2226 |
CHAPTER 27. CONTROL VALVES |
27.14.5Valve noise
A troublesome phenomenon in severe services is the audible noise produced by turbulence as the fluid moves through a control valve. Noise output is worse for gas services experiencing sonic (critical) flow and for liquid services experiencing cavitation, although it is possible for a control valve to produce substantial noise even when avoiding these operating conditions. Noise produced by a control valve also translates into vibration imposed on the piping, which may cause problems such as loosening of threaded fasteners over time.
One way to reduce noise output is to use special valve trim resembling the trim used to mitigate cavitation. A common cage-guided globe valve trim design for noise reduction uses a special cage designed with numerous, small holes for process gas to flow through. One of the ways in which these small holes reduce audible noise is by shifting the frequency of that noise upward. This increase in frequency places the sound outside the range where the human ear is most sensitive to noise, and it also helps to reduce noise coupling to the piping, confining most of the noise “power” to the internal volume of the process fluid rather than radiating outward into the air.
27.14. CONTROL VALVE PROBLEMS |
2227 |
Fisher manufactures a series of noise-abatement trim for process gas service called Whisper trim. A “Whisper” plug and cage set is shown in this next photograph:
In some versions, the holes are merely straight through the cage wall. In more sophisticated versions of Whisper trim (particularly the “WhisperFlo”), the small holes lead to a labyrinth of passages designed to dissipate energy by forcing the fluid to take several sharp turns as it passes through the wall of the cage. This allows relatively large pressure drops to develop without high fluid velocities, which is the primary causal factor for noise in control valves.
2228 |
CHAPTER 27. CONTROL VALVES |
27.14.6Erosion
A problem common to control valves used in slurry service (where the process fluid is a liquid containing a substantial quantity of hard, solid particles) is erosion, where the valve trim and body are worn by the passage of solid particles. Erosion produces some of the most striking examples of valve damage, as shown by the following photographs53:
Here we see large holes worn in a globe valve plug, and substantial damage done to the seat as well. The process service in this case was water with “coke fines” (small particles of coke, a solid petroleum product). Even ceramic valve seat components are not immune to damage from slurry service, as revealed by this photograph of a valve seat with a notch worn by slurry flow:
There really is no good way to reduce the e ects of erosion damage from slurry flows, other than to use exceptionally hard valve trim materials. Even then, the control valve must be considered a fast-wearing component (along with pumps and any other components in harm’s way of the slurry flowstream), rebuilt or replaced at regular intervals.
53A colleague of mine humorously refers to these valve trim samples as “shock and awe,” because they so dramatically reveal the damaging nature of certain process fluid services.
27.14. CONTROL VALVE PROBLEMS |
2229 |
Another cause of erosion in control valves is wet steam, where steam contains droplets of liquid water propelled at high velocity by the steam flow. A dramatic example of wet steam damage appears in this next photograph, where the cage from a Fisher valve has been literally cut in half from the flow:
2230 |
CHAPTER 27. CONTROL VALVES |
Steam may also “cut” other parts of a valve if allowed to leak past. Here, we see a valve bonnet with considerable damage caused by steam leaking past the outside of the packing, between the packing rings and the bonnet’s bore:
27.14. CONTROL VALVE PROBLEMS |
2231 |
Any fluid with su cient velocity may cause extensive damage to valve components. Small holes developing in the body of a valve may become large holes over time, as fluid rushes through the hole. The initial cause of the hole may be a manufacturing defect (such as porosity in the metal casting) or damage inflicted by the user (e.g. a crack in the valve body caused by some traumatic event). An example of such a hole in a valve body becoming worse over time appears in this next photograph, taken of a control valve removed after 40 years of continuous service:
2232 |
CHAPTER 27. CONTROL VALVES |
A close-up photograph of this same hole shows the leak path worn larger by passing fluid, allowing fluid to flow by the seat even with the valve in the fully-closed position:
In more severe process services, such holes rapidly grow in size. This next photograph shows a rather extreme example of a hole near the seat of a valve body, enlarged to the point where the valve is hardly capable of restricting fluid flow at all because the hole provides a bypass route for flow around the valve plug and seat: