ANALYZING CAVITATION
IN REFINERY PROCESSING
BY LUDWIG HABER, PHD AND
MARTIN WOSNIK, PHD
Collapsing bubbles can be disas- trous. The collapse of the dotcom
bubble in 2000 and the more recent
collapse of the housing bubble sent
shock waves through the markets,
wiping out many portfolios and creating instability in many areas of the
country.
Similarly, when operating a refinery, power plant or other industrial
facility, collapsing bubbles—which
in this case are due to cavitation—
can send shock waves through the
process, limiting performance, eroding valve and pump components,
causing vibration and noise and, if
ignored, leading to unplanned shutdowns. This article looks specifically
at cavitation in relation to refineries,
but the principles discussed also
apply to other applications where
flow and pressure changes can result
in bubbling.
While it may not always be completely eliminated, cavitation can be
effectively monitored and controlled.
This requires careful examination of
the components and parameters
involved in a particular application,
and then designing and testing a solution in numerical and/or physical
models to ensure the solution will
achieve desired results.
Refinery profitability is tied
directly to the ability to maximize
flow capacity in existing process
lines. Maximizing capacity not only
requires a detailed look at the
process chemistry, but also understanding plant fluid dynamics. One of
the limiting factors in process capacity is the onset of cavitation.
Cavitation bubbles on a valve plate on a pump.
vapor bubbles. The bubble formation process is similar to boiling water where
vapor pressure rises as temperature increases. When the local static pressure
increases above vapor pressure again, the bubbles abruptly collapse. That collapse
causes high local velocity micro-jets and pressure waves, both of which can damage
surfaces. While cavitation can be a problem in water-only systems, it becomes a
much more complex issue at refineries that deal with a mixture of liquids with
varying—and typically higher—vapor pressures, which often operate at elevated
temperatures.
CAVITATION EFFECTS AND EXAMPLES
Cavitation is of particular concern with control valves in refineries. These valves
contain flow constrictions, which produce high local velocities and low local pressures. If the local pressure drops below the vapor pressure for any of the constituent liquids in the mixture, vapor bubbles will form. As the flow path widens
downstream from the valve constriction, the local pressure increases, and the bubbles collapse. The consequent energy release can produce localized pressures as
high as 100,000 psi, which is highly erosive and can cause pitting of the surrounding metal. This energy release can result in cavitation erosion damage to valve
seats and increased vibration, which can damage pumps and piping.
If ignored, cavitation can lead to a shortened life cycle for valves or piping, and
potentially to deterioration of product quality and purity. In a worst-case scenario,
cavitation damage could lead to catastrophic process line failure.
CAUSE OF CAVITATION
Cavitation occurs when local static
pressure is reduced below vapor
pressure, leading to the formation of
ADDRESSING THE CONTROL VALVE PROBLEM
The key to eliminating the potential for cavitation in control valves is to minimize
the pressure drop through the valve, which can prevent any of the process liquid
constituents from reaching vapor pressure. Beyond this basic guideline, valve manufacturers have taken a variety of approaches to specifically address cavitation
prevention and management. For instance, one manufacturer has developed control
valves equipped with anti-cavitation features. These include attenuation plates for
the valve seat and plugs contoured to minimize low pressure regions. Another manufacturer has developed a range of valve designs specifically targeted at the reduc-
