Vacuum Breaker 2

[Engprofile]

Hi,

We are currently in process of designing a new pipeline. We wish to pump to a height of 56m (over a hill) and then the pipeline will go down the other side approximately 131m. Therefore the line will be under vacuum and I am worried the line will compress inward (as it is HDPE pipe). Is it possible to install a vacuum breaker at the top which would prevent pipeline compression? I would prefer not to pump into an open tank at the top.

Thanks in advance.

 

[zdas04]

I did a quick search on the collapse strength of HDPE and didn't find an equation, but I did find a site referencing a pipe under 115 ft of water with less than 15 psig on the line (collapse force around 35 psid). Since a perfect vacuum has a magnitude of greater than -14.73 psig I think it would be hard to collapse the pipe with your run away fluid. Or did you think you were going to get -70 psig because of the hydrostatic difference?

I've never installed a vacuum breaker on lines with a lot more elevation difference than you're talking about. I'd be a lot more concerned about the line filling up and putting full hydrostatic head of the 113 m leg on the HDPE. 113 m of hydrostatic head is 158 psig. You didn't say which HDPE you're using, but that is enough head (with zero applied pressure) to bust anything thinner than SDR 7.

David

 

[Engprofile]

Hi David,

Thanks very much for your reply. Other than the collapse pipe my other concern was that there would be a siphoning effect and the suction on the down hill would cause the water to be pulled through the pump (causing overspeeding) damaging the pump or sucking the tank pump dry. I have since selected a combination air valve for sewage (we are pumping coal slurry) which I hope will do the job.

A.I.R D-020

http://www.arivalves.com/PDF/D-020 COMBINATION SAAR.pdf

 

[micalbrch]

I think you can avoid overspeeding of your pump by installing a control valve in the downhill line (which is not a cheap piece of equipment for coal slurry). But others might know better than I do.

I'm not convinced about the air valves, especially not about valves designed for sewage and then used with coal slurry. How do you want to control the amount of air sucked into the pipeline?

 

[BigInch]

Colapse also depends on diameter.

Yes siphoning leading to pump overspeed is possible without the air valve, but since the upleg is more than 10.34 meters, the siphoning effect can be broken, further flow increase and pump overspeed being prevented, without using an air valve. When vapor pressure is reached at the pipeline's high point and siphoning will stop. 0.3 m is the approximate vapor pressure head equivalent for water (I assume your slurry is water based) at norm ambient temperatures.

Select a pump that has a differential head capacity at some flowrate Q where atmospheric head + suction head + pump differential head - friction head loss through the piping to the high point less than 0.3 m. Q will thus be the maximum possible flowrate in the system. The flowrate where vapor pressure is reached at the high point and any siphoning effect is broken.

But true, the line will have a vacuum of 10 m equivalent head at the high point, which could travel from the high point to the end of the pipeline, if sufficient backpressure is not held at the outlet, so an air valve would be prudent, if sufficient wall thickness is not provided to prevent colapse due to the resulting vacuum.

Further,

There may be a disadvantage to this type of installation, especially in a slurry line. If you do not hold backpressure at the end of the pipeline sufficient to maintain a flow across the full pipeline's cross section, (cascade flow), fluid velocities in the pipe can be high (the same as supercritical flow in open channel hydraulics) as flowrate is maintained through a lesser effective flow area. Friction factors change in those sections, you must then use the hydraulic radius, rather than the pipe diameter in determining reynolds number. It is also possible that you will experience hydraulic jumps within the pipe when slopes lessen and flow depth increases inside the cross section to values greater than supercritical depth. What does that mean in terms of operations? Cascade flow may have severe implications for internal erosion to occur when flowing a slurry at high velocity, especially in the region near the bottom of the pipe where the slurry solids may concentrate. Also you could expect to find severe erosion at any of the possible hydraulic jump locations. Another phenomenon to avoid are pipe slopes that match the critical slope, as flow is not stable, velocities will change and flowing depth will not be constant; depths will oscillate, sometimes violently around the critical depth, causing more potential for erosion to occur. In severe cases causing jumping of the pipe.

Lastly, if you have a hyraulic leak detection algorithm operating, correlating flow to pressure is very difficult and numerous false alarms will most likely occur and long term flow balancing is difficult.

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