Valve capacity rating

Manufacturers of valves, especially control valves, express valve capacity in terms of a flow coefficient C,, which gives the flow rate through the valve in gal/min of water at 60°F under a pressure drop of 1 IbFin. It is related to K by... 

In addition to the above back pressure limitations based on valve capacity, balanced bellows PR valves are also subject to back pressure limitations based on the mechanical strength of the bellows or bellows bonnet, or the valve outlet flange rating. The back pressure specified for the valve is governed by the lowest back pressure permitted by these various criteria. 

Capacity. There is little difference in the capacity rating of the three types (the diameter of the column required for a given flow-rate) the ranking is sieve, valve, bubble-cap. 

Fig. 5.1a shows the slurry packing system that 1 use. This operates at a fairly low pressure, as I am not especially interested in producing columns with very high efficiencies, but rather in saving on the cost of commercial columns. The pump and high pressure valve are rated for pressures of 500 bar (7500 psi) and 400 bar (6000 psi) respectively. The slurry reservoir is a stainless steel tube 85 cm long, with a capacity of about 50 cm3. The method is not hazardous unless there is air trapped in the high pressure line nevertheless it is advisable to use a safety screen. 

While this is the general recommendation, compliance with this is not always possible. As seen earlier in this book, higher oudet pressure drops may result in reduced valve capacity and instability. In that case, other types of SRVs as discussed in Chapter 9 need to be taken into consideration. API RP 521 also states the outlet pressure drop for individual relief valves should be based on the actual rated valve capacity consistent with the inlet piping pressure drop as discussed in Section 6.1. 

Control valves. Valve vendors publish much information about flow-control valves. Much of their information covers what is known as control valve flow coefficients Cv. It is convenient to express the valve capacity and the valve flow characteristics in terms of the flow coefficient Cv. This value is defined as the flow of water at 60°F, in gallons per minute (gpm), at a pressure drop of 1.0 psi across the valve. In a flow valve, therefore, controlling the flow of water at 60°F as the valve opens, keeping a 1.0-psi pressure drop across the valve, the C value is the rate of water flow, gpm. 

Some valve manufacturers present the capacity of their valves in graphic instead of tabular form. One popular chart, Fig. 6.9, is entered at the difference between the inlet and outlet pressures on the abscissa, or 140 — 40 = 100 lb/in2 (689.4 kPa). Project vertically to the flow rate of 2000/60 = 33.3 gal/min (2.1 L/s). Read the valve size on the intersecting valve-capacity curve, or on the next curve if there is no intersection with the curve. Figure 6.9 shows that a 1-in valve should be used. This agrees with the tabulated capacity. 

Flow-through relief valves have traditionally been analyzed by treating the valve as a convergent noz-2ie [i 29.30] discharge coefficient (kactual mass flow rate to the theoretical ideal mass flow rate through a one-dimensional convergent nozzle of exit area equal to the nominal bore of the tested valve. Measured discharge coefficients are derated by 10% (Aidr) to specify valve capacity. 

The computer program PROG52 calculates the control valve capacity coefficient (C ) or the maximum flow rate that will pass through the valve. Table 5-9 shows the input data and results of the control valve calculations for liquid, gas, and steam conditions. For the liquid service, at a flow rate of 400 gpm and the control valve pressure drop of 25 psi, the control valve capacity coefficient (CJ is 80. The gas service with a flow rate of 1200 fP/h, and control valve pressure drop of 400 psi gives the control valve capacity coefficient (C ) of 0.059. For the steam condition of capacity coefficient (C, ) = 75, and the control valve pressure drop of 10 psi, the maximum steam flow rate through the valve is 5511.4 Ib/hr. 

THIS PROGRAM CALCULATES THE CONTROL VALVE CAPACITY COEFFICIENT (Cv) OR MAXIMUM FLUID RATE FOR LIQUID, VAPOR AND STEAM IN PROCESS OPERATIONS. IF THE CONTROL-VALVE COEFFICIENT(Cv) IS ALREADY KNOWN, THE PROGRAM MILL CALCULATE THE MAXIMUM FUJID FLOW RATE (LIQUID, VAPOR OR STEAM) THAT CAN PASS THROUGH THE VALVE FOR A GIVEN SET OF PROCESS CONDITIONS. 

THIS PROGRAM CALCULATES THE CONTROL VALVE CAPACITY COEFFICIENT OR THE MAXIMUM GAS FLOW RATE FOR GAS FLOW CONDITIONS ... 

There are two main features of control valves valve capacity and valve characteristics. The most important valve parameter for calculating the size of a control valve is the flow coefficient, Cy, which gives the flow rate of water through a valve for a unit pressure drop at standard conditions. It varies with size and style of valve. For Hquids, the Cy value is calculated empirically using the equation ... 

Process-control valves have few measurable criteria that can be used to determine their performance. Obviously, the valve must provide a positive seal when closed. In addition, it must provide a relatively laminar flow with minimum pressure drop in the fully open position. When evaluating valves, the following criteria should be considered capacity rating, flow characteristics, pressure drop, and response characteristics. 

The primary selection criterion of a control valve is its capacity rating. Each type of valve is available in a variety of sizes to handle most typical... 

Variable Flow Rate Conventional variable clearance volume and valve lifting devices are impracticable at high pressures and, should it be necessary to vary the flow rate, use has to be made of variable speed electric drives or magnetic clutches. Integral steam and gas engines have been used and Burckhardt (168) developed an hydrauhc drive to provide an integrated variable capacity machine, but its efficiency is less than that of a straight mechanical drive. 

Amplitude of controlled variable Output amplitude limits Cross sectional area of valve Cross sectional area of tank Controller output bias Bottoms flow rate Limit on control Controlled variable Concentration of A Discharge coefficient Inlet concentration Limit on control move Specific heat of liquid Integration constant Heat capacity of reactants Valve flow coefficient Distillate flow rate Limit on output Decoupler transfer function Error... 

The two principal elements of evaporator control are evaporation rate a.ndproduct concentration. Evaporation rate in single- and multiple-effect evaporators is usually achieved by steam-flow control. Conventional-control instrumentation is used (see Sec. 22), with the added precaution that pressure drop across meter and control valve, which reduces temperature difference available for heat transfer, not be excessive when maximum capacity is desired. Capacity control of thermocompression evaporators depends on the type of compressor positive-displacement compressors can utilize speed control or variations in operating pressure level. Centrifugal machines normally utihze adjustable inlet-guide vanes. Steam jets may have an adjustable spindle in the high-pressure orifice or be arranged as multiple jets that can individually be cut out of the system. 

Tray active panels installed with valves under downcomer of tray above. Limited capacity. High and unstable pressure drop at moderate rates. Installation and inspection error. 

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