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Force main Modification

Force main Modification

Force main Modification

(OP)
The lift station consists of a 8' x 40' Wet well containing two Fairbanks Morse 4" 5435 MV w/ 12.6 Impellers discharging through two 6" American Flow Control 50-SC Check Valves to a 10" C-900 Force main.There are 5 separate ARV's located along the 5,600 ft force main at various locations. The over all elevation change from the pump station discharge to the discharge into the gravity system is around 113 feet.   


  Good Morning thinkers :) I wanted to give you some of the technical blah blah before I got started with the question.  I am currently working on a project that has experienced 4 separate force main failures. The failures being broken 10" pipe. The fractures have been along the pipe and never at the bell. (That's right not the bell) The splits are always located on the bottom of the pipe. The pipe that was removed was sent to private labs to test and the pipe is fine. The second thing that was done was the 3 ARV's located closest to the pump where replaced do to excessive debris. (Municipality maintained) (sheesh).

A monitor was placed at one of the ARV manholes and we sat on it for one month. The measurements never got over 85 PSI. The employer is wanting to have (GA) soft close check valves installed in place of the AFC 50-SC's and two additional inline check valves (Also GA. The pumps will be pulled and new 13.74 Impellers will be fitted. I personally feel like the Inline check valves are over kill because I feel the issue was located in the poor maintenance of the ARV's. How ever. My question is do you think that the addition of the inline check valves coupled with the head pressure will pose a problem for the pumps even with the New Impeller size.
 

RE: Force main Modification

You didn't mention the working pressure allowed on the pipe.  Long splitting would tend to point to radial pressure, rather than longitudinal stress being the cause, which I would presume would manifest as joint separation.

If that's true, then one might also suspect high transient pressure spikes too fast for an unspecialized pressure monitor to catch.  Well, that's my best guess and I'll stick to it until proven otherwise.  I would point to a cause by air in the line, esp. given you've had
ARV problems, the air compressing and driving water columns rapidly when demand is quickly increased downstream, perhaps enough to cause high velocity head pressures when the rapidly moving water colums eventually come to a stop again.

Can you check to see if the breaks were in conjunction with a pump start or stop, or a major valve opening or closing quickly, each of which would also indicate transient pressure causes, but not from air entrainment, just fast velocity changes in the piping.  

I would tend to think that inline pressure-balancing check valves would be more effective on pump starts and valve closings rather than compressed air driven velocity surges, which in that case, you should think about doing more maintaining on those ARVs.  

http://virtualpipeline.spaces.live.com

RE: Force main Modification

You post lacks so many details that few would venture to guess at a root cause for the operating problems.

Important details that must be known include:
1. Age of pipe. Age of system, new or old.
2. Pipe design pressure.
3. Pipe velocity.
4. Fluid pressure.
5. Fluid velocity.
6. Check valve type.
7. Do you have VFD's.
8. Design capacity versus actual operating capacity.

You may have a problem with the pipe design pressure. C-900 must be derated for the cyclic pressures that will occur over time.

http://books.google.com/books?id=Ij_cB-Qx3mMC&pg=PA428&lpg=PA428&dq=derating+c-900&source=bl&ots=kJF2TFi4JU&sig=u-3BkyXOpNog5SlhQC79nhaUpXo&hl=en&ei=nLTlTPHwGoednAfqv7SgDQ&sa=X&oi=book_result&ct=result&resnum=1&ved=0CBMQ6AEwAA#v=onepage&q=derating%20c-900&f=false

You should be using VFD's so that you do not have pressure spikes from the pumps starting and stopping.

You need to discontinue use of the air release valves and maintain adequate fluid velocity in the pipe to force air down the pipeline.

There have been numerous posters on this forum inquiring about problems with force mains. If the velocity is inadequate to push the air downstream, you will have trapped air in the force main. The trapped air will limit the capacity.

You need a velocity at least 4 ft/sec to force air down the force main. Since you are pumping downhill, the air will be attempting to flow backward.

Here are a couple of examples:
thread161-237329: sewer forced main problems: sewer forced main problems: sewer forced main problems
http://www.eng-tips.com/viewthread.cfm?qid=270920

One would think that you should be doing a comprehensive project evaluation rather than just looking at a few details.

http://files.engineering.com/download.aspx?folder=900d7a4d-3d64-4902-90dd-52df3ca4eba8&file=4785_001.pdf

RE: Force main Modification

Quote:

question is do you think that the addition of the inline check valves coupled with the head pressure will pose a problem for the pumps even with the New Impeller size.


Seems the question is the effect of the checks on the pumps. Not sure how effective the pumps were to begin with, since there is no data. Nor is it clear what the new impellers will do. Pump curves with both old and new impellers plus system head might be useful information along with BIMR's list of data requirements...

RE: Force main Modification

One would presume that the Owner is considering changing out the check valves and pumps because of the line breaks.

RE: Force main Modification

Quote:

"You should be using VFD's so that you do not have pressure spikes from the pumps starting and stopping."

VFDs will not function as you suggest in the event of a pump trip due to power failure.  

Isn't starting and stopping against a slowly closing/opening shutoff valve much more reliable and cost efficient than taking a 5% power cut over all operating ranges caused by use of a VFD?  

http://virtualpipeline.spaces.live.com

RE: Force main Modification

Regarding "Isn't starting and stopping against a slowly closing/opening shutoff valve much more reliable and cost efficient than taking a 5% power cut over all operating ranges caused by use of a VFD?"

If the piping is not designed for the anticipated operating pressures, he may be forced to use a VFD in order to control the pressure spikes

A force main is normally pumping sewage not clean water. It will be difficult to find a slowly closing/opening shutoff valve that is suitable for such sewage service. Such a vlave is normally used on clean water service without problems.

I would expect that this installation also has a standby power system.

RE: Force main Modification

Standby power takes a few seconds to switch in.  Far to much time to catch a pump trip transient.

Sorry.  I just hate to see VFDs touted as THE snake oil of the 21st century.  They have their uses, but this ain't one of them... IMO.



//virtualpipeline.spaces.live.com

RE: Force main Modification

So you would still recommend a slowly closing/opening shutoff valve on a sewage application? I would be interested in a product if you have one.

RE: Force main Modification

No, if such valves are impractical for the application, I'd be much more be inclined to
1, design the pipe for expected transient pressures,
2, trim the impellers to cut head & pressure output, or if that was impossible,
3, pull the existing pump and size it to deliver maximum pressures within the pipe allowable pressures,
4, attempt to replace the highest pressure/problematic pipe segments with thicker wall pipe.  

In petroleum piping running over allowables is not an option and adding problematic equipment that consumes more energy, lessens reliability and increases maintenance isn't highly desireable.   



//virtualpipeline.spaces.live.com

RE: Force main Modification

I don't disagree with any of your comments, but I would expect that a VFD is probably more attractive and practical option (after the fact) since a  mile of installed force main is going to cost $1M.

I also see little reason to argue abut the pro/cons of VFD's as has been done in other forums. The VFD is just another tool in the box.

Finally, I fail to understand the reasoning for discussing the details when the original poster has presented such little detail in the first place.

RE: Force main Modification

Let's ring it up to my "pet peve" worst nightmare.



//virtualpipeline.spaces.live.com

RE: Force main Modification

Just a crazy thought here, but was the pipe made by JM Eagle?

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