What makes Full Port Ball Valve Cv calculation challenging?
It is often the case with valve manufacturers that the valve Cv is less than other manufacturers. Many resources are spent to get the valve designed and end up with a not-so-good Cv after experimental testing.
We at simulationHub are always on the lookout for any assistance to help you with the valve problems. Let’s look at a problem involving Ball valve Cv determination that we happened to be involved with.
Ball valves are an important part of the process industries. There are different types of designs in the ball valve family. The basic valve port designs are the full port type, standard port, v-port and the reduced port type ball valve. In industry, the popular term for the full port is full bore. The terms bore or port is sometimes used synonymously in the valve industry.
Figure 1: Full Port ball valve at full opening
The full port ball valve is distinct from its other members in the ball valve family. The path of flow through this valve is very different. From the figure below, we can see the flow path for fully open conditions.
Figure 2: Flow path for full open condition(90° opening) for the reduced port (top) and full port ball valve(bottom).
Understanding the importance of Cv
To briefly introduce the readers, I have detailed the terminologies related to the valve Cv. Users familiar with net and gross Cv terminologies and who wish to dive into the core of the blog directly can jump over this part to the Full Port Ball valve section: Interpreting the value of Cv.
What exactly is Cv?
The process industry has varied applications of the valve. The operating conditions can vary significantly as well. Cv, also known as valve flow coefficient, relates the flow rate with a given pressure drop across the valve.
The formula of Cv is given as:
where:
Q = volumetric flow rate (US gpm)
SG = Specific gravity of the fluid
ΔP = pressure drop across valve (psi)
So obviously, the pressure drop will determine the Cv and the loss component to be added to the system pump effort.
The term “pressure drop across valve” is the tricky one. Experimental methods of determining the Cv follow the standards: IEC 60534–2–3 and ANSI/ISA–75.02.01 state that the “pressure measurement be taken at 2 times the valve diameter upstream and 6 times the valve diameter downstream.”
Figure 3: Pressure measurement through an experimental test
(Source: ISA-75.01.01–2007 )
So “the pressure drop considered for the Cv calculation” is actually the sum of pressure drop across the pipes attached to the valve(which is 8 times the valve diameter)and the pressure drop across the valve body itself.
Net and Gross Cv
People relate very well with the terms “Net” and “Gross” in terms of pressure drop. But these terminologies can be extended to Cv as well. For instance, if the Cv relation contains only the pressure drop across the valve body and excludes the pipe pressure drop, it can also be called “Net Cv.” The measurements that we get from flow loop measurement are “Gross Cv” as these contain the contribution of pressure drop due to pipe unless explicitly stated by the test facility report.
To know more about the Net and Gross Cv visit the link below:
Impact of pipe pressure loss in full port ball valves
Cv is inversely proportional to the square root of pressure drop for a fixed flow rate. So it is important to look at how the pressure drop is actually taking place in the valves.
Pressure drop in valves can be categorized as:
- Pressure drop due to flow obstruction
- Pressure drop due to friction
Flow obstruction is due to the valve trim, which causes flow separation leading to pressure drop. The variables affecting the frictional pressure drop are roughness of pipe, fluid properties, etc. Any change in one variable will lead to a different value of pressure drop.
A comparison of the contribution of flow obstruction and friction to pressure drop is shown below. The cases considered are a full port ball valve and a concentric butterfly valve at full opening(These figures are for illustrative purposes and may vary from case to case).
Figure 4: Illustration of the pressure loss contribution in case of a Full port Ball valve and Butterfly valve
So as seen in the figure, the contribution of friction is dominant in the case of a full port ball valve compared to that of a butterfly valve for fully open conditions.
The flow path in the case of a full port ball valve is almost like a straight, circular pipe(depends upon the schedule of the pipe), with the ID of the ball matching the pipe ID. So the contribution by pipe section will be significant compared to only the ball region in the pressure drop due to friction.
Hence it becomes necessary to distinguish the pressure drop due to pipe and that due to valve only. This is where the terms Gross and Net come into the picture.
Full Port Ball valve: Interpreting the value of Cv
A process engineer is always facing the question of “What is the right size, flow characteristics, and rating of the valve that I can add to the system?” He has to calculate the major and minor losses that the system will face and develop a system design that fulfills the requirements. Valves are a very critical part of these systems. In the selection of valves, the focal points are the valve flow characteristics, i.e., Cv. So the process engineer will look up to the valve manufacturer’s design catalogs and select the valve for given flow requirements.
This is where the challenge appears.
Figure 5: The difference between knowing which Cv values to use can be critical for plant operation
The process engineer knows the length of the pipes to be used and hence calculates the pressure drop accordingly. But when using the Cv catalog, if the manufacturer has failed to mention if the Cv value is gross or net, then the calculation might be erroneous. This might result in oversizing or under-sizing the valve, which will cause performance, safety and cost issues.
A manufacturer creates a catalog highlighting the different valves with their respective flow performances like Cv and Cdt (Coefficient of Hydrodynamic Torque) obtained from flow loop tests. Not all flow test labs provide gross and net Cv in their test report and sometimes, the manufacturer misses to explicitly mention if it is gross or net Cv in the catalog or datasheet. The main challenge is how the Cv value is represented in the manufacturer’s data sheet and interpreted by the process engineer.
This is more serious in the Full port ball valve case as the difference between Net and Gross values is huge. Hence an incorrect interpretation of gross and net Cv values by the process engineer can damage the whole system.
To avoid misinterpretation, simulationHub includes both Gross and Net Cv values in all the simulation result reports.
Case studies from simulationHub
At simulationHub, we conducted the virtual flow loop testing of a full port ball valve—the details of the test areas are mentioned below.
Valve: Full port ball valve
Size: DN 250
Pressure class rating: 150
Surface roughness:
- On the valve trim: 50 microns
- On the pipe surface: 50 microns
Figure 6: Change in ratio of net pressure and gross pressure drop with valve opening for a full port ball valve
Figure 7: The variation in net and gross Cv values with valve opening for a full port ball valve
Observations:
- The change of opening 81.43 to 90° caused the net Cv to increase 4 times while the gross Cv did not change that much. This highlights that the contribution of the frictional pressure drop due to the pipe is very significant in 90° of opening compared to 81.43° opening.
- The contribution of the valve body in the total pressure drop of 90° opening is very less compared to that of the pipe. The pipe's contribution is ≈ 90% of the total pressure drop(Cv is inversely proportional to the square root of pressure drop. Hence the mathematical value is obtained).
Thus at simulationHub, we encountered the terminology of Net and Gross Cv. We realized the vast difference between the Net and Gross Cv values, especially for Full Port Ball Valve and hence have mentioned the values of Cv specifically as Net and Gross Cv.
