Butterfly control valves

Butterfly control valve with positioner. (Courlesy of Foxboro.)... 

We have to make a distinction here between electric controllers (e.g. PID controllers) with a proportional valve as actuator and mechanical diaphragm controllers. In a regulation system w/ith electric controllers the coordination between controller and actuator (piezoelectric gas inlet valve, inlet valve A/ith motor drive, butterfly control valve, throttle valve) is difficult because of the very different boundary conditions (volume of the vessel, effective pumping speed at the vessel, pressure control range). Such control circuits tend to vibrate easily when process malfunctions occur. It is virtually impossible to specify generally valid standard values. 

Suction throttling is usually done with a butterfly control valve, which has a very low pressure drop when it is wide open. 

Three independent methods of measuring released CO2 vapor were used. The first was the measurement of carbon dioxide flow rate by a vortex shedding flow meter, located just upstream of the 4-in. and 6-in. butterfly control valves. The second method was the measurement and recording of pressures and temperature at a 4.5-in. orifice installed downstream... 

The purpose of the hot vapor bypass controller is to pump heat into the reflux drum. Obviously, if one is limited by condensing capacity, introduction of extra heat to the reflux drum aggravates the limitation. Usually, the rubber type seat in the hot vapor bypass butterfly control valve dries out with age and needs to be renewed. My experience is to eliminate the hot vapor bypass control scheme entirely and convert the tower to flooded condenser-type pressure control. 

One problem with flooded condenser pressure control is related to the need to occasionally vent non-condensables. This vent valve must not leak when closed. Since the drum is normally full of liquid, a leaking vent valve will pass liquid. Many pounds of product can be lost in this way. A butterfly control valve with a soft, rubberized seat is a good choice for a remotely controlled, non-condensable vent valve. 

B LIQUID PUMB C - LIQUID SUMP 0.3 -TRAY SECTION C WINOO S TO CENTRE TRAY f LIQUID METERINC POT e 6AS METERINO ORIFICE H VENT TO ATMOSPHERE I BUTTERFLY CONTROL VALVE J SEAL POT... 

Valve bodies are also standardized to mate with common piping connections flanged, butt-weld end, socket-weld end, and screwed end. Dimensional information for some of these joints and class pressure-temperature ratings are included in Sec. 10, Process Plant Piping. Control valves have their own standardized face-to-face dimensions that are governed by ISA Standards S75.03, 04, 12, 14, 15, 16, 20, and 22. Butterfly valves are also governed by API 609 and Manufacturers Standardization Society (MSS) SP-67 and 68. 

Figure 3.5. Vacuum control with steam jet ejectors and with mechanical vacuum pumps, (a) Air bleed on PC. The steam and water rates are hand set. The air bleed can be made as small as desired. This can be used only if air is not harmful to the process. Air bleed also can be used with mechanical vacuum pumps, (b) Both the steam and water supplies are on automatic control. This achieves the minimum cost of utilities, but the valves and controls are relatively expensive, (c) Throttling of process gas flow. The valve is larger and more expensive even than the vapor valve of case (a). Butterfly valves are suitable. This method also is suitable with mechanical vacuum pumps, (d) No direct pressure control. Settings of manual control valves for the utilities with guidance from pressure indicator PI. Commonly used where the greatest vacuum attainable with the existing equipment is desired.

Valve bodies other than those illustrated in Fig. 7.123 which are employed for special-purpose control valves are ball valves, butterfly valves and Saunders diaphragm valves. These are described in Volume 1, Section 3.5.4 and Volume 6, Section 5.3. 

Solids Metering The control valves described earlier are primarily used for the control of fluid (liquid or gas) flow. Sometimes these valves, particularly the ball, butterfly, or sliding gate valves, are used to throttle dry or slurry solids. More often, special throttling mechanisms such as venturi ejectors, conveyers, knife-type gate valves, or rotating vane valves are used. The particular solids-metering valve hardware depends on the volume, density, particle shape, and coarseness of the solids to be handled. 

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). 

Elevation Control. Elevation was controlled in the diffusion chambers by a 42-inch butterfly valve, electrically operated. The valve design included a raised seat for the butterfly. This valve, at times, opened to a certain point, then fluttered with such force as to break the driving linkage. Eventually the valve was removed from the line, the raised seat machined out, a new butterfly installed, and heavier driving mechanism provided. 

The most widely used control valves for liquid and high pressure gas streams are the various sliding stem valves such as the bevel plug, V-port, parabolic plug, and for small flows the needle valve. For corrosive materials and slurries, pinch-type valves like the Saunders patent valves are used. Rotary stem valves include plug types and for gas flows butterfly valves, dampers, and louvers. 

Nickel piping and fit-tinn s. Control valves butterfly type, others plug type. Piping furnished random lengths for fabrication by purchaser... 

Nickel piping w/monel valves, control valves butterfly type. Pipe prefab. 

Turndown devices are most often control valves (not shutoff valves) or dampers. The best valve turndown characteristic is usually accomplished with adjustable port valves or with characterized globe-type valves. Butterfly valves usually have very poor characteristics (not straight-line), but their characteristic curves can sometimes be improved by undersizing or selecting reduced port models. 

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