Field Report: Prime pump

March 22, 2003
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Progressing cavity pumps help limit waste and improve productivity at a coastal fishery

In the seafood industry, fish must be processed as quickly as possible after they are caught to maintain optimum freshness on their way to market. That's why an anchovy fishery along Peru's Pacific coast operates close to shore.

In the past, centrifugal pumps and a 20-in. pipeline were used to transfer fish from barges to the fishery's processing facility. While the pumps maintained a 70:30 seawater-to-fish ratio and a 10 percent damage rate, the fishery needed a better solution to the transfer process in order to remain competitive. Hence, it contacted progressing cavity pump manufacturer, Moyno, Inc., to explore alternatives.

Moyno's progressing cavity pump provides positive displacement, and moves a known quantity of liquid with each revolution of its rotor, which is a single-threaded screw turning eccentrically within a double threaded nut (stator). This configuration is referred to as a 1:2 element profile because there is one lead on the rotor and two leads on the stator. Although the 1:2 element profile is commonplace in most progressing cavity pumps, any configuration is possible as long as the number of leads on the rotor is N and the number of leads on the stator is N+1.

Typically, the rotor is formed from hardened tool steel, chrome plated for abrasion and corrosion resistance. The elastomeric stator creates a compression fit with the rotor to form a series of cavities defined by areas of contact known as seal lines. As material present in the pumped fluid becomes impinged on the seal lines, the elastomeric stator flexes, allowing it to pass without severely abrading the rotor or stator. As such, the PC pump is able to maintain its performance consistently over a long period of time. The pump cavities are 180 degrees apart from one another and progress from the suction end to the discharge end of the stator. With 1:2 profile elements, the first cavity opens at the same rate the second cavity closes, providing a repeatable, pulsation-less flow rate. The compression fit eliminates any clearance requirements, resulting in maximum volumetric efficiencies at both high- and low-fluid viscosities. The same technology that allows the progressing cavity pump to transfer everything from clear liquids to the most viscous, solids-laden sludges and slurries gives it the ability to handle live fish.

After a consultation between fishery and members of the manufacturer's engineering staff, a design for a customized progressing cavity "fish pump" was finalized, based upon the end user's requirements. The redesigned pump features an angled suction housing that provides the fish with a smoother path of entry into the elements of the pump. The pump transfers the seawater/fish combination at a rate of 1600 gpm (363 m3/hr) with pressures up to 60 psi (4 bar).

The new pump generates the target flow rate and operating pressure without excessive operating speed for decreased power consumption. When combined with the large pump cavities that form when the rotor turns within the stator, it allows the fish to move easily through the pump with limited damage. The spiral flow path the fish follow through the pump is not far removed from a straight line, further contributing to their gentle handling. The pump can move a higher amount of fish with less water through the long pipeline.

The pump specified includes an exclusive crowned gear-type universal joint drive train called Ultra-Drive that is only manufactured by Moyno, and is engineered to effectively manage the intense torque and thrust forces associated with moving the heavy fish/water combination. Developed in the 1960's, the joint provides continuous performance without maintenance over an extended period of time, as thousands of installations have demonstrated. Torque is transmitted through line contact between the hardened crowned teeth of the ring gear and mating, spherical gear ball. Thrust forces are transmitted through the gear ball to matching bronze thrust plates positioned on each side of the ball, making the universal joint bi-directional. The thrust plates are keyed - as is the hardened outer gear - to the rotor head or shaft. The gear ball is driven by the connecting rod through a flat root involute spline and kept in position by a lock nut. Since the proprietary Ultra-Drive crowned gear universal joint is designed to transmit torque and thrust forces across a larger surface area, it exhibits relatively low compressive stresses at the points of contact for consistent performance and long life.

The joint is almost totally encapsulated in a metal containment shell leaving very little of the bellows seal exposed to the pumpage. Long life lubrication is achieved as the joint is packed with extreme pressure grease that under proper load need not be replaced for years.

With installation of the new progressing cavity pumps completed, the barges pump at a 30:70 seawater-to-fish ratio with a 5 percent damage rate. The customer is pleased since the higher flow rate, decreased product damage and lower seawater content allow it to process more fish in less time, produce a higher quality end-product and reduce waste.

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