equipment drain chugging - vent line sizing
equipment drain chugging - vent line sizing
(OP)
We have a drain that puts water into a sump at 3000 gpm for 1.5 min. The sump drains via a 20" line which travels for 2500' with 5 manholes installed. The first manhole is located 450' after the sump.
A change was recently made to the drain line 40' downstream of the sump where a section of the 20" line was cut and replaced with two 10" lines. Now when the equipment begins to drain water boils over the sump for ~30 seconds. The current theory is that the air cannot leave the pipe fast enough which is causing the drain to chug at the sump. It does not appear that we are pipe tight because 30 seconds into the flush the drain begins to handle the flow adequately.
What do you guys think? Is this a reasonable explanation, and if so does anyone have some insight on sizing a vent pipe for the drain line? Has anyone seen this before?
Thanks ahead of time for your insight.
A change was recently made to the drain line 40' downstream of the sump where a section of the 20" line was cut and replaced with two 10" lines. Now when the equipment begins to drain water boils over the sump for ~30 seconds. The current theory is that the air cannot leave the pipe fast enough which is causing the drain to chug at the sump. It does not appear that we are pipe tight because 30 seconds into the flush the drain begins to handle the flow adequately.
What do you guys think? Is this a reasonable explanation, and if so does anyone have some insight on sizing a vent pipe for the drain line? Has anyone seen this before?
Thanks ahead of time for your insight.





RE: equipment drain chugging - vent line sizing
If you have a reasonable slope to the line (say 1 in 50) the 20" line would not run full at 3000 gpm. The air that was in the line could be "pushed" out of the pipe towards the exit because the line was not full. Now, with the 10" pipes, you have probably reached the point where the 10" lines have to run virtually full to cope with 3000 gpm and the air that was in the first 40' of line cannot be "pushed" through the 10" pipes towards the pipe exit. This air therefore has to flow backwards towards the sump, giving the appearance of boiling. Once all the air from the first 40' has escaped in this way, flow will continue normally with the air downstream of the modification being "pushed" towards the pipe exit as before.
So, in essence I am agreeing with your conclusion.
If this behavior is a problem then you could install a vent at the end of the 40' section, but if there is no problem why bother?
Katmar Software
Engineering & Risk Analysis Software
http://katmarsoftware.com
RE: equipment drain chugging - vent line sizing
RE: equipment drain chugging - vent line sizing
Two 10" diameter sewers have a combined capacity of only 32% of the capacity of the 20" diameter sewer.
What you need to do is to replace the two 10" diameter sewers with a 20" sewer. Adding a vent is not going to improve the situation.
What you are experiencing is a transitioning from gravity flow through two phase flow (air and water), to full pipe pressurized flow. Two phase flow will provide less capacity. The transition point will start at the flow blockage (where the 10" lines start) and travel backwards up the sewer line to the manhole. Manholes will most likely be surcharging. Since the flow transition point is actually traveling backward, there is no point to installing a vent. Once the sewer is completely full and surcharged, you are getting more capacity out of it because it is acting like a pressurized pipe.
RE: equipment drain chugging - vent line sizing
Richard A. Cornelius, P.E.
WWW.amlinereast.com
RE: equipment drain chugging - vent line sizing
I do not think any air will travel counter-current through the 10" section - as you say, it is in pressurized flow condition. The vent will only have to release the air from the first 40' of pipe. The vent would have to be large - close to 20" itself - and quite high to avoid it becoming a fountain.
Katmar Software
Engineering & Risk Analysis Software
http://katmarsoftware.com
RE: equipment drain chugging - vent line sizing
Since this is a gravity line, the fluid velocity is not great enough to push the air through the line. I was thinking the air would travel backwards through the pipe to escape.
The only solution that will positively fix the problem is to replace the pipe. Putting in a vent would probably cost about the same amount of money as replacing the pipe.
Of course, we do not know the story behind changing the two 10" diameter pipes.
RE: equipment drain chugging - vent line sizing
I won't get into the story behind the change, but dicksewerrat has the right idea.