Pumping Downhill

[Overbudget]

A sewer interceptor presently runs down a hill to a river bank where a pump station is located. The pump station pumps through a force main under the river which discharges into a manhole that is about 10 feet higher than the river. When the river floods, the pump station is inaccessible. The owner wants to relocate the pump station to the top of the hill where it will be easily accessible at all times.

The proposed pump station site is about 60' higher than the river and about 50' higher than the discharge point. The total length of the force main is about 7,400 lf, so the overall slope is about 0.0067. We could almost design this as a gravity sewer with an inverted siphon at the end, except for two things: 1) the last 1500 LF of the force main is gradually rising and 2)The owner wants to re-use the existing force main under the river - a single 12" heavy wall cast iron pipe. (we took wall coupons and ran a camera in it and it looks to be in good shape.)

Putting the pump station at the top of the hill would pose several issues: 1)80% of the volume of the force main will drain by gravity. With a conventional pump station, the amount of water pumped with each pumping cycle will be miniscule compared to the total volume of the force main. By the time that the batch of water reaches water at the bottom, it will have transitioned to open channel flow and we won't get a strong impulse of water to create scouring action in the force main under the river. 2)when the pump shuts off, there will be a siphon that will continue to draw water through the pump. 3)We will have to be careful about air valve placement in the force main.

Instead of a pumping station with pumps, what about a somewhat larger tank at the top of the hill, with a motorized or pneumatically acutated valve to allow a batch of water to flow out when the tank fills? I haven't looked at a hydraulic profile yet, but I believe that the flow could be driven by gravity alone, and sufficient velocities would be developed to get scouring velocity. The trick would be to size the force main correctly. Mechanically it could be very simple, and energy usage would be very low. The batch could be considerably larger than what you would pump in a typical pumping cycle, possibly large enough to fill the entire force main and give it a good flush each time. We would need a substantial air valve at the bottom.

Another idea would be to install an acutated valve at the location of the existing pump station, and allow a new "force main" to surcharge all or part way to the top of the hill. When full, the actuated valve would open and let the entire line flush through. This approach would elimminate air and water hammer issues.

We're still conceptual. In fact I haven't written the design scope of work yet. Could anyone offer any ideas or call out any issues that I'm missing?

thanks,

Ed

 

[77JQX]

Have you considered a dosing siphon?

 

[bimr]

Consider the installation of a pressure-sustaining valve on the discharge to keep the line from draining.

Ross makes a 70SWR-BP — The wastewater back pressure sustaining valve is a combination direct acting and hydraulically operated control valve that will keep the line from draining.

http://www.rossvalve.com/assets/Ross_General_Products_Brochure_2010.pdf

GA Industries, LLC used to make a similar valve.

 

[Overbudget]

The pressure sustaining valve seems like a viable option. We could set it to hold back the static pressure due to the elevation change, and then the pump just needs enough head to crack it open and overcome the friction losses. I think that this would be a job for a constant speed pump.

I haven't looked at the literature yet but I believe that these valves can be configured to be entirely hydromechanical, because there is no electricity at the discharge location. Maybe put in a solar panel for radio communications, some pressure switches, etc. We could put in a reverse pump to generate a little electricity too, but I'm getting ahead of myself.

I think that the Owner and my manager would be suspicious of any approach that doesn't involve pumps. As my manager points out all the time: in consulting, perception is as important as the facts.

Thanks

 

[LittleInch]

I've been trying to work this out and maybe it's me, but I really can't see why the pump station is there in the first place if the end point is lower than the start point? Surely the flow is going to be same whether you put a back pressure valve or not?

If you can sketch this out in terms of a profile and also explain how this works in terms of flow - continuous?, intermittent? via the pump and presumably some sort of tank/pit upstream, then it might become clearer as your 3rd and 4th paras just don't make sense to me.

I can understand that you need some sort of minimum velocity to make sure the low point doesn't fill up, but this volume / flow rate will be the same regardless of whether the whole pipe is full or not, no?

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[Overbudget]

Thanks, little inch.

It is sanitary sewage. Flow to the pump station varies widely throughout the day. With wet weather, the peak flow can be 10x the average daily flow. You have to size the pumps and force main for the peak flow. Average or below average flows are stored in the wet well until enough accumulates to run a pump cycle. During low flows, the pump may be off for 30 minutes and run for 2 minutes.

The force main requires a scouring velocity of 3.5 ft/s to mobilize grit and solids that settle to the bottom of the pipe when there is no flow.
The average flow is not enough to provide this.

