Pump Station with Gravity Connection 1

[gordini]

This is a challenge to me because this is my first pump station design, however I'm sure that this has occured once or twice before to somebody. OK, here it is:

I have a package pump station design with a peak operating flow of about 250 gal/min. The force main (4" dia.) discharges into an existing gravity main (10" dia.) about 1400' feet away. My problem is that there is an elevation peak in the line occuring midway that is about 30' higher than the pump station and gravity connection.

As far as I can tell, I have three options:

Option 1 is to terminate the force main at the peak and install a manhole connection. From there I could run a gravity line and tie into the existing gravity main. The problem with this is that it will be more expensive. It requires a deeper excavation (10'-15') to ensure service for any other properties that may want to connect to it in the future.

Option 2 is to install the 4" force main all the way down to the gravity connection, install an air release valve, and just let the last part of the line function as a gravity line during the pump off cycle. This is proably the most economical solution except for I don't know if this will cause any corrosion or pipe cleaning problems.

Option 3 is to install the 4" force main all the way down to the gravity connection, install some type of spring-loaded valve at the end that would close during the pump off cycle and then reopen during the pump on cycle. This way, the line always stays full until the pipe discharges. The only problem is that I do not know if there is a mechanical valve available for this type of application. Also, I'm sure that a larger pump is required for this since I would be inducing a restriction at the end of the line.

Any comments / suggestions on which option is best or if there is another option I am not considering?

 

[kchayfie]

I've worked with some systems that fit the description of option 1 and some systems that fit the description of option 2. As usual, there are pros and cons to both options. In terms of whole life cost, an option 1-type design might prove to be a cheaper solution as you're pumping over a shorter distance and depending on the topography of the surrounding area, future connections might not need to be pumped at all. It depends on the specifics of the system you are looking at.

 

[dicksewerrat]

If you own the system, option one will be a better fit. You can charge new properties to connect. sort of like a gov't agency has a charge to connect. It is an offset for the gov't because they haven't been able to assess the proterty for the improvement, sewer access.

 

[TerraSouth]

I'm partial to option #2. Use SDR-26 pvc pipe (or approved equal) and forget about the corrosion. If anything the air release valve might need to be replaced in a few years.

 

[bimr]

You should be thinking about using 3-5 ft/sec velocity for a force main.

Option 3 is the most economical way to go. Contact GA Industries for the discharge valve. They make such a device.

Try to stay away from air release valves on such short lines. They will be maintenance headaches and burp and spill.

 

[stanier]

Could you possibly direction drill through the high point or do you want to connect properties in the region?

 

[Chris73]

hi gordini,

I don't understand the point of the valve in option 3. Why would you want to keep the pipe full at all times? Take option 2 and off you go. Instead of a air release valve you will need a combination air valve to allow air into the pipe after the pumps stop. This will allow draining the downhill pipe section but prevents siphoning the uphill section. Since you don't have any service connections in your downhill section (I assume) - why does it act as a gravity sewer? There will be no flow after the pumps stop and the pipe section is drained. If you plan for service connections - then yes. But they need to be pumped into the forcemain. Never go smaller than 6" for a gravity sewer.

 

[francesca]

The decision probably won't rest with the design engineer, but actually with the local authority who will no doubt own the line. They're most likely to insist on Option 1, because it has the lowest long-term cost. 10-15' is not very deep for a gravity line at all.

The length of forcemain isn't long enough to be concerned that it'll go septic in the line, but if it lingers long in the lift station then it will, and that air relief valve could get pretty stinky.

 

[yogibear1]

Option 2 is very good and stainer has a good point about possibly directional drilling through the high point (to flatten out the profile). If you would drill, then use the best quality (HDPE) pipe possible since it will be deep and harder to maintain. You would not want to gravity the downhill portion since there would have to be manholes every 300 to 400 ft. If you lay the typical force main with 3 to 4 ft of cover (as we do in Florida), the ARV at the high point should be a combination air/vacuum type. We use either ASTM 2241 SDR 21 or AWWA C900 SDR 18 for this type of force main and have great success.

 

[bimr]

Automatic air-release valves should not be installed if their use can be avoided. From past experience, it has been found that the valves require frequent maintenance in order for them to function as intended. In many cases, manual air valves could be used instead of automatic valves. In general, automatic air and vacuum valves should not be used on sewage force mains, but are acceptable for use on treated wastewater. From "Wastewater Engineering: Collection and Pumping of Wastewater by Metcalf and Eddy"

There are some other considerations as well:

1. If you intend to drain the section by gravity, you will need to slope the pipe such that you maintain a velocity of 2 ft/sec or greater to prevent solids from settling out.

