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Handling Liquefied Gases Safely

Shaun Hampson, managing director of Sterling Fluid Systems (UK) Ltd, examines the advantages of a new generation of side channel combination pumps designed to handle onerous lpg applications whilst meeting the latest legislation.

Today’s generation of pumps use state of the art technology, but risks are always present when handling liquefied gas. However, the risk can be carefully considered and kept within limits that comply with safety requirements.

Future technical developments will lead to new solutions, which must be proved in practice. Pump manufacturers wish to be involved in a close exchange of experience with inspection authorities, ministries, equipment manufacturers and users, particularly when it’s necessary to determine new guidelines for lp gas plants in which pumps are installed.

The most important criteria for transfer pumps handling liquefied gases are those dictated by the characteristics of liquefied gas stored under its vapour pressure. Requirements for pumps handling liquefied gases are high-pressure generation, steep head characteristic, mixture handling, gas handling and low NPSHR (combi). The so-called combination pump, a development of the centrifugal pump consisting of side channel and centrifugal stages, fulfils these criteria.

The most common design is a horizontal construction with ring section casings and an axial suction branch. A centrifugal impeller is mounted at the suction end of the pump axially inline with the side channel stages, which form the main body of the pump up to the discharge casing.

The addition of the centrifugal stage means that a good (i.e. low required) NPSH behaviour can be achieved. Due to their multiphase pumping abilities, the inline side channel stages are capable of handling a considerable gas flow and are self-priming.

When compared with other types of pumps, the combination system requires an inlet head of only 0.5m maximum for flows of up to 35m3/h. This allows simple installation and keeps costs low. The combination of side channel stages connected inline with the centrifugal stage ensures that the flow is not interrupted when partial vaporisation occurs.

In order for large flows to be pumped economically without having to forgo the self-priming and gas handling capabilities, centrifugal combination pumps have been constructed with a side channel stage mounted parallel to the centrifugal main pumping stages. The axial inlet and a specially developed first NPSH stage leads to favourable suction heads when the centrifugal combination pump with NPSH impeller and priming stage are handling liquefied gases. The performance range currently extends up to flows of 220m³/h and discharge heads up to 380m (20 bar).

An improved barrel design increases resistance to external forces and the additional wall thickness and compact design protect the pump more efficiently against external forces. Use of a barrel with an O-ring housing seal reduces the risk of leakage and the design of pumps with sectional housings includes a requirement for additional static housing seals, each of which expects a risk of leakage. The new barrel design allows these seals to be replaced with just two or three easy to fit and maintenance friendly O-ring seals.

There are no exposed rotating shaft sections. Shaft sections in the area of rotary shaft seals have historically been mostly open and accessible to allow maintenance. In order to minimise any risk that may nevertheless be present, all rotating parts have now been completely enclosed.

A small area, at atmospheric pressure and located upstream of the rotating seal, allows the seal to be monitored for possible leakage. A small orifice directs the minimal leakage that necessarily forms part of the design into the atmosphere.

The rate of leakage goes up to indicate the end of useful service life, causing the accumulated pressure in the cooling space to increase. This can then be detected by the monitoring system, which proceeds to shut down the pump.

If stricter requirements mean that individual rotating seals are no longer sufficient, the standard configuration can include the simple addition of a second rotating seal in order to fulfil the specifications of a technically hermetic shaft seal. No modifications are needed, as the space required for installation already exists.

Sterling Fluid Systems will be displaying on stands 127 and 128 at the 2010 AEGPL Congress.

Contact Sterling Fluid Systems (UK) Ltd on 0161 928 6371 www.sterlingsihi.com

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