Valve-positioning system

The dynamics of the valve-positioning system may be described in terms of its small-signal and large-signal characteristics. The small-signal characteristics may be represented to reasonable accuracy by a first-order exponential lag, which will obey the canonical equation ... 

The large-signal characteristics of the valve-positioning system are dominated by the maximum speed/minimum stroke time of the valve. We may calculate the maximum speed by dividing the range of valve travel from fully closed to fully open by the minimum stroke time. Giving the minimum stroke time the symbol and using the convention of normalized valve travel, the maximum velocity of the valve actuator will thus be... 

Section 22.6 has accounted for one important nonlinearity, namely the velocity limit of the valve. Another nonlinearity that can cause control problems is static friction, sometimes referred to as stiction, although more properly known as Coulomb friction. Static friction acts to prevent or impede relative motion by opposing the force applied. Its effect on valve movement may be measured as the difference between the valve s demanded travel (equal to the normalized controller output) and the actual valve travel seen on the plant. This difference will lie normally in the normalized range 0.001 to 0.005 (0.1 to 0.5% of total valve travel) for valves fitted with a valve positioning system, although the author has had experience of an important control valve with a value measured at... 

Valve Positioners The valve positioner, when combined with an appropriate actuator, forms a complete closed-loop valve-position control system. This system makes the valve stem conform to the input signal coming from the process controller in spite of force loads that the actuator may encounter while moving the control valve. Usually, the valve positioner is contained in its own enclosure and is mounted on the control valve. 

Limit Switches and Stem-Position Transmitters Travel-limit switches, position switches, and valve-position transmitters are devices that, when mounted on the valve, actuator, damper, louver, or other throtthng element, detect the component s relative position. The switches are used to operate alarms, signal hghts, relays, solenoid valves, or discrete inputs into the control system. The valve-position transmitter generates a 4-20-mA output that is proportional to the position of the valve. 

Steam turbine performance is modeled using a standard steam flow versus horsepower map and valve position versus steam flow. The turbine inlet valve(s) is positioned by the governor system to maintain constant speed (or another parameter when synchronized). 

The aetual valve position is measured by the eontrol system. The step on the valve is determined as the differenee between the eurrent valve position and the new valve position. 

Step 9 Apply steps to inlet and bypass valves. Now that the new inlet and bypass valve positions are determined, the outputs to the valves can be changed. Before doing this, however, due to flexibility in the control system, it is still possible to manipulate the step on the valve. For this purpose, the control system provides scaling factors between the actual step and the calculated step. These scaling factors can help compensate for calculation errors and/or process dynamics. This is formulated as ... 

Most state laws and safe practice require a safety relief valve ahead of the first stop valve in every positive displacement compressed air system. It is set to release at 1.25 times the normal discharge pressure of the compressor or at the maximum working pressure of the system, whichever is lower. The relief valve piping system sometimes includes a manual vent valve and/or a bypass valve to the suction to facilitate startup and shutdown operations. Quick line sizing equations are (1) line connection, (i/1.75 (2) bypass, ii/4.5 (3) vent, dl63 and (4) relief valve port, cU9. 

An example of cascade control could be based on the simulation example DEACT and this is shown in Fig. 2.35. The problem involves a loop reactor with a deactivating catalyst, and a control strategy is needed to keep the product concentration Cp constant. This could be done by manipulating the feed rate into the system to control the product concentration at a desired level, Cjet- In this cascade control, the first controller establishes the setpoint for flow rate. The second controller uses a measurement of flow rate to establish the valve position. This control procedure would then counteract the influence of decreasing catalyst activity. 

The system is really unstable in the classic linear sense. The nonlinear bounds or constraints on the manipulated variable (control-valve position) keep it in a limit cycle."... 

To prevent this rapid drop, Shinskey developed a floating-pressure control system, sketched in Fig. 8.S. A conventional PI pressure controller is used. The output of the pressure controller goes to the cooling water valve, which is AC so that it will fail open. The pressure controller output is also scot to another controller, the valve-position controller (VPC). This controller looks at the signal to the valve, compares it with the VPC setpoint signal, and sends out a signal which is the setpoint of the pressure controller. Since the valve is AC, the setpoint of the VPC is about 5 percent of scale so as to keep the cooling water valve almost wide open. 

However, since Mi is more expensive than Mj, we wish to minimize the long-term steadystate use of Mi- Therefore, a valve position controller" is used to control Mi at M . What is the closedloop characteristic equation of the system ... 

We want to analyze the floating pressure valve position control system proposed by Shinskcy. Let the transfer function between cooling water flow and column pressure be... 

The scheme in Figure 9.19 shows a simplification of the separation unit, using a 2-position valve to reverse the flow through the column for load/wash and elute steps, and an additional 2-position valve as a detector diverter valve. The system also incorporated several zero-dead volume syringe pumps and several additional valves to route sample and reagents through the system. 

Back-pressure turbine control system for the generation of LP steam, provided with valve position-based optimizer. 

In operation, chromatography requires three sets of information22 operating data (pressure, flow, temperature, etc.), system status data (valve positions, pump status, etc.), and product data for quality assessment. 

The DEMCO valve was capable of being physically locked in the open position as well as in the closed position. The valve lockout system was inadequate to prevent someone from inadvertently opening the DEMCO valve during a maintenance procedure. 

Just as we approached reactor control in Chap. 4, we will start by exploring the open-loop effects of thermal feedback. Consider Fig. 5.19, which shows an adiabatic plug-flow reactor with an FEHE system. We have also included two manipulated variables that wall later turn out to be useful to control the reactor. One of these manipulated variables is the heat load to the furnace and the other is the bypass around the preheater. It is clear that the reactor feed temperature is affected by the bypass valve position and the furnace heat load but also by the reactor exit temperature through the heat exchanger. This creates the possibility for multiple steady states. We can visualize the different... 

We showed through nonlinear dynamic simulations how the process reacts to various disturbances and changes in operating conditions. We have not shown any attempts to optimize process performance, to improve the process design, or to apply any advanced control techniques (model-based, nonlinear, feedforward, valve-position, etc.). These would be the natural next steps after the base-level regulatory control system had been developed to keep the process at a stable desired operating point. 

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