Valve position

Valve-Control Deviee.s Devices mounted on the control valve that interface various forms of input signals, monitor and transmit valve position, or modify valve response are valve-control devices. In some applications, several auxiliary devices are used together on the... 

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. 

Lever-sealed plug cocks are used for throttling service. Because of the large contact area between plug and body, if a plug cock is operable, there is little likelihood of leakage when closed, and the handle position is a clearly visible indication of the valve position. 

Provide limit switches to verify physical valve positions... 

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). 

Step 6 Calculate step on the inlet valve. Now that the new inlet valve e after the breaker opening has been determined, this ean be translated into a valve position. This is a simple proeess using the valve eharaeteristie of the manufaeturer. Eigure 7-8 shows a typieal butterfly valve eharaeteristie. This eurve shows the ealeulated Using the valve eurve, the new valve position is determined. 

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. 

Figure 7-8. The new inlet valve position can be determined from the valve curve.

Step 8 Calculate step on the bypass valve. Now that the Ac bypass of the bypass valve is known, this must he translated into a new valve position. A similar process as described in Step 6 is used. However, in this case there is not an absolute value of c, but instead a Ac. Therefore, first determine the actual bypass bet- This is accomplished by taking the actual bypass valve position and using tlie manufacturer s curve (Figure 7-9). From this point, the method to determine the new valve position is the same as described in Step 6. 

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 ... 

Fig ure 7-9. The bypass valve position can also be determined from the valve s curve. 

One of the simplifieations is to assume that power is a funetion of valve position. This basieally assumes that valve and expander inlet eonditions are eonstant, whieh is not always the ease. However, knowing the proeess eonditions around the expander are fairly eonstant during a signifieant pereentage of the FCCU operating time, this method ean still yield good results. 

The next simplifieation is to assume that there is a linear relationship between a ehange in power and a ehange in valve position. This is even less aeeurate than tlie previous metliod, but ean still yield reasonable results if tlie expander typieally runs in the same operating eonditions and, for example, the bypass valve is usually elosed. However, due to the nonlinear eharaeterisities, this method ean eause signifieant over or under reaetion depending on aetual FCCU operating eonditions. 

F or the following discussion, refer to Figure 3-5, which shows an ideal indicator diagram followed by a series of cylinder illustrations depicting piston movement and valve position. The figure shows in diagram form one... 

Figure 3.22 shows, in bloek diagram form, the transfer funetions for a resistanee thermometer and a valve eonneeted together. The input X[ t) is temperature and the output Xo t) is valve position. Find an expression for the unit step response funetion when there are zero initial eonditions. 

A three-way control valve which sphts or combines two flows in a pressure relieving path is acceptable, provided that the total open cross-sectional area is at least equal to that of the adjacent piping in all valve positions. Also, the valve should be of the type that never has a fully closed port. 

BPCS/SIS functions that produce a trend analysis of input and output functions. Graphic video displays set-up to indicate process conditions and valve positions assure that the status of the process conditions is announced to the board operator, who may then verify the conditions with the field operator. 

Rising stem valves to indicate valve position. 

Close tanker valve Independent check on action. Use checklist Ensure operator is aware of consequences of failure Valve position indicator would reduce probability of error... 

The concentration mode is used when the extract from the SEE module is not concentrated enough to be directly analyzed by the CE instrument, and thus requires a concentration step, which is carried out in the concentrator device. In this mode, during the concentration step the extract from the SEE cell enters the first valve (positioned in the concentration mode (Eigure 6.11(a)), and is then directed to and adsorbed into the sample concentrator which contains an SPE cartridge. While the... 

High level of vibration Loss or gain of efficiency Changes in Control Valve position Excessive blade waar Catalyst depcelts on shroud Sulfi Chloride corrosion on blades disc roots... 

NOTE All intermittent BD arrangements require a properly adjusted throttling gate valve on the BD line (and sample coolers) for good results. It may be appropriate to lock the valve or remove the wheel to ensure that the valve position is not inadvertently further opened or closed. 

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. 

Figure 12 Schematic representation of 2D chromatography using an eight-port injection valve and two storage loops. (A) In the first position of the valve the storage loop 1 is loaded with the HPLC eluent, while the content of the storage loop 2 is analyzed by SEC. (B) In the second valve position storage loop 2 is loaded with HPLC eluent and the content of loop 1 is analyzed according the molecular size of the solute.

Figure 14 Relation Between Valve Position and Controlled...

Figure 14 shows the relationship between valve position and controlled variable (temperature) characteristics of proportional mode. Notice that valve position changes in exact proportion to deviation. Also, the proportional mode responds only to amount of deviation and is insensitive to rate or duration of deviation. At the 2 minute and 4 minute marks, when the temperature returns to its setpoint value, the valve returns to its initial position. There is no valve correction without deviation. 

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