or pressure. On LHAs, the pressure regulator is typi- cally installed after the filter. On other class ships, it may be installed before the filter. An automatic pressure-regulating system is pro- vided in all MOGAS distribution risers. The pressure regulator consists of an automatic pressure-regulating valve operated through changes of pressure in the throat of a venturi, located down- stream of the valve. The main components of the system (fig. 6-12) are as follows: Main valve (pressure regulator) (5) Pilot valve (6) Ejector strainer assembly (7) Control valve (9) Venturi tube (2) NOTE Venturi Principle. If a fluid flowing through a tube reaches a constriction or narrow- ing of the tube, the speed of the fluid flowing through the constriction increases and its pres- sure decreases. If the fluid flows beyond the constriction into a tube the same size as that of the original, the speed of flow decreases and the pressure increases. The pressure-regulating system is entirely hy- draulic in operation, using line pressure to open and close the valve. Because of this, it can be installed either vertically or horizontally in the riser. The main valve is of a modified globe design, employing a well-supported and reinforced dia- phragm. When line pressure is admitted to the cover chamber, the valve tends to close. When pressure is reduced in the cover chamber, line pressure under the disk opens the valve. The pilot valve is a direct-acting, spring-loaded valve designed with a large diaphragm and effective working area to ensure sensitive control and accurate regulation of the required delivery pressure. The pilot valve is located in the actuating line between the ejector strainer and the venturi throat. It is normally held open by a compression spring. When venturi throat pressure acting under the diaphragm increases, the valve tends to close. When venturi throat pressure decreases, the valve opens (wider). Thus, a constant pressure is maintained by balancing venturi throat pressure against spring tension. The ejector strainer assembly is installed in the actuating line between the main valve and the pilot valve. It consists of an ejector nozzle with a 1/16-inch orifice protected by a 60-mesh monel strainer to prevent clogging of the nozzle. The assembly speeds up the operation of the main valve by speeding up the evacu- ation of fluid from the cover chamber. It prevents chat- ter of the main valve by reducing the violence with which pump discharge pressure is admitted to the main valve cover chamber. The control valve is a direct-acting, spring-loaded valve designed with a large diaphragm and effective working area to ensure positive operation. The control valve, located in the ejector bypass line, is normally held closed by a compression spring. Its purpose is to close the main valve quickly when there is a sudden buildup in downstream pressure. It is opened by venturi throat pressure 10 psi in excess of the pilot valve setting. Venturi tubes are installed in the distribution riser downstream of the regulating valve. The venturi ta- pers from a 2-inch inlet to a 3/8-inch throat to a 2-inch outlet. A recirculating line on the delivery side nor- mally returns 5% of the capacity of the booster pump. Operation of the Automatic Pressure Regulator In the operation of the system, high-pressure fuel flows initially from the pump and enters the main valve body. This fuel bypasses the main valve seat and flows through the ejector strainer assembly to the pilot valve. The pilot valve is held open by its spring. From the pilot valve, this flow is directed into the throat of the venturi tube. At this point, the pressure at the throat of the venturi tube is practically nonexistent. As long as the pilot valve stays open, maximum flow through the ejector strainer assembly is permit- ted. This flow through the ejector strainer assembly creates a reduced pressure in the main valve cover chamber. (Remember that the ejector strainer assem- bly works like an eductor.) Line pressure from the pump, working under the disk of the main valve, can now open the main valve, permitting flow into the distribution riser. This flow builds up pressure in the distribution riser. The increasing pressure in the riser is transmitted from the throat of the venturi tube to the underside of the pilot valve diaphragm. When the pressure under the pilot valve diaphragm reaches a point where it is greater than the setting of the pilot valve spring, the pilot valve 6-13