Abstract: A line, especially for delivery of liquefied gas at ambient temperature, has two solenoid valves connected in series with an intermediate section of the line between them. In normal operation when both valves are closed, the isolated intermediate section contains liquid which absorbs heat from the atmosphere and/or from a heater, and the resulting increase in pressure is detected by a pressure sensor. If either valve fails to close fully to shut off flow, the intermediate section is not then isolated, so the absence of a signal from the sensor (the valves being set to close fully) indicates that at least one of them has not properly closed, and an alarm is given.