Control valve allowable pressure differential

Valves in the gas headers control the pressmes on the cells. In the usual case where pressures are close to atmospheric, pressure drops everywhere must be kept very low. Each header is sized generously in order to keep the gas pressure essentially equal on every cell in the line. The two gas pressmes can be controlled independently. However, the differential pressure between the cathode and anode chambers may be more important than the individual header pressures. In a membrane cell, for example, fluctuations in differential pressure cause vibration of the membranes. This can lead to physical damage, sometimes allowing passage of cell fluids, and to early failure. A frequent practice is to control the differential pressure directly, forcing any pressure fluctuations on the two... 

A second valve then controls compressor suction pressure by allowing gas to recycle from the discharge of the compressor. Good control maintains a constant differential... 

A similar technique, more common with air systems, is to automate the familiar double-block-and-bleed arrangement. The first step in this design might be to replace a simple check valve with a differential-pressure control valve that shuts when the pressure differential reverses. This affords two levels of protection, unless one assumes that the check valve is fundamentally untrustworthy and always allows somebackleakage. Check valves can, of course, be made more reliable by proper installation and by extra measures such as force loading. An elaboration on the diagram above is to add a second valve in the line that closes when the differential pressure is too low or even negative. Finally, the space between the two in-line check valves can be vented for more positive protection. 

A special-purpose, yet very useful, valve concept is represented by the terminal valve in Fig. 7.36. This valve, which can be opened or closed at the very end of a transfer line, allows a cryogenic container to be filled without possible contamination from air or moisture. Another type of valve, the helium-operated valve (not shown), has been very useful in the transfer of liquid hydrogen. This valve can be readily operated by remote control and opened and closed by helium gas. A magnetically operated valve has recently been developed specifically for use in small liquid helium transfer lines. This all-metal valve is opened by an energized coil and closes by gravity and pressure differential forces. 

Slide valves will have an independent low differential pressure override controller to prevent the reaction temperature controllers from opening the slide valves to the point where low differential pressure could allow feed back to the regenerator. 

The basic SFC system comprises a mobile phase delivery system, an injector (as in HPLC), oven, restrictor, detector and a control/data system. In SFC the mobile phase is supplied to the LC pump where the pressure of the fluid is raised above the critical pressure. Pressure control is the primary variable in SFC. In SFC temperature is also important, but more as a supplementary parameter to pressure programming. Samples are introduced into the fluid stream via an LC injection valve and separated on a column placed in a GC oven thermostatted above the critical temperature of the mobile phase. A postcolumn restrictor ensures that the fluid is maintained above its critical pressure throughout the separation process. Detectors positioned either before or after the postcolumn restrictor monitor analytes eluting from the column. The key feature differentiating SFC from conventional techniques is the use of the significantly elevated pressure at the column outlet. This allows not only to use mobile phases that are either impossible or impractical under conventional LC and GC conditions but also to use more ordinary... 

If the differential pressure control loop were extremely fast, we could assume that the differential pressure across the valve remained constant, and the linearization exercise would be simplified enormously. But we shall examine the case where the differential pressure control loop is slow, and needs to be allowed for in calculating the response of flow to valve opening. 

Equation (23.46) represents the change in flow to valve opening when no action is taken to control the differential pressure across the valve - a useful result in itself. Further, now that dwjdy has been found, substituting in equation (23.23) allows the total derivative dhpjdy to be found. Thus we are able to evaluate fully the response of the system with the differential pressure controller switched out, as shown in Figure 23.2. 

The fact that we have chosen a block diagram approach means that we may now proceed to evaluate also the response of flow to valve opening with the differential pressure controller switched in. Figure 23.2 includes stubs to allow for the change in differential pressure brought about by the action of the differential pressure controller. We will now connect blocks to those stubs and thus include the differential pressure control loop. The new blocks are shown in Figure 23.3. 

In a plant producing liquid chlorine, the compressed gas goes next to the liquefaction system. Rather than impose a pressure drop between the processes, the gas is allowed to flow freely into liquefaction. A valve on the uncondensed gas venting from the liquefaction unit (Section 9.1.7.2) controls the pressure on both systems. When chlorine is sent to another process without liquefaction, it would be possible to withdraw it on downstream pressure control and let the compressor outlet pressure fluctuate. This approach leads to variability in the differential pressure across the compressor recycle valve. Fluctuations in this flow can cause fluctuations in the compressor suction pressure and therefore in the cellroom chlorine header. It is better to control the compressor outlet pressure itself, even at the cost of another pressure control loop at the destination. Section 11.3.2.6 describes instrumentation hardware and the problems of transferring chlorine to more than one destination. 

Valve F is opened to allow solvent vapor into the system. The amount of solvent introduced is controlled by controlling the temperature of the solvent reservoir. Valve F is closed after each addition of vapor. Solvent vapor and polymer samples are then allowed to equilibrate. The integral and differential sorptions are observed on the respective balances and the pressure of solvent vapor is recorded. It is then possible to calculate the volume fraction of polymer in the cross-linked and uncross-linked swollen samples, v and vj respectively, at a solvent pressure p = pj and thus to know pj and pj at equal volume fractions v = vj and to calculate the ratio of activities af/aj = pj/p . From knowledge of p , p , vj and x, x(v ) is deduced through equations 3, 4 and 5. 

It is possible to automate the globe valve to avoid "over mixing." A differential pressure controller is used to control the pressure drop through the globe valve. This system automatically adjusts for changing flow rates and maintains a set pressure drop. Since this system s set point can be adjusted in the field, it allows an operator to optimize its performance. 

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