Throttling Valve Errosion

[TexasCHe]

I'm designing a recirculation circuit to flow 10 to 20 CFS through a 24-inch pipe. I was planning on using a manual 24-inch valve to throttle the flow each day where it would remain until the next day. This pipe velocity is about a 6 FPS velocity max which seems reasonable but I am told to expect valve erosion over the long-term if I plan to throttle it back to the lower range using a valve. The water has what I would consider mild to medium erosion characteristics. Has someone experienced erosion in a similar valve application and what types of valves are available that would survive long-term?

Thanks ahead of Time

 

[gerhardl]

Hello TexasCHe,

Corrosion will normally be prevented by internal good surface quality and material selection and adequate/good quality coating, some of theese more resisting also against abrasion (grinding effect by sharp particles) than others.

I presume, however, that the warnings you have got refer to possible cavitation: throtteling with crack valve openings and pressure difference that speed the liquid so high up that pressure falls below steaming temperature. Collapsing of the steam bubbels created immidiately downstream in the reverse process will cave out valve and pipeline material in the pinpont hits of up to 10000 bar.

Your figures indicate that flow and pressure differences will give for example a double eccentric butterfly valve a reasonable opening (more than 10-15%) and well away from cavitation problems.

NOTE! This has however to be controlled with exact valvetype and dimension given, mass flow figures (already given) and pressure. Full pipeline presumed to avoid crack opening when filling and to minimize delta P over the valve.

Technically a regulating valve is a better choice with much wider and better regulating range both for pressure, exact regulating and pressure variatons.

 

[TexasCHe]

Gerhardl,

Thanks for the quick response. My back-up plan, which it sounds like I may have to strongly consider, was multiple lines. Since this branch will be short (less than 50 feet) I have considered, at the advice of a friend, installing either two 18-inch lines and valves or even three 12-inch lines to minimize throtteling. With the three lines in some cases one or two lines may could be opened 100 percent. Sounds like this would allow me to avoid cavitation issues.

Again, thanks for the quick response.

 

[gerhardl]

Before commenting on detailed regulating pipe and valve layout, it is necessary to have the full application picture with flow requirements (all possible variations) and pressure immidiately upstream and downstream of the valves for all cases (or nearest guesstimate).

And why not consider a 'traditional split range solution' one smaller pipeline and valve (could be a bypass) to cower low flow situations, and one main flow pipeline and flow to open and control large-flow situation?

The cost has to be considered against the cost of one pipeline and one regulating valve, where the regulating valve could possibly (by correct sizing of regulating valve and for certain tyoes) land up in a dimension several diameter units below pipeline size, and with abrupt reduction flanges at valve inlet and outlet (In place of perhaps cone reduction and cone increasing of pipe before and after the valve).

Suggestion: ask one or two local suppliers of valves for suggestions for the two types of solutions with suitable size and type of valves and rough budget price, your final selection based on cost and estimated cost/lifetime (both).