Control valve coefficients

The control valve coefficients in Table 4 are for full open conditions. The control valve must be designed to operate at partial open conditions for good control. Figure 3 shows partial open performance for a number of trim types. 

Subcritical Condition. The control valve coefficient, C , is defined by (W)(K)... 

Calculate the control valve coefficient for gas flow under the following conditions ... 

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. 

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... 

The valve cannot control if it is at either end of its travel. To ensure controUabiflty, a valve is generally chosen in such a way that at the maximum design flow rate the flow coefficient required is no more than 85% of the wide-open valve flow coefficient, and at the minimum anticipated flow rate requiring control, a flow coefficient of about 10% of the wide-open valve flow coefficient is required. Whenever practical, control valves are located at grade or at platforms, to assure adequate working space for servicing. 

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... 

Another example of pressure control by variable heat transfer coefficient is a vacuum condenser. The vacuum system pulls the inerts out through a vent. The control valve between the condenser and vacuum system varies the amount of inerts leaving the condenser. If the pressure gets too high, the control valve opens to pull out more inerts and produce a smaller tube area blanketed by inerts. Since relatively stagnant inerts have poorer heat transfer than condensing vapors, additional inerts... 

For positive displacement pumps, a bypass-type control valve should be furnished to set the primary lube system pressure. The valve should be able to maintain system pressure during pump startup and pump transfers, which includes relieving the capacity of one pump, while both are running. The valve should provide stable, constant pressure during these transients. Flow turndown of 8 to 1 is not unusual. Multiple valves in parallel should be used if a single valve is not suitable. The valve should be sized to operate between 10 and 90% of the flow coefficient (Cv). Additional pressure control valves should be furnished as required to pro ide any of the intermediate pressure levels. 

Table 10-3 Example Flow Coefficient Values for a Control Valve with Various Trim Characteristics... 

Cg control valve discharge coefficient for gas flow, L4tT1,2/M]... 

Figure 3.14. The lower ends of fractionators, (a) Kettle reboiler. The heat source may be on TC of either of the two locations shown or on flow control, or on difference of pressure between key locations in the tower. Because of the built-in weir, no LC is needed. Less head room is needed than with the thermosiphon reboiler, (b) Thermosiphon reboiler. Compared with the kettle, the heat transfer coefficient is greater, the shorter residence time may prevent overheating of thermally sensitive materials, surface fouling will be less, and the smaller holdup of hot liquid is a safety precaution, (c) Forced circulation reboiler. High rate of heat transfer and a short residence time which is desirable with thermally sensitive materials are achieved, (d) Rate of supply of heat transfer medium is controlled by the difference in pressure between two key locations in the tower, (e) With the control valve in the condensate line, the rate of heat transfer is controlled by the amount of unflooded heat transfer surface present at any time, (f) Withdrawal on TC ensures that the product has the correct boiling point and presumably the correct composition. The LC on the steam supply ensures that the specified heat input is being maintained, (g) Cascade control The set point of the FC on the steam supply is adjusted by the TC to ensure constant temperature in the column, (h) Steam flow rate is controlled to ensure specified composition of the PF effluent. The composition may be measured directly or indirectly by measurement of some physical property such as vapor pressure, (i) The three-way valve in the hot oil heating supply prevents buildup of excessive pressure in case the flow to the reboiier is throttled substantially, (j) The three-way valve of case (i) is replaced by a two-way valve and a differential pressure controller. This method is more expensive but avoids use of the possibly troublesome three-way valve.

A cylindrical tank is fed with water at a flow rate of 2.3 m3 /hour and is equipped with an output control valve at the bottom. The steady-state height in the tank is 2 m. What is the valve coefficient (and its units) under these conditions The tank diameter is 3 m and the total height of the tank is 5 to. 

The first strategy appears to be very effective in fact, as shown by (2.30) and (2.31), the direct (linear) effect of increasing the flow rate is augmented by the increase of the jacket-side heat exchange coefficient. This control action can be realized without a noticeable time delay by a simple control valve, the only drawback for its quantitative assessment being the nonlinear relationship between the overall heat exchange coefficient and the flow rate of the heat exchange fluid. 

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. 

A control valve can also be viewed as variable-area flow meter. Therefore, smart valves can measure their own flow by solving the appropriate valvesizing equation. For example, in the case of turbulent liquid flow applications, where the valve capacity coefficient... 

For a complete treatment of the many aspects of control valve sizing, refer to the Instrument Engineers Handbook. For some basic equations to calculate the required valve coefficients (C ) for various liquid and gas flow applications using different valve types, refer to Tables 2.66 and 2.67. 

Compute the valve flow coefficient The valve flow coefficient C is a function of the maximum steam flow rate through the valve and the pressure drop that occurs at this flow rate. When choosing a control valve for a process control system, the usual procedure is to assume a maximum flow rate for the valve based on a considered judgment of the overload the system may carry. Usual overloads do not exceed 25 percent of the maximum rated capacity of the system. Using this overload range as a guide, assume that the valve must handle a 20 percent overload, or 0.20(1500) = 300 lb/h (0.038 kg/s). Hence, the rated capacity of this valve should be 1500 + 300 = 1800 lb/h (0.23 kg/s). 

With these data available, compute the valve flow coefficient from C = W K/3( ApP2 )0 5. where W is steam flow rate, in lb/h, K equals 1 + (0.0007 x °F superheat of the steam), p is pressure drop across the valve at the maximum steam flow rate, in lb/in2, and P2 is control-valve outlet pressure at maximum steam flow rate, in psia. Since the steam is saturated, it is not superheated, and K = 1. Then, C = 1500/3(8 x 94.7)0 5 = 18.1. 

Related Calculations. The flow coefficient C of the usual 1-in-diameter (2.5-cm) double-seated control valve is 10. For any other size valve, the approximate C valve can be found from the product 10 x d2, where d is nominal body diameter of the control valve, in inches. Thus, for a 2-in-diameter... 

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