Issue link: http://intechdigitalxp.isa.org/i/1079769

K bv = velocity head loss coefficient for the ball valve ƒ bv = turbulent pipe friction factor associated with the ball valve P 1 6 d P 2 p p d INTECH JANUARY/FEBRUARY 2019 45 AUTOMATION BASICS Use the analytical adjustments discussed above to make a more meaningful comparison. Flow coefficient values can be estimated considering only the losses in the valve (equation 4). As shown in table 2, these re- sults are a close match to the values for brand "X." When the analytical piping losses are also con- sidered (equation 8), the resulting capacity is much closer to the tested brand. From this comparison, it can be concluded that even though the published flow coefficient values for these values are very dif- ferent, they will both pass the same flow rate under the same conditions. Flow capacity Representing the flow capacity of full-bore ball valves in the wide-open position presents some challenges. The flow coefficient is very sensitive to whether or not associated test piping frictional losses are included. Published values for different manufacturers may be based on different approaches. To ensure a correct selection, understand how the the valve manufacturer determined the flow coefficient. n ABOUT THE AUTHORS Marc L. Riveland is retired from Emerson Automa- tion Solutions. Riveland is chairman of the ISA75.01 working group on control valve sizing and is on other ISA75 working groups and IEC TC65B WG9. Andrew Kinser (Andrew.Kinser@emerson.com) is the manager of test and evaluation engineering at Emerson Automation Solutions. Figure 1. Ball valve loss model Figure 2. Modified loss model including piping losses FBBV. The loss coefficient estimate may be as simple as assuming the FBBV behaves as a straight pipe and determining friction losses, or a more complex ap- proach based on using a specific handbook model. An example of the latter is offered by Crane, which suggests using the loss coefficient model: Where K bv = velocity head loss coefficient for the ball valve and ƒ bv = turbulent pipe friction factor as- sociated with the ball valve. The friction factor may be evaluated from a number of methods and resources but will generally fall in the range of 0.01 to 0.02. The conversion to flow coefficient C is given by the derived equation Where d = inside pipe diameter (or in this case ball valve diameter). This analytical method does not in- clude the effects of upstream and downstream test piping integral to the empirical method. Comparison of methods To make a meaningful comparison between the two evaluations, the effects of loss from piping must be treated the same in both models. One approach is to use the analytical method, but to employ a modified model that includes the effects of straight pipe, as shown in figure 2. 1. Convert reported flow coefficient to a loss coefficient. Rearranging equation (4): 2. Add line losses. The equivalent loss coefficient for a given length of pipe of is: Where l = length of straight pipe; d = inside diam- eter of pipe; and ƒ p = pipe turbulent friction factor. Therefore, 3. Convert back to the flow coefficient. Calculate the revised coefficient based on the total loss: Published C v values for two different ball valve designs are shown in table 2. Even though the two valves are different brands, the flow paths through them are virtually identical, so the flow coefficients should be nearly equal. However, brand "X" con - siders only the losses in the valve, so the flow coeffi- cient is considerably larger than brand "Y," which is tested and includes the test manifold piping losses. Table 2. Comparison of published and calculated flow coefficients Valve size ƒ p d, inches Published C v Analytical method † C v Brand X Brand Y w/o piping loss w/ piping loss 8 0.014 7.981 9,000 6,040 9300 4,810 12 0.013 12.00 22,500 12,300 21,800 11,500 16 0.013 15.25 37,200 19,900 35,200 18,700 † Assume ƒ bv = ƒ p . Actual value may be less than the pipe friction factor because the interior surface is machined.

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