By-pass valve unit for a high pressure liquid delivery unit

A by-pass valve unit for a high pressure liquid delivery unit, the delivery unit comprising:

• a motor-driven high pressure pump (11) having a delivery side (13) and a suction side (14), and
• a delivery gun (15) connected to the pump delivery side (13) by a delivery conduit (16) and having a flow on-off valve, a source of liquid being connected to the pump suction side (14),
• the valve unit comprising a by-pass valve (25, 33) arranged to close or alternatively, when in the open state, to enable direct flow passage between the delivery side (13) and the suction side (14), and having a control means (31, 32) sensitive to the liquid pressure in the delivery conduit (16),
• characterized in that said control means comprises two valving elements (31, 32) which are normally joined together to form one piece in contact with the liquid of the delivery conduit (16), but which separate when the pressure in the delivery conduit (16) exceeds the normal delivery value (P1) following flow interruption by the delivery gun (15), to hence provide direct passage for the liquid from the delivery conduit (16) to the suction side (14).