How to select a wastewater check valve
In water & wastewater applications, there’s an ever-increasing demand for dependability, efficiency and energy savings. With the increases in demand, engineers must show greater care when selecting valves for a job as it needs to be the correct one! The following guidelines will help you gain more confidence when it comes to picking your valves.
Check Valve Selection Criteria
There are four criteria which should be looked at during check valve selection.
- Non-slamming characteristics
- Headloss characteristics
- Application and selection
The first three have a close interrelationship and engineers have generally taken two strategies:
- Design a valve accounting for all three in equal consideration
- Emphasise on one or two of the criteria
The mixture of both these approaches has led to the vast range of check valve designs available on the market today.
Non-Slamming Characteristics of the Check Valve
Two major aspects affect a Check Valves potential for slamming and creating surge. The amount of time it takes for the Check Valve to close and the way in which the disc travels from the open to the closed position.
Factors Affecting Rapid Closure
- Velocity or Reverse Flow – It is not advised to let the reverse flow close the valve as slamming will occur. This situation is to be avoided if slamming and surges are a concern.
- Mechanical Assistance – Rapid closure of a check valve can be achieved through the use of springs, weights and levers. Swing Check Valves have successfully integrated these in many applications. However, this has often resulted in the expense of efficiency in term of turbulence and increased head loss.
- Length of Disc Stroke – how far the disc must travel to reach the closed position. Shorter stroke gives faster valve closure.
Factors Affecting Slow Closure
A valve designed for slow closing will allow velocity of reverse flow to build-up. The speed at which the valve closes can be calculated and controlled to avoid slamming and surges. Ways to control the closing speed of a slow closing Check Valve include dashpots, oil accumulators, cushions and power actuators.
Headloss in Check Valves
All Check Valves create headloss. Headloss costs the user money through increased power consumption by pumps. The importance of headloss varies depending on the system.
The key way to reduce headloss is through design. Design factors impacting the amount of headloss are:
- Total flow area through the body
- Body geometry
- Interaction between disc and flow
- Disc design
Check Valve Body Design
A turbulence-creating design will generate headloss. Design should avoid large changes in flow direction. This can be reached by having a body that has a smooth contour allowing the flow to take as smooth a path as possible. Extended or globe style bodies are most beneficial as they return flow to normal gradually.
Check Valve Disc Design
Location is a key factor here. If a disc is located in the flow, headloss will be generated. A valve’s non slamming characteristics can be improved by use of springs etc to create rapid closure with the expense of headloss. This design forces the disc into flow causing turbulence and therefore headloss. If slamming and headloss are of concern then a swing type valve is most recommended, this allows the disc to ride above the flow instead of being forced into it.
Cost of Check Valves
Aspects the engineer should consider in terms of cost are:
Is the initial purchase price competitive?
Are energy consumption costs high due to headloss?
Lower headloss valves are generally more expensive to purchase, however difference in cost is generally met within a year or two.
Saving a little on initial cost price can end up costing a lot more in the long run. The main things a buyer should consider are system downtime, valve location and cost of parts and labour.
Application & Selection
There are trade-off’s to be made when selecting check valves. The importance of each selection criteria must be weighed to make an informed selection on the valve best suited for the application. Once this is decided it is important to review the designs of various manufacturers to assure getting the best valve possible.
What Check Valves are you currently using for your water application?
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