Air vents in a steam main

[Zapster]

I am a bit confused with a design/build specification requirement for sizing air vents in a 50 psi main steam distribution line.

The project specifications call for air vents with a thermostatic element to be sized for normal operation conditions, and not for startup condition. Startup is to be supervised and air is to be released through manually controlled valves. The location of the steam air vent in question is on a 12” distribution main, with a thermodynamic trap located 5’ upstream from the indicated location of the air valve. The requested location of the air vent is in the middle of the distribution run with many lateral takeoffs down stream. It is not possible for this location be considered a stagnant area of the line.

Because the system is to have a manual startup, the thermodynamic trap should not air bind. So, during normal operation, there should be no air in the main distribution line and hence no need for a thermostatic air vent.

Question: Am I looking at this wrong? If so, what criteria should be used for sizing the air vent for normal operation (not startup)?

 

[25362]

Have a look at

http://www.spiraxsarco.com/learn

and look for modules
10.5, 11.12, and 11.13

 

[Zapster]

Thanks for the links 25362. I glanced at the links and I still cannot understand why the owner would want an air vent in the middle of a distribution line unless it was for startup. I have no problem sizing the air vent for startup; however, the owners explicitly states in their specification that the air vent be sized for normal operation and not start up. It would makes sense for air vents at the end of the laterals off the main distribution line or even at the end of the distribution main; however, having one in the middle does not make sense unless it is for startup. I will think about this for a while before I contact the owner and find out what his intention was so I can get the air vent properly sized.

 

[TBP]

The thermodynamic traps won't get air bound - they just get rid of air quite slowly, as compared to some other trap styles. This system is designed as a supervised start-up, so that won't be a factor anyway. The only time some steam systems (like batch operations) need vents is to get the air out when going into service. There is no requirement for venting air for "normal operating conditons". The location seems odd. Most system vents, when they're needed, are located near the ends of the mains. Note: not right AT the ends of the mains, as any little bit of even minor, annoying water hammer will kill the vent mechanism, and make it blow through. They should be located back a few feet, and mounted on long nipples - maybe 10" or 12" to protect them. Mount them right at the end of the mains on 6" nipples, and see how long most of them last