Globe control valve

In this chaftter, we wilt discuss control valves mainly for automatic process control purpose. 

Most control valves used for automatic process control are either globe or rotary control valves. Their parts are briefly discussed here. 

ACTUATOR SPRING ACTUATOR ITCH SPAMG SEAT SPRHG ADJUSTOR TEM CONNECTOR yCKB  

The bonnet assembly is connected to the valve body. It provides a passage for the valve stem to move up or down. The valve seat ring is for the valve plug to rest on it. The cage is used to guide the valve plug movement. The packing box is used to prevent fluid leak out from the valve stem. 

Some bonnet has a long extension between packing box and bonnet bottom oonnection. The purpose is to prevent packing box exposed to the extreme hot or cold process fluid inside the control valve body. 

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In addition to the normal manual valve between a compressor and the common discharge header, there should be a fail-closed remotely actuated globe control valve with Monel trim and an equal percentage flow characteristic (HV-65 on Fig. 11.26). It should handle the full capacity of the compressor with a pressure drop of about lOkPa. When, during startup, the pressure instrument referred to above shows that the compressed gas pressure is equal to the discharge header pressure, the operator can open this valve slowly. 

Per hardware globe control valve (sin e port or double port), rotaiy ecwjtrol valve (ball, plug, or butterfly), angle control valve. 

III.l Globe control valve Globe control valve has following parts See Figure 2 [1] for details. 

Figure 2 Major components of a globe control valve. [Courtesy of Fisher Controls International LLC, Emerson Process Management.]...

Figure 7 Installed flow characteristic curve and installed gain curve fora butterfly and a globe control valve. [1] Other factors affecting control valve perfonnafice ...

Fr is a control valve s Reynolds number factor, Cvs is laminar flow control valve Cv. Cv is turbulent flow Cv. Fs is laminar or streamline flow factor. Fd is valve type modifler. Fd equals 1.0 for globe control valve and ball type rotary control valve. It equals to 0.71 for butteifly type rotary control valve. 

Try 1/ -inch (i.d. = 1.61), since 114-inch (i.d. = 1.38) is not stocked in every plant. If it is an acceptable plant pipe size, then it should be considered and checked, as it would probably be as good pressure drop-wise as the 114-inch. The support of 114-inch pipe may require shorter support spans than the 114-inch. Most plants prefer a minimum of 114-inch valves on pressure vessels, tanks, etc. The valves at the vessels should be 114 inch even though the pipe might be VA inch The control valve system of gate and globe valves could very well be lA inch. For this example, use 114-inch pipe, Schedule 40 ... 

To estimate the control valve, take a globe valve to be half open ... 

All three types of valves described above are on-off type devices (binary devices). A basic requirement in the design of these valves is that they offer minimum resistance to flow when open. For many types of calculations it is often justifiable to neglect pressure losses through such devices. But such a simplification cannot be applied to flow control valves. With these devices the control of flow is accomplished either by a constriction or by a diversion. In either case an additional resistance to the flow is introduced. Globe, angle, cross, and needle valves are typical devices of this type. Specific pressure drop correlations should be developed for such devices and used whenever possible. One such correlation used by Stoner (S5) takes the form... 

Fig. 7.119. Pneumatically operated control valve (a) double-spring actuator with single-ported globe valve (b) exterior view of double-ported control valve with valve-positioner fitted on the side...

Fig. 7.123. Valve bodies showing typical trims used for control valves (a) double-port globe valve with top and bottom guiding (b) single-port globe valve with top and...

Other types can be used. Butterfly valves are often used for the control of gas and vapour flows. Automatic control valves are basically globe valves with special trim designs (see Volume 3, Chapter 7). 

A control valve is a specially designed globe valve that acts as a variable resistance in the line to control the flow rate. Closing down on the valve decreases the area between the plug and the... 

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