Positive displacement pump systems with a variable control orifice

A positive displacement pump system comprising:

• a first delivery passage for transferring a first flow portion of pumped fluid;
• a second delivery passage for transferring a second flow portion of pumped fluid;
• a main discharge passage communicating with said first delivery passage and said second delivery passage so as to receive the first flow portion and the second flow portion therefrom as a main flow;
• a non-return valve located between said second delivery passage and said main discharge passage;
• a pressure-sensitive variable control orifice in said main discharge passage for receiving the main flow, said pressure-sensitive variable control orifice being adapted to automatically open an amount based on a pressure of the main flow;
• an overspill passage;
• and
• a control valve for apportioning the second flow portion from said second delivery passage between said main discharge passage and said overspill passage, and for controlling a by-passing of the main flow from said main discharge passage to said overspill passage, said control valve comprising:
• a valve body including a bore therethrough, an overspill port communicating with said bore and said overspill passage, an annular by-pass port communicating with said bore and said second delivery passage upstream of said non-return valve, and a spring chamber, said bore having a first end in communication with said main discharge passage upstream of said pressure-sensitive variable control orifice, said spring chamber communicating with said main discharge passage downstream of said pressure-sensitive variable control orifice;
• a valve member slidably mounted within said bore and having a first metering land between said first end of said bore and said overspill port, and having a second metering land between said spring chamber and said overspill port, said valve member being slidably mounted within said bore such that said second metering land variably obstructs said annular by-pass port;
• a spring arranged in said spring chamber so as to urge said valve member towards said first end of said bore, wherein said annular by-pass port and an axial end of said second metering land nearest to said overspill port are arranged such that when said valve member slides within said bore against said spring, a communication between said annular by-pass port and a space in said bore at an axial side of said second metering land nearest to said first end of said bore is initially less than fully annular.