Avoiding a vacuum when draining a pipe with no High point vent

[AngusH]

Hi all,

I have a problem I cannot get a simple answer to.

I need to drain a pipe (diameter 450mm) that has been brought over a structure and back to working level where it will be drained. This structure is creating a head of around 35m from the top to the draining level. Due to design there is no way to drain it in any other configuration.

The end of the pipe has a pullhead which has 2 ports (1x 2", 1x 1/2") both of which can be changed to add valves prior to the head difference being created.

with a rough pressure of 4 bar at draining level due to the weight of water from the head difference there will be an initial force pushing the water out however as I understand this will create a "gluging" release of the fluid inside. something I am trying to avoid as much as possible.

Is there any other way to remove this?


Cheers,

 

[EmmanuelTop]

Can you connect a pressurized source (e.g. Air, Nitrogen) to the front end of the pipe, and keep pushing water by Air/N2 pressure? This will effectively purge the entire line.

Dejan IVANOVIC
Process Engineer, MSChE

 

[hydtools]

Air has to get in from somewhere, therefore the gluging. Can you crack open a fitting that is up high on the piping?

Ted

 

[AngusH]

The only access to tie in to the pipe is through the pullhead at the location of the draining area. The issue with this is that I think if the 1/2" was used to inject pressurised air the 2" would just suck it back out as they are close together (opposite side of the diameter). Or am I overthinking this and the air would find its way up to the high point in the pipe?

I do not need to purge the entire line. Only as much as will flow out if the pullhead is removed.

There is no other inlet higher up in the line. The only 2 ports available are on the end fitting.

 

[EdStainless]

I have seen lines with a small diameter tube running up inside of them for venting. It doesn't need to go clear to the top, just high enough that the suction doesn't get too great during draining.

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P.E. Metallurgy, Plymouth Tube

 

[AngusH]

The pipe is in place already and without removing the pullhead there is no way to insert a new pipe or tube. The 1/2" port has a 1m long tube on the inside but I feel this won't be long enough.

 

[LittleInch]

A drawing would help to see what it is you're trying to do, but I think you have it all there. Connect a pressurised air line to the small port. open the second port and let some water flow out, equivalent to about 3-4m of pipe length. The top of your pipe will be at a negative pressure to atmosphere, but do you have a limit?

Then turn the water flow off then blow air into the pipe - you'll need in excess of 3.5 bar. Monitor pressure on the 2" valve port - when it reaches sat 4-4.5 barg, open it up again and then either match water out with air in - in a pipe of this size the flow velocity shouldn't be so high that the air bubbles are blown down, basically anything less than about 1m/second in your big pipe will allow the air bubbles to migrate upwards - work out what your flow rate from the 2" pipe is but at that pressure I can't see your velocity being in the 20-30m/second range coming out out the 2" pipe

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

 

[bimr]

As long as the entry end is open, one would think that you can blow air backwards through the pipe to break the siphon.

 

[pennpiper]

AngusH,
I have to ask, How did the pipe get full of water in the first place if there is no vent to let the air out?

Sometimes its possible to do all the right things and still get bad results