1, Overview Highly hydrophobic for thermal power plants, reheater hydrophobic and hydrophobic superheater system, such as the closure of the drain and drain valve sealing requirements to be fast open and close quickly, and with fault protection, automatic control and high The reliability. Therefore, the cut-off valve of the power plant (power plant, commonly known as traps, traps) become the system of choice for such products. Pneumatic control globe valve seal than the pressure lasting stability, simple commissioning, low maintenance requirements, low failure, high reliability, automatic loss of energy loss of power protection, quick opening and closing, the price is relatively low, energy saving, and therefore suitable for thermal power plant Drainage condition. 2, the selection of valves and pneumatic actuators Pneumatic control valve is normally closed full-closed. From the flow characteristics of consideration, the globe valve and ball valve with a short stroke opening and closing, fast speed, reliable sealing, opening and closing of small static torque characteristics, which have been applied to both products. However, reliability considerations, the mainstream product is still the pneumatic valve. Pneumatic control device for the stereotypes cylinder products, depending on the mode of action can be divided into single-and double-acting two. Single-acting products with a reset cylindrical spring, with loss of automatic reset function, that is, when the air cylinder piston (or diaphragm) under spring action, the drive cylinder push rod back to the initial position of the cylinder (stroke original position). Double-acting cylinder without reset spring, push rod advance and retreat must rely on the change of cylinder gas source import and export location. Gas source from the piston into the chamber, the push rod down. Gas source from the piston into the lower chamber, push rod upward movement. Without the return spring, the double-acting cylinder has more thrust than the single-acting cylinder of the same diameter but does not have the automatic reset function. Obviously different intake positions make the push rod have different direction of movement. When the intake position is in the back cavity of the push rod, the intake air advances the push rod, which is called the positive cylinder. On the other hand, when the air intake position is on the same side of the push rod, the air intake makes the push rod reverse, which means that the reaction cylinder is in this way. Pneumatic globe valve due to the general need for loss of protection, usually use a single-acting cylinder (Figure 1). From the analysis we can see that when the valve protection is lost when open, you must choose the role of a single positive cylinder. When the valve protection is lost when closed, you must choose a single reaction cylinder. The positive role of single-acting cylinder in the valve must be closed to maintain a stable source pressure, the stability of the gas source and the durability of the higher Normally closed valve as far as possible to avoid the choice of this design, this structure is suitable for normally open valve . The single-acting reaction cylinder is suitable for normally-closed valves, but requires a high degree of spring force and should have a stable and long-lasting elasticity. 3, the cylinder thrust calculation In addition to the choice of mode of action, calculate the selected cylinder thrust is an important part of the work, with a common normally closed pneumatic shut-off valve (Figure 2) as an example analysis. Because of the high pressure, in order to choose the smallest possible cylinder, the use of upstream flow (reverse flow) of the valve design. To improve the pressure ratio, the use of a tapered sealing surface. More keyword search: Globe valve