Primary loop start-up method for a high pressure expander process

A method for start-up of a system for liquefying a feed gas stream comprising natural gas, the system having a refrigerant system comprising a primary cooling loop and a sub-cooling loop, the method comprising:

• (a) pressurizing the refrigerant system,
• wherein step (a) comprises:
• a1. providing the feed gas stream at a pressure less than 1,200 psia, and introducing a first portion of the feed gas stream to the primary cooling loop as a primary loop refrigerant;
• a2. pressurizing a sub-cooling refrigerant in the sub-cooling loop to a sub-cooling loop pre-circulation pressure;
• and
• a3. pressurizing the first portion of the feed gas stream in the primary cooling loop to a primary cooling loop pre-circulation pressure;
• (b) starting and establishing circulation of the primary loop refrigerant in the primary cooling loop, the primary loop refrigerant passing through at least one primary cooling loop compressor unit and reaching a primary cooling loop discharge pressure that is higher than the primary cooling loop pre-circulation pressure;
• (c) starting and establishing circulation of the sub-cooling refrigerant in the sub-cooling loop, the sub-cooling refrigerant passing through a sub-cooling loop compressor unit and reaching a sub-cooling loop discharge pressure that is higher than the sub-cooling cooling loop pre-circulation pressure;
• and
• (d) after starting and establishing circulation in the primary cooling loop and in the sub-cooling loop, ramping up a flow rate of the first portion of the feed gas stream to the primary cooling loop and ramping up circulation rates within the primary cooling loop and the sub-cooling loop;
• wherein a second portion of the feed gas stream undergoes indirect heat exchange with the primary loop refrigerant and the sub-cooling refrigerant in a heat exchanger zone.