If the valve is opened then the diaphragm is lifted out of the flow path and the fluid will flow streamlined. When the valve is closed then the diaphragm is tightly seated against the seat in the body known as the weir. The valve will be closed completely when the diaphragm is forced downward.
There is no need for stem packing in this valve because there is no valve stem in the flow path. While compared to other conventional valves these valves require less maintenance. The material which is used to construct the diaphragm is a mostly rubberized compound so these valves can be used where highly corrosive materials are present.
In the diaphragm control valve, opening air from the pilot acts on the valve diaphragm. The structure in which the diaphragm is contained could be direct-acting in some valves or it can also be reverse acting in others. If the structure is direct-acting then the operating air pressure from the control pilot is applied to the top of the valve diaphragm. If the structure is reverse acting then the air pressure will be applied to the underside of the valve diaphragm. This can be simply described as the diaphragm control valve structure could be reverse acting or direct-acting and the diaphragm control valve may be upward seating or downward seating.
These factors, as well as the purpose of the installation, determine how the diaphragm control valve and its air-operated control pilot are installed in relation to each other.
Weir in the flow passage is designed to reduce the flexing of the diaphragm to a minimum level, but it still provides a smooth and streamlined flow passage. So that the flexing stress in the diaphragm is minimal and this would increase the diaphragm life.
The solenoid valve in this photograph has a brass body and a green-painted solenoid coil. Recommended Articles. More Articles. Control Valves. How Diaphragm Valves Work? Send this to a friend. The straight-through diaphragm valve is structurally similar to weir-types; however, it does not contain the characteristic saddle and instead has a completely straight pathway.
Their membranes are typically more flexible and allow for farther travel distance, as it must touch the very bottom of the valve. Straight through diaphragm valves are used for slurries, viscous fluids, and other applications where blocking must be reduced. They are also useful for bi-directional flow regimes, as there is no saddle to impede swift changing from inlet to outlet.
There are no industry standard diaphragm valves; as a result, specifying the right valve for your application can be tricky. Note this list is not exhaustive but should at least point your engineers and suppliers in the right direction.
Because these valves are used in throttling applications, their size has important fluid effects. To determine the correct size of your valve, first determine the desired volume of process fluid through the system.
Then, tables can be used to determine the right ratio of inner to outer diameter for your valve. What is the desired drop in pressure across the valve?
Ensure that this value is specified, or if unsure, choose a percentage that is large enough to make a difference in fluid flow. Next, determine the range of pressures the valve will experience when the system is operating so that you choose a valve that can handle the entire range.
Using the previous section, estimate which diaphragm type will be most useful for your process fluid. Then, determine if the body and stem should be exposed to media known as a wetted valve , or if these components must be separated known as non-wetted valves.
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