Instrumentation A new type of pneumatic actuator and a new thermocouple for use in reducing atmospheres are featured this month. bg Ralph E.Mundc
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used to position control valves or other types of final control elements in proportion to the output pressure from pneumatic control instruments. Air pressure, applied above the diaphragm of a pneumatic motor such as that showri in Figure 1, forces it down, compressing the spring until it exerts an upward force equal to the downward force exerted by the diaphragm. Thus there is an equilibrium position for the pneumatic actuator for each applied pressure. Since the deflection of the spring is a linear function of the force applied to it, the motion produced by the motor is a linear funct,ion of the pressure applied to the diaphragm. Most pneumatic actuators are designed to produce a full stroke when the pressure is changed from 0 to 15 pounds per square inch. The above analysis holds true only as long as the load applied to the motor is small compared t o the forces which can be exerted by the diaphragm and spring. I IWhen a load which is always in the same direction is applied to the pneumatic motor, it produces a constant displacement in the relation between applied air pressure and motor position. If frictional resistance constitutes the load, the equilibrium position will depend on the direction from which the equilibrium is approached. For control applications where simple on-off control action is required, the effects of load on pressure-position characteristic of the pneumatic motor are usually not serious. However, when proportional type control with wide proportional band adjustment is required, these effects may prevent successful control. The usual solution of this problem is to use a valve positioner. A valve positioner is a pneumatic relay which will apply any required air pressure up to the full line pressure in order to make the valve stem position correspond to that called for by the controller. Use of a I valve positioner permits a diaphragm motor of given size to handle from five ? to ten times the load which it could handle alone, or to position a given load five or ten times as accurately. R. B. Werey of the Conoflow Corporation, 2100 Arch St., Philadelphia 3, Pa., has described a new method for handling heavy loads with a pheumatic operator in a bulletin entitled “Characteristics of Pneumatic Diaphragm Motors.” His solution to the problem (see Figure 1)is a springless diaphragm motor. Figure 1. Conoflow B-22’ Pneumatic Motor a3 A
chemical and petroleum industries, pneumatic control systems are the most widely used type. They are dependable, and easy to service if maintenance is required. I n addition, they are sufficiently versatile to handle almost any type of control problem, from the simplest to the most complex. Their cost is reasonable. Finally, they can be used safely where danger of fire or explosion would make it difficult or impossible to use electrical systems. One of the components of a pneumatic control system which should be more clearly understood by the average user is the pneumatic diaphragm motor. The drawing on page 84 A shows the construction of a typical spring and diaphragm lever type pneumatic actuator. Units such as this are almost universally N THE
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REDUCING RELl
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Instrumentation
Lever-Type Spring and Diaphragm &Totor
I n this device a constant air pressure applied below the diaphragm is used to secure return motion of the load instead of a spring. Because such a motor by itself would move to one end or the other of its stroke, a valve positioner is an integral part of the device. This arrangement makes possible useful forces more than ten times those which can be obtained from a spring and diaphragm motor of the same area used with a valve positioner in the customary manner. It is good practice to design pneumatic control systems so that in event of air failure the valves will be returned to safe positions by the diaphragm motor springs. Where necessary, a light-weight spring can be incorporated in the valve. A new nickel-nickel molybdenum thermocouple which will stay on calibration in reducing atmospheres at temperatures as high as 2100" E'. has been announced by the Industrial Heating Division of the General Electric Company. Applications of the new thermocouple include measuring temperatures in protective atmospheres in heat-treating of steel and malleable iron, and in copper brazing. The thermocouple element, supported by ceramic insulators, is sheathed in a special alloy protection tube which is welded a t the hot end t o make it airtight. To assure rapid response, the couple makes physical contact with the end of the tube. A special glass seal was developed t o make the terminal end of the assembly airtight. A gastight adapter is welded to the alloy tube a t the terminal end. The adapter screws onto a 1-inch pipe which is welded to the steel furnace casing to make a gastight connection with the furnace. Thus it is not necessary to pack the thermocouple or use a junction box. The addition of a terminal connector with a die-cast housing completes the thermocouple assembly. Before the thermocouple is assembled in the alloy tube, the nickel and nickel moly wires are butt-welded together, forming a strong joint that is free from foreign material. Standard Chromel-Alumel thermocouple extension lead wire is used for connecting the thermocouple to the temperature-indicating instrument, as this wire matches the characteristics of the thermocouple. 84 A
