sewer forced main problems
sewer forced main problems
(OP)
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.
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.





RE: sewer forced main problems
As to your other idea about closing a valve. You can shut a valve a considerable amount (50% or more depending on the type of valve and the system conditions) without seeing decrease in flow and notable increase in head.
You also can't assume that a closed valve will give you an indication of any percentage of blockage. A partially closed valve will act like an orifice and cause an isolated increase in headloss. Clogged piping can act the same if it is a clog in a small area; but, a 100 foot long clog will act more like a section of smaller pipe. (ie you might get 10 feet of headloss with small constriction of 75% pipe diameter; but, you could also get the same amount of headloss over a long constriction of 10% pipe diameter.
The only way to check if it is clogged is to cut a section out and see.
RE: sewer forced main problems
Actually we did open our air release valve yesterday after some other work and found quite a bit of air. It was brought up that since the location of the air valve almost always has pressure on it (beacuase the air valve is located at the lower elevation hill and has the water column pressure on it from the next higher hill) that our release valve may not work right even after it was torn down and cleaned. The facility staff is aware of this and is keyed in to do a manual drain at least for a while to see if release valve is working or not. We did see a slight increase in GPM (about 20 GPM) after bleeding but still missing about 140 GPM. Since we opened up the line at low point the same day we bled air valve we are not sure which action actually made the difference (I hate it when that happens).
Thanks again for the comments, appreciate everything we can get.
RE: sewer forced main problems
RE: sewer forced main problems
The pigging may very well be our final shot. Would have been far nicer if cleanouts had been installed. Be intersting to see how this all turns out.
A last tid bit for anyone reading this, we have two different design firms and two different responsible parties (city and state) invloved, one for the state from pump station to the low spot and then one for the city from the low spot to the gravity sewer. Of course the problem is in the "other guys" part
RE: sewer forced main problems
You do not mention the velocity in the force main. Velocity criteria for force mains are derived from observations that solids do not settle out at a velocity of 2 ft/sec or greater. Solids do settle out at lower velocities or when the pump is stopped. A velocity of 3.5 ft/sec or greater is required to resuspend the deposited solids.
Try pumping periodically at a higher velocity to clear the line. Get a flow meter to document the flow.
RE: sewer forced main problems
If so, bring in a larger temporary pump to connect and increase the scour velocity to see if that helps.
Also, I don't know how old the system is or what type of pumps you have; but, have you checked the pump to see that it is still operating on it's curve? If it has one and the wear plate is adjustable, any excess gap between it and the impeller will also cause the pump to lose efficiency (ie flow).
RE: sewer forced main problems
Our engineer on the state side of the project (we were responsible for the pumps, commissioned on 3-27-07) noted that we should be getting 3.7 FPS at design flow of 522 and even with the slow down at 380 GPM we should be getting 2.6 FPS,(we have a Siemens meter in the system which only shows GPM and total pumped, upgrading to a better model but hasn't happened yet, we should have a third party come in and verify FPS rate) which if I read above correctly should keep solids in suspension (we run the material through a grinder that reduces solids to 2mm). The design built in a redundant system with two of the same sump and house pump, and pump set A and pump set B cylce trade off run times. With this design we do not have hook-up for an emergency pump but that would be a good idea.
We have a 4x4x11 submersible 40 hp sump pump with a 103' tdh at 1750 and a 4x4x14 torque flow with 170' tdh at 1750 for the house pump. One other item that makes the whole system interesting is the wet well (collection tank) is small enough that it takes approximately 6 run cycles before the water that has left the tank reaches the gravity sewer. The system is designed for a prison facility that we finished a year ago, and the flow during the day is fairly constant and the pumps cycle quite a bit, but at night time there is not much activity. The pumps are not that old and have not been run a lot. throw another log on the fire, the drop in flow began while project was still under the GMGC one year warranty so needless to say there is some third party finger pointing in here, trying to figure out whether a design issue (I don't think so) or a construction issue)
Our engineer for the project was laid off several months ago from the firm that did our civil design, he was nice enough to help us out a bit after lay off but no one here (state) wants to hire him back on to help with this problem. I think it looks closer and closer to going back to the "pigging" web page.
