Abstract: (1) A wall cleaning unit including a housing with an open side for placement in contiguous relationship to a wall, upper and lower spray bars in the housing for directing high pressure water against the wall, sealing members on the housing around the opening for effecting sealing relationship with the wall, an outlet on the housing for conducting liquid away from the housing, and a high pressure liquid jet arrangement for creating a venturi effect to produce a suction for pressing sealing members against the wall. (2) A system for washing vertical walls including a vehicle mounted for movement along the top of the wall, a boom on the vehicle, a reel on the boom for moving a spray housing vertically on the wall, a source of high pressure water, a second reel on the boom for mounting a hose conducting high pressure water to the spray housing, and a motor on the vehicle for selectively moving the boom to cause the spray housing to move toward and away from the wall.

Abstract: A pressure relief valve including a housing, first and second bores in axial alignment in the housing, first and second plungers in the first and second bores, respectively, a third bore in the first plunger, a seal between the first and second plungers with the end of the second plunger being located in communication with the third bore, a first conduit for placing the housing in communication with a source of fluid pressure to expose the first plunger thereto and also expose a portion of the second plunger thereto through the third bore, a shear pin for holding the second plunger in position whereby the fluid pressure on the first and second plungers is exerted on the shear pin, a plurality of radial bores in the housing proximate the seal between the first and second plungers, and a second conduit in communication with the radial bores for conducting high pressure fluid from the housing upon shearing of the shear pin and after movement of the second plunger away from the first plunger.