Special Section: Flow & Level
A butterfly out of water
Researchers reduce noise, erosion; add fins to butterfly valve
By Kazuhiko Ogawa
The industry sees widespread use of butterfly valves because they are compact and simple to install compared with other types of valves. However, depending on the conditions, cavitation may occur around a butterfly valve. When severe noise and vibration occur because of cavitation around a butterfly valve, the valve body and pipe wall are subject to erosion.
To reduce cavitation noise around a butterfly valve using a simple method, we proposed attaching fins to the valve body. A high-speed camera and numerical analysis allowed us to see how the intense vortex cavitation clouds were suppressed in the valve body by adding fins. This is because the fins suppress interference of the flow from the orifice side with the flow from the nozzle side.
We investigated the effect of the fins based on experimental results and numerical analysis. The results of the experiment, wherein the two semicircular fins were attached to the downstream side of the valve body, showed the inception of cavitation around the valve with two fins was earlier than that around a normal valve, with the fins under the constant pressure loss coefficient. It also showed the fins could suppress the maximum cavitation noise just before flashing condition. The cavitation noise reduced by about 5dB just before flashing condition when the valve opened 45°.
Butterfly valve applications
Butterfly valves sometimes see use inside the piping of air-conditioning facilities, and users can sometimes mistake the noise and vibration caused by cavitation for mechanical trouble. The need to prevent such noise and vibration is increasing from an environmental standpoint, and the prevention or suppression of cavitation itself is very important. Accordingly, other researchers have proposed quite a few products to prevent or control cavitation around many types of valves, from reports on characteristics of a control valve with tortuous paths to using a multi-perforated cone to prevent cavitation from occurring around an orifice.
These methods have already seen application in actual products, and those products proved successful in reducing noise. However, tortuous path valves are applicable only to cases in which the fluids are clean and the shapes of the piping arrangements around the valves are complicated. Moreover, the air injection method that is effective in reducing cavitation is limited to cases in which you can ignore the effect of air.
As such, we proposed in earlier research that enlargement of a pipe downstream of a butterfly was much simpler than the conventional methods. However, sudden enlargement of the pipe is not adequate for flows containing particles because the particles accumulate in the enlarged section of the pipe. Our current study proposes the attachment of fins to the valve body in order to further reduce cavitation noise around the butterfly valve. You can use this method for flows containing particles because of the simple shape of the valve body. Cavitation occurs intensely around the butterfly valve because of the interference of the flow from the nozzle side with the flow from the orifice side. To avoid this interference, we attached semicircular fins to the valve body.
During intense cavitation, we would not normally use a butterfly valve, but this cavitation appears when a valve repeatedly opens and closes. In this case, we can easily presume separation of the flow and the vortex region around the butterfly valve during light or moderate cavitation.
Interference from both flows causes intense cavitation and brings about the erosion of the wall surface. Therefore, we attached fins to the valve body to avoid interference the flows cause.
Butterfly valve models
We measured cavitation noise in a closed-type cavitation tunnel, using water as the fluid. There was a pump on the downstream side of the test section. We kept the upstream pressure at atmospheric pressure and fixed the valve opening during the experiment. We increased flow velocity by controlling the frequency of the pump using an inverter. We measured cavitation noise at each flow velocity and created a visualization by a high-speed camera. We measured cavitation noise using a noise meter placed close to the outside surface of the test section duct. The frequency range of this noise meter was 20-8000Hz.
In this study, we called a test valve TYPE-A when a semicircular fin attaches to the downstream surface of the valve. When two semicircular fins attach to the downstream surface of a valve, the test valve is TYPE-B. On the TYPE-C test valve, three semicircular fins attach to the downstream surfaces of the valve. For each valve, each fin fixes perpendicular to the valve stem.
Cavitation number, pressure loss coefficient
We define the cavitation number in this study as follows:
where p is upstream pressure, pv is saturated vapor pressure, ρ is density of water, and U is average upstream flow velocity. The following formula defines pressure loss coefficient ζ:
where ΔP is the differential pressure across the test valve.
Fin effects on cavitation noise reduction
In other studies, when using measurements of cavitation erosion and noise, the cavitation around the butterfly valve occurs most intensely when the valve is halfway open (40°-45°). Therefore, we measured noise around halfway-open valves, and investigated the degree of the cavitation reduction based on these measurement results. In the experiments, we measured cavitation noise at each flow velocity, while gradually increasing flow velocity by controlling the frequency of the pump.
In the case of the normal valve without a fin, cavitation occurred at σ = 46; but in the case of TYPE-B, cavitation occurred at σ = 39. These results proved fins suppressed cavitation occurrence. Moreover, the maximum noise just before flashing was 5dB lower in the case of TYPE-B than in the case of the normal valve.
In the cases of TYPE-A and TYPE-C, the cavitation number at cavitation inception was larger than in the case of the normal valve. The fin promoted the occurrence of cavitation because the middle fin was fixed on the orifice side, where cavitation occurrence was intense.
To get these results, we measured cavitation noise at each flow velocity, while increasing flow velocity using a pump controlled by an inverter. We kept upstream pressure at atmospheric pressure in each experiment, and the effect of the saturated steam pressure was minimal. Therefore, the flow velocity was also almost the same when the cavitation number was the same.
However, in many cases, changes in frequency of the pump did not control the flow rate. But adjusting the opening of the valve did. The head curve of the pump and the pressure loss of the piping system as a whole determined the flow rate in the actual plant. Accordingly, it is important to investigate the effect of cavitation control under the constant pressure loss coefficient condition.
When a fin attaches to a valve body, you can change the flow rate by variation in the pressure loss of the valve.
We used numerical analysis to examine the effects of the fin. The numerical analysis code was STAR-CD, and we used the barotropic model as a model for cavitation. The calculation conditions were θ = 45o and σ = 22.
We examined velocity vectors in the cross section near the fin to clarify the effects of the fin.
For the normal valve, the flow from the orifice side and the flow from the nozzle side interfered with one another about 1 diameter from the valve stem. This interference made the downstream flow of the valve swell and brought about intense vortices, which correspond to a cloud cavitation.
The swell of the flow for the TYPE-B valve with two fins was moderate compared to that of the normal valve. The swell of the flow is the smallest for the TYPE-C valve. However, it is probable the cavitation intensified since the fin was located in a position where the contraction flow was severe. Accordingly, in TYPE-C, two fins on either side of the valve suppressed the flow interference effect, which was canceled out by the intensification of cavitation due to the central fin. Therefore, the effect wherein two fins on either side of a valve suppress flow, interference is offset by the effect wherein the fin in the center intensifies cavitation.
ABOUT THE AUTHOR
Kazuhiko Ogawa (email@example.com) is a professor at Osaka Sangyo University, Nakagaito, Daitoh, Osaka, Japan.