Safety relief valve having a low blow-down value and spring therefore

A method of preparing and using a coil spring in a pressure relief valve comprising:

• a) measuring the spring rate of the coil spring;
• b) modifying the spring after measuring its spring rate so as to modify its spring rate to be within ±2% of a target spring rate;
• and
• c) building a pressure relief valve having an inlet comprising an inlet valve seat, a disk member closable on the inlet valve seat and a mechanism biasing the disk member on the inlet valve seat, a body, and an outlet, wherein the disk member and inlet valve seat are configured to provide a huddling chamber, with the modified coil spring being used in the biasing mechanism, and wherein the relief valve further comprises a secondary orifice between the inlet valve seat and the outlet, the inlet being configured so that gas flows from the inlet valve seat in a sonic flow when the valve opens due to a pressure in the inlet exceeding the set pressure, and the secondary orifice being sized so that gas flows through the secondary orifice in a sonic flow when the valve opens due to a pressure in the inlet exceeding the set pressure.