Steam pressure drop and line sizing

[acurry]

I have a 3" steam line (sized for 4800 #/hr) coming off the header, 120 psig, 350 degF, going to a 3" pressure reducing valve for an outlet pressure of 8 psig. The line after the valve is sized for 6" to maintain the same 4800 #/hr, given the pressure drop.

My question is: If I have a few feet of 3" after my valve, before I go to a 6", will this have a negative effect on my line capacity? Should I immediately go to a 6" instead?

Thanks for any insight into this.

-Adam

 

[Joesteam]

Your answer depends upon what type of reducing valve you have. If it is self contained, you will get a nominal drop with the smaller pipe on the outlet, which also may effect capacity.

If you have a pilot operated or pneumatic valve, then if you take your reference pressure AFTER the steam is in the 6" valve, then the pressure drop through the 3" portion will not really effect anything (unless the valve used is already at full capacity).

 

[BronYrAur]

Joesteam is right on the money. Pressure drop in the 3" line with 4,800 #/hr is 10.1 psi per 100 ft, so 3 ft isn't too bad. Go with a pilot operated valve to be safe. Just be aware of potential noise issues. Steam will be streaming through that small piece of 3" pipe at 27,000 FPM. Is that too high from an errosion standpoint?

 

[TBP]

Oversizing is THE great sin in steam systems. It causes more trouble than everything else combined. Everyone is worried about going "too small". They overestimate the steam load, then compound their error by subsequently oversizing the piping and components for that oversized load. The installed cost is much greater than necessary, and control valves hunt themselves to death long before their time.

In a former life, I sized a LOT of steam PRV stations. A pretty good rule of thumb is size the PRV. The inlet piping is typically one pipe size larger, and the outlet piping is double the inlet piping size. An 1-1/2" Spirax Sarco pilot operated PRV will JUST be big enough. 2" is LOADS. A few feet of 2" pipe (or 3") won't be long enough to matter. This includes the downsteam isolation valve, if there's a bypass around the PRV. For what it's worth, I dislike bypasses. They get abused by the unknowing, and have caused me far more problems than they've ever solved.