7 replies to this topic

[black friday]

Hi all,
The problem I am seeking help with is regarding a water line. We operate a water treatment unit operation which is supplied by accepts from a DAF unit. In times when the DAF is down, the unit is supplied by general service water.
In such an arrangement I witnessed the feed to the treatment unit vibrating wildly. Pressure was fluctuating by about 20kpa perhaps, and the feed line (the auxiliary GSW line is perpendicular to the main feed line) vibrating quite alarmingly.
The vibration and noise were periodic, lasting anywhere from a minute or so to 10 seconds. The line comes of a 400m header supplied by the sites main GSW pump. A number of other services are supplied by this pump and no such problems have been reported. This unit is at the end of the header in question.
If I had seen this phenomenon in a refinery I would have suspected two phase flow, but the fact that it's a water service line with relatively low pressure (sub 150kpa) to my mind makes that implausible. The periodicity seems to indicate some build up of pressure or vibration in the line. I checked the pump in question and while noiser than a sister pump on a different line, I did not see any vibration or excess noise sufficient for alarm.
At the current time my thinking is that the problem is either an issue with the pump, which an inspection has convinced me is not to likely,or potentially a case of water hammer.
Does the forum consider either of these to be likely? Is it possible water hammer is localised in the end of the supply system by a restricting valve or other flow restriction? Or is a pump vibration issue more likely? Or am I completely off base?
Any and all input would be greatly appreciated.
Cheers all.

[riven]

My idea would be water hammer. The cause is unclear.

Water hammer is usually caused by a sudden impediment to flow e.g. a valve closing. The water before the valve is still moving and may cause a pressure buildup resulting in a shock wave. See if there are restrictions/multiple bends and compare with the other lines of the header for comparison.

However equally a sudden disturbance into an unfit system could be the case. For example the piping may be too small or the velocity too large. Make sure that the piping in the line is adequate for the header flow and pressure.

Check valves can also be an issue. The sudden deceleration due to the obstruction (or fluid expanding into space) may cause the valve to shut though I have not encountered this.

[Qalander (Chem)]

My idea would be water hammer. The cause is unclear.

Water hammer is usually caused by a sudden impediment to flow e.g. a valve closing. The water before the valve is still moving and may cause a pressure buildup resulting in a shock wave. See if there are restrictions/multiple bends and compare with the other lines of the header for comparison.

However equally a sudden disturbance into an unfit system could be the case. For example the piping may be too small or the velocity too large. Make sure that the piping in the line is adequate for the header flow and pressure.

Check valves can also be an issue. The sudden deceleration due to the obstruction (or fluid expanding into space) may cause the valve to shut though I have not encountered this.

Dear.

Although you have not described the

detailed piping arrangements/line-up sketches for the two cases

as these might have been real help for problem's insight as well as analysis.

I give my Wild guess to start with any screens or strainers available in 2nd circuit might not be OK.

• Source water might have solid particulate crried along amd hitting inside unknowingly and producing vibrations to these Noticeable levels.
• Their might be some air or vapours entry unknowing making somewhat two phase or slug flow conditions;thus inducing The vibrations.

Hope this shows some way forward for problem resolution
Best of Luck.

[black friday]

Hi all,
Thanks for the replies, definitely some food for thought.
I'd have a drawing of the piping details up but unfortunately the line is undocumented in the sites drawings. This site is relatively lax in updating technical drawings which of course is a pain. I'll see if I can rough something up.
Thanks again.

[katmar]

I have seen similar phenomena where the visible vibration is quite remote from the source. It seems vibration induced by a pump can travel along the pipe until it reaches a section where the natural frequency of an unsupported length matches the frequency of the vibration and then you suddenly see the pipe swinging about. Another pipe support in the middle of the vibrating section would stop this, although the better solution is to stop the vibration at source.

I would not suspect water hammer in the normal sense because the way you describe it is that it occurs during normal running, and not at shut down or start up.

Let me match Qalander and throw in a wild theory of my own. I have seen an instance where the vibration seems to be caused by a globe valve with a flat, non-rotating (i.e. loose) plug. The water flowing past the plug pulls it down in a similar fashion to the way a thermodynamic steam trap works. Once the flow decreases the water pushes the plug open again and the increasing flow pulls the plug down, over and over again causing the vibration. I have not been able to open this line to confirm my theory yet.

[Qalander (Chem)]

I have seen similar phenomena where the visible vibration is quite remote from the source. It seems vibration induced by a pump can travel along the pipe until it reaches a section where the natural frequency of an unsupported length matches the frequency of the vibration and then you suddenly see the pipe swinging about. Another pipe support in the middle of the vibrating section would stop this, although the better solution is to stop the vibration at source.

I would not suspect water hammer in the normal sense because the way you describe it is that it occurs during normal running, and not at shut down or start up.

Let me match Qalander and throw in a wild theory of my own. I have seen an instance where the vibration seems to be caused by a globe valve with a flat, non-rotating (i.e. loose) plug. The water flowing past the plug pulls it down in a similar fashion to the way a thermodynamic steam trap works. Once the flow decreases the water pushes the plug open again and the increasing flow pulls the plug down, over and over again causing the vibration. I have not been able to open this line to confirm my theory yet.

Principally I second the situation envisaged by 'our' katmar ; with reservation about these phenomenal vibrations increase induced by change of supply pump and source to such a great/Noticeable extent


only on account of inadequately or less supported pipeline. This should rather show up in other case as well with a somewhat lesser magnitude.


However the resonating frequency idea might be a possibility, I agree.

The other seemingly credible scenario is some loose/ dis-oriented globe valve's disk or Block valve's gate or other fitting's issue.

Request black friday to check and come back to update us on the issue.

Edited by Qalander (Chem), 11 March 2010 - 01:41 AM.

[unsig]

If this pipeline is ok for supports and running with full pipe, I'm doubting the way of 'water treatment unit operation' using sevice water.
I suggest turning on parameters to get a smooth change of flowrate to the water treatment unit or just adding a small bypath to allow continous flow.
The cause is water hammer.

Best regards

Lu Yongxiong

[Qalander (Chem)]

If this pipeline is ok for supports and running with full pipe, I'm doubting the way of 'water treatment unit operation' using sevice water.
I suggest turning on parameters to get a smooth change of flowrate to the water treatment unit or just adding a small bypath to allow continous flow.
The cause is water hammer.

Best regards

Lu Yongxiong

Hello/Good Morning Dear black friday.

Any updating feedback!

Wishing Good luck.