The role of check valves
in water lines
What are check valves?
Check valves are fluid control devices that restrict the flow of media in a piping system to one direction. This broadly defined check valve function plays different roles in various industries. Also known as non-return valves, one-way valves, or backflow preventers, check valves prevent flowing media from flowing back upstream.
Applications of check valves
Some of the applications of check valves are as follows:
Protection of upstream equipment
Backflow can easily damage meters, compressors, filters, pumps, and other devices built for unidirectional flow. Water hammer is another potentially dangerous phenomenon. Check valves can help reduce the effect of water hammers, especially when the valve is equipped with an end stage damper.
Preventing media contamination or mixing
By preventing reverse flow, check valves prevent the mixing of different media between two reservoirs. For example, in a water treatment plant, check valves prevent treated water from flowing back into the untreated water reservoir.
When media is being pumped from a suction reservoir to a discharge reservoir, reverse flow is highly likely to occur when the pump is stopped. Check valves are used to prevent this. The valve type commonly used for this is the foot valve.
How do check valves work?
A check valve comprises two ports - an inlet and an outlet - and a shutoff/closing mechanism. The unique characteristic of check valves that distinguishes them from other types of valves such as ball and butterfly valves is that, unlike these valves that require some form of actuation to operate, check valves are self-operating. Check valves function automatically, relying on differential pressure to effect control. In their default position, check valves are closed. When media flows in from the inlet port, its pressure opens the closing mechanism. When the inflow pressure drops below the outflow pressure due to flow being shut off, or the pressure on the outlet side becomes greater for any reason, the closing mechanism immediately closes the valve.
The closing mechanism in check valves is usually held in place by a loaded spring, a counterweight, or gravity. For the valve to open, the media has to overcome the force of the spring or closing mechanism.
Check valves do not operate based on differential pressure alone. If that were the case, a trickle would be enough to open the valve. Therefore, specific minimum pressure is required to open a check valve. This is known as the cracking pressure. The cracking pressure of a check valve is the specified minimum inlet pressure required to open a check valve, just enough to allow detectable flow.
Types of check valves
Various types of check valves are categorized according to different characteristics such as design, mode of operation, valve material, and size, with the most popular characteristic for classifying check valves being design/build. The following are the commonly used types of check valves.
Swing check valves
A swing check valve is built with a disc hinged, at one edge, to the valve body. When fluid flows into the valve with sufficient pressure, it forces open the disc, allowing flow. The disc shuts off when there is reverse flow or when the inflow pressure is no longer sufficient. These valves are suitable as water check valves and are widely used in water distribution systems.
Tilting disc check valves
These valves are built with a double-eccentric disc and are usually designed to be installed between two flanges. They are suitable for large diameter flow and require relatively high pressure to open. Tilting disc check valves are not suitable as wastewater check valves as debris can get stuck in the portions of the valve that are in the flow path.
Ball check valves
Ball check valves feature a ball as the closing mechanism. The inflow pressure lifts the ball off its seat and into a dedicated chamber. When the inflow pressure drops, the ball returns to its seat, shutting off the flow and preventing reverse flow. A ball check valve will perform excellently as a sewage check valve because the closing mechanism doesn't interfere with the flow in the open position.
Foot valves are commonly used in water pumping lines to prevent water from draining in the event of a pump stop. They are installed in the pump's suction line and usually feature a screen that filters out debris from the reservoir. This feature makes the foot valve a suitable check valve for pump applications. Since this valve type works with gravity, the installation must be vertical in the pipeline.
Axial Silent Check valves
These valves close with the help of a spring. Axial silent check valves are fast closing valves and prevent pumps from rotating in the reverse direction when the backflow occurs, as this can damage the pump.
Check valve selection criteria
Check valves are vital in keeping piping systems equipment safe. They are practical, easy to install, cost-effective, and can be used in various applications and working conditions. However, all these benefits would be unavailable if the wrong check valve is selected for an application. Proper selection is also crucial for cost savings. The following factors must be considered before choosing a check valve.
A check valve may be used as a wastewater check valve or sewage check valve, chemical check valve, or water check valve, among other applications. The application determines the design and mode of operation of the selected valve. For example, ball valves are suitable for wastewater treatment as they do not interfere with the flow. However, they perform poorly in chemical mixing applications due to slow shutoff speeds.
- Cracking pressure or minimum operating pressure
It is imperative to install a properly-sized check valve with the appropriate cracking pressure. If the valve's cracking pressure is excessive, the flowing media may not be able to open it. On the other hand, the check valve would not be functional if the cracking pressure is insufficient.
Valves are available in numerous sizes. Of course, the selected check valve's size must fit the sizes of the pipes and other associated equipment.
Check valves are designed to be installed vertically or horizontally and cannot be installed in an orientation other than the intended one. Note the installation that your piping system requires before selecting a valve.
The check valve material, both valve body and seating, must be compatible with the flowing media. Check valves may be made of ductile iron, stainless steel, brass, polypropylene, polyvinyl chloride, etc., and material compatibility is vital. Hawle's check valves are made of ductile iron and come with a GSK-certified high-quality epoxy coating that provides corrosion resistance.
Operating conditions (temperature, pressure, moisture, etc.)
Different check valve builds and materials are designed for different operating pressure, temperature, and flow velocity levels. Careful consideration of these factors is crucial to selecting the correct valve type.
The initial purchase, the assembly, the maintenance, and the running costs must be considered before selecting a check valve. Cost is usually an important consideration, ranking just after quality and functionality. Note, however, that low purchase costs can bring high running/maintenance costs. Also, a cheap check valve failing to perform its duty can cause serious problems at the pumping station.
Some check valves may require costly pipeline modifications, raising assembly costs. Regarding maintenance costs, the more complex a valve is, the more costly its maintenance and repairs will be. Some complex solutions require the professional assistance of the supplier. Hawle's professional technical support is readily available to help with any problems you may encounter.
All types of Hawle check valves
Hawle and Özkan produce check valves of the highest quality that meet the rigorous standards of international certifying agencies. Our check valves for water come with a 10-year guarantee, while their wastewater (sewage) counterparts come with a 5-year guarantee.
Do you need a check valve for your water distribution network? Talk to our experienced specialists to make sure you're buying the right one.