2. If you allow the air to enter the pipe, when you purge the air out, you will create H2S odors at the discharge point.

3. Allowing air to enter the pipe will create the potential for water hammer as the air cavity collapses. In this case, the elevations are not very great so it should not as big of a concern as with larger force main pressures.

To address Chris73's comment on the valve in option 3, the positively controlled valve is used to minimize the potential for water hammer that can occur if the force main elevation rises sharply from the pumping station. During the initial downsurge following power failure or pump shutdown, the absolute pressure may drop to nearly complete vacuum, resulting in the formation of vapor cavities. The two portions of the water column on either side of a cavity would then behave independently of each other, coming to rest or slowing down and reversing. If any of the reversing columns should come against a closed valve or stagnant water column, the resulting water hammer pressure increase could be high. Therefore, the answer is that the valve is used to prevent water hammer.

 

[yogibear1]

ARV's are necessary and recommended at high points in any pipe line to release entrapped air (meaning any gas). If the air is not released, a pocket will form and prevent further pumpage. Indeed, sewage ARV's are bothersome, esp if the ports are small. Manual ARV's do work well as they are large ported. This force main will have two dowhnill components: one part going back to the pump manifold and the other part will drop into the receiving manhole - right? So the drop into the manhole will naturally release air and odors at that point. The run back to the pumps should stay full because of the check valves at the pump discharge. You will not have a water hammer when there is such a drop into a manhole. Lastly, depending upon conditions in the downhill run a vacuum could be generated, which if enough can implode the pipe (granted this rare, but it does happen every now and then); hence it would be prudent to have a combination ARV at the high point.

 

[bimr]

yogibear1,

Not really sure why you would argue with Metcalf & Eddy's recommendations regarding automatic air release valves. Metcalf & Eddy wrote the book.

Spilling wastewater may be more than just bothersome, it happens to be a reportable NPDES violation. For a 4" line, with limited elevations, why would you bother with an air release valve.

 

[stanier]

If we are talking about a 4" line get on and direction drill it and solve all your problems. I thought this was a real pipeline.

I am currently working on a DN1000 line with the same problem. Luckily not sewage but raw water so air valves can be used but the solution is a dispersion cone valve (Glenfield and Kennedy) at the end of the line to keep the HGL above elevation.

Air valves on sewage are used successfully in S Africa, Australia and SE Asia but use real ones with science and engineering behind them.. Ventomat RGX type. Yes any valve will need maintenance. Fats and greases build up on floats and block orifices. With due respect to Metcalf and Eddy USA technology has always lagged behind the rest of the world. Consultants are highly influenced by local manufacturers and write specifications to support Fortress America.

Yes odour is a problem depnding upon detention times in the reticulation sytem and pumping station. We use odour scrubbers or eliminators to slve the problem.

"Dilution is the solution to polution"

See the technology at

http://www.ventomat.com

that was developed by Australian and South African engineers and scientists.

 

[Chris73]

to bimr,

I understand the systematic of waterhammer and what to do normally. And I agree that the upslope section of this forcemain can be subject to waterhammer that needs attention. But I was questioning the usage of the mentioned: (cit) "...spring-loaded valve at the end that would close during the pump off cycle and then reopen during the pump on cycle. This way, the line always stays full until the pipe discharges..." The downhill section of this forcemain will never contribute to any waterhammer. And there is no reason for keeping the pipe full in the downhill section. Agreed?

There are pretty smart sewage-air valves on the market, even here in North America and I would be cautious to regard M&E as gods. I know why...

 

[rcooper]

gordini,

I have heard of a solution if the pumps are fixed speed rather than variable speed, but have not tried it. If you can maintain a high enough velocity in the downward section of pipe, the water will entrain the air and carry it through the pipe to the discharge, hence no air release valve or manhole. I have seen an equation for the minimum velocity based on the pipe slope. Has anybody tried/gained experience of this?

We have put a pressure sustaining valve on a wastewater rising main in the past and it caused no end of problems.

 

[bimr]

Chris73,

The ideal solution is to generally keep air out of pipelines and keep the pipelines full at all times. Go back and reread the comments earlier.

It seems obvious that it is difficult to get air out of any pipeline. If you have air in a pipeline, you will get a situation that the Chem E's call two-phase flow where you get unpredictable flow situations. The capacity of the pipeline will also be reduced in a two-phase flow situation.

You might also consider that in the real world, all pipe installations are not perfect. You may have high or low spots in the pipeline that will trap air.

One final option for the designer is to use a variable speed drive and operate the pump station continuously. You can attempt to match the flow rate with the pumping capacity and eliminate most of the start/stop piping problems.