Replacing 8" Valve with 6" one

[devaxrayz]

Hi All,

There is a problem where my 8" Shut Down Valve (ball) is leaking. Since we dont have the spare for 8" valve, my boss considering of using 6" ball valve which we have in warehouse, so I need to evaluate it first. this is definitely a Control Valve sizing case.

The fluid is steam 580 F, 970 psig, 207000 lb/hr.

I evaluate it using fischer CV Handbook equation for compresible fluid to determine the valve max capacity, and determine whether it will be choked or not at flowrate above. Unfortunately i dont have Cv value for the 6" valve (RICHARDS) so I using Fischer 6" ball valve for the Cv value.

My boss also insist me to conduct a velocity evaluation to this case.

Is there any velocity rule for control valve selection?? Maximum and Minimum flow?? I do not notice there is such velocity criteria for Control valve.

How do you think about using other valve manufacturer Cv value for this case? Is that can be used for rough estimation??

Thanks all

-Rayz-

 

[TD2K]

This is just an on/off valve?

Looking at 6" sch 80 piping for this flow, I estimate the steam flow as about 175 ft/sec. This is higher than I would use for continuous service in piping but it's not out of the range for a control valve (not that this is really a control valve). Control valves are typically limited to an exit velocity of about 0.3 Mach which you are still a ways away from. Biggest question is 'is this 6" ball valve a full port or reduced port' as a reduced port will further increase velocity.

As far as the Cv, Fisher's value would be a starting estimate but if you need the Cv, call the manufacturer or representative. They should be able to give you that immediately and you likely have to do that in any event, wny waste time? If both are a 'full port' ball valve I would assume the Cvs are close (though Fisher uses Cs for steam) but if they are reduced port, you could have significant differences.

 

[davefitz]

It looks like you can use the 6" ball valve, but its normal steady state full open pressure drop will be greater than the 8" and you will pay for this over time with higher boiler feed pump power .

For the conditons provide, steam spec vol=0.502 ft3/lbm, flow area= about 0.1963 ft2 * 0.64 ( vena contracta if no gradual reducer used) =0.1256 ft2, velocity = 300 fps, frictioal loss for 2 velocity heads = 22 psid, vs the 8" ball valve friction loss of about 3 psi.

The 300 fps is about as high as one would want to go to avoid noise or toher issues, espescially if there is a tee or fitting downstream.

 

[JimCasey]

I punched the numbers into the Flowserve sizing program and used a full-port 6" ball valve. I get 2.4 psi pressure loss, Velocity in the valve of Mach 0.09, velocity in the 8" pipe of M=.05, and less than 70 dB. Certainly your specific results may vary depending on the specific valve used, but you appear to be a long way away from velocity or noise troubles in this installation. Just make sure your ball valve is rated for the pressure and temperature: ANSI 600 or higher, metal seats, high-temperature packing and seals.

 

[BSQUAREDBUCKEYE]

Full ball type ball valves usually have two seats. Can you remove the valve, turn the valve body 180 degrees and reinstall it making the upstream seat now the downstream seat? It may save you the cost of the pipe modifications.
Please be sure the valve is bi-directional.

 

[stanier]

Richards were bought by Tyco many years ago and you may find data on their valves from their website or ask the representative for a CD . Website is pretty good for information.

If not ring Keith Barter on +61 =2 9632 9533. I assume you are in Australia or New Zealand as Richards were made here and not exported to many countries.