Max length of vent from drip pan elbow

[mrev23]

Manufacturers of drip pan elbows seem quick to "highly recommend" their use, but they may not be practical when a vent line must run through several stories from a basement mechanical room to a roof.

The following may be optimistic maximum lengths for vent lines from drip pan elbows for saturated steam:
[pre]
Vent size Max length
(inches) (equivalent feet)

2 12
4 25
6 38
8 51[/pre]

The table above results from the observation that Chart II-1, Compressible Flow Analysis, in Appendix II of ASME B31.1 doesn’t plot values of f(L_max/D) greater than 1.0.

To clarify one of the values above:
[ul]
[li]B31.1 recommends f = 0.013 for saturated steam.[/li]
[li]Ignoring the difference between nominal and actual pipe size, the maximum vent length for a 6” pipe and f(L_max/D) = 1 is:[/li]
[/ul]

L_max = 1 / f * D = 1 / 0.013 * (6 / 12) = 38 feet.​

This suggests that if a 6” vent is longer than 38 feet, then we can’t use Chart II-1 to even begin a check for blowback from a drip pan elbow into a mechanical room -- because f(L_max/D) would be greater than 1.0 and off the chart.

If we can’t even check for blowback, then we might expect it to occur -- because the vent line is already "too long" based on the maximum length anticipated in Chart II-1.

If blowback is not tolerable, then we can’t use a drip pan elbow with a 6" vent line longer than 38 feet.

The simple calculation above might be an initial check to see whether a proposed vent length is close to workable -- before spending more effort on calculations in Appendix II.

There may be exceptions, but is this a reasonable first look when considering whether to use a drip pan elbow?

 

[PEDARRIN2]

The way I understand sizing vents for relief valves is to minimize pressure drop (which would equate to back pressure on the relief valve diaphragm)based upon the rated relief flow of the regulator. You do not mention what that flow would be so I would think it would be difficult to determine what the maximum vent distance for a pipe size would be.

While I have not sized relief valves for steam, I have sized them for natural gas and laboratory gases. I have used the vent length formula found in ASHRAE 15 Appendix H.

 

[mrev23]

The flow rate through the vent pipe is used to calculate value of V/V* for Chart II-1 in Appendix II of ASME B31.1.

But Chart II-1 doesn't plot values of V/V* for values of f(L_max/D) greater than 1.0.

The implication seems to be that if the value of f(L_max/D) is greater than 1.0, then the vent line may be too long.

No guidance is given in Appendix II to evaluate the potential for blowback where f(L_max/D) is greater than 1.0, though there may be a way to do it with formulae here:

https://en.wikipedia.org/wiki/Fanno_flow

I think manufacturers do us a disservice by encouraging the use of drip pan elbows without providing guidance on where they should not be used.

This thread seems to provide an example of where a drip pan elbow should not have been used:

http://www.eng-tips.com/viewthread.cfm?qid=390819

Based on the simple calculation suggested above, a more detailed check for blowback should have been done if the length of the vent line approached 64 feet:

L_max = 1 / f * D = 1 / 0.013 * (10 / 12) = 64 feet.​

The vent pipe that was installed is more than three times the length calculated above.

The problem of "Steam Escaping from Drip Pan Elbow Gap" might never have happened if manufacturers of drip pan elbows provided guidance for a sanity check on running 200 feet of 10" vent pipe.