Force Main Pumping Downhill 8

[tbs]

I have an unusual problem. I am designing a 10 MGD Lift Station and 30” DIP Force Main that is pumping downhill. I am a licensed engineer and experienced in pipeline design, but pumps and force main systems are not my specialty.

The Details--The proposed 30” DIP epoxy lined force main is 12,000 feet long and will connect to an existing 30” DIP mortar lined force main that is 6,500 feet long which discharges into a gravity line. There is a 17 foot drop from the lift station to the final discharge with an intermediate high point that is 7 feet higher than the lift station. The high point is only 1,800 feet downstream from the lift station. From the highpoint to the discharge there are several intermediate peaks that will prevent typical gravity flow. The pipe is epoxy lined to help prevent corrosion and air/vacuum release valves have been place at the appropriate peaks.

When the pumps are on, the line will be flowing full. When the pumps shut off, air pockets will develop at the peaks in the line. The potential for corrosion at the air pockets is high but has been minimized buy the use of epoxy lined pipe.

The concern is the pump performance. If the pipe remained full when the pumps shut off, the pumps would see a constant head when they came on. Unfortunately that is not the case. The pipe will have several air pockets which will have to be evacuated from the line which will cause the pipe to see highly varying heads and could produce shock waves in the line.

I have investigated two possible solutions to keep the line full when the pumps are off.

Solution 1 was to install a valve on the discharge that would close when the pumps are off thus preventing the line from continuing to discharge by gravity flow. This proved to be a possibility, but the criteria are the valve must be mechanical, no electricity, pneumatic, or hydraulic systems will be accepted by the client, and the failure mode of the valve must be open. Red Valve makes a spring loaded valve that fits the need, but they only make it up to 12”.

Solution 2 is to build a vertical “gooseneck” that will create a highpoint in the line to prevent it from draining. The force main would go vertical out of the ground just prior to the discharge to an elevation that was equal to the highpoint of the line and then turn 180 degrees to go back into the ground and discharge into the gravity line. This solution would create a manmade high that would keep the line full at all times which would prevent corrosion in the line and would allow the pumps to see a constant head when they came on. A decent solution, but no one likes the idea of a 30” DIP Force Main sticking up over 20’ feet in the air.

I have been told by some that pumping down hill in this situation is not a problem. If it is, then I have a hard time believing that this problem has not been encountered before and that with modern technology there is not a better solution than having a 20 foot tower of 30” pipe sticking out of the ground.

Any input would be appreciated.

Thanks
TBS

 

[gibfrog]

I have only designed small package lift stations. I have designed about 30 over the last 18 years. I have no experiance in larger lift stations, like a 10 MGD, but this is how I would proceed.

Lets us consider the pumps only, as per your request. The pumps are normally designed to operate over a range because the head is not normally constant in a Waste Water Force Main (There will be other pump stations that will operate intermittantly and change your system pressure). Check both the min and max head conditions when selecting your pump. It would seem apriori that 1,800 LF of 30" line plus a 7' static rise should be more than adequate to maintain a minimum backpressure on the pumps (I assume that there will be multiple lead pumps). If not, consider using a short section of 24" or 18" pipe in the pump station to insure a minimum backpressure for your pumps. (Really a non optimum choice, better to select a pump that can operate within the desired ranges)
I do not see any pressure waves (as long as the air release valves function.) You are pumping at 10-20 fps and the speed of sound is ?400? fps.
If the pipeline partially empties and you can deal with the corrosion problems and your pumps operate with the mfg spec ranges so what if the head varies.??
Remember, to verify the minimum pump on running time at max flow/min head and your design cycle condition for max pump starts per hour.

I would purchase the ASCE manual of practice for pump stations and I would speak to the pump mfg reps. I woulod also search the web. I can help you with a web search if necessary. (Within the next 18 months, I will propably have to design a 5-10 MGD waste water lift station, so I am not completely an altruist.)

I hope that this helps.....

 

[gibfrog]

I forgot to mention tow items -
1) Check out the design of the existing pump station for the existing 6,500 FM (it is probably public record). Speak to the operations/maintenance crew, Get their feedback. Find out what pumps are being used and their operational history. Speak to the mfg rep for the current pumps, get his suggestions. Speak to the engineer who designed the previous pump station (it is on the plans). consider offfering him a consulting fee for a few hours of guidence.
2) Consider using two (or more) different size pumps. For example. Use a small 2 MGD lead pump (this may fill the pipeline and raise your system friction head), then had another small 2 mgd pump kick on, next as the level control trips on, have a 10 mgd pump kick on and possibly have the two smaller pumps shut down (or that can be the fourth level control. The next level control turn on another 10 mgd pump and the last level control, triggers an alarm and turns on all pumps, including the emergency/spare 10 mgd pump. This would be a 5 pump LS. This is one hypothetical method.

