sewer forced main problems 10

[kentgolding]

A forced main sewer was installed on my last job and is having some issues.
The line is about 1.5 miles long and goes from pump station over a small rise and then drops lower than pump station into a draw and then goes up over another hill higher than pump station and then drops into a gravity sewer on the top of the hill, a built in air bleed here. We have one vacumn/air valve at the top of the first hill and a blow out at the lowest point but for some reason no clean outs were installed; deleted through a higher authority so we have what we got.
We have been losing GPM and a slight increase of pressure has occured since start up, about two years ago. A lot of different ideas have come up and a lot of checking of system has been done and all things point to a block in the line.
We do have valves in the system and one engineers idea was to shut one of the valves down in known degrees (25 turns to go from open to close so 8 turns would be 1/3, 16 turns 2/3 and 25 turns full shut) and check pressure and GPM. If the pipe is 1/3 clogged then we will see no change when valve is closed 1/3, and so on. From the other side of the room we are hearing that this won't work due to variables in design(?). Seems like it would work to me, and if so our test idicates that our line is about 2/3 clogged. We have dropped from a specified 522 GPM to 330 with a pressure increase of almost 15 PSI.
We have a sump pump in the collection well and a centrifigual pump in the pump house to push over the hill. At start up we were at 522 GPM (although documentation is rather weak).
The big question is should the valve test idea work? Seems pretty nice and simple to me, but I am a "simple is good" type person. Any ideas on this would be great, thanks to everyone. Also if you wish some more info let me know.

 

[kentgolding]

Thanks Rw. you notice that the valve listed is a vacuum breaker/air relief and APCO even says right in their page that if you want air removal during operation you need another type of a valve,which they do show on their "system" diagram on the last page. We were given only the vacuum break/air relief type valve and we are installing a piggyback air release valve (actually it should be a gas relief valve as I doubt if much breathable is comming out of our sewer ).
Sorry the slide show doesn't work for you, shows a really big steel pipe collapsed under the vacuum that should have been handled by a vacuum break valve.
And yes, the name is the same as a hop type, too bad I can't get a percentage from using my name . I believe the hop was originally grown in the "Kent" area of England, and probably by someone named Golding, so the "Kent Golding Hop. Understand it is a good stout brew.

 

[bimr]

In general, automatic air-release valves should only be used as a last resort on sewage force mains. From past experience, automatic valves require frequent maintenance to function as intended.

The valves must be specially designed to keep the valve operation mechanism free from contact with sewage to inhibit clogging and the resulting malfunctions.

You should consider the use of manually operated air release valves. In addition, if you can get your velocity up above 4 ft/sec, the pumped fluid will force the air bubbles through the force main.

 

[kentgolding]

Thanks Bimr. The APCO valve is one of the types that (according to the manufacturer) keeps the operating mechanism above the fluid levels.
I believe that our "normal" flow is up in the 3.7 GPM range (at least not above this) so if the system is operating correctly it will not meet the >4 GPM you mention. But thanks to you and everyone here, even though I will probably never have to mess with one of these again, if it should happen I know a lot of what NOT to do.
Still waiting for the valve to arrive and be installed, at this point more like a soap opera, so tune in next week!

 

[alexcmmi]

So the original designer did not spec out combination air release / air/vacuum valves at the high points?

BIMR - while I respect your opinions, I would have to disagree with you on the not using an automatic air release valve. They are crucial for venting air on long force mains, especialy in undulating areas. Yes, they might get clogged by solids - but they DO work...

Real World - If a maintenance person isnt going to service an automatic air release valve even once a year, do you really think they would MORE FREQUENTLY take the time to open a manual air release valve??? I personally dont think so. Especially if the climate of the area is rainy and/or extremely cold in the winter...

EX - I conducted a structural integrity investigation of a long, common force main in the northeast several years ago. It was a very thorough study. The FM had about 5 manual area release 'blow offs' and two automatic release valves. The automatic valves probably hadnt been serviced since they were installed (25 years - and you could still hear them vent when the FM was flowing). The manual valves - NONE of the staff even knew they existed (i found them on the A/Bs) - and every single one of the curb boxes to get to the air had been paved over. we had to locate them all w. metal detectors and chizel them out. Then the curb boxes were all silted up, and none of the actuators even moved! Not good.

IMO, when it comes to sewers, if it can be done automatically with out humans, its better to do that to rely on that than an operator / DPW / Sewer Dept staff to do anything...

 

[kentgolding]

I love the automatic systems but we still have to maintain them. Really not hard as long as scheduled checks and maintainence is carried out.

<P>It appears that the higher ups, had a bit of a SNAFU on the catalog for air release valves. Some brainiac figured that a series 400 valve ment any APSCO valve that had 40x was the same. The 400 valve we just put in was a air release valve while the 401 valve (installed originally) was the vacuum/air valve for filling and draining the pipe, and the 402 is a combination valve (I think).

<P>The 401 is not meant to operate under pressure which our system has. If you had a single hill to go over the 401 would have been fine, let the air out as the water moves up hill and once done pumping vacuum break allows water to flow out of pipe, pretty easy.

<P>Anyway nothing fantastic on GPM at this time, new valve was installed 4-16-09, and really have not seen a change yet. I'm for sending a pig through and see if it comes out the other side. More news as I hear it, and thanks again for the help here.

<P>(funniest thing I've heard all day, covering up the boxes with asphalt, and we are the most intelligent species on the planet).

 

[scmurphy]

Whoa- that pump configuration is interesting, but I'm inclined to believe the problem is an air locked force main. Just looking at the detention times in the force main and the number of cycles the station has to turn to purge the main, there's going to be substantial biological activity and subsequent gas production. Plus on a 1.5 mile force main, there are probably a number of pipe sways a as a result of installation that are not necessarily reflected in your elevation numbers. We have one force main in particular that was laid to our WWTP from a nearby water and sewer authority that had similar issues. We solved the problem by putting in a few quick saddle taps along the main until we found the problem. For the time being, we manually vent the valve about once a week to restore flow. Also, periodically we use a nearby fire hydrant to continuously fill the lift station while the pumps run to supply enough water to "turn over" the main. Hope this helps.