Harmonization of European laws has increased the stringency of requirements covering design, safe use, and leakage control in pumps. When handling inherently hazardous liquefied petroleum gases (LPGs) pump designs should tick all the regulatory boxes as well as bring economic benefits to operators.
Liquefied hydrocarbon gases include propane, butane, and their derivatives. Other industrially important liquefied gases are carbon dioxide, ammonia, and other refrigerants.
Storage, transportation, and pumping of such liquefied gases present the key success factors to handling this media. It is vital that the difference between this set of fluids, when compared to conventional media, is fully understood when specifying equipment. In simple terms, pumping a flammable liquid at its boiling point needs greater care than, say, water at 60 °C.
Regarding the pumping equipment, there is a set of criteria that the user or specifier ought to consider as fundamental to the long-term, trouble-free operation and ownership. The high vapour pressure of the liquid determines the most important specifications for a liquefied gas pump:
- low net positive suction head (NPSH);
- high differential pressures;
- steep characteristic (head-flow) curve;
- ability to pump gas as well as liquid;
- two-phase pumping without a significant reduction in capacity; and
- Atex rated in the case of flammable substances, which make up the majority of liquefied gases.
The technologies used to pump liquefied gases 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 pumps provide effective gas/vapour/liquid pumping with exceptionally low NPSH(r) and a very straight performance curve. They are robust enough for decades of trouble-free service, readily available in sealless magnetically coupled designs, and can run dry for periods of time. They are, nevertheless, limited by relatively low maximum flowrates and efficiency.
- Centrifugal pumps provide higher flowrates, can offer high efficiencies and are available sealless. However, their NPSH(r) is nothing special and vapour locking is a risk. Vertical pumps with submerged suction overcome NPSH limitations, but can cause vapour to collect at the highest point of the pump, preventing cooling and seal lubrication by the medium. Pressurised back-to-back double seals solve this problem, but at the expense of a maintained system.
- Positive displacement sliding vane pumps self-compensate for wear, which enables a peak performances to be maintained over the life of the vanes. They are well suited to thin, volatile liquids and have a steep performance curve. NPSH requirements are high, however, and cavitation can mean short pump life if the NPSH is compromised for even short periods of time. Sliding vane pumps are noisy, which can exclude them from domestic areas, and consequently autogas filling stations.
- Combination pumps combine the benefits of side-channel and multi-stage centrifugal pumps, yet avoid their drawbacks. They provide high flows and pressures, low NPSH, and limited tolerance to dry running. Combination pumps are robust enough to provide decades of trouble-free operation and can be supplied sealless with magnetic coupling.
More about side channel pumps? Read an article On Special Assignment
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