Control valves analysis

For FCC applications, a rigorous analysis typically involves transient evaluations of expander coupling failures, generator load drops, compressor and surge system operation, and control valve malfunctions. The results of these evaluations permit optimum selection of control valves and control strategies. 

A vapor poeket on the exchanger s low-pressure side can create a cushion that may greatly diminish the pressure transient s intensity. A transient analysis may not be required if sufficient low-pressure side vapor exists (although tube rupture should still be considered as a viable relief scenario). However, if the low-pressure fluid is liquid from a separator that has a small amount of vapor from flashing across a level control valve, the vapor pocket may collapse after the pressure has exceeded the fluid s bubble point. The bubble point will be at the separator pressure. Transient analysis will prediet a gradually inereasing pressure until the pressure reaches the bubble point. Then, the pressure will increase rapidly. For this ease, a transient analysis should be considered. 

Failure of Individual Control Valve - The following individual control valve failures should be included in the analysis of control systems for determination of pressure rehef requirements ... 

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.)... 

Since one of tlie purposes of a fault tree analysis is tlie calculation of tlie probability of the top event, let A, B, and C represent tlie failure of pump A, tlie failure of pump B, and tlie failure of the control valve, respectively. Then if T represents the top event, no water flow, we can write... 

Combustion equipment can be set to give optimum efficiency at the time of commissioning but this condition will not be maintained. Wear and tear on control valves, partial blockage of filters, sooting of surfaces, etc. will all cause a fall in efficiency. To counter this, regular maintenance is desirable, and must include routine flue analysis and burner adjustment. 

Most combustion equipment is not controlled by means of a feedback from flue gas analysis but is preset at the time of commissioning and preferably checked and reset at intervals as part of a planned maintenance schedule. It is difficult to set the burner for optimum efficiency at all firing rates and some compromise is necessary, depending on the control valves used and the control mode (e.g. on/off, fully modulating, etc.). 

It is very important to note that in this loop system the parameter To, which must be kept constant, is measured, though all subsequent action is concerned with the magnitude of the error and not with the actual value of To. This simple loop will frequently be complicated by there being several parameters to control, which may necessitate considerable instrumental analysis and the control action will involve operation of several control valves. 

In this chapter we will illustrate and analyze some of the more common methods for measuring flow rate in conduits, including the pitot tube, venturi, nozzle, and orifice meters. This is by no means intended to be a comprehensive or exhaustive treatment, however, as there are a great many other devices in use for measuring flow rate, such as turbine, vane, Coriolis, ultrasonic, and magnetic flow meters, just to name a few. The examples considered here demonstrate the application of the fundamental conservation principles to the analysis of several of the most common devices. We also consider control valves in this chapter, because they are frequently employed in conjunction with the measurement of flow rate to provide a means of controlling flow. 

Fig. 1.23. Monitor AW 2. In the foreground right Sample vial with measuring electrodes and resistance thermometer, behind to the left the control- and analysis unit. The storage of LN2 and its control valve are not shown. The resistance in the measuring head has to be large compared with the resistance to measure e. g. 1011 1 (photograph AMSCO Finn-Aqua, D-50354 Hiirth).

There was one instance of a pipe leaking near the pressure control valve. This was readily fixed but resulted in one day of downtime. The leak was attributed to having retrofitted an alternative pipe routing without a thermal expansion analysis. 

The mass spectra of the gases evolved from the deuterated SWNT sample heated in vacuum were measured with the MI 1201V mass spectrometer. Gas ionization in the ion source of the spectrometer was produced with a 70-eV electron beam. To obtain the gas phase, the sample was placed in a quartz ampoule of a pyrolyzer that was connected to the injection system of the mass spectrometer through a fine control valve. Then the ampoule was evacuated to a pressure of about 2-x 10-5 Pa in order to remove the surface and weakly bound impurities from the sample. After the evacuation, the ampoule was isolated from the vacuum system and the sample was heated to 550°C in five steps. At each step, the sample was kept at a fixed temperature for 3 h then the fine control valve was open and the mass-spectrometric analysis of the gas collected in the ampoule was performed. After the analysis, the quartz ampoule was again evacuated, the valve was closed, and the sample was heated to the next temperature. The measurements were carried out over the range 1 < m/z < 90, where m is the atomic mass and z is the ion charge. The spectrometer resolution of about 0.08% ensured a reliable determination of the gas-phase components. 

Because of the enormous diversity in components it is difficult to describe a straightforward design-path for components for the MCB concept. Here we focus on the modeling and the design of the fluid control modules and specific on the thermo-pneumatic actuated micropump used (twice) in the demonstrator. An elaborated model of this micropump is given by van de Pol et al. [21]. The main functions of the fluid control in micro analysis systems are the switching function and the direct flow and/or pressure control. Building blocks are hydraulic inertances, resistors, capacitors and passive and control-valves. Very often an active element like a micropump is needed. 

We can make a similar classification around U and W. For instance, control valves belong to the set UB. whereas the regeneration of a packed-bed catalyst would be classified as JX,. Similarly, measurements of the reactor feed flow and temperature belong to W while a once-per-shift analysis of the reactor feed composition belongs to W,. 

In a great majority of applications, including sliding-stem control valves, the implications of cost analysis are plain rather than arbitrarily specifying a tight-shutoff class, one should generally pick the lowest class required for the application. This will help ensure that the most cost-effective valve is chosen. 

The sample system is responsible for collecting a representative sample of the process and delivering it to the analyzer for analysis. Obviously, the reliability of the sample system directly affects the reliability of the overall composition analysis system. The transport delay associated with the sample system contributes directly to the overall deadtime associated with an on-line composition measurement. This difference in sampling deadtime can have a drastic effect on the performance of a control loop. Table 15.2 summarizes the dynamic characteristics and repeatability of typical control valve systems and several different types of sensors. 

When a very small change is made in the composition of the Fqb stream (zoB.B changed from I to 0.999 and Zqb,a changed from 0 to 0.001), the process can barely handle it. The steady-state value of in the third column changes 15 percent for this very small disturbance. Thus, the steady-state analysis predicts that this structure will not work. Dynamic simulations confirmed this very small disturbances drive the control valves on / 3 and V3 wide open, and product quality cannot be maintained. 

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