Multistage high-pressure pump

A multistage high-pressure pump for pressurizing a fluid, comprising:

• a rotational shaft;
• impellers secured to said rotational shaft;
• a casing configured to house said impellers therein;
• a mechanical seal constituted by a first pair of rotary seal member and stationary seal member in sliding contact with each other and a second pair of rotary seal member and stationary seal member in sliding contact with each other;
• a stuffing box secured to a side end of said casing and housing said mechanical seal therein;
• a seal chamber surrounded by a periphery of said rotational shaft, said stuffing box, and said first and second pairs of rotary seal members and stationary seal members;
• an oil reservoir configured to store oil therein;
• an oil supply line providing a communication between said oil reservoir and said seal chamber;
• an oil pump configured to pressurize the oil from said oil reservoir and supply the oil to said seal chamber, a gap being formed between said rotary seal member and said stationary seal member of each pair for discharging the oil from said chamber;
• a pressure retaining mechanism configured to retain pressure of the oil in said seal chamber such that the pressure of the oil in said seal chamber is higher than pressure of the fluid;
• and
• an oil outlet line for discharging the oil from said seal chamber, said oil outlet line being connected to said seal chamber at a site different from the gap.