One of the group left thinks that the sump pumps might be running in reverse, so they are going to have a submersible pulled and run just to see if that might be the case (at this point everyone feels we should do all we can to make sure the problem does not involve pigging), luckily the sumps are set up with a fast release mount, just pick it up along guide rails and set it back in the same way.
Again thanks to everyone for the input and ideas, I will pass them onto the rest of the group.
(I mainly do QC work during the construction and warranty period but I enjoy reading this site and was hoping we could get some ideas here, thanks, a lot of good stuff. This is my first big job and I hope they all don't go this way, every other warranty item I have fixed, this is driving me crazy)
RE: sewer forced main problems
RE: sewer forced main problems
RE: sewer forced main problems
good luck
RE: sewer forced main problems
RE: sewer forced main problems
It looks like you have silent checks on the pump discharg. These will plug. You should have a free flow discharge type of check valve.
Is the pump suction plugged? If so, the pumping rate will be reduced.
How long is the suction piping?
Are both pumps pumping at the same pumping rate?
Is it possilbe that the pump check valves are leaking backward?
Obtain the pump curve. The pump should be operating on the pump curve. If not, there is a problem with the pump.
RE: sewer forced main problems
Civilperson, good point about the pumps cycling so often, unfortunately that was a design item that I will look for next time around.
bimr, I am hoping you are wrong about the pipe size, hopefully if we get an outside firm in they will find that pipes are correct or I guess we will have to figure a way to change them. Both pump sets pump the same, and we have checked the valves in the pump house. Yes the checks are spring and we have seen debris already, but not enough to cut us back 140 GPM. Someone said that they thought a regular flapper type valve would beat it's self to death rapidly because it was so close to the pump? We do have flapper checks in the piping from the sump.
The piping from the sump pumps in the wet well is about 40 feet, I guess it is suction line although it runs about 35 PSI when the sump pumps are supplying water to the house pumps which also are running. I do need to have the piping from the sump to the house pumps checked before we do a lot more work (like digging up pipe or running a pig) and I hope we can do that soon.
Everyone who has answered here has been a great help, thanks to all of you
RE: sewer forced main problems
Our those pumps just hanging out in the middle of a room - the discharge header seems to be in the middle of nowhere w. plenty so space on either side...just seems very strange use of space.
RE: sewer forced main problems
The buildings were all pre-cast panels and the pump house is about the smallest design on the whole site, probably something to do with minimum size of pre-cast and design. We also have a huge bioxide tank in the building and there is room for storage and good access for working on equipment. Being a sewer system it is nice to have the room to avoid drips and what not.
Not sure what you mean by NPSHa or NPSHr. As far as sound of the equipment running we don't here grinding noises or cavitation type sounds, and both sets of pumps sound the same with the same output.
One of the ideas with the design is supposed to be ease of maintenance. Being a 1800 person correctional facility the sewer system has to work correctly or there is a lot of trouble. Piping and pumps were placed to be easily serviced, unfortunately finding this issue so far has not been easy. We have a heavy grinder that reduces solids to 2mm, and from what I have heard inmates flush all kinds of items into the system. The area we are having trouble with is after the grinder, so not an inmate flushing issue.
Side note: one of the "tricks" that inmates like to pull is flush all toilets in faciltiy at the same time, haven't ever seen it but hear a lot about it
RE: sewer forced main problems
This may seem like a stupid question but are you sure all the valves are open. We just finished investigating a similar situation for one of our clients and the were 100% sure all the valves were open. After looking at the as-builts they noticed they had a buried valve and it was about 1/2 open.
Good Luck!
RE: sewer forced main problems
a = available
r = required.
NPSHa > NPSHr for you pumps to work.
I am still not following you, but it sounds like you have an inline situation where the sumbersible pumps in the wet well feed those horizontal close coupled pumps in your picture? I am guessing you did this b.c of the high heads? If this is the case, you get into a tricky operating situation and your problem may very well be that your submersibles are not feeding the horizontal pumps w. enough volume and they are essentially sucking the suction header dry...
I may be way off here, but I have a hard time following what you are saying. Diagram of the pumping situation might help.
RE: sewer forced main problems
One additional point to check is the clearance below the submersible intake. It should be installed 7-8 inches off the wetwell floor.
