End of Steam Main Collection Leg Analysis

When ever a steam main is started up and it is heated up from a standby cold temperature a large amount of condensate is formed and can work its way to the end of the steam main where a main steam trap is provided to remove all of this discharge.

If an initial warm-up is assumed to take place with a start temperature of 0 deg. F with a condensate forming temperature of 212 deg.F, pressure in the pipe will start to increase as the main stabilizes at the 212 deg.F temperature.

For this analysis we will assume 500 feet of 4" diameter steel main, with the temperature scenario indicated above you will generate 55 # of condensate per 100 ft. Using a factor of 1.1 to take into consideration fitting warm-up and the wetness of the steam the following will apply:

Volume of condensate will equal: 55 #/100 ft. X 5 X 1.1 = 300 #’s of total condensate will develop.
Volume of the condensate = 300 #/62.4 lbs per cu. ft
Volume = 4.8 cu.ft

The collecting leg should therefore be at least equal to 5 cu.ft in volume.
The following considers some alternate pipe sizes for possible collection leg sizes:

4" Pipe:
A = 3.14(Dia.sqd)., = 3.14(16.21)/144 = .355 sq.ft
Vol. = Area (Length) = .355(L) = 5 cu.ft
Length of the collection leg (L) with 4" pipe should equal 14 feet.

6" Pipe:
A = 3.14(Dia.sqd)., = .81 sq.ft
Length = 5/.81 = 6.2 feet

At this point you have several choices you can make; you could simply install the 14 foot 4" collection leg. In most cases however installing very long collection legs can present problems of there own. It is advisable to select the smallest collection leg that will do the job.

A popular way to reduce the size of the required leg is to assume that in long mains the main itself will act as a form of collection leg especially taking in the time factor for the condensate to work its way along the main. When utilizing this method it is often assumed that approximately 50% of the condensate will still be in the main as the collection leg collects the remaining condensate and removes it through its trap. If this is the case then you can utilize either seven feet of 4" pipe or 3 feet of 6" pipe.

As with any analysis of this type you must consider what the site conditions will be before making a final determination as to the proper size of the actual collection leg. Additional information can be found in my book “Steam Distribution and Flow”. 

Hal