Vapor control valve sizing

For a given vapor How rate (W), in lb/Tir and control valve pressure drop (dp), in psi, v wr control valve sizing coefficient (Cv) can be calculated by following equation  

Fp is piping geometry factor, defined by Eq. (3) P) is control valve inJet pressure, in psia Y is expansion factor X is control valve pressure drop ratio, which is the ratio of control valve pressure drop to control valve inlet pressure (absolute) M is vapor molecular weight Tl is control valve inlet temperature in °R (Rankine)  

Z1 is inlet vapor compressibility factor Fk is ratio of specific heat ratio of vapor to air, see Eq. (16) XT is control valve pressure drop ratio for air at sonic flow k is specific heat ratio of vapor at inlet conditions Cp is vapor (constant pressure) molar heat capacity at inlet conditions, in btu/lb mole- F. 

max is selected control valve Cv at full open position. 

Like liquid control valve, there is a flow capadty limitation for vapor control valve. The difference is that for vapor control valve, its flow capacity is limited by sonic flow. For sonic flow, X is caiculated by Hq. (15b), and Y equals to 0.667 per Bq. (14). The value ofY will never be less than 0.667. For subsonic flow, the value of Y is between 0.667 to 1.0. By comparing the value of X and Fk XT, we can tdl whether vapor flow at control valve is sonic or not. If X is greater or equal to Fk XT, sonic flow at control valve is reached. Increasing X (pressure drop across the control valve) will not cause flow increase through the control valve. Therefore, for sonic flow, X is set equal to Fk XT, 

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The careful selection and design of control valves is important good flow control must be achieved, while keeping the pressure drop as low as possible. The valve must also be sized to avoid the flashing of hot liquids and the supercritical flow of gases and vapors. Control valve sizing is discussed by Chaflin (1974). 

Cv I iquid or vapor control valve sizing coeffl cient. 

V. Control Valve Sizing Control valve sizing for liquid or vapor service will be discussed in this section. 

For sizing a flashing control valve add the Cv s of the liquid and the vapor. 

So it is important to vent off some vapor to get rid of the ethane, but not too much. The vent valve (AC) is split-range, so that it is closed when the pressure controller output signal is 83% of full scale. It is wide open when the controller output is at 0%. The sizing of the steam, chilled water, and vent valves is critical to the safe and efficient operation of this batch reactor. Figure 4.42 gives a sketch of the reactor, the controller, the setpoint generator, and the three control valves. 

To size control valves for vapors other than steam, use the relation C = W(v2/Ap)ns/63A, where W is vapor flow rate, in lb/h, v2 is specific volume of the vapor at the outlet pressure P2, in ft3/lb ... 

Equation 5-29 is valid for liquid flowing below its saturation temperature in the turbulent zone with a viscosity value that is close to that of water and size of pipe. Also, the control valve must be the same. Equation 5-29 can also be applied, if the vapor pressure of the liquid at the flowing temperature is equal to or less than one-half the upstream pressure. Eor this case, the vapor pressure of the liquid is substituted for downstream pressure, P2 and the valve coefficient is calculated. The calculated must be corrected by a critical flow factor, Cj, where C corr is defined by... 

Control system. For subsequent selection and sizing of pumps and compressors, we need to map out the number and location of the control valves. Since the number of control valves is related to the number of control degrees of freedom, identify the control degrees of freedom. For example, a typical hydrodealkyllation process with a reactor, furnace, vapor-liquid separator, recycle compressor, two heat exchangers, and three distillation columns has 23 control degrees of freedom (Luyben et al., 1997). This requires 23 control valves whose location affects the rest of the design and the safety and hazards (see Section 16.7). 

It has been recommended to design a forced-circulation reboiler for a high pressure drop (134). In some cases (68), a restriction is placed in the vapor line downstream of the reboiler and sized to prevent vaporization in the reboiler. This restriction is often placed at the column inlet nozzle (68), but this may generate an undesirable high-velocity jet at the column inlet (see Sec. 4.1, guideline 5). Further, a restriction downstream of the reboiler may interfere with the action of a control valve located in the liquid line to the reboiler. The author is also familiar with a case where such a restriction experienced erosion at an intolerable rate. 

An Aspen Flash model is used for the reflux drum with pressme set at 1 bar and design specification of a vapor fraction of 10 , which makes the drum essentially adiabatic. A small vapor flow rate is necessary so that the control valve in this vent line can be sized. In the Aspen Dynamics simulation, the valve is completely closed. The liquid holdup in the drum is set to give 5 min at 50% full (diameter 3 m and length 6 m). 

Excess-liquid heat pipes operate in much the same manner as gas-loaded heat pipes, but utilize excess working fluid to block portions of the pipe and control the condenser size or prevent reversal of heat transfer. Vapor-flow-modulated heat pipes utilize a throttling valve to control the amount of vapor leaving the evaporator. In this type of control scheme, increased evaporator temperatures result in an expansion of the bellows chamber containing the control fluid. This in turn closes down the throttling valve and reduces the flow of vapor to the condenser. This type of device is typically applied in situations where the evaporator temperature varies and a constant condenser temperature is desired. 

The precursor should be either a liquid or a solid with sufficient vapor pressure and mass transport at the desired temperature, preferably below 200°C. Liquids are preferred to solids owing to the difficulty of maintaining a constant flux of source vapors over a non-equilibrium percolation (solid) process. Such nonbubbling processes are a function of surface area, a non-constant variable with respect to both time and particle size. The upper temperature limit is not dictated by chemical factors rather it is a limitation imposed by the stability of the mass flow controllers and pneumatic valves utilized in commercial deposition equipment. It must be stressed that while the achievement of an optimum vapor pres-... 

With the equipment sized and the flowsheet pressure checked, it is exported into Aspen Dynamics. Figure 8.37 shows the initial flowsheet that opens in Aspen Dynamics. Pressure controllers are already installed on each column. In column Cl, pressure is held by manipulating the position of valve VI3 in the overhead vapor line upstream of the condenser. Note that the Reflux recycle loop is not closed. 

The reflux drum and column base are sized to provide 5 min of liquid holdup when half full. The file is pressure checked and exported to Aspen Dynamics. The initial control scheme that opens is shown in Figure 11.26. Note that there is a level controller (LCW1) that pulls off free water from the reflux dmm. The other level controller (LC12) manipulates reflux flow rate to hold the liquid level of the organic phase in the reflux dmm. Since the reflux ratio is only 0.344 in this column, we change the control structure to hold reflux-dmm level with the NAPHTHA flow rate (valve V12). Pressure controller PCI manipulates the valve V14 in the vapor line. 

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