Jan. 05, 2023
We will focus on the most common type of non-return valve, the swing check valve. It is available in a wide range of sizes, pressure ratings and materials. Three basic variants exist, as shown in examples a, b and c below.
a) unassisted gravity swing check with cap
b) Double wafer body
c) Single baffle wafer body
All types of check valves operate on the same principle. The traditional method of selecting the body size to match the pipe size usually results in the valve not opening fully at minimum or even normal flow rates. Although this practice is widely used, in most cases this will not result in optimum performance or minimum cost of ownership.
The reasons for this are similar to those for control valves. To obtain a properly sized control valve, you must solve for the required Cv (determine the size of the valve or orifice); and once you have solved (usually Cv Min, Cv Norm and Cv Max) for the minimum, normal and maximum flow conditions of the process, you can then select the appropriate valve body size for the identified internals (internals) that best fits those Cv values.
Swing Check Valve
Cv, the flow coefficient of a device (SI unit Kv flow coefficient) is a relative measure of its allowable fluid flow efficiency. It describes the relationship between the energy (as pressure drop) passing through an orifice plate, valve or other piping component and the corresponding flow rate.
The installed Cv of any valve is the Cv that occurs when the valve is operating in the piping system and the process is running. if the check valve is not fully open, the published Cv of any check valve is irrelevant. As most check valves provide an inlet and internal valve internals size for each size and class, the likelihood that it will match the application in which it will be installed and not be sized is low and will only become apparent when it may be too late after installation.
The difference is that the flow rate is not sufficient to fully open the gravity swing check valve and it does not open enough to produce the Cv that it is capable of producing. this also has a detrimental effect on the service life of the valve as it means that the baffle will be unstable and constantly moving and prone to wear; if properly sized, the service life will be shorter than expected.
The single flapper wafer check takes the 7 ft/sec water and momentarily increases the velocity at the disc through a decrease in orifice size to lift the flapper fully, a position where it is stable and achieves the highest installed Cv. This requires an engineered and sized inlet orifice, what we call a Venturi inlet.
Lift Check Valve
If we install pipe size check valves that are not sized for low and normal flow conditions and they do not fully open the flap, we should expect a shortened life. This is due to the mechanical movement of the valve flap, as there is little change in flow during the life of the valve, and the resulting turbulence and vibration. If we fit a properly sized check valve for minimum flow and obtain 100% Cv, then we will always achieve 100% Cv at higher flows. once fully open, we should expect a longer service life through the stable position of the flap - 100% open is 100% open.
In order to size the check valve correctly, you must determine the minimum speed required to fully lift the flap. Then find the average flow rate in the pipework and ensure that it is greater than or equal to the required minimum flow rate.
v = average flow velocity (ft/s)
r = weight density of the fluid (lb/ft3)
V = specific volume of fluid (ft3/lb)
Q = velocity of flow (usgpm)
d = internal diameter (in)
Lift Check Valve
To determine the minimum velocity required to fully lift a butt swing check valve flap, we use the following formula: (note we use Hy-Grade Inc, T series venturi inlet single baffle as the formula because of their extensive flow test results)
vmin = 55 √(V), where V = 1/ r
Determine the average flow rate in the pipe using the following equation.
v = 0.4085 (Q / d2)
The data required to correctly size a check valve may not be readily available, but the effort required to obtain them will be rewarded by a significant increase in service life and operational performance. Our analysis of many check valve installations has shown that most check valves are oversized.
In summary, when you select the right check valve size for your application, you will usually or always gain the following advantages.
- Smaller valves at lower cost
- Longer valve life due to baffle stability
- Longer valve life due to baffle and seat
- Higher performance due to less wear and tear
- Fewer downstream piping problems
Youyi Machinery is an industrial valve manufacturer. There are about 100 series of products: GB, API, DIN, JIS, GOST standard gate valve, globe valve, check valve, ball valve, butterfly valve, bottom valve, drain valve, strainer, flange and etc. For more information and consultation, contact us here!
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