The hydraulic laws governing level control valves are often overlooked. Incorrect sizing or application, especially at high pressures, can cause early valve failure. The two main enemies of level control valves are pressure and flow. Although related, each requires a different solution.
This article outlines the rules governing reservoir inflow and how to apply them correctly to prevent premature failure.
Pressure
What constitutes high pressure depends on the application. For reservoir level control valves, anything above 4–5 bar is high. Pressures above 6–8 bar can be destructive if precautions are not taken.
The primary risk is cavitation. Cavitation occurs when pressure drops below the vapour pressure of water, forming vapour bubbles that collapse as pressure recovers. It’s what you’d see in a boiling kettle, but more more violent.
This collapse creates high-velocity micro-jets. If this occurs near metal surfaces, erosion results. In severe cases, cavitation can erode holes through valve bodies within weeks.
A valve’s resistance to cavitation is commonly expressed as the allowable upstream-to-downstream pressure ratio.In self-actuated globe control valves used in the water industry, a 3:1 ratio is typical. For example, with 12 bar upstream, the valve can safely discharge to 4 bar. Some diaphragm-actuated globe valves achieve 4:1 due to modified internal flow paths.
Flow Rate
High velocity is the second major cause of damage.
Each control valve has a maximum recommended flow rate based on acceptable velocity for controllability, service life, and noise. For diaphragm-actuated globe valves, continuous operation is typically limited to 6 m/s, with short periods at up to 20% higher acceptable.
The standard sizing relationship is:
Q = Cv √dP
Where: Q = flow rate Cv = wide-open valve capacity dP = differential pressure
For a given valve, flow increases as differential pressure increases when the valve is fully open.
In practice, most valves reach their maximum recommended flow at around 2 bar differential pressure. Allowing a valve to operate fully open above this value results in excessive velocity.
For example, consider a diaphragm-actuated globe valve capable of a 4:1 pressure ratio. With 4 bar upstream and 1 bar back pressure, a 200 mm valve may pass 329 l/s, although its recommended maximum is 200 l/s. Operating under these conditions will significantly shorten valve life.
The Solution
Two conditions must be met:
- The pressure drop ratio must remain within the valve’s cavitation limits.
- The flow rate must not exceed the recommended maximum.
The solutions to these requirements are interrelated.
Flow Limiting
Where dynamic head exceeds 3 bar and reservoir static head is 10 m or less, a rate-of-flow control feature should be fitted. This prevents excessive velocity and protects both the valve and the wider network.
Pressure Drop Control
Maintaining a safe pressure ratio requires sound engineering judgement.
Artificial Back Pressure
After fixing the flow rate, an artificial back-pressure device such as an orifice plate may be used.
Example: Upstream pressure = 9 bar Reservoir head = 1 bar Valve limit = 3:1 ratio
To meet this ratio, total downstream pressure must be 3 bar. An additional 2 bar pressure drop must therefore be created across an orifice plate.
This method only works at the design flow rate, as orifice plates produce a specific pressure drop at a specific flow.
A limitation arises during slow opening and closing. Reservoir level control valves often require long stroke times to prevent water hammer. During these transitions, the orifice plate might not maintain adequate back pressure, allowing temporary cavitation. Engineering judgement is required to assess whether this exposure is acceptable.
Pressure Reducing Valve
The most robust solution is to install a pressure reducing valve upstream, combined with rate-of-flow control. This ensures safe operation across the full operating range, including opening and closing cycles.
With a 3:1-rated valve and 1 bar reservoir back pressure, a 9 bar upstream pressure can be safely managed without reducing service life.
Conclusion
Control valves are designed to dissipate energy and may handle pressures up to 250 bar in industrial applications. However, in low-pressure reservoir level control systems, they are highly vulnerable to cavitation and excessive velocity.
It is the engineer’s responsibility to ensure valves operate within their design limits.
