Valves

 GATE VALVE:
Introduction


Gate valves are used in most of chemical process industries, predominantly in Petroleum industries, because of the attention that it has gained as a good flow isolation valve with very little or negligible leakage. Nearly 70% of the valves in petroleum industries are gate valves. They are suitable for services which require ON/OFF application. Throttling is not preferred with this valve type.
Construction / Working:
                                          Rising stem


The stem rises/lowers with opening/closing the valve.
The stem in closed position is not in contact with the process fluid, hence corrosion issues are NIL in this case.
Requires more space relatively.
All fluids are compatible, usually recommended for high temperature and corrosive fluid application, where its counterpart, non-rising stems is not applicable.
Usually made of cast or forged steel.
                                         Non-rising stem:


·         Stem in this case is static.·         Screw threads of the stem are exposed to process fluid so stem damage/contamination (fouling, scaling) is an issue.·         Requires less space.·         Used only when the process fluid is a clear liquid.·         Usually made of bronze, cast iron, or brass.

GATE / DISC type:
        Wedge type
                                            Solid  wedge:


Does not allow for expansion or contraction of valve body and stem  material with large change in temperature.
Not used of high temperature application.
Can handle any type of fluid.
Available in following MOC:  bronze, cast iron, carbon steel (CS) etc.
                                           Split gate
It allows for expansion and contraction.
Disc has better fit in valve seat over wide range of pressure and temperature.
Appropriate to handle non-condensing gases and liquids.
                                          Flexible gate:






For very high temperature processes, it gives very tight seal on expansion and contraction on the valve seat over wide range of pressure and temperature.
Usually applicable for steam application.
Advantages of Gate valve:
Lowest pressure drop across valve when fully opened
High Cv (flow coefficient)
Low fluid resistance (no obstruction to flow)
Bubble tight shut off / Negligible leakage
High capacity.
Multiple liquids allowed (clean, oil, gas, steam, slurry, corrosive, erosive etc.)
Good sealing performance when fully opened.
Can be operated manually even for larger pipes.
Applicable for wide temperature range and varied line sizes.
Relatively low cost valve.
Disadvantages of Gate valve:
Not suitable for high pressure applications, since gate acts as a diaphragm and can bend or burst under high pressures.
Should be only used for ON/OFF purpose. Throttling causes vibration and loud noise.
Valve disc and seat undergoes a lot of wear & tear, making it necessary to replace them more often.
Possibility of foreign material blocking the gate from sealing.
Maintenance is difficult
More space is required
Long time to open or close valve.