Manual flow controls

For the controlled removal of the liquid phase of a liquefied gas, a manual flow-control valve... 

Manual flow-controls may be used when an intermittent flow is needed and an operator will be present at all times. A manual flow-control is simply a valve connected directly to a cylinder valve, which incorporates a needle stem design for precise flow control and is operated manually to deliver the proper amount of gas. (See Fig. 5-3.)... 

Fig. 5-3. Manual flow-control valve (needle type).

For the controlled removal of the liquid phase of a liquefied gas, a manual flow-control valve is used. Special liquid flow regulators are also available. Removal of the liquid must be done at the vapor pressure of the material. Care must be taken to prevent blockage of the gas line downstream from a user s heat exchanger, which would cause excessive pressure buildup in the heat exchanger and the cylinder. Pressure relief devices should be installed in all transfer lines to relieve any sudden and dangerous hydrostatic or vapor pressure buildups. 

For air-flow control, the system may contain a control valve or damper that automatically or manually modulates system pressure drop. The dotted curves in Figure 4a on each side of the system resistance curve might represent operating extremes of the system resistance as the control valve is varied from maximum to minimum opening. These curves also intersect the fan curve at desirable operating portions of its range both for efficiency and flow control. 

Air-Flow Control Process operating reqmrements and weather conditions are considered in determining the method of controlling air flow. The most common methods include simple on-off control, on-off step control (in the case of multiple-driver units), two-speed-motor control, variable-speed drivers, controllable fan pitch, manually or automatically adjustable louvers, and air recirculation. 

The flow controllers are often used to set desired flows for the fresh feed, stripping steam, and dispersion steam. Each flow controller usually has three modes of control manual, auto, and cascade. In manual mode, the operator manually opens or closes a valve to the desired percent opening. In auto mode, the operator enters the desired flow rate as a set-point. In cascade mode, the controller set-point is an input from another controller. 

Before the retrofit, the exhaust system was manually controlled, by turning off fans. There were no dampers to close the inlets and no flow controls for the fans. Since these are local exhaust, heat recovery was not... 

The flow control system provides facilities for injecting air, steam, oxygen or other fluids into the reactor, and is governed by d/p cell transmitters and pneumatically-activated valves which permit automatic or manual regulation of flow rates and pressures. 

Except for continuous weighing, control of the flow of solids is less precise than that of fluids. Several devices used for control of feed rates are shown schematically in Figure 3.7. They all employ variable speed drives and are individually calibrated to relate speed and flow rate. Ordinarily these devices are in effect manually set, but if the solid material is being fed to a reactor, some property of the mixture could be used for feed back control. The continuous belt weigher is capable ordinarily of 1% accuracy and even 0.1% when necessary. For processes such as neutralizations with lime, addition of the solid to process in slurry form is acceptable. The slurry is prepared as a batch of definite concentration and charged with a pump under flow control, often with a diaphragm pump whose stroke can be put under feedback control. For some applications it is adequate or necessary to feed weighed amounts of solids to a process on a timed basis. 

Figure 3.15. Control modes for the upper sections of fractionators, (a) Pressure control by throttling of the overhead vapor flow. The drawbacks of this method are the cost of the large control valve and the fact that the reflux pump operates with a variable suction head. The flow of HTM is hand set. (b) Applicable when the overhead product is taken off as vapor and only the reflux portion need be condensed. Two two-way valves can replace the single three-way valve. The flow of HTM is hand set. (c) How rate of the HTM is regulated to keep the pressure constant. One precaution is to make sure that the HTM, for example water, does not overheat and cause scaling. The HTM flow control valve is small compared with the vapor valve of case (a), (d) Pressure control is maintained by throttling uncondensed vapois. Clearly only systems with uncondensables can be handled this way. The flow of the HTM is manually set. (e) Bypass of vapor to the drum on PC ...

For a direct material balance, the inlet liquid is flow controlled, the level is maintained by adjusting the steam flow and the pressure is then controlled by adjusting the vapor flow, as shown in Figure 3.13(B). If the feed pump limits capacity, the pressure should be allowed to droop by putting the controller into manual with a fully open valve. Then the feed flow controller will run with its valve fully open and the pressure will droop to an equilibrium point, where inlet flow equals the flow through the downstream equipment. 

Both modes usually are conducted with constant vaporization rate at an optimum value for the particular type of column construction. Figure 13.9 represents these modes on McCabe-Thiele diagrams. Small scale distillations often are controlled manually, but an automatic control scheme is shown in Figure 13.9(c). Constant overhead composition can be assured by control of temperature or directly of composition at the top of the column. Constant reflux is assured by flow control on that stream. Sometimes there is an advantage in operating at several different reflux rates at different times during the process, particularly with multicomponent mixtures as on Figure 13.10. 

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