Valves, control

A mechanical device embodying a bellows-sealed needle valve with a lever reduction movement for fine control is shown in Fig. II, 23, 5. The needle is of stainless steel. This fine control valve assembly is useful for pressures ranging from 20 to 100 mm. of mercury when used in conjunction with a good water pump. 

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. 

The next step is to apply a number of loss control credit factors such as process control (emergency power, cooling, explosion control, emergency shutdown, computer control, inert gas, operating procedures, reactive chemical reviews), material isolation (remote control valves, blowdown, drainage, interlocks) and fire protection (leak detection, buried tanks, fire water supply, sprinkler systems, water curtains, foam, cable protection). The credit factors are combined and appHed to the fire and explosion index value to result in a net index. 

IB High 1. Control valve fails open 1. Reactor cools, reactant cone 1. Instruct operators and update JFL 1/93... 

Control valve fails to respond 3. Same 3. Place valve on critical instrumentation JFL 1/93... 

Figure 11 shows a system for controlling the water dow to a chemical reactor. The dow is measured by a differential pressure (DP) device. The controller decides on an appropriate control strategy and the control valve manipulates the dow of coolant. The procedure to determine the overall failure rate, the failure probabiUty, and the reUabiUty of the system, assuming a one-year operating period, is outlined hereia. 

Following the invention of the hydrauHc press in 1795 (3), the use of hydrauHcs expanded rapidly during the nineteenth century. The weight-loaded accumulator, invented ca 1850, was used to store energy in hydrauHc systems. The elementary press circuit has several parts that are common to all hydrauHc systems a reservoir, a pump, piping, control valves, a motor, which in this case is a hydrauHc cylinder or ram, and the hydrauHc fluid. By ca 1860 hydrauHc presses were used for forging, and an adjustable-speed hydrauHc transmission was perfected in 1906 (2). The manufacture of hydrauHcahy actuated machines attained industrial importance after 1920. 

Dry inlet gas that has been dehydrated by molecular sieves (qv) or alumina beds to less than 0.1 ppm water is spHt into two streams by a three-way control valve. Approximately 60% of the inlet gas is cooled by heat exchange with the low pressure residue gas from the demethanizer and by external refrigeration. The remainder of the inlet gas is cooled by heat exchange with the demethanized bottoms product, the reboiler, and the side heater. A significant amount of low level refrigera tion from the demethanizer Hquids and the cold residue gas stream is recovered in the inlet gas stream. 

The two portions of the feed stream recombine and flow into the high pressure separator where the Hquid is separated from the vapor and is fed into an intermediate section of the demethanizer with Hquid level control. The decrease in pressure across the level-control valve causes some of the Hquid to flash which results in a decrease in the stream temperature. The pressure of the vapor stream is decreased by the way of a turboexpander to recover... 

The valve cannot control if it is at either end of its travel. To ensure controUabiflty, a valve is generally chosen in such a way that at the maximum design flow rate the flow coefficient required is no more than 85% of the wide-open valve flow coefficient, and at the minimum anticipated flow rate requiring control, a flow coefficient of about 10% of the wide-open valve flow coefficient is required. Whenever practical, control valves are located at grade or at platforms, to assure adequate working space for servicing. 

Once the desired control action has been transformed to an analogue signal, it is transmitted to the final control element over the transmission lines. However, the final control element s actuator may require a different type of signal and thus another transducer may be necessary. Many control valve actuators utilise a pressure signal so a current-to-pressure (I/P) transducer is used to provide a pressure signal to the actuator. 

Control Valves and Other Final Control Elements. Good control at any hierarchial level requites good performance by the final control elements in the next lower level. At the higher control levels, the final control element may be a control appHcation at the next lower control level. However, the control command must ultimately affect the process through the final control elements at the regulatory control level, eg, control valves. 

Fig. 9. Control valve and actuator (a) nomenclature (b) fail-open (F/O) or air-to-close (A/C) action and (c) fail-closed (F/C) or air-to-open (A/O)...

Various accessories can be suppHed along with the control valves for special situations. Positioners ensure that the valve stem is accurately positioned following small or slowly changing control signals or where unbalanced valve forces exist. Boosters, which are actually pneumatic amplifiers, can increase the speed of response or provide adequate force in high pressure appHcations. Limit switches are sometimes included to provide remote verification that the valve stem has actually moved to a particular position. 

Loop two or more elements where a signal passes between them for measurement and/or control, eg, sensor -i- transmitter -i- control + valve device two or more elements combined for a discrete state condition, eg, block valve -i- solenoid valve -i- actuator -I- limit switches... 

If the allowance for control can be reduced, it should be. One option is the use of variable-speed drives. This eliminates the control valve and its pressure drop and piping. Its best appHcation is where a large share of the head is required for friction and where process demands cause the required flow to vary. 

Steam Purity. Boiler water soHds carried over with steam form deposits in nonreturn valves, superheaters, and turbine stop and control valves. Carryover can contaminate process streams and affect product quaHty. Deposition in superheaters can lead to failure due to overheating and corrosion, as shown in Figure 6. 

Fig. 1. Fat sphtter. TRC, temperature recorder controller FLIC, Hquid level indicator controller PCV, pressure control valve and HCV, heat control valve.

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