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Types of Valves
Gate Valves
These are the valves which have small pressure at the opening. These valves are of two types: Wedge Shaped Gate Valve and Double Disk Parallel Parallel Seal Type Valve. The wedge shaped gate though solid may be either split or flexible. Galling of the sealing surface is minimized by flexible and split wedges. In double disk parallel seal type valves, there is an inclined plate equipment which is installed between the disks. This helps in coverting stem force to axial force pressing disks against the seat after the disks have been positioned for cleaning. The gate assembly is automatically adjusted in order to match both angular misalignment of the seats and longitudinal shrinkage of the valve body on cooling.
The design of globe valve is based on two Patterns-inside Screw Non Rising Stem or Outside Screw Rising Stem. Small valves are are normally of the inside screw type whereas outside screw type is preferred for larger type. In some of the designs, disks rotate freely on the stem to prevent galling between the disk and the seat. With respect to other valves the pressure drop through globe valve is much higher. In horizontal lines, Globe valves prevents complete drainage.
These valves can tolerate pressure up to 7kg/cm cube. Diaphragm that are reinforced from fabrics are made up of natural rubber, synthetic rubber, natural or synthetic rubber faced with Teflon fluorocarbon resin. Plastic bodies are considered as practical in diaphragm valves because the alignment and the distortion are very minor problems. Diaphragm valve can be installed in any position and work very well for those fluids that contains suspended solids. The maintenance of the valve is very easy. The maintenance required is diaphragm removal that can be done easily and quickly without removing the valve from the line.
Ball Valves
In a ball valve, ball is moves axially very easily. In this type of valve, sealing element is ball hence its alignment with the axis of the stem is not essential to tight shutoff. The pressure difference across the valves forces the the ball in a closed position against the body. They are used up to that temperatures that have very little effect on their plastic seats.
With respect to other valves, butterfly valves occupy very less space in the line. They consist of tight sealing without excessive operating torque. The seat water is accomplished by various methods like resilient seats, piston rings on the disk inclining the the stem to limit the contact between the portions of the disk closest to the stem and the body seat to a few degree of curvature. The valve is closed by fluid- pressure distribution. Pressure drop is higher as compared to gate valves.
Swing check valves are used in those lines where the force on the disk is maximum at the start of the closing and minimum at the end of the closing. However, like other valves, swing check valves tend to leak at lower pressure. Due to this reason elastomere is install on the disk. These valves are available with low cost insert bodies.
Lift check valves come in three styles. Vertical lift check valve, globe check valve and angle check valve. Vertical lift check valves are installed in vertical lines whereas globe check valves are installed in horizontal lines. Angle check valves on the other hand are installed where a vertical line with upward flow turns horizontal.
