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]

Oops, here are the two drawings if this works.

 

[kentgolding]

and the sump pumps

 

[alexcmmi]

Wow, that design is 'interesting' to say the least. I think i would have done things JUST A BIT differently (read: it wouldnt look like that all).

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....

 

[RWF7437]

Kentgolding,

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.

 

[RWF7437]

please also include the design operating "rules". Which is the lead pump ? Which is the lag pump. At what elevation do the lead, lag and both pumps come on and go off ?

 

[RWF7437]

Also, also.... Did the Oregon DEQ review and approve these plans ?

I live, and have worked in Salem for forty years so suspect the answer must be yes.....or should be yes.

 

[RWF7437]

http://www.dewberry.com/uploadedFiles/Wastewater Pump Station Design Problems and Solutions.pdf

 

[alexcmmi]

RWF7437 - Its not the two WEMCO pumps that are operating in series - its the submersibles (listed as 4x4x11 a few posts above) and the horizontals (WEMCO 4x4x14) that are in series...

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...

 

[RWF7437]

alexcmmi,

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.

 

[RWF7437]

http://www.deq.state.or.us/wq/rules/div052/guidelines/designwwps.pdf

 

[kentgolding]

Thanks for all the info so far, sorry I have been out and about a bit. At this time we still have not found our problem. A litttle clarification on what I know of the project. Being a prison we are not willing to risk a grinder/pump unit, as inmates send far more items down the system than the normal public (although I bet some would argue this). As far as the sump pump and the above ground pump instead of just one pump I cannot give you an answer on that. The system is set up to alternate between set A (house pump and sump pump) and set B (house pump and sump pump) the idea is so that both sets of pumps will wear out at the same time (well not really but that is the general consensus of the install contractor and I sort of have to agree). If a sump pump fails then the other sump pump (SP) would kick on but the same house pump (HP) would continue pumping ( example, HP A and SP A are running and SP A fails then SP B would kick on with HP A still running). As far as approvals and what not goes, I am in a QA inspection position with the state, so the upper paperwork stuff was done and gone long before the issue settled toward me.
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.

 

[RWF7437]

Thanks for the update Kentgolding.

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://www.pump-zone.com/valves/valves/theory-applications-and-sizing-of-air-valves.html

 

[bimr]

"Being a prison we are not willing to risk a grinder/pump unit, as inmates send far more items down the system than the normal public (although I bet some would argue this)."

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.

 

[kentgolding]

Actually we have a seperate grinder unit that is in the sewer before it gets to the pumps. It is a "Muffin Monster" grinder that pulverizes everything before it even gets into the wet well. The pumps will "grind" and pass large objects (up to 3" I believe) but should not ever have to do this. More redundancy in the system, but sort of the way all our facilities have been built.

 

[alexcmmi]

The muffin monster is a comminutor. They are common at these facilities and at pump stations.

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?

 

[bimr]

After reviewing your posts again, one can only conclude that the force main is plugging with solids. As stated in the Oregon Standards for Design and Construction of Wastewater Pump Stations May, 2001:

"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://www.pumpuniversity.com:8080/...a.jsp?id=7&name=Pipeline+velocity+calculation

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.

 

[kentgolding]

[tab]Thanks again for the input. Alex, at this time we have a pressure gauge on the piping between the sump (wet well) pumps and the house pumps and whenever the pumps are running we have 35 PSI showing on the piping between the pumps. Next week we plan on opening up this section of pipe to TV the line between the sump and the well house.
[tab]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.
[tab]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. The auger dumps material into a dumpster so it is not pumped. I do think that we could still have a suspended solid issue but not sure how to completely do away with it. Anyway I believe we have used the auger/muffin monster set up at all the facilities. Not an easy item to change.
[tab]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.

 

[bimr]

The objective in wastewater treatment is to remove the materials, not necessarily to grind up the materials.

It would be better to remove the solids than to try to pump the ground up solids 1 ½ miles.

 

[kentgolding]

Been a bit since I had anything to add here; we are still fighting the issue. Bimr, we use the auger to remove most of the solids that go through the grinder and they are dumped into a dumpster which then goes to the local landfill. Sounds appealing doesn't it though.
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.

 

[RWF7437]

Thanks for the update Kentgolding.

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