3 replies to this topic

[rohollah]

Hi every body


How can I predict cavitation in a control valve,by cavitation index which is obtained from below formula



cavitation index=(P2-Pv)/(P1-P2)


For example we have a control valve which has a v-ball valve body with below conditions:

P1=11bar P2=8bar vapor pressure=0.5bar
does it cause to cavitation?

[fallah]

Hi every body


How can I predict cavitation in a control valve,by cavitation index which is obtained from below formula



cavitation index=(P2-Pv)/(P1-P2)


For example we have a control valve which has a v-ball valve body with below conditions:

P1=11bar P2=8bar vapor pressure=0.5bar
does it cause to cavitation?

rohollah,

As far as i know if P2-P1 would be greater than delta Pmax (maximum allowable liquid sizing pressure drop) then choked flow would be occured. Then flashing or cavitation should be realized.

Fallah

[GS81Process]

Please refer to Fisher Control Valve Handbook, Fourth Edition, p.115-116, 136-138.

http://www.documenta.../book/cvh99.pdf


I think it is important to note the difference between "flashing" and "cavitation".

In either case, the fluid pressure in the valve will be reduced to below the fluid's vapour pressure (at the "vena contracta"). In the case of flashing, the outlet pressure will remain lower than the vapour pressure and the fluid will stay as a vapour downstream of the control valve. Flashing damages the valve by erosion caused by high fluid velocity as the liquid expands into vapour. In the case of cavitation, the fluid pressure will recover to above the vapour pressure downstream of the vena contracta and result in the vapour collapsing back into liquid. This collapsing of vapour bubbles is what causes cavitation damage.

Valve manufacturers offer trim designs for flashing or cavitation service, however these only attempt to minimize cavitation or flashing damage and are generally ineffective. It is best to change the process pressure drop to avoid flashing or cavitation across a control valve. To avoid cavitation it is common to use multiple valves in series, so that the minimum pressure at the vena contracta for each valve is increased.

Edited by GS81Process, 19 December 2011 - 11:38 AM.

[kkala]

Agreeing to post by fallah in general terms, following clarifications could be useful.
1. Liquid velocity increases across a valve, reducing pressure. If pressure gets lower than vapor pressure, flashing occurs. In case that pressure is then recovered above vapor pressure, bubbles will collapse. This is cavitation.
2. Cavitation will not occur, if P1-P2=ΔP is lower than Km*(P1-Rc*Pv), where Km=recovery coefficient (depending on valve type), Pv=liquid vapor pressure, Rc=critical pressure ratio (depending on Pv/Pc, Pc= abs critical pressure of fluid). Refer to attached "cavitation.xls" for estimated values, noting that Km from manufacturer would be more precise.
3. ΔP higher than Km*(P1-Rc*Pv) will not increase flow rate, restricted by cavitation. If unavoidable, select material to withstand these severe conditions.
4. Example: P1=11 barg, P2=8 barg, Pv=0.5 bara, conventional buterfy valve: Km=0.75, Km*(P1-Rc*Pv) = 8.25-0.375*Rc. Its value is greater than ΔP=11-8=3 bar for any possible Rc (see cavitation.xls); so the valve will not cavitate under these conditions, whatever liquid passes from it. E.g. for water Rc~0.95
5. Consequently condition for no cavitation is P1-P21/Km. Reported cavitation index=(P2-Pv)/(P1-P2) is a bit different. However above seems to give a result too.
6. Searching the web, http://www.stealthva...ng-Equation.pdf describes a similar method, cavitation index is mentioned, but probably this index is related to a chart. A chart found in http://www.cla-val.c...ation_Guide.pdf indicates no cavitation for index <0.5 and risk of cavitation damage for index>0.8. It does not seem to represent the general case (any valve opening, type, liquid handled). But as a rule of thump is simple. More info on this cavitation index (or a general chart) would be welcomed.
Method of Rc is probably more precise (having been used locally).

Attached Files

•  cavitation.xls   26.5KB   175 downloads

Edited by kkala, 19 December 2011 - 11:54 AM.