re: waterhammer Problem 1

[TurbineBlade]

Hi,

I am trying to understand waterhammer problem in a process system with focus on void formation and column separation. Can anyone suggest a good source of information/books?

The system is re-circulating water; fluid travel in a U-shape loop circuit, reaching at the highest elevation of 25 m from the pump suction, and finally discharge fluid at elevation 12m high. Both the suction and the discharge point are to atmospheric pressure.

1) I was told when the flow travels upward and turns horizontally, a column separation could occur?

2) Void formation could occur at the high elevation point.

Thanks

 

[BigInch]

Column sep occurs when the pressure of the fluid reaches its vapor pressure. This is usually at the high point, as it is the point of lowest pressure, however that is really only true for a static system. In a dynamic system with transient pressure waves, low pressure points can be anywhere at any given time and could of course even be at several points at the same time.

When fluid in motion is stopped by closing a valve or restricting flow ahead of the motion, increasing pressure is at first generated, but as the fluid compresses and rebounds away from a closed valve, a low pressure point is created as the fluid vacates that space. The opposite is true if fluid motion is stopped from behind by closing a trailing valve. The fluid moving away from the valve reduces pressure. If the fluid has enough momentum, it will continue moving away from the valve and the vacated space reduces in pressure. With sufficient momentum, the reduced pressure can reach the vapor pressure of the fluid, whence the trailing fluid will vaporize and pressure is not reduced further. If the reduced pressure is not sufficient to reverse the direction of fluid motion, given sufficient momentum, the fluid is said to achieve "column separation" since it may not recombine, depending on the momentum to reduced pressure ratios.

If the fluid is not, for example, running down a sloping pipeline where momentum is continually increasing, it is possible that it may eventually slow down and reverse direction from climbing up the next hill or to the top of a process column. Should that occur, the fluid column may fill the vapor space and reconnect on increasing the pressure above the vapor point. Collapse of the vapor pocket can bring on a rapid acceleration of the fluid column and a collision of the fluid ahead of and that trailing the vapor pocket, resulting in a very high pressure increase. The resulting wave traveling down the pipe is the "water hammer" effect.

Thus, vapor pocket formation and column separation can happen at any point where pressure is below the fluid's vapor pressure, not only at the top of a column.

BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com

 

[BigInch]

PS there are a number of water hammer cause and effect blogs on the internet, my page is only one. Search "water hammer" and quite a few will pop up.

BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com