Non-Stop Pumps Help Keep Coffee Production Flowing

In June 2000, Michael Smith Engineers supplied six Nikkiso canned motor pumps to Star Refrigeration that operate as part of the process of manufacturing instant coffee. Since June 2001, half of the pumps have operated almost continuously for 24 hours a day, seven days a week, without any servicing, repair or breakdown. All six pumps are pumping liquid carbon dioxide, which is non-lubricating and has a viscosity of around 0.1cP, at a temperature of -54C. Four of the pumps are pumping 50m3/hr at 25m differential head, while two of them are pumping 44m3/hr at 35m differential head.

All the pumps are said to be operating with a hydraulic efficiency of around 60 per cent. Michael Smith Engineers recommended the Nikkiso BQ Reverse Circulation design for these duties, which can be mounted vertically with the motor over the pump to ensure quick and efficient venting of any vapour/gas in the rotor chamber. This design uses the process fluid to lubricate the sleeve bearings. Liquid carbon dioxide from behind the pump impeller passes through the rotor chamber, lubricating between the rotating pump shaft/rotor and the stationary sleeve bearings.

This process fluid flow also dissipates any heat building up in the rotor chamber. As the liquid carbon dioxide will have warmed up in the rotor chamber, there is a chance that it could flash off or boil and so the lubrication stream is taken from the rear bearing back to the vapour zone of the carbon dioxide storage vessel. The reverse circulation flow is around 1m3/hr, less than two per cent of the total process flow. Nikkiso canned motor centrifugal pumps are a hybrid between a centrifugal pump and an electric motor.

The pump impeller shaft and the motor's rotor are combined into one pump shaft/rotor. This shaft/rotor is hermetically sealed from the motor stator windings and rotates within the process fluid. The canned motor design has its advantages over conventionally sealed and magnetically coupled centrifugal pumps. For example, they are leak free by design and offer secondary containment as standard so in the unlikely event of a pump failure, the process fluid will be contained within the stator housing.

Nikkiso canned motor pumps only incorporate two bearings, reducing the number of wearing parts and helping eliminate potential alignment problems. They occupy a very small space and usually operate at less than 80dB. The pumps are manufactured from 316 stainless steel and are fitted with carbon graphite sleeve bearings with a wearing surface that has been lined with silicon carbide to a depth of approximately 4mm.

This silicon carbide/graphite combination has the wear characteristics of pure silicon carbide at a reduced cost. The pumps are fitted with Nikkiso's electric bearing monitor, which monitors the wear on the bearings, both axially and radially. This device uses eight sensors to continually check any deviation on the rotation of the pump's rotor and gives early indication of wear, either as a visual indication locally or remotely via a 4-20mA signal, pre-empting any major pump failure.

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