Abstract: A header pipe has an external sealing surface. A diaphram type control valve is externally mounted to the header pipe. An outlet pipe is mounted to the header pipe, enters and crosses the flow passage in the header pipe, and has a threaded end. The body of the control valve has a skirt which bears against the sealing surface, and a tube which is threaded onto the threaded end of the outlet pipe to mount the valve and to bring the skirt against the sealing surface. A peripheral seal fits between the skirt and the sealing surface. It may be a cup-sectioned seal which is pressure-biased to form the seal.