Two different approaches are used to rate the capacity of full-bore ball valves. It is important to understand the difference between these approaches to make meaningful comparisons.
By Marc L. Riveland and Andrew Kinser, Emerson Automation Solutions
Marc L. Riveland, retired from Emerson, and Andrew Kinser, manager of test and evaluation engineering at Emerson Automation Solutions in Marshalltown, Iowa, write that depending on the methods ball valve manufacturers employ for determining their published rated flow capacities, the resulting value may be different, even though the valves are virtually identical.
Their article, Understanding the Rated Capacity of Full-Bore Ball Valves, in the Jan/Feb 2019 issue of InTech magazine, points out that the rated flow capacity of a valve is typically expressed in terms of some type of flow coefficient (C), and the preferred method of evaluating these coefficients is by conducting a flow test. But full-bore ball valves (FBBVs) in the fully open position present some challenges, they say.
First, they typically do not fall within the scope of conventional control valve sizing methods. A second challenge related to the ultra-high capacities of these valves is that they may exceed the flow capacity of a test lab.
Attempting to represent the flow capacity of a FBBV in terms of the control valve standards methods is confounding and can be very misleading, especially when comparing different ball valve units. Published data would suggest that a much bigger FBBV is needed to match the desired capacity of an isolation-type valve. In reality the two valve types should be the same size.
Engineers specifying FBBVs should ask the valve supplier how the rated C was determined to make sure they are purchasing the correct size valve. While the flow coefficient C has conditional utility, it is important to understand the definition and evaluation methods for this term as presented in the standards.
Following are descriptions of two different approaches to evaluating the flow coefficient of a line size FBBV in the fully open position. (Note that complete descriptions of the equations and analytical methods can be found in the InTech article.)
Standards Based Method (Empirical)
This method is based on actual flow testing of the FBBV according to industry standards. The test method presented in these standards prescribes a test manifold, along with the methods for measuring flow rate and pressure drop to allow direct calculation of the flow coefficient. The test manifold includes the valve and lengths of straight pipe upstream and downstream of the valve. The inlet (P1) pressure is measured two pipe diameters upstream of the valve, and the outlet (P2) pressure is measured six pipe diameters downstream of the valve
Figure 1: ISA/IEC valve flow test manifold.
It is important to note that the pressure drop includes the additional losses associated with the eight diameters of straight pipe. This effect is typically minimal for most control valves within the scope of the standard because the valve produces the dominant loss compared to the piping loss. However, the FBBV valve is essentially a very short piece of straight pipe, and the test piping losses can actually exceed the losses strictly attributable to the valve in some instances.
Flow Model Based Method (Analytical)
The basis of the analytical approach for computing the flow coefficient C starts with an estimate of the static head loss coefficient associated with the FBBV. The loss coefficient estimate may be as simple as assuming the FBBV behaves as a straight pipe and determining friction losses, to a more complex approach based on utilization of a specific handbook model.
The friction factor may be evaluated from any one of a number of methods and resources, but will generally fall in the range of 0.01 to 0.02.
This analytical method does not include 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.
Analytical adjustments can be employed to make a more meaningful comparison. When the analytical piping losses are also considered, the resulting capacity is much closer to a valve that was flow tested, they say:
From this comparison, it can be concluded that even though the published flow coefficient values for these values are very different, they will both pass the same flow rate under the same conditions.
Representing the flow capacity of full-bore ball valves in the fully open position presents some challenges, the authors say, so care must be taken.
The flow coefficient is very sensitive to whether associated test piping frictional losses are included in the evaluation or not. Published values of the flow coefficient for different manufacturers may be based on different approaches. To ensure a correct and meaningful selection, it is important to understand the method by which the flow coefficient was determined by the valve manufacturer, and to consult with the manufacturer if clarification is required.
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