Process Line Highpoint - column separation?

[jonelv]

Hello


I'm looking at a design where I'm pumping from approx ground level (say 0m), over a pipebridge (7m elevation) to a vessel (1m) which is open to the atmosphere. The downhill part of the pipeline is a near vertical drop. My thoughts are to treat the headloss design for the pump duty point as for a total head of friction loss plus 1 m of static but to ensure that the pump can generate a closed valve head at least 7m + friction to ensure the system will prime when the pipe is empty.

My intention is to fit an air valve at the high point to remove air on filling but not to admit air. My question - can I assume that I will get the full benefit of the syphon effect caused by the downhill section or would the fluid acceleration as it drops down the pipe cause me vaccuum / column separation problems. Should I therefore select a duty point based on the high point static head?

Cheers

Jone

 

[quark]

You must consider discharge static head of 7 meters as delivery is open to atmosphere. If you are taking water from the same tank(recirculatory kind of thing) then 6 meters is sufficient.

You should check final velocity of free fall to know whether there is pressurized flow at the highest point. For ex. if the water from the highest point falls just by gravity then the velocity at the exit will be (2gh)^1/2. Apply Bernoulli's theorem to check the pressure at the highest point.

Regards,

 

[katmar]

Hi Jone,

Always be very wary of vertical downflow of liquids. It is often neglected because liquids naturally flow downwards, but it can be very troublesome, and even dangerous.

You have not given any indication of your flowrates. If the flowrate is steady, and the friction pressure drop down the vertical leg is more than the increase in static pressure then you will recover the head. But if the flow is less than this critical value, you may have vaporisation problems depending on your fluid and temperature. Apart from losing the head, vaporisation can cause vibration and all the problems that go with it.

The safe way to do it, is to size the pump to give 7m plus friction, and to install a syphon break at the top of the vertically downwards section. The syphon break should rise 1,0 to 1,5 metres above the line. The vertical down section should be sized for self-venting flow to avoid drawing air down into the vessel.

Rather play safe.

regards
Katmar