The series ZJP roots vacuum pump is with overflow valve. The figure-of-eight rotors are counter-rotating at a constant speed inside the pump housing for suction and exhaust of gas. Two rotors are supported by two bearings and synchronized by a pair of gear, which ensure these two rotors in certain relative positions. They are near to one anther and to the housing without actual contacting ,so lubrication is unnecessary in the working housing. The carefully balance operating parts and high precision bevel wheels ensure the pump be operated stably and continuously under the condition of high-pressure difference. Dynamic seal part use our patent technology and imported oil seals.The vibration of shaft at the shaft seals is controlled to less than 0.02mm against oil leaking. A gravity valve is installed between the suction and exhaust part of the pump. The function of the gravity valve is as follows,when the pressure difference between the suction and exhaust part is over the weight of the valve, the valve opens automatically,which makes the pressure difference always keep in a fixed controllable valve,This valve is the allowable highest pressure difference to ensure the pump work normally and so that in fact, the roots vacuum pump with overflow valve is a kind of overload self-protective pump. Series ZJP roots vacuum pump with overflow valve has substantially higher speed at relatively lower inlet pressure and it is possessed overload self-prtotecive function .Because it is a pump of dry clearance seal construction.if a certain pumping speed rate and an ultimate vacuum need to be obtained, it is necessary to supply a lower inlet pressure for reducing the back flow therefore, a pump must be backed in use, roots vacuum pump should be started soon after its inlet pressure reaches a permissible value for economization.It is permit to select different kinds of pump as the backing pump for factual requirements ,such as oil seal pump and liquid ring vacuum pump.When pumping the gas containing large amount of vapors, the liquid ring vacuum pump is the ideal backing pump.