Control valves defined

A control valve consists ot a valve, an actuator, and possiblv one or more valve-control devices. The valves discussed in this section are applicable to throttling control (i.e, vviiere tlovv through the valve is regulated to anv desired amount betvv een maximum and minimum limits). Other valves such as check, isolation, and relict valves are addressed in the next subsection. As defined, control valves are automatic control devices that modify the tliiid tlovv rate as specified bv the controller. 

Also, the electronic control-valve device s level of immunity to, and emission of, electromagnetic interference (EMI) can be an issue in the chemical-valve environment. EMI requirements for the control-valve devices are presently mandatory in the European Community but voluntary in the United States, Japan, and the rest of the world. International Electrotechnical Commission (lEC) SOI, Parts I through 4, Electromagnetic Compatibihty for Industrial Process Measurement and Control Equipment, defines tests and requirements for control-device immunity. Immunity and emission standards are addressed in CENELEC (European Committee for Electrotechnical Standardization) EN 50 081-1 1992, EN 50 081-2 1993, EN 50 082-1 1992, and prEN 50 082-2 1994. 

You can quickly identify these plant sections by reviewing process flow diagrams and valving arrangements. Isolation points are defined by control valves or powered block valves that can be remotely activated. Process hazard analysis techniques help you identify the maximum credible accident scenarios. (Note that manual valves should not be considered reliable isolation points unless they are located to be accessible following a major accident. However, remotely-activated valves can only be considered reliable isolation points if there are adequate reliability engineering and maintenance programs in place.)... 

From the basic process-containing flowsheet other engineering specialties develop their own details. For example, the instrument engineer often takes the requirements of the process and prepares a completely detailed flowsheet which defines every action of the instruments, control valves, switches, alarm horns, signal lights, etc. This is his detailed working tool. 

For the multivariable case, an uncertainty matrix A must be defined which contains the uncertainty in the various elements of the plant transfer function (and also could include uncertainties in setting control valves or in measurements). 

TTie pH signal is fed via a pH meter to a potentiometric recorder and to a neutrahzation controller, which compares the electrode e.m.f. with pre-selected values and opens or closes gas-control valves accordingly. Depending on the pH, it is possible to obtain a flow of air, ammonia or a mixture of both gases. By careful adjustment of the potentiometers and the flow-rates of ammonia and air, it is possible to control the final pH and also to keep the digest within a fairly closely defined pH range during the neutrahzation process. 

Control valves. Valve vendors publish much information about flow-control valves. Much of their information covers what is known as control valve flow coefficients Cv. It is convenient to express the valve capacity and the valve flow characteristics in terms of the flow coefficient Cv. This value is defined as the flow of water at 60°F, in gallons per minute (gpm), at a pressure drop of 1.0 psi across the valve. In a flow valve, therefore, controlling the flow of water at 60°F as the valve opens, keeping a 1.0-psi pressure drop across the valve, the C value is the rate of water flow, gpm. 

This is a simple way to define control valve characteristics, including their capacity. Using this relationship to the K value derivation herein, the following equations derived by numerous vendors may be applied. 

Interlock testing is performed on a periodic basis, usually every 12 months, following detailed written test procedures. The test procedure for each safety interlock defines the actions, including the approval system and the mechanical work required to ensure reliability. These procedures cover the testing activity from the process measurement device to the final element, which is often a control valve. [26]... 

A typical cbmbined-detail flow diagram shows the location of temperature and pressure regulators and indicators, as well as the location of critical control valves and special instruments. Each piece of equipment ds shown and is designated by a defined code number. For each piece of equipment, accompany-... 

The control equipment model is composed of three temperature transducers models and two control valves models, each model defined by... 

Equation 5-29 is valid for liquid flowing below its saturation temperature in the turbulent zone with a viscosity value that is close to that of water and size of pipe. Also, the control valve must be the same. Equation 5-29 can also be applied, if the vapor pressure of the liquid at the flowing temperature is equal to or less than one-half the upstream pressure. Eor this case, the vapor pressure of the liquid is substituted for downstream pressure, P2 and the valve coefficient is calculated. The calculated must be corrected by a critical flow factor, Cj, where C corr is defined by... 

For critical flow, the downstream pressure of the control valve is less than one-half the upstream pressure. Therefore, the term (P, - P2)(P2) reduces to 0.5P, in Equations 5-32 and 5-33. C, is then defined by... 

Subcritical Condition. The control valve coefficient, C , is defined by (W)(K)... 

Control valves form an important equipment item in process plants, and therefore, a lot of money can be saved by ensuring that the right valves are installed. Selecting the right control valve for any application starts with defining the valve s functions. For example, if an on-off interlock valve is to be used, then the primary criteria for selection are reliability and simplicity. If very fast and precise modulation is required, such as for a compressor antisurge vent, a high quality valve is chosen. 

An emergency response team, under the control of the emergency response coordinator, should be assembled with a clearly defined chain of command. The members of this team should be selected and trained for specific mitigative and communication tasks. Specific duties might include spill cleanup, first aid, employee evacuation and headcount, incipient fire response, fire pump/sprinkler control valve/fire door monitoring, firefighting, fire/emergency responder communications, and rescue. 

Figure 13.15 shows the operational scheme of this automatic tltrator. The heart of the unit Is an INTEL 8080 microprocessor mounted on the central processing unit (CPU) board. The rotary reaction cell assembly can accommodate up to three different sensors for multiple measurements on the same processed sample. Each stepper burette board controls up to two burette dispensing assemblies. Function boards such as the colorimeter board, air burette board, E/I output board and RS-232 printer Interface boards are available optionally. The optional D/A and E/I board is used for closed-loop applications where the tltrator controls the final element such as a control valve. The RS-232 printer Interface board Is useful for troubleshooting the equipment and editing user-defined programs. The Instrument accuracy, repeatability and response time vary widely and depend on the particular type of measurement concerned. The system requires a.c. power, a 75-psl air supply and a dilution water supply for proper operation. The air flow-rate required is of about 50 cm3/mln... 

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