Globe Valves (Screw Lift Valves)
Valves are used for flow control at higher pressures and temperatures and where larger dimensions are involved. The most commonly used valves falling to the following categories; globe valves, gate and slide valves, butterfly valves, check and non-return valves and control valves.
Nut for Hand Wheel
Valve Stem (Attached to Valve Disc)
Valve Disc (Attached to Stem)
Disc Stem Nut
Gasket for Bonnet
Nut for Bonnet Studs
Stud for Bonnet
Valve Stem (Not Attached to Valve Disc)
Valve Disc (Not Attached to Valve Stem)
The globe valve has a bulbous body, which houses a valve seat and a plug or valve disc arranged perpendicular to the axis of the pipe.
Valve disc and valve seat are coated with stellite (very hard alloy which can withstand both erosion and corrosion) There is an alternative design in which valve seat is threaded and screwed into the valve body, so that in the event of failure it can be easily replaced with a new spare and no need to change the whole valve which is costly. Also in some cases these renewable valve seats are not threaded, but are interference fitted to the valve body, which also can be replaced easily. The valve spindle have a square or 'V' thread, above or below the stuffing box. This thread is guided through a yoke bush for opening and closing the valve.
The valve spindle can be fitted to the valve disc providing some play for the disc to align itself while seating. A stuffing box with suitable gland packing is provided to prevent leakage of the liquid through the spindle. Normally valves are fitted in line with the pipe line. But there are some cases say taking suction from a well and then changes the direction of the pipeline. In such cases valves with change in flow direction are used, called as angle valve. In order to create a low pressure on the gland packing, pressure side is given to the bottom of the valve disc as shown. In all the illustrations shown, liquid enters through the left hand side, if valve is open, it enters through the bottom of the disc and flow to the right hand side.
Alternatively the valve stem or spindle is found unattached with valve disc. These are called as screw down non-return valves (SDNR). In this case the valve disc construction is different. The valve disc must have a spindle rod at the bottom which passes through the bush fitted in the valve body. It act as a guide to the valve disc and allows it to sit properly in position when valve closes. Such types of valves are called as non-return valves. Even in the open condition, these valves can prevent back flow of liquid. Used in boiler feed check valves, bilge suction lines valve, suction valves for pumps operating parallel, etc.
In ships there are cases demanding presence of a non-return valve only. In such cases using an SDNR valve is costly. There comes the advantage of flap valves (swing check valves) or simple check valves, which does not have any valve spindle or hand wheel, but only valve disc or a flap hinged to the body. In the former leather or rubber lining is provided at the sealing surface between flap and seat.