Valve coefficients

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 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. 

Kj = valve coefficient of discharge (from valve manufacturer) Farris and Consolidated spring-operated,... 

K(j = valve coefficient of discharge = 0.92 for pilot-operated Pi = flowing pressure, psia MW = molecular weight of gas = 23.2 Z = compressibility factor = 0.9334 C - gas constant based on ratio of specific heats Cp/C ... 

C = standard valve coefficient from manufacturer s catalog... 

C. = valve coefficient of flow, full open (from manufacturer s tables)... 

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. 

If the feed to the tank increases from 2.3 m3/hour to 3.4 to3/hour and the valve opening remains the same, i.e., the valve coefficient remains the same, calculate and plot the change of height with time. Find the final height using the dynamic model and the steady-state model and make sure that they both give the same result. 

Because of the AP, the flow is not choked, but subsonic or subcritical. This obviously has an effect on the sizing of the valve (coefficient). Subsonic can occur at 25% to 30% backpressure Always check first ... 

If test certifications for spring-loaded valves on gas show the valve coefficient (API coefficient of discharge, Kd) to be less than 0.95, vendors proposals for that particular valve design shall be submitted to the end user for verification and approval of valve sizes. 

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. 

Multiply Equation 8.5 by A to obtain the volumetric flow rate through the valve, and let p = pwftj where r is the specific gravity of the fluid. If a valve coefficient, Cy, is defined by... 

The power requirecT to accomplish mixing in a motionless mixer is provided by the pump used to force the fluid through the mixer. The pressure drop through a motionless mixer is usually expressed as a multiplier K of the open pipe loss or as a valve coefficient Cy- The value of the multiplier is strongly dependent on the detail geometry of the mixer, but is usually available through information from the supplier. Fluid properties are taken into account by the value of the Reynolds... 

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... 

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

Sizing a control valve for liquid-gas mixtures introduces incorrect valve coefficient, C, if for each of the two fractions is calculated separately, and then added. This method assumes that the liquid and gas are passing through the valve orifice independently and at greater different velocities. 

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. 

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