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Lpg Gas Pumping Combination

By Shaun Hampson, managing director, Sterling Fluid Systems (UK) Ltd

Harmonisation of European law has resulted in some changes to lpg pump specifications with regards to their design and safe use, and the risk of environmental leakage.

Safety directives now refer to the risk of explosion emanating from not only the electrical elements of an installation, but also mechanical equipment, such as pumps, fans and compressors.

This modified legal position requires manufacturers to make greater efforts to ensure safety. Additionally, new international ambient air quality regulations (European Directive 96/62/EC) have been released to protect the environment against seal leakage.

Where liquefied hydrocarbon gases, including 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 that encompasses all safety requirements.

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.

Liquefied Hydrocarbon Gases
Not only do liquefied gases vary in terms of their industrial and domestic uses, but also the way in which they are produced. Liquefied gases, as defined by DIN51622, include media, such as propane, butane and their derivates. However, liquefied gases also include others, such as carbon dioxide, ammonia and other refrigerant-based components.

There are various technologies that have been adopted for pumping liquefied gases. Generally, these include side channel pumps, centrifugal pumps and sliding vane positive displacement pumps. Although each of them has the ability to cope with the demanding nature of such boiling media, they differ in their ability to cope with the demanding nature of the product, application and site.

Side channel
Side channel pumps offer very effective gas/vapour/liquid pumping, with an exceptionally good NPSH(r). They are highly robust and have the ability to give decades of trouble free service.

They are also blessed with a very straight performance curve, which lends itself to pumping liquefied gases, and are readily available in seal-less, magnetically coupled designs with which to remove the inherent mechanical seal issues. They can also run dry for periods of time.

So why are side channel pumps not the ideal choice? Because they are limited by relatively low flow rate and efficiency.

Centrifugal
Centrifugal pumps address the issue of minimal flow rate and offer some high-end efficiency. Seal-less designs are also available, which again removes the inherent leakage risks. Often, this technology represents an adaptation of a standard liquid pump, and is consequently limited by its design.

Centrifugal pumps generally offer very ‘normal’ levels of NPSH requirement but, critically, are susceptible to the problems associated with entrained gas and/or vapour, such as reduced performance and even vapour locking. NPSH limitations are sometimes overcome by the employment of long vertical designs, which are excavated into the ground in order to give additional liquid head at the point of suction entry.

Concerns arise with vertically oriented centrifugal construction, as entrained/released vapour/gas naturally finds its way up through the liquid to the highest point – which is often the seal chamber. As the seal area can fill with vapour, the (product) lubrication and cooling of the mechanical seal can be detrimentally affected. Again, this can be overcome with pressurised back-to-back double seals, but this is at the expense of a maintained system.

Sliding vane
Positive displacement sliding vane machines benefits from self-compensating wear adjustment, which permits peak performance to be maintained over the life of the vane. The sliding vane pump is well matched to very thin volatile liquids, and their construction materials allow them to be manufactured in accordance with ATEX requirements. They too have a steep performance curve, which permits small quantities of liquefied gas to be pumped at relatively high pressure.

Principally, sliding vane technology can be susceptible to cavitation because of its significant demand for process NPSH. Consequently, lifespan can sometimes be quite limited if the NPSH is compromised for even short periods of time.

Additionally, its adoption is compromised in domestic areas because of its noisy operation. Autogas-centric choices can often be driven by such a noise limitation, as fuel filling stations are often in the close vicinity of domestic housing.

Combination
Finally, there is one choice that combines the benefits of the aforementioned technologies, while relieving the user from their weaknesses. For obvious reasons, this machine has been termed the ‘combination’ pump.

Taking all of the side-channel benefits and integrating them with those of a multi-stage centrifugal pump results in high flow; high pressure; arguably unbeatable NPSH at varying duty points; limited tolerance to dry running; robustness, where decades of trouble free operation are the norm rather than exception, and sealless with magnetic coupling. Equally, these units are available with double back-to-back (pressurised seal system), or tandem (unpressurised seal system) seals with leak detection facilities.

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

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