Abstract: Disclosed is a method and apparatus for reducing the level of noise emitted by air turbine machinery, particularly a pneumatically driven hydraulic pump for an airplane, wherein a modulating valve meters the flow of air from a source of pressurized air through a conduit to an air turbine to control the speed of the turbine. A pressure regulating valve is located in the air conduit upstream of the modulating valve to regulate and maintain the upstream pressure on the modulating valve at substantially the minimum pressure required in order to permit operation of the air turbine over its full range of speed as the amount of airflow is varied with the modulating valve. Reducing the pressure across the modulating valve in this manner minimizes the production of supersonic airflow and attendant noise by the modulating valve, which noise is otherwise propagated to the atmosphere through the turbine exhaust stream and is additionally radiated by the turbine casing.

Abstract: An improved fluid flow control valve of the type wherein a spool is slidable within a sleeve cavity provided in the valve body. Primary fluid passageways through the body access the sleeve cavity at main orifices which form metering surfaces with lands on the spool. The improvement comprises secondary fluid passageways which carry parallel flow with the primary passageways. The secondary passageways access the sleeve cavity at sacrificial orifices which form sacrificial metering surfaces with the spool lands. The sacrificial metering surfaces carry all of the flow through the valve at the valve neutral position thus reducing flow erosion at the main metering surfaces. Valve leakage is limited to a small value determined by the total flow capacity of the secondary passages.