 

[gibfrog]

Final Advise - Visit

http://WWW.WEF.ORG

(Water Environmental Federation) bookstore and check out the manual of practice (written in conjunction wiht the American Society of Civil Engineers)
Design of Wastewater and Stormwater Pumping Stations MOPO FD-4. $35 plus s/h. Clifford H Laubstein
FL Registered PE 58662

 

[dangerous]

Howdy,
This is out of my field but a simple solution might be to install a small check valve open to atmoshphere on the high point of the discharge line. When the pump is off, the portion of the line downstream of the check valve would drain but the seven-foot head would be maintained (the pump would remain flooded). The next on cycle would close the check valve. Not sure what the gas implications would be, however.

 

[stanier]

I have designed a similar system for Sri Lanka but for drinking water. When the troubles were on there was no possibility of telemetry between sites 12km apart. The waterhammer analysis definitely shows a problem. The main problem however was the ingress of air when the pump stoppped. Solution was a use of cylinder type control valve Singer, ClaVal, Dorot. This can be set up as a presuure sustaining valve. It maintains an upstream pressure. However you are burning up energy all the time you pump across the valve. Also I take it that its a sewer main so there could be a problem with this type of valve from contamination. Eccentric plug valves are made up to 54" but the contaminated fluid prevents using the head to power the unit. Have you considered manifiolding the line and having 8 off 12" Red Spring loaded valves in parallel?

An alternative could be a tall reservoir at the pump station. Pump up to the resrvoir and turn the force main into a gravity main. Same problem as the gooseneck, who wants one poking out of the ground. Also you are of couse using as much energy.

Could you consider variable speed pumps and a holding tank/pond at the pump site? Balance the flows so that the line is always full?

 

[ejlow]

Have you found a reasonable solution to the problem. If not I can offer a few suggestions. Please advise.

 

[dicksewerrat]

You may want to use a pump that comes online slowly. By slowly I am talking about coming up to full speed in 30 seconds not 10 minutes. This will fill the airpockets slowly and then go to max. pumping. I thought there were some varible speed motors out there. Call a Flygt Pump rep near you or call the offices in Milwaukee for help.

 

[TBS1]

Hello all,
Sorry for the lack of responding. Turns out there was a problem with my log-in. Now back to the problem. Thanks for all the input. The problem still exists and a reasonable solution has not been found. In fact, the project is on hold and the client is not happy. In addition, we have a new lift station project witht he same situation. We are slip-lineing an existing gravity sewer and turning it inot a force main....pumping down hill. This is being done due to inadequate capacity in the existing gravity line. But the engineer is wanting to construct a 30' verticle tower to insure a constant backpressure on the pumps. Again, I have newver seen a lift station with a standpipe. Still looking for solutions??? Thanks

 

[piolom]

There is no need for a standpipe. The hydraulic analysis should tell you what to do. The line will probably change form gravity to pressure during pump on - off cycles, so adequate flushing velocities need to be maintained periodically - should not be an issue. We have engineered systems that operate this way. Create a model of the system, understand how it operates and have a monitoring system in place.

 

[DavidCavender]

The water hammer problem can be soved with a pump control valve system at the pumping station. Use an air over oil check valve to slowly bring the flow through the force main up on pump start and similarly bring it slowly down before pump stop. The only electronic components are a simple PLC and a pressure sensor, since these are located at the pumping station, the owner should approve.

The mortar lined segment of the force main concerns me. The seweage will likely be septic and the mortar will not hold up over a long period, even if the line drains fully between pump cycles.

 

[BobPE]

David:

You have to be careful with those types of valves as they are not pump control valves. They should not be used for transients in shutoff service unfortunately either. Their design is to reduce valve fluctuation during operation. All to often I see them installed when I go to investigate pump failures. People put then in and they do not allow the pipe to check flow in the opposite direction quickly enough since they are damped to the maximum. The goal of a check valve is to close against reverse flow as quickly as possible, addressing transients should be done by other means.

BobPE

 

[aussiemike]

G'day All.

Allow me to throw in my ten cents worth....

I have just finished the design of a 590L/s sewage pumping station near Sydney - currently under construction. The pumps are 180kW (2 duty / 2 standby) with variable speed drives. The flow is level controlled within a range 120L/s to 590L/s.

The pressure main in our system is primarily 750mm MSCL and is around 7000m long. It contains two HDPE HDD sections, where we drilled the pipeline through hill and under a harbour. The discharge point is at the inlet works of a sewage treatment plant, which is around 6m lower than an intermediate high point. The end of the pipeline (inside the treatment plant) has a 6m high "barometric loop" or "gooseneck" in order to ensure the pipeline stays full at all times. The downgoing leg of the barometric loop is designed as a tangential entry drop structure in order to dissipate some of the energy upstream of the inlet works.

Downhill pressure mains are always a pain in the backside - particularly if they carry sewage. My advice to you is solve the problem hydraulically with a "gooseneck" and forget your problems. We spent about $2 million drilling through a hill near the pumping station in order to keep our "gooseneck" down to a reasonable height. We could not quite eliminate it, but the next highest "high point" was much lower than the hill. Any chance of doing this in your system?

