Pulsator

In a pulsator comprising first and second valve devices, each of said first and second valve devices having a pulsation outlet, a pressure inlet to be connected to a first source of pressure for generating a first pressure, and a pressure outlet to be connected to a second source of pressure for generating a second pressure, said second pressure being lower than said first pressure, said pulsator further comprising means for adjusting each of said first and second valve devices for alternately connecting the pulsation outlet of each said valve device to said first source of pressure via the respective inlet of the valve device and to said second source of pressure via the respective pressure outlet of the valve device, wherein the improvement comprises a third valve device having a valve housing forming a chamber, a first inlet to said chamber connected to the pressure outlet of the first valve device, a second inlet to said chamber connected to the pressure outlet of the second valve device, first and second valve seats extending around respective openings of the first and second inlets in said chamber, an outlet from said chamber connected to said second source of pressure, and a valve member which is displaceable in the chamber between a first position at which the valve member abuts sealingly against the first valve seat, and second position, at which the valve member abuts sealingly against the second valve seat.