Can you connect two sanitary sewer force mains with a "Y"? 4

[JeromeCDNP]

I am "newbee" designing a sewerage system for a small subdivision where all of the sanitary sewer system is gravity feed with the exception of 9 estate lots. These lots are located down a hill from the main gravity sewer system on two different roads that intersect before the gravity system. The maximum elevation difference between the furthest homes and the gravity system are 25' and 19'. I was told that a 3" force main down each road would be sufficient if each home utilized a E/One Grinder pump tapping into the respective 3" force mains and each force main flowed into a sewer manhole at the roadway intersection then gravity fed to the main system (there is approximately 1%of fall at this point-and about 3ft of cover over the pipes). This might work but I don't have elevation difference for the typical (4' diameter 6' deep gravity manhole at the itersection. Would it be possible to use a "Y" at the intersection and go farther up the hill to a depth where a "typical" manhole will fit, then gravity feed to the main system? Any insight would be appreciated...

 

[Maury]

If you could count on only one pump operating at a time, it would be no problem. But if both are running, each pump would see a higher head condition. Evaluate the pump curves for each situation. It may not be possible to come up with a pump that can handle both situations.

 

[BobPE]

Yes you can. But I read your comment and the most immediate concern I see is the size of the forcemains you are considering. With that small of a loading, those mains will go septic and will most likely plug from lack of scouring velocity.

The trick to grinder pump design is determining core loading, that is how many pumps will be on at any one given time. You are pumping to a gradient established by the pumps, so as long as you are using pumps of a similar head/flow curve family, there will be no problem with pumps coming on and off while other pumps are on or off.

If you are considering environment1 then I suggest you look at their web site, I believe they have low pressure wastewater designs hints there for people to consider.

BobPE

 

[stanier]

The Eone pump is a quasi positive displacement pump that uses thermal overload to trip if the pressure is too high. I believer that they may have changed this for a cutreent overload. In any event the number of pumps operating will depend on the head in the system. When one pump trips as it overloads then the others will continue. A timer prevents them restarting. After a time the pumps retries.

The Eone design parameters use a statistical analysis of the number of pumps operating ina system at any one time. You look at each leg in a dendritic system to determine the size of pipe for scour and slime control as well as maximum velocity. However you dont add up all the pumps operating in the sub systems to get the total flow. The statistical average is used at every section ( they call them zones)too determine the number of pumps and hence flow. Each zone must meet the maximum velocity and slime control criteria.

A recent system I looked at with 350 pumps has a total of 38 pumps operating when you count up all the zones but only 15 when you consider the 350 pumps.

The flow rates are surprisingly close to the flows determined using the EP and peaking factor design parameters for gravity system. of course you dont need to allow for strom flows or gravity infiltration in these LPS systems.

E One are not the only manufacturers of grinder pumps. There are Barnes, ITT Flygt, Mono Pumps, ABS and others. The key to using them is who will service them in your area!

 

[KRSServices]

Has someone else designed the forcemain sizing? With only 9 homes in total, the forcemain may be oversized because it is unlikely that the systems are going to be discarging at once, therefore there may be a small risk of the line going septic. Notwithstanding that, you asked about connecting the lines with a Y - connector. Alas, I am slightly confused, are you referring to the connections of the "service" pipe to the forcemain, the connection of one forcemain to another or the connection of the forcemain to the gravity system?

KRS Services

http://www.krs-services.com

 

[JeromeCDNP]

thank you all for your insight to this quandry and to answere a few questions and shed some light on this dilema...

we were contracted to complete the infrastructure for the subdivision; however, each estate lot owner will be required to complete the service connection to the sewer system (force main) including the sizing of the grinder pump. Therefore, we can only recommend what type of pump to use and pumping curves.... based on similar designs utilzed in the 1st phases of the infrastructure design by another engineer we were directed to maintain a simimilar grinder pump (as is the Eone was used in the earlier effort)...

to answere another question... two force mains will connect at the road intersection (hopefully utilizing a "y")and with then flow uphill to a previously designed gravity system...

Based on Stainier's and BobPE's responses, I am a little concerned that the 3" diameter pipe may be a bit oversized and I will achieve the flow rates required to scour the pipes... I will review the information on the website is there another location or other resources that I may read to learn more about low flow force main design...

Thanks again for all your help,

JeromeCDNP

 

[BobPE]

JeromeCDNP:

You brought up another interesting question is your most recent post. The use of different pumps in the system. Although this will work on papaer, you should know that the Eone pump is capable of rather high heads that can cause other pumps to approach shutoff head if it is the E1 pumps that are establishing the system gradient. What this means is that those users that have other pumps will have long run times because of the low flows as their pumps attempt to overcome the gradient established by the E1's. Ususally there is adequate volume in the wet wells that the non E1 pumps clear retained flows over night when the gradient is lower. The problems I have seen is premature non E1 pump wear out and residential wet well overflows....

Now regardless of how the pumps got in the system, the design engineer WILL be blamed...keep that in mind and make strong recommendations to users based on engineering.

BobPE