Method of preparing a system for a maintenance operation

A method of preparing a system for a maintenance operation, said system comprising a first subsystem a second subsystem, a third subsystem and a bleed conduit, wherein

• the first subsystem contains a flammable fluid under super-atmospheric pressure,
• the second subsystem is located between the first subsystem and the third subsystem, and the system is capable of allowing for the fluid to flow from the first subsystem to the third subsystem via the second subsystem;
• with
• a first sealing block between the first subsystem and the second subsystem capable of being in an open state for passing fluid from the first subsystem to the second subsystem and in a closed state for blocking said flow,
• a second sealing block between the second subsystem and the third subsystem capable of being in an open state for passing fluid from the second subsystem to the third subsystem and in a closed state for blocking said flow;
• and
• said second subsystem being provided with the bleed conduit;
• said method comprising the step of
• ensuring that the first sealing block and the second sealing block are in a closed position, and in arbitrary order
• bringing the third subsystem to atmospheric pressure open to the atmosphere, and
• applying sub-atmospheric pressure between the first sealing block and the second sealing block for removing flammable gas from the second subsystem via the bleed conduit before performing said maintenance operation on the third subsystem;
• wherein the step of removing gas from the second subsystem via the bleed conduit comprises applying the sub-atmospheric pressure using a venturi-pump into which an anoxic gas to drive said venturi-pump is introduced to mix with and remove gas from the second subsystem, said venturi-pump having a maximum venturi ratio Mvr, where
• Mvr
• =
• Sf
• ×
• 2
• -
• 2
• ⁢
• (
• (
• 1
• -
• LADOC
• )
• ⁢
• 3
• 1
• -
• 2
• ⁢
• O
• 2
• -
• 3
• 3
• 1
• -
• 2
• ⁢
• O
• 2
• -
• 3
• )
• 2
• ⁢
• (
• (
• 1
• -
• LADOC
• )
• ⁢
• 3
• 1
• -
• 2
• ⁢
• O
• 2
• -
• 3
• 3
• 1
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• 1
• wherein
• Sf is a safety factor having a value of 2 or less,
• O2 is the volumetric oxygen concentration in ambient air,
• LADOC (Lowest Air-Derived Oxygen Concentration) is an O2 concentration in vol. % that can be determined with a equilateral triangular flammability diagram, said flammability diagram having
• a horizontal base of the equilateral triangle indicating the oxygen concentration in volume percent with 0 vol. % at the right and 100 vol. % at the left,
• a right side of the equilateral triangle indicating the concentration of the anoxic gas in volume percent, with 0 vol. % at the top and 100 vol. % at the bottom, and
• a left side of the equilateral triangle indicating the concentration of the flammable gas in volume percent with 0 vol. % at the bottom and 100 vol. % at the top;
• and
• a flammability area where a mixture comprising the flammable gas and oxygen is flammable, and
• defining an air line running from the apex of the equilateral triangle to the horizontal base at 21 vol. % oxygen where the ratio of the oxygen concentration and the anoxic gas is equal to the concentration of oxygen to other gases in air;
• wherein the steps of determining the LADOC are
• moving a line parallel to the air line until it has a single point in common with the flammability area, and
• determining the oxygen concentration (LADOC) at the intersection of said line and the horizontal base.