What are the selection requirements for Forged Steel globe valves? For low pressure check valves below DN50mm, butterfly check valves, vertical lift check valves and diaphragm check valves should be selected.        止回阀是指依靠介质本身流动而自动开、闭阀瓣，用来防止介质倒流的阀门，又称逆止阀、单向阀、逆流阀、和背压阀。止回阀属于一种自动阀门，其主要作用是防止介质倒流、防止泵及驱动电动机反转，以及容器介质的泄放。止回阀还可用于给其中的压力可能升至超过系统压的辅助系统提供补给的管路上。止回阀主要可分为旋启式止回阀（依重心旋转）与升降式止回阀（沿轴线移动）。    Check valve refers to the valve that automatically opens and closes the disc depending on the flow of the medium itself to prevent medium backflow, also known as check valve, one-way valve, counterflow valve, and back pressure valve. Check valve is an automatic valve. Its main function is to prevent medium backflow, reverse pump and drive motor, and release of container medium. Check valves can also be used in pipelines to supply auxiliary systems where pressure may rise above system pressure. Check valves can be divided into swing check valves (rotating according to the center of gravity) and lift check valves (moving along the axis).        对于止回阀的选用要求，必须知道以下几点：    For the selection requirements of check valves, the following points must be known:        对于DN50mm以下的高中压止回阀，宜选用立式升降止回阀和直通式升降止回阀；    For high and medium pressure check valves below DN50mm, vertical lift check valves and through lift check valves should be selected.        对于DN50mm以下的低压止回阀，宜选用蝶式止回阀、立式升降止回阀和隔膜式止回阀；    For low pressure check valves below DN50mm, butterfly check valves, vertical lift check valves and diaphragm check valves should be selected.        对于DN大于50mm、小于600mm的高中压止回阀，宜选用旋启式止回阀；    For high and medium pressure check valves with DN greater than 50 mm and less than 600 mm, swing check valves should be selected.        对于DN大于200mm、小于1200mm的中低压止回阀，宜选用无磨损球形止回阀；    For medium and low pressure check valves with DN greater than 200 mm and less than 1200 mm, wear-free spherical check valves should be selected.        对于DN大于50mm、小于2000mm的低压止回阀，伊选用蝶式止回阀和隔膜式止回阀；    For low pressure check valves with DN greater than 50 mm and less than 2000 mm, butterfly check valves and diaphragm check valves are selected.        对于要求关闭时水击冲击比较小或无水击的管路，宜选用缓闭式旋启止回阀和缓闭式蝶形止回阀。    For pipelines requiring less water hammer or no water hammer when closing, slow-closing swing check valve and slow-closing butterfly check valve should be selected.