Slide 1
A valve changes the passage available for flow
A valve changes the opening available for fluid flow. Its effect depends on both the valve position and the pressure conditions around it.
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An opening of half the maximum area does not necessarily deliver half the flow. The valve interacts with every other resistance in the connected system.
Slide 2
Throttling controls flow by adding resistance
A throttling valve controls flow by adding resistance. Mechanical energy is dissipated as fluid passes through the restricted region.
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This can be a useful and simple control method, but it carries an energy cost. If demand stays low for long periods, changing pump speed may avoid repeatedly wasting excess head.
Slide 3
Shutting off and fine control can need different shapes
Some valves are designed mainly for isolation, while others suit continuous adjustment. A valve that shuts securely may still regulate poorly near closure.
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The relation between opening and flow capacity is a design characteristic. Installed behaviour can differ from that characteristic because the pressure drop across the valve changes with the system.
Slide 4
A sudden closure can send a pressure wave through the pipe
Suddenly stopping a moving liquid can create a pressure wave through the pipe. Valve timing therefore affects loads beyond the valve itself.
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Pipe elasticity and fluid compressibility allow the disturbance to travel and reflect. A steady-flow calculation does not capture this temporary event, often called water hammer.
Slide 5
Pressure, temperature and fluid chemistry constrain the choice
Pressure, temperature, fluid chemistry and required control range all influence valve selection. The smallest fitting that passes nominal flow may be unsuitable.
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A large pressure drop can cause local vapor formation in a liquid service. Avoiding damaging cavitation can require staged pressure reduction or a different valve design.
Sources
- Improving Pumping System Performance: A Sourcebook for Industry
U.S. Department of Energy