There are many options and several conditions that need to be considered when purchasing a valve. The more information from the field, the better the choice will be. The ultimate goal is to identify the best valve for the job required at the most economical price. While sizing actuated control valves isn’t rocket science, there is some fundamental information that needs to be taken into account.
Here are some key questions to ask:
- What is the purpose of the valve?
- and what does the valve need to accomplish?
- What are the required flow rates? Both minimum and maximum need to be considered.
- What type and size pipe will the valve be located on?
- Will the valve be used continuously, intermittent, or momentary?
- What are the operating pressures that the valve will be subjected to?
|Figure 1. Performance Curve for Flat Diaphragm Valve|
It is also important to understand that while choosing a valve with special features may cost more at initial purchase, advanced features may make the valve more cost effective over the long run with a longer service life, easier maintenance, and/or the need for fewer replacement parts. The initial cost of a valve versus its true cost over its lifetime, can vary significantly if valve options are not considered carefully.
Sizing actuated control valves
Generally speaking, the aim is to choose the smallest valve that can do the job required, thus minimizing cost. To effectively size a valve, it is important to determine the operating conditions that the control valve will be expected to endure, as well as the maximum and minimum flows that the valve will see. In many cases the person choosing the valve does not take into account the low-flow requirement, which can be dramatically different form manufacturer to manufacturer. A valve asked to work below its minimum flow requirement may hunt and fluctuate, causing pressure spikes downstream, which can lead to pipe bursts.
Valves require pressure differential to work correctly. A good guide is to have 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. This is why it’s important to know the operation pressure to be sure the valve will work as required with the pressures available.
There are several ways to properly size a valve; manually using the Cv method, using performance curves, or using the tables in a manufacturer’s catalogue.
The following formula can be used to measure the flow through an open valve:
Q (gpm) = Cv x Square Root of pressure differential.
Cv = Q/Square Root of pressure differential
Cv is the flow across a valve when there is 1 PSI pressure differential across a fully open valve. This can be determined by looking at the performance curves of the particular model of valve. Follow the straight line of the performance curve until it hits the 1 PSI intersection of the X axis. For any model and size of valve, the Cv of the valve of the y-axis at the 1 PSI intersection. (See Figure 1 for an example.)
If the valve vents to downstream and differential pressure is low, then flow through the valve will be less than fully open. When the bonnet is the same pressure as the downstream, there is no opening force from the diaphragm assembly. The weight of the inner valve and spring tends to close thevalve. Flow starts when the differential is able to overcome the weight and spring, and this increase in differential increases the opening force.
The valve is fully open at approximately 10 PSI differential. The drooping portion of curve shows how flow increases as differential pressure increases from zero. When the droop hits the straight line of the curve, the valve is now fully open. Not all valve manufacturers show the valve going open on their curves, but it is certainly a good feature to have.
Consider continuous, intermittent or momentary flow when sizing actuated control valves
When one looks at the performance curve of Figure 1, each size of valve also has three letters located on the curve, C, I, and M. Any position to the left of the C allows the valve to be used “Continuously,” which is up to a maximum of 20ft/sec. Any position on the curve to the left of the ‘I” allows the valve to be used in a Intermittent operation, which is up to a maximum of 25ft/sec. And finally, any position to the left of the “M” allows a valve to be used “Momentary,” which is up to maximum of 45ft/sec.
Manufacturers have performance graphs for all models of valves. These differ depending on valve types, full-port globe valves, reduced-port globe valves, full-port angle valves, and reduced-port angle valves. In these examples we are using Singer Valve graphs and charts to demonstrate.
Sizing example using manual Cv method for a pressure-reducing valve
Requirements: To size a valve to handle 3,500 GPM maximum flow, inlet pressure is 100 PSI, outlet pressure is to be 70 PSI. Here are the three steps:
1. Solving for Cv
Pressure differential = 100 PSI – 70 PSI = 30 PSI
Flow(Q) = 3500 GPM
Solving for Cv = 3500 / √30
Cv = 639
2. Compare Cv and Flow Frequency Capabilities for Full Port (106) and Reduce Port (206) valves. (See Table 1 & Table 2)
3. Valve Selection:
Cv = 639,
Maximum Continuous Flow = 3500 GPM
In this example the 10- inch 206 body is the best selection, as it meets the Cv requirement, as well as the application meets the continuous flow recommendation for this valve. While the eightinch S106 body meets the Cv requirement, the continuous flow recommended is below what is required for this application. Therefore, if using the S106 full-port body, one would require the 10-inch size as well. Full port valves cost more than a reduced-port valve of the same size, therefore using the 10-inch 206 meets all the requirements, as well as being the most economical option.
Sizing example using performance curves
|Figure 2. Sizing a valve for minimum inlet pressure|
Requirements: A city main supplies water at varying pressures throughout the year, from 70 PSI to a high of 110 PSI. A large industrial user requires a water supply of 1,000 GPM at 50 PSI so a pressure-reducing valve is required. What size valve should be specified?
The worst case scenario would be when the valve has the smallest pressure coming into the inlet of the valve, so we want to choose the 70 PSI inlet pressure to size the valve.
Pressure differential = 70 PSI – 50 PSI = 20 PSI
Flow required = 1,000 GPM (Fig. 2)
Using this curve diagram, it is possible to run a couple of different scenarios:
1. With 1,000 GPM at 20 PSI, by marking the spot on the graph it shows that any valve above that will be the right size. So size equals six inches.
2. When using a four-inch valve size, we look at 1,000 GPM, and then drop down to the bottom to see that the pressure drop is 25 PSI. So while it is possible to get the flow, it’s not at the right pressure. This means more pressure is being lost through the valve than is necessary, and at that flowrate it will only be able to provide 45 PSI (70-25 = 45). At this point, it is important to consider if this slight variance in the downstream pressure requirement is acceptable.
Ultimately, in order to size a valve correctly one must 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 considerations 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 a good thing, especially when sizing specialty valves such as surge anticipation valves, where over sizing can be very detrimental.
In any valve-sizing exercise there is always an element of choice by the system designer. Is the flowrate 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.
Jody Malo is an international sales manager for Singer Valve. He has worked in the water industry for over 20 years and sits on the certification board for IIABC in Canada, which sets certification guidelines in Western Canada. Mr. Malo can be reached at email@example.com.