Force Main Pumping Downhill
Force Main Pumping Downhill
(OP)
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
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





RE: Force Main Pumping Downhill
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.....
RE: Force Main Pumping Downhill
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.
RE: Force Main Pumping Downhill
Design of Wastewater and Stormwater Pumping Stations MOPO FD-4. $35 plus s/h.
Clifford H Laubstein
FL Registered PE 58662
RE: Force Main Pumping Downhill
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.
RE: Force Main Pumping Downhill
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?
RE: Force Main Pumping Downhill
RE: Force Main Pumping Downhill
RE: Force Main Pumping Downhill
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
RE: Force Main Pumping Downhill
RE: Force Main Pumping Downhill
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.
RE: Force Main Pumping Downhill
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
RE: Force Main Pumping Downhill
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
RE: Force Main Pumping Downhill
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.
RE: Force Main Pumping Downhill
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
www.krs-services.com
RE: Force Main Pumping Downhill
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.
RE: Force Main Pumping Downhill
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
RE: Force Main Pumping Downhill
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.
RE: Force Main Pumping Downhill
RE: Force Main Pumping Downhill
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
RE: Force Main Pumping Downhill
Variable speed pumps are not the answer. To illustrate this point if you plot your system curve against the pump curve with the HQ parameters for various speeds you will notice that there are portions of the system curve that are relatively flat near low flow rates. Now imagine trying to control the speed of the pump at a low flow rate where a small change in speed gives you extreme changes in flow.
The rule of thumb has been variable speed drives should be used where the static head represents 50% of the total head. Impossible to do when you have negative heads.
In porevious postings there are references to goose necks to provide the head on the pipeline. This is ok if the head required is small. turning the main into a pressure/gravity combination by having a vent at the high point is common however as Aussiemike mentioned there will be odour problems. But if you have a control valve at the end of the gravity section to maintain the line in a flooded condition then the pipe will remain full and the amount of air reduced considerably. On a very large main you could consider using reverse rotating pumps to generate power as a means of keeping the line full. Warman designed a such system for Bougainville Copper Mine to obtain energy from the tailings line.
Control valves at the discharge are another alternative in getting the hydraulic grade line above the pipeline profile.
Merry Christmas and a happy New Year
RE: Force Main Pumping Downhill
Thanks for your response. Much appreciated. One other question. How would you design the pump head for such a system (assume the orginal scenario with no gooseneck, control valve etc). Would you assume the scenario where the pipe is fully pressurised (ie steady state equations). Or would you model it using a package like SewerCAD. I have been told that you would model this using pressure main to the high point (as its always full) and then gravity main (with bolted manholes) onwards from the high point. In this case the required pump head will be the difference from the high point to mean water level in the wet well plus friction losses in main to the high point. Is this right ?? I guess, what im asking is how 'best' to work out the design head of the pump under such a potential unstable hydraulic scenario.
Cheers and have a Happy and Safe X-Mas.
Mark
RE: Force Main Pumping Downhill
If you had to live with no gooseneck or control valve and you were going to use bolted manhole covers then the calculations need to be two fold. One to be able to fill the line for there will be a point where it will drain from the high point. Air ingress through rubber ing joints leaks, incorrectly fitted manhole covers etc. thus the near stalled head of the pump must be great enough to get liquid into the line. The when the line is in operation your system curve will be depressed by the gain in head as the liquid runs downhill.
Really it is not a desirable way of designing the pipeline. All sorts of consequences arise. column separation causing extreme surges greater than predicted by the Joukowsky equation causing pipe bursts, damage to pump and valve seals, damage to valve seats, failed linings of pipes and pipe collapse.
If your client is insisting on this have the good grace to walk away from the job for the embarassment now will be far less than when the pieline fails.
RE: Force Main Pumping Downhill
Thanks again for your feedback. Wasnt really thinking about actually constructing the downhill portion with bolted manholes but only simulating the partial full situation by using fake bolted manholes. See the following and in particularly Tom Walskis notes:
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The problem is that if i use this method and adopt a pump to pump under this situation, when i put the same pump under a full pressurized situation the pump head is vastly different (ie higher head, lower flow). Off course this latter situation is the worst case (ie during wet weather) but i wont achieve the required pump flows. And i guess the same is inversely true: if i design a pump assuming a full oressure situation and then use this pump under the part full case (ie simulated with bolted maholes in SewerCAD) i get the pump pumping against a very low head (ie the pump may be pumping past its allowable range). Im not sure if im missing something in my logic or that im going around in a circle !!
The control valve option is not totally out, but i still am determined to work out what to do with a situation like the above. Im sure im missing something simple im my thinking !!!
Cheers
Mark
RE: Force Main Pumping Downhill
I have a license for Sewercad & Watercad but prefer to use AFT's Fathom and Epanet for that matter. Haestad have too much glitz and glamour for my liking.
What you describe is what is happening. If there is no vent in the line the benefit of the hydraulic head (syphon)in the gravity section will be seen by the pump. As I said you have to fill the line. After stopping the line will empty , albiet slowly, depending upon the amount of air that can get into the system.
I have just designed a negative system. DN250 4.2 km long. The only way I could get stable operation was to lift the HGL above the pipeline elevation. To do this I added a pressure sustaining valve to the outlet of the line. Once you do this the added head allows the use of a variable speed drive.
RE: Force Main Pumping Downhill
Ahh...so you had a PSV on the end of your main which basically prevented the main from emptying ie if the pressure starting dropping, the valve started to close and then it actually closes when the pump stops (or thereabouts). I guess you modelled this using a water hydraulic package like EPANET to work out what upstream setting you required to maintain the HGL above the pipe. With the pump starting (or pump at low flow), the valve may not open until the upstream pressure reached a certain pressure.
This is starting to make making more sense.
Cheers
Mark
RE: Force Main Pumping Downhill
1. Design for 1800LF of Forcemain and the lift to reach the peak you described. Under this condition ignore the remainder of the force main. This ignores the "recovery head" and simulates the pump starting with an empty force main.
2. Check the pump using the elevation of the discharge and the full force main length for friction loss. This condition assumes the pipe is full and ignores the peak. This is called using the "recovery head" and will be true only after the pump runs long enough to completely fill the force main.
3. Check the pump using the elevation of the peak but still use the full length of the force main for friction. This is the worst case for pressure head.
If your selected pump and motor is ok for these three conditions, and if the rate of flow is acceptable for all three conditions (For condition 3 flow will be low, for conditions 1, & 2 flow will be higher), you are done.
We designed a pump for 1/2 recovery head for GUC in 1986 and it worked like a charm. Don't create the site conditions by implementing that gooseneck idea. Deal with the conditions you have.