The Right Pump for Liquefied Gases is Crucial for Safety and Efficiency

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Sterling Fluid Systems (UK) Ltd

Latest Trends for LPG Pumps

The latest liquefied gas pumps from Sterling-SIHI incorporate a number of other developments to improve safety and reliability. One is the addition of a “barrel” or shell around the pump. The main benefits are secondary containment of leakage from the seals, and extra protection against mechanical stresses.

Secondary containment is often sought with liquefied gas pumps, to give the static stage seals the same attention as secondary mechanical sealing. This is achieved by constructing a barrier around the length of the pump so that any inter-stage leakage is contained.

To reduce the risk of explosion, liquefied gas pumps must be protected from dry running. High-end pumps typically include a sensor to ensure that the pump housing contains liquid. Side-channel pumps offer additional protection against dry running because they recirculate some liquid internally. Although they cannot run dry indefinitely, this design provides security under mixed-flow conditions and during priming.

The section of the pump shaft just outside the seal area has traditionally been left exposed to simplify maintenance. For increased safety, however, all rotating parts are now enclosed. An added benefit is the shorter distance between the bearing and the seal, resulting in lower shaft deflection and longer seal life.

Some new pumps feature leak detectors: A chamber between the seal and the outboard bearing contains a pressure sensor that shuts down the pump in the event of seal failure.

This arrangement is easily converted to double back-to-back sealing, since the necessary space already exists on the upstream side of the original single seal.

More aboutSpecial Horizontal Dry Running Pumps

Additional Information

Combination Pumps

Two Types of Combination Pumps

The most common type of combination pump is a horizontal design with a large axial suction branch, followed by an oversized centrifugal impeller and multiple ring-sections. Their extremely low NPSH requirement — just 0.5 m in some designs for flowrates up to 35 m³/h (see figure below) — reduces installation costs. For flowrates up to around 200 m³/h, more centrifugal stages can be added. When the pump has more centrifugal than side-channel stages it is known as a centrifugal combination pump.

Designing a Pump for Liquefied Gases

Where Liquefied Hydrocarbon Gases (LPG) are concerned, the pump manufacturer is faced with a combination of directives that surround explosive, pungent, and sometimes toxic media. Arguably, the safest position is to develop a standard pump family which encompasses all of the appropriate essential safety requirements. Where an inherent risk still exists, the user is clearly notified through the installation, operation and maintenance manual. Consequently, today’s generation of liquefied gas pumps are significantly different from yesterday’s ancestors. Priming ability, seal technology, design orientation, materials, and internal clearances are examples of evolutionary development. Although there might not be one pump that fits every application, a lot of typical problems can be overcome by carefully selecting the right pump, seal and mounting.

* Henning Mollenhauer is LPG Market Manager, Sterling SIHI GmbH, Germany. Shaun Hampson is Managing Director, Sterling Fluid Systems (UK) Ltd.

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