We live in a consumer-driven economy. Very few people purchase even a box of breakfast cereal without doing plenty of online research and pricing comparisons. Before you invest in anything for your home, you want to make sure the product is safe, fulfills your needs and, perhaps most importantly of all, you want to make sure you are getting a good value. Choosing control valves is really no different. At the end of the day, you want to ensure that you are selecting a product that is high quality, is economical and will get the job done for many years to come. The best place to start is by considering the most pertinent detail of any valve: the size.
The goal for any process control engineer is to find the smallest valve necessarily for the purpose. Often, the smallest valve is the least expensive. However, it is imperative that you know exactly the size needed for the pipe. Choosing the wrong size can lead to inefficiency and operating problems.
The best way to determine the right size for your needs is to use the Cv method.
• First, find the minimum and maximum flows that the valve will experience. It is best to be as accurate as possible. A valve working below the minimum flow expectation will often hunt and fluctuate, which leads to pressure spikes downstream and, ultimately, a burst pipe.
• Next, calculate the pressure differential between the lowest possible upstream pressure and downstream pressure required.
• Input the minimum and maximum flows, as well as the pressure differential, into the following equation:
• Q (gpm) = Cv x square root of pressure differential
• Cv = Q/square root of pressure differential
• Utilizing a manufacturer catalogue, choose a product with a Cv value that is at least the number calculated using the formula above. By choosing a valve with a slightly higher Cv than you have calculated, you have the flexibility for higher flows in the future.
• Lastly, be sure the given flow rate is realistic and the pressures are accurate and proven. Always opt for exact calculations over educated estimates to ensure maximum efficiency.
Sizing example
We need to size a valve to handle 3500 gpm maximum flow, inlet pressure is 100 psi, outlet pressure is to be 70 psi.
• i. Solving for CvPressure differential = 100psi – 70psi = 30psi Flow(Q) = 3500 gpm Solving for Cv = 3500 / √30 Cv = 639• ii. Compare Cv and Flow Frequency Capabilities for Full Port (106) and Reduce Port (206) valves:
• iii.Valve Selection: Cv = 639, Maximum continuous Flow = 3500gpm
By applying the Cv formula, we find that the 8-in. S106 body meets the Cv requirement. However, the continuous flow estimate is below our requirement. Therefore, you would need to use a 10 in. on the S106 body.
The 10-in. S206 body, however, meets both the Cv measurement requirement and the continuous flow requirement. It is a clear winner for this job. You have successfully chosen the right pressure-reducing control valve for your needs.
When it comes to selecting a valve, there is really no such thing as too much research. By doing all your homework before you begin reviewing manufacturing catalogues, you’ll have a better understanding of the scope and scale of the project and can more accurately estimate your costs.
