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

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Pumping Liquefied Gases: Suction Lift

Liquefied gases are often stored in underground tanks. To avoid the need to install underground pumps and piping, pumping is generally done from ground level — with boiling liquids this raises obvious problems with suction lift.

One solution is to mount the whole pump, or the wet end, inside the tank. Side-channel combination pumps with gas separators and recirculation lines are another alternative: During priming, where NPSH is not readily available, the pump moves a large quantity of vapour.

The resulting evaporation in the submerged part of the suction line lowers the liquid temperature to the extent that boiling ceases and liquid is pulled into the pump. From this stage on, a small fraction of the pump discharge is allowed to vaporise before being fed back to the vapour space at the top of the tank, where the subsequent pressure increase ensures stable pumping.

For propane and propane/butane mixtures with at least 20 percent propane, a rule of thumb is that suction lift should not exceed 4 m and tank volume should be less than 200 m³. For storage tanks with diameters no greater than 6 m, an alternative solution is a side-channel combination pump mounted vertically with an external motor (see figure on p. 32). If the tank bottom is fitted with a suitable well, the pump inlet can remain flooded even when the tank is practically empty. These pumps generally use magnetic couplings to eliminate seal maintenance and the risk of seal damage through vapour locking.

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Mechanical Pump Seals: Pros and Cons

Generally, pumps are rotated by an external driver of some description. Therefore, the drive-shaft needs to be sealed from gas, vapour, and liquid leakage to the external environment. The most common type of mechanical seal in liquefied gas pumping today is the single balanced type, fitted at the discharge end of the pump so that pressurised liquid is available to lubricate and cool the seal.

One issue with single seals is their inability to fully contain pungent odours, vapours and gases. This is because the pumped liquefied gas must be present across the seal face in which to provide lubrication and cooling.

More serious is the fact that any mechanical seal will inevitably fail in time, typically as a result of excessive shaft displacement caused by bearing wear or cavitation. Because the single mechanical seal has no backup, there is always a risk of spillage. Condition-based monitoring (CBM) systems can reduce the risk by monitoring cavitation and bearing wear, but will not eliminate it totally.

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