Practical control valve sizing

Driskell, L.R., Practical Guide to Control Valve Sizing, Instr.Techn., 14, p.47,1967. 

Although it has been common practice to specify the pressure loss in ordinary valves in terms of either equivalent length of straight pipe of the same size or velocity head loss, it is becoming more common to specify flow rate and pressure drop characteristics in the same terms as has been the practice for valves designed specifically for control service, namely, in terms of the valve coefficient, C. The flow coefficient of a valve is defined as the volume of Hquid at a specified density that flows through the fully opened valve with a unit pressure drop, eg, = 1 when 3.79 L/min (1 gal /min) pass through the valve... 

Direct feed of compressed chlorine to another process is possible only when the quality of the chlorine meets the user s needs. The receiving process determines and controls how much chlorine is taken from the header, but another control valve is necessary at the compressor discharge header in case the user attempts to take more chlorine than is available. The liquefiers normally handle the chlorine not taken by the direct user. It has been common practice to design the liquefaction plant for full cell output, so that the cells can operate at full rate during short upsets in the user s process. This approach may be modified to suit restrictions on maximum chlorine inventory. In any case, the liquefiers should always have some chlorine gas fed to them to keep them operational and ready to handle full chlorine production should the direct user suddenly stop taking gas. In addition, a supply of liquid chlorine may be needed for a suction chiller. A bypass line around the control with a restricting orifice sized for about 10% of full capacity at the control valve drop can meet this requirement. Any chlorine not taken... 

The maximum AT at the cold end of the heat exchanger would be realized if a controlled continuous expansion and heat exchange could be accomplished in the high-pressure stream from 15 K to the expansion temperature. This, of course, presents practical problems, but placing just one expansion valve between the 15 bath and the final expansion raises the AT at the cold end of the exchanger to a value that can be obtained in practice with reasonably sized heat exchangers. 

Capillary burst valves, which are rotationaUy actuated, stop flow by the counterpressure induced at a capillary feature such as a hydrophobic constriction or a sudden expansion in a (hydrophilic) channel (Fig. 4). If the substrate material is hydrophobic, the valve is typically formed by a constriction in the microchannel. These burst valves yield once the spin rate passed a certain threshold governed by the valve geometry, its radial position, the contact angle, and the surface tension of the liquid. Mainly due to minimum feature sizes and manufacturing tolerances, there is a practical limit which, on the one hand, restricts the maximum burst frequency and, on the other hand, smears out the definition of the nominally discrete burst frequencies into bands. In many applications the number of independently controllable burst valves and thus the number and vigorousness of LUOs are significantly capped [4]. 

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... 

Thus the piston performs an advance and return motion and the control gear normally required with a constant delivery pump, can therefore be omitted. This simple circuit arrangement calls for equal-sized power- and return surfaces of the press piston which also results in equal speed rates in either direction. When these areas are of different size - as it is mostly the case in practice - and idling- and return strokes are to be performed at considerably higher speed rates than the power stroke, it is necessary to provide the press with a filling valve and a check valve. The corresponding circuit diagram is illustrated in Fig. 171. 

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