Pressure dictates the direction of flow through pipes, but it's not always reliable. Sudden pressure changes – like when someone opens a faucet – can cause unexpected events, such as a siphon that pulls fluid back through the system. Consequences can range from a minor inconvenience to putting people's health at risk.
Check valves prevent flow reversal and stop systems from draining when you want them to stay full. Here's more information:
Check valve basics
A check valve closes automatically unless held open. Holding it open takes a certain amount of force, and that's provided by the flowing liquid or gas. When the flow drops below a minimum “cracking” pressure, the valve closes.
The simplest check valve is the swing design. This is a flap that is held closed over an orifice until flow pushes it open. When flow drops, the flap swings shut, preventing any backflow.
Two more complex designs are ball and diaphragm check valves. Ball check valves use a spring to push a ball into a conical seat. They only open when the fluid pressure exceeds the force exerted by the spring.
A diaphragm check valve uses a membrane over the inlet opening. Spring pressure holds it closed, opening only when cracking pressure is exceeded.
Four things to know
1. When check valves are needed
There are two types of applications: when you don't want a system to drain and when you can't risk flow reversal. A good example of avoiding drainage is an irrigation system. When the pressure drops, a check valve will stop water draining out, improving water usage and speeding start-up.
Flow reversal is another risk with irrigation systems, as it could lead to cross contamination. A check valve – more accurately, a double check valve – prevents this.
Heating and other pumped systems also need check valves to prevent liquids from flowing the wrong direction. They can prevent convection effects in vertical pipe runs or stop condensate flowing back into a boiler.
2. Selecting the right size
While low pressure drop (Cv) is desirable in many valve applications, it’s not true for check valves. In these, a low Cv may mean there’s not enough pressure to hold the valve open properly. That can increase Cv and lead to fluttering, which accelerates wear.
3. They are directional
Check valves have two ports: an inlet and an outlet. An arrow cast onto the valve body shows which is which.
Vertical pipes present a special challenge. Swing check valves may not work at all, while ball and diaphragm valves will operate at pressures different to the design values.
4. A double check valve may be needed
Whenever there’s a risk of backflow or siphonage into a potable water line, a single check valve may not work fast enough. A double check valve, essentially two check valves in series, prevents problems.
Simple, but important
Never rely on a pump or gravity to make fluid flow the way you want. In anything more than the simplest system, opening or closing valves or even just temperature changes can lead to backflow. Incorporate check valves to ensure this doesn’t become a problem.