PR

WorldPumps #433 - Oct. 2002

Multifunction pump for handling gaseous liquids in process engeneering

Pumping liquids with a high gas content can be problematic, causing sharp losses in head and flow or even total cessation of pump flow. German pump manufacturer SERO believes its Multifunction SRZS Pumps provide the ideal solution for such challenging situations, offering a combination of the advantages of side-channel and radial flow centrifugal pump hydraulics. Together these achieve high pump heads, even at relatively high gas entrainments, combined with low NPSH values. Since its introduction in 1988, some 8000 SRZS pumps have been produced, and the company reports continuing growth in demand.

Many applications in processing plants require pumps that are – at least temporarily – able to handle liquids with gas or vapor entrainments, without risk of flow interruption.

Liquids pumped near their boiling point, such as condensates, liquified gases, hydrocarbons, aerosols or refrigerants, demand a lot of a pump. The operator should attempt to eliminate all risks. Radial centrifugal pumps allow only limited gas entrainments in the pumped liquid; with slight gas contents of ca. 5% their head and flow decline dramatically. Higher gas contents cause total interruption of the pump flow.

The SERO Multifunction Pump
SRZS, a side-channel pump of special construction, has no problems with gas inclusions, even under extreme conditions. For example, during venting of the suction line, the SRZS has the outstanding ability to reliably deliver the gas with the help of only a small amount of liquid left
in the pump.

Design features and operating principles
The multifunction pump combines the advantages of two pumping systems:

• the side-channel pump hydraulics for gas transport allow the pump to attain pump heads that are up to four times greater than that generated by radial flow centrifugal pumps; the radial flow centrifugal pump;

• hydraulics of the inlet stage helps achieve extremely low NPSH values.

Priming process
During the process of priming, operation of the side-channel pump hydraulics is similar to that of a gas handling vacuum pump or a compressor. The side-channel pump has the ability to work temporarily as a liquid ring vacuum pump and therefore may also be considered as a rotary displacement pump. Similar to the operating principle of a liquid ring vacuum pump, a circulating liquid ring has a displacement effect, the auxiliary liquid entering (piston-like) the impeller and the side channel and exiting during each circulation. The centrifugal effect causes collection of the liquid in the outer region of the side channel and impeller cells and thus forms the liquid ring, which is necessary for the priming process. The gas builds up in the inner region.

The special arrangement of the side channel gives the liquid ring an eccentric course, bringing about the displacement effect of the liquid ring vacuum pump. The vacuum produced allows an independent venting or degassing of the suction line and a priming of the liquid. The operation of the multifunction pump as a mere vacuum pump is only possible over a limited period. Non-supply of further auxiliary liquid at normal temperature leads to a gradual heating and finally to a vaporization of this liquid.

The multifunction pump has gained widespread use because of its unique advantage of insusceptibility to vapor or gas inclusions in the pumped liquid and at the same time its ability to achieve substantial heads. Gas bubbles are entrained through the pump along with the liquid, with a maximum allowable gas content of 50%. Liquid–gas mixtures can be handled problem-free with no need of any auxiliary devices. Gas contents of 10%, for example, have virtually no impact on the steeper characteristic curve compared with radial flow centrifugal pumps.

Commonly a side-channel pump is called ‘self-priming’, whereas ‘self-venting’ would be a much better description. The self-venting effect is a great advantage in the process of  partial vaporization. Even at the very beginning of cavitation, the performance curve of a normal centrifugal pump drops steeply, whereas the flow of the multifunction pump does not break down.

The ability to handle such a high gas content is not common for all pumps of this type. Minor constructional modifications may have a great impact on the gas-handling ability. For example, the US type of side-channel pump – known as the ‘peripheral’, ‘regenerative’ or ‘turbine’ pump – was modified to be more efficient, yet can only handle small gas contents. In contrast, the multifunction pump is capable of handling relatively high gas volumes.