CV and orifice or better CV for flashing steam? 2

[MattNCSU03]

I am checking a calculation where about 160000lb/hr of water from a tank 120°F/60psia drains through around 100' of 6" SCH40 steel pipe and a level control valve to another tank at 1.5psia. The saturation temperature at this pressure is about 116°F so I would expect flashing. The originator recognized this and sized an orifice downstream of the CV to take the last 10psi of pressure drop and prevent flashing in the valve.

My question is, would you consider this to be the best practice or would it be better to go to the valve manufacturer and have them quote a valve with trim to handle the flashing?

Thanks!

 

[JLSeagull]

The greatest dp is at the vena contracta within the valve body. Another 10 psi drop after the pressure recovery could worsen the condition. I would select hard faced trim and forget the plate.

 

[TD2K]

I get cavitation warnings for this service at about 40 psi dP (depends on your valve characteristics of course) and you will run close to 50 psi with your design. Even with the orifice you will have some cavitation within the valve as JL points out.

What is the min and max flow of your system compared to the 160,000 lb/hr design rate? The problem with orifices is that they don't have a lot of turndown. Increase the flow and your valve goes wide open as you run out of dP. Decrease the flow and the valve effectively takes all the dP.

I'd prefer not having the orifice and select a valve capable of handling a flashing service.

 

[MattNCSU03]

160,000 lb/hr would be considered the max expected under normal operation

I estimate 156,000 lb/hr as normal operating condition. During startup of the cycle, the flow will increase proportionally to power output: 35,000 lb/hr at 25%, 90,000 lb/hr at 60%, 122,000 lb/hr at 80%, etc.

I am in agreement when it comes to using an engineered valve. Adding an orifice was going against my instinct to keep things as simple as possible. Still relatively "new at this" so I appreciate the advice.