维持费: cost maintenancecost of maintenancecost of upkeepmaintain the status quomaintenance chargemaintenance costsmaintenance feeoperation and maintenance costupkeep cost
The valve remains open while the operating pressure is within the normal operating limits 当操作压力维持在正常工作界限内时，阀保持开的状态。
2.
Requirements : the pressure differential relief valve must operate to maintain the " zone " between the two check valves at least 2 psi less than the supply pressure 要求：压差安全阀必须具有将两个止回阀之间的“区域”压力维持在比供应压力低至少2psi的功能。
