to the difference in temperature between the pumping fluid and the external ambient temperature. This differential in operating temperatures caused shaft binding issues due to the thermal expansion/contraction properties of the materials used in construction.  The idea of a fully submersed pump/motor combination was then designed and implemented by the J.C. Carter Company in 1961 and installed in the French carrier vessel Beauvais in 1961. This technology was able to be realized due to two principles. First, due to the nature of LNG storage, the vessel in which the liquid was carried and stored created an oxygen inert environment. This meant that the risk of combustion had been removed, eliminating any explosion due to electrical or mechanical arcing. Secondly, hydrocarbons are dielectric fluids, thus they provide a medium for insulating the motor and cabling system, allowing the equipment to be safely submerged in the fluid.

This new technology proved itself aboard the Beauvais and was soon approved by the current marine classification societies. It soon became the standard aboard future LNG transportation vessels. SMPs slowly began to replace deepwell style pumps in all aspects of LNG production and distribution, ultimately making Cryogenic SMPs the standard at LNG facilities around the globe.

Design of Submerged Motor Pumps

Submerged motor pumps utilize a design that is considerably less complex than that of a deepwell external motor type pump. SMPs use an integral shaft for both the hydraulic set as well as the motor, creating a compact simple design. The same base design is used for many different configurations of SMPs, allowing the pump to be easily installed in a tank column, vessel (pot), or permanent tank mounting in a variety of tank styles. Figure 1 shows two typical SMP pump configurations.  This compact design also allows for a much reduced installation footprint particularly for column mounted pumps, reducing structure size for both land based and floating/vessel installation. Figure 2 shows a SMP vs. an external motor type pump.

Typical Submerged Motor Pump (Column & Vessel Configurations)

Figure 1: Typical Submerged Motor Pump (Column & Vessel Configurations)

Size comparison of Submerged Motor Pump & External Motor Pump of equal power/hydraulic requirements

Figure 2: Size comparison of Submerged Motor Pump & External Motor Pump of equal power/hydraulic requirements

Advantages of Submerged Motor Pumps.

The simplified design of the SMP not only offers the advantages as previously listed, but also eliminates many of the shortcomings of the external motor pump which lead to downtime and high maintenance needs and costs.

Greatly extended motor life
One of the largest departures in design when changing to a submerged motor pump is exactly as the name implies, the motor is designed to be completely submerged and operated in the pumping fluid. This design allows the motor to be constantly cooled with a flow of pumping fluid through the rotor air gap. In typical SMP operation the heat rise of the operating motor is only 1-3* of that of the pumping fluid. With heat being one of the largest threats to motor life (it is generally estimated that for every 10* rise in operating temperature that the life of the motor insulation is reduced by half), this heat sink effect of the cooling fluid virtually eliminates the risk of motor failure due to heat.  A secondary benefit of this much lower operating temperature is that the required motor size is also greatly reduced due to the efficiency of motor materials in this low temperature environment. This helps contribute to the smaller footprint of vessel mounted SMPs and the reduced column diameter of in-tank mounted pumps. The location of the motor now being inside of the tank or vessel also greatly increases the life of the motor by eliminating the effects of the outside environment. Many installations where petrochemical/hydrocarbon pumps are required expose the external motors to severe conditions such as high heat, humidity, airborne contaminants on land based sites, and salt/brine/water on ship board installations.
With safety being a paramount concern at every facility, an SMP offers a significant safety advantage over an external motor by placing the motor into an oxygen inert environment in the tank or vessel. With no oxygen available to allow ignition of gasses the area of the pump installation is now considered a non-hazardous environment, ensuring safe operation of the pump and eliminating the risk of spark hazard. Installing the motor in a non-hazardous area also eliminates the need for complex hazardous area certifications for not only the motor, but the power cabling associated with pump operation.

Elimination of seals
The leading cause of pump downtime are seal related issues, either by replacement at end of life or failure due to lack of required maintenance. Even with proper maintenance, mechanical pump seal life can only be extended with an expected lifetime of 36 months. With an SMP design there are no seals required for operation. This removes of the risk of unexpected downtime due to seal failure, the need for ongoing maintenance of seals, the risk of creating a hazardous environment due to excessive seal leakage, and contamination of the pumping fluid due to leakage. While seal life can only be extended through proper maintenance, the only way to completely mitigate their risks is to remove them from the pump altogether, a solution only possible with SMPs.

Removal of thrust bearings and bearing lubrication system
Just as with mechanical seals, external motor pumps have an additional high maintenance item in their thrust bearings as well as their external bearing lubrication systems. In the initial development of SMPs for LNG service it was found that thrust bearings performed poorly in cryogenic service, so a new solution was needed. Initially a balance drum/disk configuration was used, but it was found to have some problems in properly balancing the rotating assembly across the entire pump operating range. In 1973, Cryodynamics (a division of today’s Ebara International Corporation, EIC) developed a new solution to thrust balancing which did away with the drum/disk balancing assembly. The solution used a fixed and variable orifice system integrated into the last stage impeller via an offset wear ring and a thrust plate, with a floating rotating assembly. This advancement has since become the Thrust Equalizing Mechanism (TEM®), and is used on all EIC submerged motor pumps as well as liquid expanders. (See Figure 3). This configuration allows for excellent rotating assembly balancing across the full operational range, and removes the risk of thrust bearing failure as well as the need for continual maintenance.

TEM-min-gap

Figure 3: TEM Design

Much like the cooling of the SMP motor, the pump design utilizes the pumping fluid to both cool and lubricate the rotational bearings. With an SMP there is no longer a need for an external lubrication system and/or constant maintenance of the pump/motor bearings, as well as maintenance of the lubrication system itself. Coupled with the equal balancing of the TEM, this system provides for extremely extended bearing life compared to external motor pump, with greatly reduced maintenance needs and related downtime.

No shaft couplings or alignment issues.
As described earlier SMPs use and integral shaft between both the hydraulic set (impeller and inducer) and the motor rotor. With this design there is no need for any motor couplings in any portion of the pump and motor system, again greatly reducing the potential for downtime, need for maintenance, and requirements of on-hand spare parts. Without the potential for shaft mis-alignment from improper installation or thermal variances, bearing life is also greatly increased due to reduced load and vibration.

Economic impacts of Submerged Motor Pumps

When considering the purchase of new equipment for any business it is always important to not only review the initial cost of the equipment, but also the long term cost associated with maintenance, personnel, and spare parts. In the case of an SMP vs. an external motor pump, a cost-benefit analysis should be done between the two pieces of equipment that take into account:
Direct repair costs:

  • Material
  • Labor

Indirect Costs:

  • Admin cost for creation of purchase orders
  • Inventory Capital
  • Inventory Management
  • Personnel Management
  • Loss of Production

When considering that SMPs remove many of the high maintenance requirement and failure prone components that are present on external motor pumps,  it is easy to see that the long term operating costs can quickly make this style of pump the most economical choice for petrochemical/hydrocarbon needs.

Summary

When breaking out of the box of what is the “standard equipment” for any process or construction, all items must be considered. Cost, reliability, safety, etc. should all be part of the equation. When compared to external motor style pumps, submerged motor pumps offer a modern, reliable, and safe solution to petrochemical/hydrocarbon pumping needs that provide the user with lower lifetime operating costs, reduced maintenance requirements, and increased safety.

Bibliography:

Flood, S., “Mechanical Seal Reliability – What Realistically Can Be Achieved” IMechE Mechancial Sealing Technology Seminar, London, UK, April 2007
Cords, M. “The History of Submerged Motor Pumps” Hydrocarbon Engineering, February 2011

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