Most hydraulic pumps are air or natural gas driven reciprocating pumps. Electrically driven pumps are occasionally used. As shown in Figure 1700-12, the gas power end has a larger area than the liquid end. The pump reciprocates due to the action of a cycling gas supply.
The maximum discharge pressure of the liquid at no flow is approximately equal to the pneumatic supply pressure times the area ratio plus the liquid suction pressure. When this balance of forces is reached, the pump stalls and ceases pumping without consuming any supply gas. The pump will automatically restart when the hydraulic pressure drops below 97% of the design pressure.
Normally, a backup manually operated hydraulic pump is used for maintenance or emergency use to keep the SCSSV open.