Written by Prathamesh Badadal
Tuesday, May 05, 2020
What makes Full Port Ball Valve Cv calculation challenging?
By
Prathamesh Badadal
It is often the case with valve manufacturers that the valve Cv is less as compared to other manufacturers. A lot
of resources are spent to get the valve designed and then end up with 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 have a 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 ports designs are the full port type, standard port, v-port and the reduced port type ball valve. In industry the
popular terms for full port is full bore. The terms bore or port are sometimes used synonymously in the valve industry.
Figure 1: Full Port ball valve at full opening
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 condition.
Figure 2: Flow path for full open condition(90° opening) for reduced port (top) and full port ball valve(bottom).
Understanding the importance of Cv
To give a brief introduction to the readers I have detailed out the terminologies related to the valve Cv. Users familiar with net and
gross Cv terminologies and who wish to directly dive into the core of the blog can jump over this part to the section of Full Port Ball valve : 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 is a sort of number which relates the flow rate with given pressure drop across the valve.
The formula of Cv is given as:
where:
Q = volumetric flow rate (US gpm)
SG = Specific gravity of fluid
ΔP = pressure drop across valve (psi)
So obviously the pressure drop is going to 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 experimental test
(Source: ISA-75.01.01-2007 )
(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 well 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 as well, 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 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 case of full port ball valve as compared to that of butterfly valve
for fully open conditions.
The flow path in 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 is going to be significant as 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 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 come up with a system design that fulfills the requirements. Valves are a
very critical part of these systems. In 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 catalogues 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 catalogue
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 over sizing
or under sizing of the valve which will cause performance, safety and cost issues.
A manufacturer creates a catalogue 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 both 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 catalogue or data sheet. The main challenge is how the Cv value
is being represented in the manufacturer's data sheet and that 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 the 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 are as 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
| Valve Opening | Gross Cv | Net Cv |
|---|---|---|
| 30.00 | 181 | 181 |
| 38.57 | 345 | 345 |
| 47.14 | 603 | 605 |
| 55.71 | 996 | 1003 |
| 64.29 | 1643 | 1674 |
| 72.86 | 2792 | 2949 |
| 81.43 | 5163 | 6421 |
| 90.00 | 8241 | 26045 |
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:
1
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 pipe is very significant in 90° of opening
as compared to 81.43° opening.
2
The contribution of the valve body in the total pressure drop of 90° opening is very less as compared to that of the pipe. It can be
said that the contribution of the pipe is ≈ 90% of the total pressure drop(Cv is inversely proportional to square root of pressure
drop, hence the mathematical value is obtained).
Thus at simulationHub we encountered the terminology of Net and Gross Cv. We realised the vast difference between the Net and Gross Cv
values, especially for Full Port Ball Valve and hence have come up with mentioning the values of Cv specifically as Net and Gross Cv.
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