Canned Motor Pumps for the Conveying of Liquefied Gases — Alternative for High Pressure

Related Vendors

Lutz Pumpen GmbH

EDL Anlagenbau Gesellschaft mbH

ProMinent GmbH

HERMETIC-Pumpen GmbH

Employment of Multistage Pumps

As a rule, multistage canned motor pumps are used for lower flow rates and higher differential heads. They can be designed as multistage pumps or as so-called barrel-type pumps. Preferentially, multistage canned motor pumps are employed if the necessary differential head can no longer be achieved at 2.900 min^-1 or if the factor of maximum efficiency is the priority.

The efficiency of centrifugal pumps is dependent on the specific rotation speed nq and the flow rate Q. At around n_q = 40, the efficiency reaches a peak that increases with an increase in the flow rate. Therefore modern boiler feed pumps are also designed for conventional power stations with performances of up to around 50 MW in this area.

The specific rotation speed n_q, an important characteristic number of the impeller geometry, referring to the pumping data within the best efficiency point of the impeller, in multistage pumps referring to the pumping data of only one stage:

n_q = n·Q_opt^½ / H_opt^¾ equation (1)

If the necessary total head H_tot is divided into several stages i, a rise in the specific rotation speed can be achieved in comparison with single-stage pumps and consequently a rise in efficiency:

n_q = n·i^¾·Q_opt^½ / H_opt^¾ equation (2)

(n_q , n in min^-1, Q_opt in m³/s, H_opt in m)

This can be shown with the example of a hermetic canned motor pump. An optimum efficiency pump is sought for the following pumping data:

• medium to be pumped: Methylchloride,

• flow rate: 25 m³/h,

• differential head: 110 m,

• density: 890 kg/m,

• temperature: 30 °C.

Firstly a single-stage canned motor pump in accordance with a chemical hydraulic standard (EN 22858; ISO 2858) CNF 40-315 with 2.930 min^-1 is selected. The total power requirement is 20 kW, according to the characteristic curve, at an efficiency of 33.2% and n_q = 7.2 min^-1. If the total head is divided into several stages, a four-stage canned motor pump Type CAM 32/4 with a rotation speed of 2.955 min^-1 and equal pumping data can be selected. At the same time the specific rotation speed rises to n_q = 20.4 min^-1 in accordance with equation (2).

In accordance with the characteristic curve, the corresponding total power requirement amounts to only 12.3 kW because the total efficiency of the pump increases to 54.2%.

The power difference to be saved is 7.7 kW. The saving energy is more than one third (39%).

* The author was Sales Manager Hermetic Pumps, Gundelfingen/Germany.

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