I would recommend you put your standpipe / gooseneck at the discharge end if at all possible. If you put a standpipe at the upstream end (which we are doing on an ocean outfall on the same project) you take the risk of having a shower of s*** if the pipe becomes blocked or someone leaves a valve shut. Our outfall standpipe is higher than the shutoff head of our outfall pumps (low head high flow) so there is no chance of an embarrassing incident.

If you have a large diameter sewage pressure main that wants to flow part full you are asking for trouble. One major problem we found was the scary amount of odorous air that would be evolved from the airvalves each time the pumps started. Each high point will drain downstream to the next high point while filling through the air valves.

Another significant problem will most likely be water hammer. Have you done a full dynamic water hammer analysis? Fluid column separation could be a real problem in a large diameter pipe and you could have some very unpleasant pressure spikes. This will almost certainly be much much worse if you have some parts of the pipe flowing part full, but run a model just to be sure.

If you don't put in a gooseneck, check out the volume of your wet well and see if it has enough volume to fill the pipe on each pump cycle. With your size pipe I'll bet it doesn't. This means you will never get your pipeline full and you will always have trouble with partial flow and unstable hydraulics.

Oh yes, I should also add the problem you will have with corrosive gases being evolved from the free surface and attacking your cement lining. This of course, will be greatly accentuated if you allow suction pressures to be pulled on the sewage as the air screams into the pipe through the air valves every time your pump stops.

My advice to you is keep it simple and keep your hair.

Cheers
Aussie Mike

 

[stanier]

Hi Aussie Mike,

Guess your working on the Illawarra project. Are you with Vivendi, PPK, Walters? or other. Who has done the transient analysis for you? I'd be interested in what program you use.

I was part of one of the consortiums that didnt get that job so know well the hills you speak of.

 

[KRSServices]

I have read with interest regarding some of the threads posted here. I have not had experience with large 30" systems, but have had plenty of war stories with 12" and smaller systems. Aussie makes a very good point with air evacuation...either evacuate well away from complaintive noses or invest in an odour eliminating device.

Secondly, with reference to the "gooseneck", I have utilized the services of an in-line check valve to keep the system charged. Further, I utilize soft start/stop control pumps to reduce the impact of hammering. This was with a 12" sewer forcemain. More importantly, the hyraulic analysis should be able to identify the correct direction your solution takes. Good luck. KRS Services

http://www.krs-services.com

 

[aussiemike]

Got it in one.

I'm with Parsons Brinckerhoff (formerly PPK). I was design team leader for the transfer systems, which includes all the major pumping stations and pipelines. Also had fairly significant input to the ocean outfall and outfall pumping station.

Illawarra was/is a great job to work on!

We did our own water hammer analyses using a program called WatHAM. I don't actually know how to use it myself so I'm certainly no expert on how it works.

The dynamic simulation function is very useful. It is interesting to watch how the surge waves bounce around and occasionally superimpose to create huge spikes. As soon as you get column separation the surge waves go ballistic!

It is also interesting to observe the effect of different pipe materials on the peak surge. Cast iron and ductile iron can give you some pretty hefty surge pressures compared to softer materials.

We didn't end up using any special surge arresting air valves because we didn't need them.

I'm actually designing another "up and down" sewage rising main at the moment in Queensland. Luckily it is only 150mm, although it is 10km long. I reckon we'll use a control valve on the end of this one to stop it draining when the pumps stop. The hills are too high for a loop!

Bye All.

 

[stanier]

Hi Aussie Mike

I am doing the waterhammer analysis for the Sewerfix Pumping Stations Upgrade, over 80 stations so far, all materials etc as well as a steady stream of work from other consultants and contractors. If you ever need any QA or second opinions on your waterhammer send me a message blenrayaust@yahoo.co.uk

 

[NumberCruncher]

Two important notes:
1. My region (South Mississippi, USA) has experienced the failure of a 30" force main due to corrosion of mortar lined pipe. Get the best interior coating that you can for the high spots. Use regular lining for non-high spots.
2. Perform a good hydraulic design. Use extensive manual calculations or try PS-1 Pumping Station Design software. Get a full featured trial version at CivilWorld.com.

 

[Gordonba]

As long as you have a flooded suction, you should be able to deal with all head variables using a VFD with your motor.Limit the RPM in relation to head pressure.

 

[mou]

Hi all,

Ive got something very similar whereby i am pumping downhill from the high point (which is at the pumping station). There are intermediate high points along the way along with a river crossing.

The only time the force main will be running full will be during ultimate wet weather conditions otherwise it will be partially full (ie pump cycles are not enough to fill the main). If you design the pumps assuming full pressure main i get a flow at a certain head. But if i use this same pump under partial flow conditions i assume the head will be much lower and therefore the pump could be pumping near (or past) its maximum allowable. So how the hell do you design the pump ??. Would variable speed pumps solve this problem ??. Designing barometric loops is out of the question, operators are not keen on a control valve, the route cannot be modified. The only way i can see to account for the possible unstable hydraulics is to have variable speed pumps. Is this logic correct. Any comments would be highly appreciated.

Cheers

Mark