The check-valves, or non-return valves, allow the flow in one direction only, the vertical check-valves, in particular, allow only an up-warding flow.
The obstruction action on the reversing flow takes place automatically, without external interventions.
The check-valves may be divided in two categories: standard shutter type and swing type. In the former the shutter may be differently shaped: head, frustum of cone, piston, disc with gasket, sphere etc.
The valves must always be installed so that the shutter can move vertically and, more precisely, they must move upward in the opening phase and downward in the closing one.
These valves, suitable for vertical piping, are named intermediate valves.
In the swing valves, the shutter consists of a hinged and swinging door which seals on a body seat obtained on a plane approximately perpendicular to the piping axis.
These valves may be assembled horizontally, obliquely or vertically.
In any case the valve opening is caused by the fluid pressure difference on the two shutter faces and, when there is a directional flow, the opening is maintained by the dynamic action of the same fluid.
When the flow stops and tends to reverse its course, the shutter locks because of the gravity or the flow pressure and dynamic actions.
If the locking has to take place when the direct flow becomes slow or before coming to a stop and tending to reverse its course, the locking may be made easier by means of springs or weights working on the shutter.
The swing check valves, winch are very quick in the opening and closing operations, may be supplied with an outside hydraulic shock-absorber against water hammers.
The valves with normal shutter for horizontal piping oblige the flow to follow a tortuous way. This causes pressure drops which can be compared with those of the directional flow or globe interception valves. Some of the swing check valves have been designed so that the swing could be fully raised up by the flow in order to have a nearly rectilinear passage with negligible pressure drops.
In order to make the shutter perfectly fit to the seal seat, it is guided by a stem or it is provided with fins winch do not leave the seat even when the shutter is fully raised up.
The sphere shutter is advantageous because it can be fully free, thus ensuring a perfect sealing any way it may return on the seat. Moreover, the same contact casualness between sphere and seat causes a uniform sphere wear with an unalterable sealing in the time.
The dead-weight valves are particular check-valves foreseen to be installed on the lower end part of exhaust piping for well and tanks.
They have only one outlet, because the other has been replaced by a cage or perforated metal grid in order to avoid the suction of possible solid impurities which could reach the pump.
The dead-weight valves often have a venturi-metric passage because of the particular necessity to have negligible pressure drops in the suction piping.