Cavitation is a damaging phenomenon that occurs within and downstream of control valves under certain well understood temperature, pressure and media velocity conditions. At its very worst, cavitation creates high noise levels along with vibration, pitting and erosion of valves and piping.
The noise may be annoying or severe enough to be a health threat. However, the noise is only a symptom of the underlying problem — the process conditions. These conditions also may cause flashing, i.e., formation of vapor bubbles in the media; it doesn’t always lead to cavitation and poses less-severe erosive effects.
The damage done by cavitation can impair control-valve performance, which may require reducing the process throughput (and profits) to compensate. Ongoing cavitation will result in progressive deterioration of valve components, ultimately leading to leakage and premature valve failure.
This progressive damage undermines the life of the control valve and may mandate performing unscheduled maintenance to replace the valve and nearby eroded piping, incurring substantial product loss and labor costs.
Most of the consequences and costs of cavitation are avoidable. However, fending them off requires understanding the conditions that produce cavitation, choosing appropriate control valves and other line components, and sizing them accordingly.
FLASHING AND CAVITATION
The first step to a flashing or cavitating condition is the vaporization (or boiling) of the media. This occurs when reduction in the area available for flow (caused by the throttling effect of the control valve) leads to a large increase in fluid velocity, which in turn causes a drop in the fluid pressure due to the law of conservation of energy (Figure 1). The pressure is at its lowest level at the vena contracta, which is located a little downstream of the physical valve orifice.
When the pressure at the valve vena contracta is lower than the liquid vapor pressure, gas bubbles form in the media. These vapor bubbles are similar to the media boiling — except they’re caused by a drop in pressure rather than an increase in temperature. It may have a very erosive effect and should be controlled if possible.
In certain conditions these vapor bubbles are very short-lived because recovery of pressure after the vena contracta raises the pressure above the liquid vapor pressure, causing collapse of the vapor bubbles. This is cavitation — and it’s far more damaging.
The collapsing bubbles cause extremely violent shock waves, which are caused by liquid microjets that form when the vapor bubble collapses upon itself. If the bubble is very close to, or in contact with, a solid wall, the microjets impact the wall with enough intensity to crack or chip the surface. The intensity of these impacts has been measured to be over a million pounds per square inch, and they all occur in a confined area. So, it’s little wonder why cavitation can produce so much damage within a control valve. If a bubble implodes further away from solid walls in the body of the liquid, it generates spherical pressure shock waves that produce unwanted noise.
Although cavitation damage can be a purely mechanical phenomenon, it’s often related to corrosion or erosion due to the breakdown of the passivating film that’s supposed to protect the base material from these forms of attack. Corrosion and erosion in the presence of cavitation can be minimized by using stainless steel and other hardened materials, particularly in the trim and, if possible, selecting trim components that progressively reduce flow velocity.
Sometimes, flashing alone occurs. When the downstream pressure of a control valve is less than the liquid vapor pressure, part of the liquid is vaporized and remains as vapor downstream of the valve. So, fluid downstream of the valve is part liquid and part vapor. Flashing flow may cause mechanical difficulties like erosion and vibration. However, the problems stem from the high velocity of the two-phase flow stream due to the larger vapor volume compared with that of the liquid, rather than bubble implosion.
Because flashing generally is the result of unalterable line conditions, it can’t be avoided by selection of valve type. High velocity can erode valve body internal surfaces downstream of the throttling point, so use of more wear-resistant materials is recommended.
In practice, to minimize the length of piping downstream, the valve usually is located as close as possible to the receiving vessel. Vertical upward flow should be avoided to prevent any slug flow, which may cause strong vibrations.
Typically when noise is present in liquid service, so is cavitation, and roughly to the same degree. Cavitation bubble implosions are the main source of disturbing noise; noise level is directly related to cavitation intensity. In the early stages of cavitation, the noise can sound like sand going through the valve. When the pressure difference across the valve rises, the intensity of the cavitation increases, as does the severity of the noise.