Check valves are generally installed in pipelines to prevent backflow. A check valve is basically a one-way valve, in which the flow can run freely one way, but if the flow turns the valve will close to protect the piping, other valves, pumps etc. If the flow turns and no check valve is installed, water hammer can occur. Water hammer often occur with an extreme force and will easily damage a pipeline or components.
Check valves are used in many different applications. For example they are often placed on the outlet side of a pump, to protect the pump from backflow. Centrifugal pumps, the most common type of water pumps, are not self-priming, and therefore check valves are essential for keeping water in the pipes. Also, check valves are very often used in HVAC-systems (Heating, Ventilating and Air Conditioning-systems). HVAC-systems are e.g. used in large buildings, where a coolant is pumped many storeys up. The check valves are here installed to make sure that the coolant is not flushing back down.
There are various types of check valves available for water and wastewater applications. They work in different ways but serve the same purpose. AVK offers a wide range of swing check valves, ball check valves, tilted disc check valves, slanted seat check valves, nozzle check valves and silent check valves. The most common types of check valves for water and wastewater are swing check valves and ball check valves:
A swing check valve is mounted with a disc that swings on a hinge or shaft. The disc swings off the seat to allow forward flow and when the flow is stopped, the disc swings back onto the seat to block reverse flow. The weight of the disc and the return flow has an impact on the shut-off characteristics of the valve.
A ball check valve functions by means of a ball that moves up and down inside the valve. The seat is machined to fit the ball, and the chamber is conically shaped to guide the ball into the seat to seal and stop a reverse flow.
The tilting disc check valve is similar in appearance to an eccentric butterfly valve. The valve body is double‐flanged and of a short length. The disc is held in place via a shaft which is positioned eccentrically from the body centreline in both the horizontal and vertical axes.
The double eccentricity of the shaft results in the lower section of the disc occupying a greater area in the flow path. Consequently, the disc begins to open at very low flow rates. The tilting disc check valve is therefore commonly used in pumping systems with low flow rates and also for pulsating flows.
During flow reversal, the disc closes due to gravity and the upper part of the disc above the shaft centreline pushes against the flow thus acting as a hydraulic brake which cushions the disc as it returns to the closed position. Tilting disc valves can be supplied with optional levers and weights to adjust the closing characteristics of the valve. As such, the tilting disc check valves are well suited to reduce the risk of water hammers.
Some of the things you may need to consider are fluid compatibility, flow characteristics, headloss, non-slam characteristics and total cost of ownership. For optimal performance, it is of course important to choose your valve respecting the characteristics of each specific installation.
All check valves are designed to handle water and treated wastewater but handling of raw waste water/sewage may cause some issues. When selecting a valve for these fluids, you should probably consider how the presence of solids may potentially affect operation of the valve.
If a check valve closes very fast, it may prevent slamming. However, the rapid closure will not protect against the surges caused by pumps being started and shut down. If the valve opens (and closes) quickly, the flow will be changed suddenly and surges will more likely occur.
Headloss is a function of fluid velocity and the valve headloss is affected by the flow conditions of the system and the internal surface of the valve. The geometry of the valve body and the closure design determines the flow area through the valve and thereby also affects the headloss.
The headloss to be considered is a combination of static head (caused by elevation difference) and friction head (caused by the internal of the pipes and valves). There are a number of formulas for headloss and rating of valves based on this. The most common is probably the flow coefficient of the amount of water passing through the valve at a certain pressure drop during a certain amount of time. However, for comparisons the resistance coefficient Kv is considered the best choice.
The costs for your check valve may consist of more than just the purchase price. For some installations, the most important costs may be purchasing and installation but in other cases maintenance or energy costs may be equally or even more important. When considering costs as a selection criteria for your check valve, the total costs over the life of the valve should therefore be considered. In general, the more simple the valve construction is, the lower are the maintenance requirements.
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