Gravity flow / Siphon

[wilfully12]

All

I have a problem with cooling water flow, and am getting confused as to how the system works hydraulically.

I have a high flow seawater pump (7000 m3/hr) cooling a steam turbine condenser. The Pump is mounted in a caisson approx 30m below process deck. The condenser is mounted at around 15m above the process deck. Sea level provides suction head and this is normally around 20m.

So the pump is pumping seawater to the condenser mounted around 45m above the pump, water outlet is above the condenser again. In this way the inlet pipe of the condesner will always be flooded. At the high point is a vent / siphon breaker before the water flows back down to sealevel (once through cooling system) through a 36" pipe.

The flowrates result in a Froude number of >0.7, higher than that recommended for self venting flow. The question is: What will happen here? I expect that during normal flow conditions the pressure will sit on the vent (i.e. atm pressure) and during a break in flow air will be sucked in by the vacuum generated. Would air be entrained during normal operation from the vent? On what criteria do I need to design the vent for? Essentially I am not sure how this system is going to operate on the plant.

Any assistance on gravity flow or siphon breakers appreciated

Regards

 

[wilfully12]

Having read through some more answers (i.e. thread378-81608) I can just add a couple more things:

When the "syphon" point is mentioned, I guess this is referring to the point where Froude = 1. Self venting occurs if FR<0.3. In my situation I cannot change out the pipework, and I cannot reduce flow due to cooling duty requirement. I may be able to increase flow to above FR=1.

Thanks

 

[katmar]

The section of 36" pipe from the vent down to sea level could probably cope with 80 to 100 thousand m3/h of water before it started backing up. Your actual flow is way below this, but still above the self venting point so you will definitely draw air in through the vent.

My approach to calculate the quantity of air drawn in and to size the vent would be to calculate the pressure drop down the 15 m section in two-phase flow and get that pressure drop to balance with the head generated by 15 m of fluid at the average density in the two phase regime. A rough estimate tells me that 100,000 m3/h of air could be drawn in. To keep the air velocity reasonable in the vent you would need 26 to 30" diameter pipe, so you might as well make it full size and 36" like the rest of the pipe.

I have no experience with two phase flow at these velocities in such large pipe and I would recommend you find a consultant who has hands on experience with this sort of problem. Otherwise you could find all sorts of problems with vibration etc. The worst problems would be the type that we don't even know that we don't know.

It's a pity that you posted in this forum - you would probably get more knowledgeable comments in the Pipeline forum in the Mechanical Section. If you don't get good answers here perhaps request the site management to move your post. Simply double posting is not allowed.

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[wilfully12]

Katmar

Thanks for your advice. On that basis I calculate 105,000 m3/hr air so you were pretty close!
Unfortunately my already installed vent is not that large! So a restricted vent will result in this system running under vacuum?

Perhaps I should have been more careful which forum I chose. I would be interested to find people's opinion on the likely physical effects of this flow in the pipework.

Best regards,

 

[cloa]

Actually, he put a new post with a link to this thread. That is permitted.

 

[katmar]

wilfully - here is a link to someone who has a similar problem to yours (unless your alter ego is daryon). Perhaps you can help each other - but please give feedback so that we can all learn.

http://www.cheresources.com/invision/index.php?/topic/10639-sizing-a-drop-line-vent/

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com