CHECK VALVES

FUNCTION OF CHECK VALVES
       The prime function of a check valve is to protect mechanical equipment
in a piping system by preventing reversal of flow by the fluid. This
is particularly important in the case of pumps and compressors, where
back flow could damage the internals of the equipment and cause an
unnecessary shutdown of the system and in severe cases the complete
plant.
         Generally speaking check valves have no requirement for operators, and
so the valve is operated automatically by flow reversal; however, in very
special circumstances this uni-directional facility has to be overridden.
Check valves either can be fitted with a device that allows the closure
plate(s) to be locked open or alternatively can have the closure plate(s)
removed. The latter alternative requires dismantling the valve, removing
the plates, and re-installing the valve.
        Check valves are automatic valves that open with forward flow and close
against reverse flow. This mode of flow regulation is required to prevent return flow,
to maintain prime after the pump has stopped, to enable reciprocating pumps and
compressors to function, and to prevent rotary pumps and compressors
from driving standby units in reverse. Check valves may also be required
in lines feeding a secondary system in which the pressure can rise above
that of the primary system.
           Grouping of Check Valves
            Check valves may be grouped according to the way the closure member
moves onto the seat. Four groups of check valves are then distinguished:
           1. Lift check valves. The closure member travels in the direction normal
to the plane of the seat
           2. Swing check valves. The closure member swings about a hinge, which
is mounted outside the seat
           3. Tilting-disc check valves. The closure member tilts about a hinge,
which is mounted near, but above, the center of the seat
           4. Diaphragm check valves. The closure member consists of a
diaphragm, which deflects from or against the seat.
          
 Operation of Check Valves
Check valves operate in a manner that avoids:
             1. The formation of an excessively high surge pressure as a result of the valve closing.
            2. Rapid fluctuating movements of the valve closure member.
  
           However, the speed with which forward flow retards can vary greatly
between fluid systems. If, for example, the fluid system incorporates a
number of pumps in parallel and one fails suddenly, the check valve at the
outlet of the pump that failed must close almost instantaneously. On the
other hand, if the fluid system contains only one pump that suddenly fails,
and if the delivery line is long and the back pressure at the outlet of the
Figure 4-14. Diaphragm Check Valve, Incorporating Flattened Rubber Sleeve
Closure Member. [Courtesy of Red Valve Company Inc.)
pipe and the pumping elevation are low, a check valve with a slow closing
characteristic is satisfactory.
            Rapid fluctuating movements of the closure member must be avoided to
prevent excessive wear of the moving valve parts, which could result in
early failure of the valve. Such movements can be avoided by sizing the
valve for a flow velocity that forces the closure member firmly against a
stop. If flow pulsates, check valves should be mounted as far away as practical
from the source of flow pulsations. Rapid fluctuations of the closure
member may also be caused by violent flow disturbances. When this situation
exists, the valve should be located at a point where flow disturbances
are at a minimum.
          The first step in the selection of check valves, therefore, is to recognize
the conditions under which the valve operates.