If we use an 8" pipe from the top of the hill down, the volume inside that pipe would be about 14,000 gallons, which will be empty between pumping cycles. An 8" pipe needs a flow of 550 gpm to get 3.5 ft/s. If the pump runs for 3 minutes, it delivers 1650 gallons. 1650 gallons in an 8" pipe that is 5500 feet long is going to transition to open channel flow and spread out over a longer lenght of pipe. When this flow hits the staionary water in the pipe at the bottom, the only energy it has is from the velocity rather than from the elevation, and that will not produce much velocity in the much larger volume of water laying in the low spot. That's why I used the term impulse. (Mass x velocity)

 

[bimr]

Littleinch:

Regarding "I've been trying to work this out and maybe it's me, but I really can't see why the pump station is there in the first place if the end point is lower than the start point? Surely the flow is going to be same whether you put a back pressure valve or not?"

It looks like the OP's client is asking the same question, why was a lift station constructed in the flood plain instead of just running a force main the entire length?

 

[Overbudget]

Yeah, I don't have the history. It was built in the 1970's. I'm speculating that the present PS location would have allow other connections into the gravity sewer coming down the hill, but that never materialized.

 

[LittleInch]

Ok, that makes much more sense now.

I used to use a valve from FMC controls which was entirely hydro mechanical - no electricity required.

http://www.fmctechnologies.com/en/M...roducts/ValvesLineAccess/200SeriesValves.aspx

I'm sure there are others or you can use the actuator on a different valve - looks similar to the one linked by bimr. Making sure the sensor lines don't get blocked can be a bit of an issue though so you would need to come up with some sort of filter / flushing mechanism to avoid this.

Only other option is to maybe use the gravity sewer as your "tank" and put a valve close to the river let it fill up to part way up the hill then open it and let it flush away before closing it. Would probably need too much instrumentation / power etc but you might be able to do it via pilots to open at one pressure (high) and close at another (low). Jut a thought.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[bimr]

LittleInch, the valve used in these applications has to be able to handle the solids. There are many valves that can accomplish this task on clear fluids, but not many are designed for wastewater. It is a good idea to put a bypass around the valve in case it needs to be serviced.

 

[Overbudget]

I'm looking at two possible solutions.

1. I did find a mechanical relief valve for sewage. It uses a spring to oppose the line pressure, but still has pilot pipng to control the closing speed. The pilot piping is oil filled, and it uses a diaphragm chamber to transmit line pressure to the pilot piping.

http://www.singervalve.com/pressure-relief/a106-dl

This valve could go right at the end of the force main to keep the force main full of water all the way up to the pump station. Then the pump station would only have to create enough pressure to open the valve and overcome friction losses in the force main.

A mechanically operated valve is necessary because there is no eletricity at the discharge point.

2. The other solution would be to have an acutated valve at the location of the existing pump station at the bottom of the hill, before the force main crosses the river. There is electricity at this location. In this scenario we might not even need a pump station! We could let the force main fill up with water from gravity flow, and then open the valve and let the entire force main flush through. To keep it simple, this could be a pneumatically acutated pinch valve. It could be configured with normally closed solenoid valves in the air lines, so that if there is a power failure, everything will fail open, allowing continuous flow at the rate that flow is coming down the hill. You wouldn't get scouring velocity under the river, but you could live with that for a time until a problem could be corrected.

 

[bimr]

My opinion is to go with the pump station.

Option 2 sounds attractive but will come with operating problems. Unless you have a tank at the top of the hill, then you will drain the forcemain which will allow air to enter and then you have to remove the air. The flush cycle will only be for a few minutes as well. It is also difficult to push air/air bubbles downhill. Downstream processes will also have to deal with the variations in flow. Should you have solids settle in the forcemain, you have no pump to push the solids out.

Forcemains can be operated continuously at 2 ft/sec velocity. If you stop the forcemain and allow the solids to settle, then you need to restart the forcemain at 3.5 ft/sec or higher to resuspend the solids.

 

[katmar]

Perhaps you could combine your latest Option 2 with the tank at the top of the hill, which you mentioned in your first post. The pinch valve could still be positioned at the site of the existing pump station (or maybe a bit upstream to be above the flood line) and control the level in the tank between 2 limits. In this way the line coming down the hill would always remain full of water and you would have no concerns about air in the pipe.

If it is important that the section between the river and the manhole should remain full of water it can be done with an inverted p-trap just before the manhole - a cheap and obstruction-free pressure sustaining device. Don't forget the siphon breaking vent at the top of the p-trap.

This should work without any pump at all, but if your boss and client really want a pump you could put it under the tank and pump down the fill - still controlling between the high and low limits in the tank.

Katmar Software - AioFlo Pipe Hydraulics

http://katmarsoftware.com

"An undefined problem has an infinite number of solutions"

 

[bimr]

There are problems associated with a tank:

[ul]
[li]The problem with a tank is that the solids will settle out unless mixed.[/li]
[li]A tank also has the possibility of odors and freezing weather may be an issue.[/li]
[li]You would also need pumps to put the wastewater in the tank so the tank option also has the highest electrical operating cost.[/li]
[li]There is a security issue as well.[/li]
[/ul]