2BV series water ring vacuum pumps are suitable for pumping the gases and steam. Its suction pressure can reach 33mbar (ABS) (i. e. 97 % vacuum degree). If the transformer oil is used as the operating liquid, these pumps are called oil ring vacuum pump and the suction pressure can reach 6.7mbar (ABS) (i. e. 99 % vacuum degree). So the oil ring vacuum pumps can replace the reciprocated vacuum pumps completely. When the liquid ring vacuum pumps work under the condition near the limited vacuum for a long time, it is necessary to couple with the cavitation protection pipe in order to get rid of the screaming and protect the pump.