Abstract: A lockout valve comprises a body with an inlet passage and an outlet passage which define a longitudinally-extending flow path through the body. A flow interrupter device, or slide, is mounted for transverse movement within the body. The slide can be located in a closed position, whereby fluid is prevented from flowing from the inlet passage to the outlet passage, but fluid downstream of the lockout valve is allowed to vent to atmosphere; to an intermediate bleed position, whereby restricted fluid flow is allowed from the inlet passage to the outlet passage through a bleed aperture; and to a fully open position whereby fluid is allowed to flow substantially unimpeded through a flow aperture from the inlet passage to the outlet passage. A latch normally prevents the slide from moving from the bleed position to the open position while fluid pressure rises across the lockout valve. The latch can be manually manipulated to allow the slide to be moved to the open position.