RE: sewer forced main problems
Right now we are planning on doing a check on the piping from the wet well to teh pump house, easiest place to look (and the last place) that we don't have to spend a lot of money or use the pig.
RE: sewer forced main problems
RE: sewer forced main problems
RE: sewer forced main problems
I just dont understand the double pumping (pumps in series) in this scenario? Why not just use biggers submersibles and cut out the horizontals all together? Your paying for a lifetime of double energy cost, double O&M, etc. There MUST have been a reason for this. I think if we understand that reasoning, we might be able to better assess the problem.
Also, solids grinder and auger (comminution)? Why not a mech bar screen and REMOVE solids as opposed to grinding? It doesnt look like space / site constraints were an issue...
WITH THAT SAID - after looking at that, I stand by my initial thoughts. I think the problem is suction side, NOT force main side. With the pumps in series like that, you get into a finicky control scenario, ie.. if the submersible feed pumps are NOT pumping at the same flow as the horizonatals, you will suck your suction dry... Not trying to be rude and dont take this the wrong way, but there are a million different things that could be wrong here..
Good luck my friend....
RE: sewer forced main problems
It appears some respondents believe that the two Wemco main pumps are pumping in series. If I correctly understand the drawings and photos you've posted they are pumping in parallel.
The arrangement is indeed strange, as some have noted, so this may be what is contributing to the confusion.
Please clarify this for us all. You can do this most easily by sketching a schematic diagram of the system.
Any additional information, such as the original design assumptions and criteria may also help to track down the problems you are having.
RE: sewer forced main problems
RE: sewer forced main problems
I live, and have worked in Salem for forty years so suspect the answer must be yes.....or should be yes.
RE: sewer forced main problems
RE: sewer forced main problems
The suction line of the horizontals is filled by the submersibles. The submersibles are located in the 10' dia wet well. Technically speaking, this would put you into a flooded suction scenario, but since your suction is a pressure conduit, you essentially lose all atmosperic pressure which would have contributed to NPSHa...
RE: sewer forced main problems
That's understood, now. The confusion arises in part from the fact that the submersibles are NOT shown on either of the drawings provided. A schematic would really clarify much of this as would the elevations (MSL) of the pumps, wetwell, discharge manhole, top of first hill, bottom of 'draw', top of next hill, etc.
Let's hope the O.P. favors us with this essential information.
RE: sewer forced main problems
RE: sewer forced main problems
The height difference between the bottom of the sump pumps and the house pumps is about 18'.
Right now we are really questioning the air relief valve system, may be the next point of work.
RE: sewer forced main problems
I'll save my rant about government agencies exempting themselves from their own regulations for another time.
Meanwhile, here's some useful infotmation on the proper location of air vacuum valves.
http://w
RE: sewer forced main problems
Whether your pump is installed in the wetwell or mounted on the floor (like your system is installed), you still have a grinder pump in your prison. What is the difference?
One can argue that it is less risk to have the grinder pump in the wetwell because it is easier to remove for repairs than one mounted on the floor.
Having the two (2) pumps installed in series the way your system is installed will be less reliable than having one pump.
My understanding is that most prisons have grinder pumps. Most of these applications are probably in wetwells.
RE: sewer forced main problems
RE: sewer forced main problems
If you had space, the better option would have been a mechanical bar screen, but that generates a entire other waste stream which needs to be discarded. Hence probably why the original designer went w. a comminutor rather than mech. bar screen. But thats neither here nor there...
Has anyone looked at an overal HGL diagram of the system? If you guys are sure the problem is force main side, this would be the place to start...Does the ground profile every pass above the HGL? That is likely where you problem is
However, you still have not indicated if anyone has taken a look at the suction side hydraulics of the house pumps...They way these pumps are set up in series, you have to be very sure that the sumps keep up with the house pumps, if not, you suck the suction line dry. (third time i have made this recommendation) do you understand that?
Also, where is the original designer / design firm in all this mess?
RE: sewer forced main problems
"The design engineer may select variable-speed pump drives as a non-standard feature, with the Owner's approval. Variable-speed drives shall be designed and programmed to provide a flushing velocity in the force main of at least 3.5 feet per second at the beginning of each pumping cycle.
