There are many options and several conditions that need to be considered when purchasing the right valve for a job. The more information from the field, the better the choice will be. The ultimate goal is to choose the best valve for the job required at the most economical price. While this is not rocket science, there is some fundamental information that needs to be taken into account to ensure the proper valve is chosen.
Questions to Ask Before Getting Started
- 1. What is the purpose of the valve and what do you want the valve to accomplish?
- 2. What are the required flow rates? Both minimum and maximum need to be considered.
- 3. What type and on what size pipe will the valve be located?
- 4. Will the valve be used continuously, intermittently or momentary?
- 5. What operating pressures will the valve be subjected to?
The ultimate goal is to choose the smallest valve that can do the job required, which, in turn, will minimize cost, making sizing an important aspect to get right. One of the most common ways to size a valve is to use the Cv method. Cv is the flow across a valve when there is a 1-psi pressure differential across a fully open valve.
The Selection Process
1. Determine the maximum and minimum flows that the valve will see. This is important, as a valve working below its minimum flow requirement may hunt and fluctuate, causing pressure spikes downstream, which can lead to pipe bursts.
2. Determine the pressure differential. A good guide is to have a minimum of 5 psi between inlet and atmosphere if the valve bonnet is being vented to the air, or 10 psi if the bonnet cover is connected to the downstream.
3. When you have this information, the most common method for calculating size is the Cv method, using performance curves. To measure the flow through an open valve you can use the following formula:
Q in gal per minute (gpm) = Cv x square root of pressure differential
Cv = Q ÷ square root of pressure differential
4. After calculating the Cv for the application using the formula above, go to the manufacturer’s catalog or data tables for the Cv values of each size and model of valve. Always choose a valve that has at least the Cv value calculated using the formula and that meets your application requirements. It is always better to have a little safety, so the Cv of the valve you choose should have a higher value to give flexibility for the future if higher flows are ever desired.
Here is a sizing example using the manual Cv method for a pressure reducing valve:
To size a valve to handle 3,500 gpm maximum flow with inlet pressure of 100 psi and outlet pressure of 70 psi, the calculation for Cv would be as follows:
i. Solving for Cv
Pressure differential = 100 psi – 70 psi = 30 psi
Flow(Q) = 3,500 gpm
Solving for Cv = 3,500 ÷ √30
Cv = 639
ii. Compare Cv and flow frequency capabilities for full port (106) and reduced port (206) valves (see tables 1 and 2).
iii. Valve Selection:
Cv = 639, maximum continuous flow = 3,500 gpm
In this example, the 10-in. 206 body is the best selection, as it meets the Cv requirement and the application meets the continuous flow recommendation for this valve. While the 8-in. S106 body meets the Cv requirement, the continuous flow recommended is below what is required for this application. Therefore, using the S106 full port body would require using the 10-in. size as well. Full port valves cost more than reduced port valves of the same size, therefore, using the 10-in. 206 meets all the requirements and is the one that is most economical.
5. In any valve sizing exercise there is always an element of choice by the system designer. Is the flow rate given realistic? Are the pressures given accurate and proven? As the person responsible for sizing (while graphs, charts and even software calculators are necessary), having a clear understanding of the issues certainly makes for a more educated choice.
In order to size a valve correctly, one must be able to understand what the valve is required to do, what the system parameters are; and then use this information to pick the valve that will do a proper job while keeping the economic desires in mind. Basically, pick the smallest valve that can do the job properly. While in many instances making a valve larger than required is a safe choice, this is not always ideal, especially when sizing specialty valves such as surge anticipation valves, where over-sizing can be detrimental.
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