equipment drain chugging - vent line sizing 1

[Hotown]

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.

 

[katmar]

The one bit of information that is missing that would enable a complete analysis is the slope of the line. But in principle I think I understand what is happening. The basic problem is that two 10" lines have much less capacity than a single 20" line. At the most simplistic level, the two 10" lines have half the cross sectional area of the 20" line. Add to this the additional friction because of the increased pipe wall surface and you have decreased the capacity of your line by more than 50%.

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

 

[Hotown]

Thanks for the response katmar. I'm glad to see you came to the same conclusion we did. The problem is that the boiling floods the equipment room about 3" during each backwash. We are going to install a vent at the end of line.

 

[bimr]

You are wasting your time putting in a vent.

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.

 

[dicksewerrat]

Can you lengthen the 'dump' time to 5 minutes? Which genuis decided that 2 10's equals a 20? I bet it was a contractor talking to a bean counter.

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[katmar]

@bimr - I don't think it is a total waste of time to install a vent, but it may not be the best solution to the problem. It depends on how long the 2x10" section is. If it is short it would be better to restore the 20" pipe. But if it is long it may be cheaper and easier to install a vent at the end of the first 20" section.

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

 

[bimr]

Entrapped air causes will cause unpredictable phenomena to occur.

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.

 

[Hotown]

Thanks for the input guys. The more we looked at the vent idea and the new line up the less comfortable we were. We will be digging up the piping and re-designing it. Unfortunately the 10" diameter is still a requirement, so we're not out of the woods yet.

I won't get into the story behind the change, but dicksewerrat has the right idea.