After an initial flushing of the maximum practical duration, depending on wetwell volume, the pumping velocity may be reduced. Velocities shall not be allowed to
fall below 2 feet per second, due to solids settlement, eventual plugging of the force main, and station failure. All variable-speed drives on raw sewage pumps shall be
programmed to maintain a fluid velocity of at least 2 feet per second in the force main after initial flushing at minimum 3.5 feet per second."
The Oregon Standard calls out the velocity of 3.5 fps to flush and resuspend solids that settle out in a force main when the pumps are shut off. The high initial velocity of 3.5 fps is required on pump startup to resuspend and flush the solids. The velocity of 3.5 fps is recommended by a number of sources and will work with typical domestic wastewater.
The prison's initial pmp capacity of 522 gpm was stated as a velocity of 3.7 fps. My calculation indicates that the velocity is only 3.33 fps when operating at 522 gpm.
http://w
The 3.5 fps velocity is probably not enough to resuspend and flush the solids considering that this application is a prison. Prisons are notorious for having considerably more solids than domestic wastewater. Considering the increased solids that one would expect in a prison application, a higher velocity than 3.5 fps is warranted.
It is recommended that you review this situation with the pump supplier to see if there is any way that the pump capacity (velocity) can be increased. Perhaps you can operate both pumps at the same time.
This will have to be reviewed carefully by the pump supplier to determine if the pumps have adequated HP when pumping in parallel.
The use of the muffin monster in this application is probably not a good idea. You should investigate the feasiblity of installing screening equipment to remove the solids. Removal of the solids will assist in keeping the force main from plugging.
You will probably have to have the force main cleaned. Have a sewer contractor clean one section of the force main and then make recommendations based on the results.
RE: sewer forced main problems
As far as the engineering firm involved there are actually two different design teams, one for the city and one for the facility. The actual design engineer for the facility was laid off last fall, of course. I think politics have a lot to do with the mess, and who pays who before any work is done. If we have a design issue we can go to the engineers but we need to know for sure we have no stupid problems (rag in line, valve not open, instrumentation not right) first.
Bimr, I am a bit confused on why the muffin monster is not a good system. We grind up material so it is no larger then 2mm and the auger takes out all material that does go through the grinder and is not fully pulverized i.e. fiber material and other items that squash flat but are still of some size, more on what actually rides the auger if we really want to know
We probably will be looking for a pigging contractor to come in and clean the line if we can't find anything in the next week or so. We will be doing the last checks we can without pigging or digging in the next week or two. We are hoping that we can get the system back up to flow that the design said it would be, and that the system will work, but will not know for a bit longer.
RE: sewer forced main problems
It would be better to remove the solids than to try to pump the ground up solids 1 ½ miles.
RE: sewer forced main problems
One of the first items that was mentioned in this forum on the GPM drop issue was air in the line. Originally the air relief valves were all O.K.'d and declared correct for the system. Now the fun part.
If anyone here is familar with APCO Willamette Valves they already know that these people have some great information on their site on valves. It appears that at the high point in the pressure main (not where it drops into gravity sewer but the high hill in the middle of pressure main) a vacuum/air release valve was installed, an APCO 402. If you go to the APCO page this valve only works once, when filling (air release) or draining (vacuum release). It does not do anything about releasing air when the system is working and under pressure. We are going to install a 400 series added to the 402 and see if that makes a difference. If our issue is trapped air it is probably trapped in a few areas. Does anyone have any ideas as to how soon we should see the system come back to normal flow? We have manually bled the high point a few times and we get a slight increase but we lose it over a day or so. We have just ordered the valve so it is not yet in place. While we need to install the valve to make the system correct I am not 100% sure that it is all the problem.
Anyway here is a pretty neat slide show from APCO, I really like the steel pipe part.
RE: sewer forced main problems
I could not view the slide show with my browser ( Firefox ).
If you don't have this please take a look at it:
http://www.apcovalves.com/pdfs/601.pdf
Note please that sometimes, when the high point is difficult to locate exactly, two valves are used; one in each side if the high point.
Is kentgolding a variety of hops ?
good luck
RE: sewer forced main problems
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
RE: sewer forced main problems
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.
RE: sewer forced main problems
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!
RE: sewer forced main problems
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...
RE: sewer forced main problems
<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).
RE: sewer forced main problems