Method and a control arrangement for determining the status of a fluid tank

A method for determining the status of a primary fluid tank or one or more secondary fluid tanks, each of the primary and secondary fluid tanks being arranged to contain a fluid or a mixture of fluids available in liquefied form and vaporised form, wherein

• the primary and secondary fluid tanks are parts of a pressurized part of a fluid system, wherein
• each of the primary and secondary fluid tanks has a first intake line having an inlet located inside a respective one of the primary and secondary fluid tanks, the first intake lines being part of the pressurized part, wherein
• the inlets of the first intake lines are arranged to mainly receive a liquid of the fluid, wherein
• the primary and secondary fluid tanks are in vapour communication with one another via a first pressurized fluid conduit arrangement such that a vapour of the fluid can travel from one of the primary fluid tank and the secondary fluid tank to the other one of the primary fluid tank and the secondary fluid tank, the first pressurized fluid conduit arrangement being part of the pressurized part of the fluid system, wherein
• the first intake lines are arranged to be fluidly connected to a second pressurized fluid conduit arrangement and arranged to provide a fluid flow from the primary and secondary fluid tanks to the second pressurized fluid conduit arrangement, the second pressurized fluid conduit arrangement being part of the pressurized part of the fluid system, wherein
• the fluid system comprises one or more sensors for determining one or more pressures of the fluid in the pressurized part, wherein the method comprises:
• determining, by usage of the one or more sensors, one or more pressure drops (Δp) associated with the fluid flow through the second pressurized fluid conduit arrangement;
• when a pressure drop (Δp) exceeds a threshold value, blocking the fluid flow from one of the primary and secondary fluid tanks to the second pressurized fluid conduit arrangement while the fluid flow from the other one of the primary and secondary fluid tanks to the second pressurized fluid conduit arrangement continues;
• and
• determining, by usage of the one or more sensors, a pressure drop (Δp) associated with the fluid flow through the second pressurized fluid conduit arrangement when the fluid flow from said one of the primary and secondary fluid tanks to the second pressurized fluid conduit arrangement is blocked while the fluid flow from the other one of the primary and secondary fluid tanks to the second pressurized fluid conduit arrangement continues,
• wherein the method further comprises:
• if the pressure drop (Δp) returns to a value below the threshold value when the fluid flow from said one of the primary and secondary fluid tanks to the second pressurized fluid conduit arrangement is blocked, determining that said one of the primary and secondary fluid tanks is empty of liquefied fluid, or
• the method further comprises:
• if the pressure drop (Δp) remains above the threshold value when the fluid flow from said one of the primary and secondary fluid tanks to the second pressurized fluid conduit arrangement is blocked, blocking the fluid flow from the other one of the primary and secondary fluid tanks to the second pressurized fluid conduit arrangement while allowing the fluid flow from said one of the primary and secondary fluid tanks with the previously blocked fluid flow to the second pressurized fluid conduit arrangement;
• determining, by usage of the one or more sensors, a pressure drop (Δp) associated with the fluid flow through the second pressurized fluid conduit arrangement when the fluid flow from the other one of the primary and secondary fluid tanks to the second pressurized fluid conduit arrangement is blocked while the fluid flow from said one of the primary and secondary fluid tanks with the previously blocked fluid flow to the second pressurized fluid conduit arrangement is allowed;
• if the pressure drop (Δp) remains above the threshold value when the fluid flow from the other one of the primary and secondary fluid tanks to the second pressurized fluid conduit arrangement is blocked, determining that the primary and secondary fluid tanks are empty of liquefied fluid;
• if the pressure drop (Δp) returns to a value below the threshold value when the fluid flow from the other one of the primary and secondary fluid tanks to the second pressurized fluid conduit arrangement is blocked, determining that the other one of the primary and secondary fluid tanks is empty of liquefied fluid.