Pumping downhill with a high point 2

[JohnWeal]

We have a 5km pipeline that is transferring sewage from an elevation 15m above the discharge. The pumping station is a sewer network pumping station with coarse screens. The discharge is the inlet screen chamber at the treatment plant.
The reason we cannot gravity flow to the inlet works is that we have a 40m hill to run the pipe.

Has anyone any experience of this kind of pipeline profile and what potential pumping or flow problems may arise?

The screens are 25mm bar screens and the pipeline is 1400mm. The flows vary from 0.3 drry weather flow to 6 x dry weather flow.

Regards
John

 

[LittleInch]

You need to plot the elevation versus length and post it here to get some more help.

In essence you need to create the static height plus frictional losses from the pump to the high point.

what happens after that can be either:

a full pipe if you either have a lot of friction losses or you maintain the head at the end point to 40m by using a valve which control on back pressure.

A pipe which could vary from full to nearly empty as the water basically falls off over the high point and the pipe acts like a free flowing channel. That's not necessarily a bad thing, but flow is likely to be quite variable and you could get surges and flow which gurgles with entrained air.

You need to plot the line using head losses for your different flows.

What is the arrival pressure / head??

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[JohnWeal]

Many thanks Littleinch.
Your responses are exactly my thoughts. It is the control of the downhill section for all the possible flow condions.
As we can have screened sewage to 25mm in 1 direction, it would have to be a plug valve if back pressure is needed.
The pressure at the inlet works arrival chamber is the head of water. It is not enough to provide the necessary back pressure.

I will carefully check the pipe gradients and use some gravity flow software.
Also, I will check the plan view / pipe routing to see if the peaks and any troughs can be eliminated.

Regards
John

 

[MFJewell]

You may get vacuum conditions at the high point. For the utility I used to work at, we had several power stations along the Mississippi river in Louisiana. All of the plant cooling water was brought in from and returned to the river. The piping/pumping systems were designed to have vacuum over the return section that went over the levee. Every plant had a vacuum pump system installed on the levee with suction points along the line as it went over the levee. These vacuum systems were used to prime the system during initial start-ups, aid in low river level operation (this is when the vacuum conditions were the highest), and remove air during normal operation. Additionally, all the piping (66-84" dia) had reinforcement rings to mitigate pipe collapse under vacuum. At two different units we found pipe buckled at 45 degree bends due to erosion from cavitation in the section of line that was under vacuum.

I believe it was designed that way have the lowest discharge pressure possible going back to the river due to environmental impact requirements.

 

[LittleInch]

Designing and pumping as a "slack line" as it's known is not normally recommended, but you need to look closely at the elevation plot to see what will happen. Subsequent dips and high points less than 40 m could result in vacuum or low pressure conditions. As flow rate increases you could get collapse of those vacuum or low pressure sections and some very high surge pressures as the water columns hit each other.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[bimr]

A few observations:

The force main should have a minimum continously operating flow rate of 2 ft/sec to avoid solids deposition.

If you operate the force main intermittently, the flow rate when the pump starts should initially be 3.5 ft/sec to resuspend the solids that settle out when the flow stops.

The best solution is to install a back pressure-sustaining valve on the discharge. The valve will stop siphonage. The valve needs to have a full port for wastewater applications.

You can get one from Red Valve, GA Industries or Ross Valve:

http://www.rossvalve.us/products-1.html

Similar discussion on another site:

https://communities.bentley.com/pro...ogy-forum/14911/system-head-curve/96988#96988

 

[JohnWeal]

Many thanks Bimr. A plug valve is what I was considering.

As the pipeline is conveying as low as 0.3 x dry weather flow, the velocity will be low as the pipeline is sized for maximum flow.

How would the back pressure valve be controlled? By a pressure switch with a minimum pressure set point which would close the valve a certain amount?

I have some software that will model this so will contact redvalve.

Is it advantageous to have two smaller pipes? I think septicity would be an issue either way.

 

[LittleInch]

Any pipeline system with an 18:1 turndown ratio will run into all sorts of problems, from how you actually control flow or run in batch mode to bimrs issue of sediment settling etc.

Back pressure is controlled either by springs indie pilot valves controlling valve position or electronically via a pressure transmitter controlling the position of a control valve.

I think for this system you should work on keeping the line full and then operating in a pulse or batch mode to keep a minimum flow velocity as recommended above.


Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Compositepro]

Do not overlook the possibility of routing the pipeline around or through the hill to avoid the need for a pump.

 

[JohnWeal]

That's exactly my next Task compositepro.
Many thanks for all your valued experience.

 

[bimr]

f) Vertical alignment.
(1) Uphill pumping is preferred in a force main, where the force main discharge point to the
gravity sewer is at a higher elevation than the rest of the system, so as to keep the force main
under pressure.

(2) If an intermediate high point in the force main lies above the downstream point of the gravity
discharge, a partial vacuum condition can be created at the high point, when the force main drains
after pumps shut off and when the HGL profile drops below the high point.

(3) Downhill pumping, vertical profiles which are conducive to siphoning at high points and gravity
drain/air locking in downhill pumping conditions will require special analysis to ensure proper
hydraulic performance. These types of force main profiles are also conducive to potential severe
waterhammer pressures caused by rapid velocity change in the force main
resulting from pump start up or shut down. It is therefore recommended that force main profiles
which can generate downhill flow be avoided. If downward pumping condition cannot be avoided,
then proper hydraulic performance of the force main should be ensured based on sound engineering
and design principles. Consider the following, when downhill pumping is required.

(a) The downward sloping force main section following the high point may not flow full during
initial line start up because the flow carrying capacity exceeds the line filling rate. The
elevation of the high point, in this case, will give the highest static head that the pump must
overcome during initial start up.

(b) The downward sloping force main section may not flow under pressure at some pumping rates
during normal operation of the pumping station and when pumps shut down. Consider whether and how
the pressurized pipe flow should be achieved and maintained.

(c) The extent and effects of partial vacuum condition/siphon action on force main hydraulic
performance. Consider allowing the partial vacuum condition during normal conditions and how is
it maintained.

(d) The trapping of air/sewer gases at the high point and the downward sloping section, and the
effects on pumping head and removal of the air/gas from the force main.

(e) Potential waterhammer pressure due to pump shutdown or power failure.

https://www.wsscwater.com/files/live/sites/wssc/files/Design Manuals/S-24-2008_52075.pdf