valve industry. He graduated in Process Engineering and. Mechanical Engineering in Stuttgart,. Germany, holds a MBA degr
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Cavitation In Process Valves? In this issue I am using a well-known appearance in water lines commonly known as Cavitation and looking at the causes of its development and measures for prevention in a water distribution piping system.
T
he question in the title can be answered by describing the theoretical causes of this phenomenon. If the criteria causing cavitation are considered in piping, fittings and valves, then actions can be concluded which help to avoid the consequences, namely damage to the construction material. The use of corrosion-resistant materials at potential points of cavitation shows good results. A very simple demonstration can be used to show the development of cavitation: English scientists discovered that the noise which is heard when pulling a finger firmly is cavitation. The cause of this cracking is imploding of gas bubbles which form as a result of the lowering of the pressure in the space between the finger joints which then burst noisily in the adjoining pressure fluid. According to the law of liquid continuity and to Bernoulli’s energy equation, velocity changes or turbulence can reduce the pressure in a liquid down to that of vapor pressure. Critical areas for the development of cavitation in a piping system are sudden changes in flow such as throttling gaps, changes in flow direction and crosssection enlargements. Within the areas of such pressure drops, small vapor and gas filled bubbles form in the liquid. These locally restricted changes in the fluid are commonly referred to as cavitation or cavity bubble formation. From the water vapor table (Fig. 1) it can be seen that the vapor pressure is reached also at low temperatures if the pressure within the liquids is accordingly low. www.valve-world.net
Fig. 1: Extract from the water vapor table
Carried by the flow, these cavities reenter a region of higher pressure and suddenly collapse, at velocities of up to 1000 m/sec., the vapor condensing. On the occurrence of this conversion, which is also denominated as “micro-jet impact”, very high implosion pressures arise and, depending on bubble size, these may reach more than 10,000 bars. This pressure drop can cause the pressure to drop below the atmospheric pressure and reach the vapor pressure of the liquid, producing the conditions for cavitation beginning. As a rule, the pressure drop can be influenced by streamlining the flow inside and outside in the region of the crosssection change. However, one has to realize that the measures for streamlining can be an optimum for one determined flow condition only. The throttling region of a valve has a similar
effect as the non-uniform cross-section contraction shown in Fig. 2. The sudden changes in cross section cause a transverse contraction in the jet which causes a pressure drop. This pressure drop can cause the pressure to drop below the atmospheric pressure and reach the vapour pressure of the liquid, thus producing the conditions for cavitation beginning. As a rule, the pressure drop can be influenced by streamlining the flow inside and outside in the region of the crosssection change. However, one has to realize that the measures for streamlining can be an optimum for one determined flow condition only. Cavitation shows up in three ways: 1. Noise molestation due to radiation of a portion of the converted energy in the form of noise 2. Critical oscillations which can lead to instability of theactuator, loosening of bolted connections, damage to foundations and fatigue cracks in construction materials 3. Destruction of metallic materials Cavitation as such may not necessarily lead to damage: This depends on the cavitation intensity, i.e. the life span of the bubble from its formation to its implosion. Therefore the pressure travel gradient gains importance which is closely related to the shape of the flow passage.
Fig. 2: Pressure distribution at the throttling point. November 2011
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Fig. 3: Material destruction as a consequence of cavitation
The intensity diminishes with the increase in the life of the bubble. If this is considered in the construction, geometrical shapes can be found which, despite cavitation, do not lead to damage, i.e. the bubble implosion occurs without the possibility of touching the material surfaces.The result of this is that only bubble implosions near the wall are destructive. If the bubbles contact material surfaces, the destruction mechanism conforms with that of liquid droplet erosion (as seen in wet steam) (Fig. 3). From the point of view of metal physics, what happens is a high-velocity deformation of the metal as a result of the bubble implosion. As construction materials which have been shown to be less prone to cavitation austenitic, single-phase copper alloys (bronzes), stainless steels and stellite armoring have been successful.These materials are largely resistant to corrosion so that they are not subject to this additional attack. Typical potential points of cavitation damage are: a. S uction pipes of pumps b. N arrow flow spaces, leakage and bearing gaps c. S udden changes in flow area d. C hanges in flow direction as in bends and pipe tees e. C hanges which lead to turbulence f. Downstream of throttle and control valves; components built into the flow stream g. L ow back pressure
Fig. 4: Flow guided by vaned ring in a needle valve 82
November 2011
From this the following measures can be concluded by which disadvantageous cavitation consequences can be avoided: 1. Avoidance of turbulence by proper streamlining of flow, e.g. by means of a vaned ring in a needle valve (Fig. 4) 2. Prevention of wall contact after areas of pressure drop by sudden enlargement of the pipeline. Developing cavitation bubbles implode in the water space. Cavitation arises in a space not endangering the material. 3. Letting pressure drop occur over sharp edges. 4. Dissipation of the kinetic energy not solely through turbulent mixing but a. through built-in resistance, i.e. by increasing the friction-causing wall surfaces. This causes an increase in the back pressure after a point of throttling. A pressure drop below the atmospheric pressure can be avoided. b. p artitioning of several resisting bodies in series (multi-stage pressure drop). In approximation the number of stages required can be calculated as follows: n = -6.45 x 1g
in which
p1 = upstream pressure p2 = downstream pressure 5. Principle of flow partition into small single cross sections, e.g. hollow cylinders. The division into single jets beneficially influences the movement behaviour of the medium flowing off downstream of the throttling devices. In addition, the partition into small cross sections achieves a more uniform downstream flow in the pipe already after a short flow path. From the above examples of cavitation prevention can be seen that for throttling and control duties special valve types are required which are designed with special seat and exit configurations. In general, valves are limited only according to the nominal pressure rating. Within the standard design
requirements, the special dynamic loadings in the flow passage of the various types of valves are not considered. In the case of mere shut-off valves such as gate and butterfly valves, the necessary adaption by means of design cannot be achieved. They are, therefore, not suitable for pronounced throttling and control duty but, because of the low permanent pressure drop, are ideal for on-off duty. Of course, such valves are suitable for short-period throttling duties as, for example, during shut-off in the case of a burst pipe and free exit respectively. However, when dimensioning these valves, the limits must be considered which result from the energy head: Butterfly valves: PN 25 < 7.5 m/s PN 16 < 5 m/s PN 10 < 4 m/s PN 4 < 2.5 m/s PN 2.5 < 2 m/s (Flow velocities referred to valve nominal diameter.) If butterfly valves are used as safety devices in the case of a burst pipe, the responsible manufacturer considers the stresses in the case of a burst pipe and factors in valve and operator designs in a correspondingly strong manner. To enable a control valve to fulfil its duty, e.g. continuous throttling of the rate of flow, the valve must be properly dimensioned.
Meet Günter Öxler Günter Öxler is a freelancer to the Process Industry and has a long history within the valve industry. He graduated in Process Engineering and Mechanical Engineering in Stuttgart, Germany, holds a MBA degree in VWA as well as a controlling degree and is a REFA specialist. For more than 25 years, Günter has worked for several companies in the valve business. He is also a member of the IWA, ISA and VDI German Engineer. Günter can be contacted under
[email protected]
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