Automatic control valves

Modulating The control action of minute increments and decrements of adjustment in a system, such as in automatic control valves. 

Once filling is complete, filtration begins [Fig. 22.3(b)]. Treated brine is pumped into the vessel at the desired flow rate through an automatic control valve. The flow of brine is from the outside to inside of the filter candles. Filtrate clarity is achieved immediately no recycling of flow is required. As the filtration proceeds at constant flow, solids accumulate as a filter cake on the outside surface of the filter socks. While... 

The interface with the process at the other end of the control loop is made by the final control element. In a vast majority of chemical engineering processes the final control element is an automatic control valve which throttles the flow of a manipulated variable. Most control valves consist of a plug on the end of a stem that opens or closes an orifice opening as the stem is raised or lowered. As sketched in Fig. 7.5, the stem is attached to a diaphragm that is driven by changing air pressure above the diaphragm. The force of the air pressure is opposed by a spring. 

Hydraulically operated linear plunger pumps with automatically controlled valves are also applied for the transport and metering of highly viscous adhesives (> 10s mPas) against pressures up to several 100 bar [10] for various -partly robotic - production processes. 

The mercury bubbler C acts as a safety vent and controls the pressure in the apparatus. A ball valve near the top of the inlet tube prevents the mercury being sucked into flask B in the event of the automatic control valve D becoming blocked. 

A high-capacity water-seal vacuum pump downstream of an automatically controlled valve provides for chamber evacuation and pressure... 

The preneutralizer slurry is pumped at a controlled rate to a rotary-drum, ammoniator-granulator there it is distributed onto a rolling bed of recycled solids. The most commonly used metering system for the preneutralized slurry is a variable-speed centrifugal pump with automatic control from a magnetic flowmeter. Some plants have had success with a magnetic flowmeter and automatic control valve of a special ball type. 

This process operates m short cycles, with three main phases. in a series of adsorbers laid out in parallel, containing the molecular sieves in fixed beds, and capable of being switched in their operation by a set of automatically controlled valves. 

Automatic valves are part of a control loop, which is shown in Figure 8.6. The loop contains a primary element, which measures the controlled variable, such as temperature, pressure, flow rate, and liquid level. The operation of a control loop is the same regardless of what variable is controlled. In the case of flow-rate control, the controller obtains the flow rate from transmitter a flow meter and compares the measured flow rate with a value that has been preset in the controller. If the flow rate is greater than the preset value, the controller increases air pressure on top or bottom of a diaphragm in the valve. Then, the valve partially closes to reduce the flow rate. On the other hand, if the flow rate is below the preset value, the controller will act to reduce the air pressure on the diaphragm, and hence the valve opens wider. Electric motors can also operate automatic control valves. 

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). 

Fig. 1 illustrates the automated venting system. The dual-probe unit has a long probe and a short one. When the long probe is not covered by slurry, the relay opens an automatic control valve, which vents the tank. When slurry contacts the short probe, the valve clo.ses and the venting is completed. The length differential betw een the probes determines the venting interval. 

Regenerator pressure is controlled by throttling the flue gas (305). A manually operated valve is sometimes used for rough adjustment and supplemented by an automatically controlled valve in a by-pass line for precise regulation (330). The automatic control may be arranged to maintain constant differential pressure between the reactor and the regenerator (68). Reactor pressure is controlled by holding a constant pressure at the suction side of the compressors that remove uncondensed vapors from the fractionator (202,330). 

The fixed bed column is essentially a simple pressure vessel. Each vessel requires a complexity of ancillary equipment. Each column in a cascade will require several automatic control valves and associated equipment involving process computer controls to sequence the proper flow of different influent streams to the resin bed. 

Instrumentation. Standard API RP521 (10) recommends "that any automatic control valves, which are not under consideration as causing a relieving requirement and which would tend to relieve the system, (are assumed to) remain in the position required for the normal processing flow. Although this policy may appear conservative (it usually isX it is justified. Controllers are often operated on "manual at other times, they are tuned extremely slowly. Taking credit for controller action may lead to overpressure if the controller does not respond fast enough. 

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