A stop check valve, illustrated in Figure 27, is a combination of a lift check valve and a globe valve. It has a stem which, when closed, prevents the disk from coming off the seat and provides a tight seal (similar to a globe valve). When the stem is operated to the open position, the valve operates as a lift check. The stem is not connected to the disk and functions to close the valve tightly or to limit the travel of the valve disk in the open direction.
      A stop-check valve includes inside a valve body a swing check valve disc and a contacting projection. A vertically mounted valve rod provided with a handwheel and having at the lower end a structure of a cylindrical contact portion is further included. The contact portion is vertically movable in coupling arrangement with the valve rod and capable of contacting with the contacting projection on the valve disc.

     The check valves are thus restricted in uses and for this reason,it is frequently required that a stop valve be fitted during the design of passageways in order to open and close and regulate the fluids, and hence an increase in expenses for equipment and fitting. Such a drawback requires thus the attention for an improvement and solution.


Relief  and  Safety  Valves

     Relief and safety valves prevent equipment damage by relieving accidental over-pressurization of fluid systems. The main difference between a relief valve and a safety valve is the extent of opening at the setpoint pressure.

      A relief valve, illustrated in Figure 28, gradually opens as the inlet pressure increases above the setpoint. A relief valve opens only as necessary to relieve the over-pressure condition. A safety valve, illustrated in Figure 29, rapidly pops fully open as soon as the pressure setting is reached. A safety valve will stay fully open until the pressure drops below a reset pressure. The reset pressure is lower than the actuating pressure setpoint. The difference between the actuating pressure setpoint and the pressure at which the safety valve resets is called blowdown. Blowdown is expressed as a percentage of the actuating pressure setpoint.