DIRECTIONAL FLOW VALVES
A third function of valves is to direct flow.
Check Valve Figure 4. Unit prevents reverse flow and can come with
Check valves (Figure 4) ensure flow in only one direction. In most designs, the upstream fluid force pushes a spring-loaded poppet open, allowing flow. An increase in downstream or back-pressure force drives the poppet back into the seat, stopping reverse flow. Check valves are available with fixed or adjustable cracking pressures.
Some ball and diaphragm valves are designed with multiple ports. In most multi-port valves (Figure 5) fluid enters through a single inlet but may exit through one of several outlets, depending on the position of the actuator. Multi-port valves may or may not have a shutoff position.
OVER-PRESSURE PROTECTION VALVES
These devices prevent buildup of system pressure beyond a certain setting. They come in two types: relief valves and rupture discs.
One type of relief valve is a proportional relief valve (Figure 6). It contains a vent to atmosphere that opens when system pressure exceeds a set point. A spring-loaded poppet enables the measured release of fluid. The vent closes when pressure returns to a point below the set point.
A safety relief valve is designed to open very quickly, releasing a large amount of system media. Safety codes require use of such valves in certain applications.
Don't use safety relief and proportional relief valves interchangeably with check valves -- the three serve different functions.
Rupture discs are found mainly on sample cylinders to protect against over-pressurization, which may occur, for example, when temperatures rise during transport. Like relief valves, rupture discs vent to atmosphere. A metal diaphragm bursts when pressure reaches a value preset by the manufacturer. Once activated, the rupture disc must be replaced. A rupture disc is an economical choice where transportation codes require equipping compressed gas cylinders with a pressure relief device.
Multiple-port Valve Figure 5. Some ball and diaphragm valves can direct flow
EXCESS FLOW VALVES
Designed to stop uncontrolled release of system media if a downstream line ruptures, excess flow valves use a spring to hold a poppet in the open position under normal conditions. When excess flow arises downstream, the poppet moves to a tripped position, preventing almost all flow. When the system is corrected, the valve returns to its open position. These valves are available with fixed tripping values.
A GOOD START
Once you have matched valve type to function, you're well on your way to selecting the right valve for your instrument system. However, you still must grapple with many details, including:
• installation issues, maintenance schedules and access;
• safety and code requirements; and
• system parameters, such as pressure, temperature, flow rates and system media.
Ultimately, you'll need to determine:
• valve size and type of actuation; and
• materials of construction (including O-rings and seals), which must be compatible with the chemical composition of the system media, pressures and temperatures.