3.66 Pump influence of gas

Pump influence of gas

Pump influence of gas. dissolved and undissolved (entrained) gases, depend on pump design.The pumped fluid can contain certain quantities of dissolved gas (mostly air). Lubricating oil, for example, at atmospheric pressure and room temperature can contain up to 10 % air by volume, petrol can contain up to 20% air by volume. The ability of a liquid to dissolve gas reduces with reduced pressure and with increased temperature. Gas can therefore separate out in areas of localized low pressure and re-dissolve when the pressure increases again. This process is similar in some ways to the formation and implosion of vapour bubbles (cavitation), but is considerably less violent and causes no mechanical damage. When gas separation and cavitation occur simultaneously the gas has a dampening effect on the implosion process and in this way helps to limit the extent of damage caused by cavitation.

The pumped fluid can also contain undissolved gas (usually air). This entrained gas has considerable influence on pump performance. Both the delivery head and the efficiency are noticeably reduced by increased quantities of entrained gas.

Figure 3.66a The influence of entrained air in water on a centrifugal pump Q-H curve

As shown in figure 3.66a, the originally stable Q-H curve becomes unstable due to the influence of undissolved air. For the system curve shown there are, for a given concentration of entrained air, two alternative intersection points and there is risk of unstable operating conditions.

Certain conditions can cause air to collect in the pump impeller, which can lead to failure of the pumping function. The risk of air collecting is greatest for flows which are less than the designed flow. It is normally possible to pump without other difficulties arising, other than reduced efficiency, for air concentrations of up to 2-4% by volume measured at the pump intake connection

Displacement pumps usually cope with larger concentrations of air better than centrifugal pumps. For displacement pumps operating with a suction lift the theoretical volume flow is reduced considerably (up to 50%) if the fluid contains undissolved or dissolved gases. Undissolved gas will expand and dissolved gas will separate out and partly fill the displacement.

The capacity loss factor f according to equation 3.66a, i.e the volume flow in relation to a gas-free liquid, becomes, for a liquid containing entrained (undissolved) gas:

where

g_a1 = concentration by volume of entrained gas at atmospheric pressure (-)
f₁ = capacity loss factor for entrained gas only (-)
p_a = absolute atmospheric pressure (Pa)
p = absolute intake pressure (actually measured inside the displacement pump chamber (Pa)

The capacity loss factor in the case of dissolved gas is dependent upon the solubility of the gas at various pressures. Assuming that the solubility is proportional to the absolute pressure, then the capacity loss factor in the case of dissolved gas only becomes:

g_a2 = concentration of dissolved gas at atmospheric pressure (-)
f₂ = capacity loss factor for dissolved gas only (-)