pipe vibration in stainless steel pipe

[ake71]

I have a pipe vibration issue. Each time the submersible pumps starts the stainless steel pipe vibrates. I have two static mixers, which I can not remove. They are required because we have found that although removing the mixers has more or less eliminated vibration, it has affected the chemical mixing efficiency dramatically. Therefore I need to eliminate the vibration using other methods. I thought of bolting the pipe supports to the pipe flanges, however, I was advise that that merely transfers the vibration elsewhere along the pipe.
I was even thinking of putting in flexible couplings, but once again this may reduce vibration but not eliminate.
Does anyone know how to eliminate pipe vibration on stain less steel pipes? The current pipe support are about 1.5 m appart.
Regards
AKE71

 

[GregLocock]

There's no such thing as eliminate. What reduction in vibration do you need? What frequency. etc etc. The more details you give the less it looks like 20 questions.



Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[ake71]

I dont have the tools to measure frequency as I am in remote location. All I can say is that you can see the pipe vibrating.

 

[desertfox]

Hi ake71

Can you upload a photograph or sketch so we can understand a bit more your situation.


desertfox

 

[btrueblood]

Can you tell us the type of pump you are running, and at what rpm. # of cylinders, or # of vanes in the pump would also help.

 

[ake71]

here is a drawing

 

[btrueblood]

Two pumps pumping from an open (vented) vault/tank? Or are there suction inlets on one of the pumps that don't show? Are the two pipes entering the vault/tank in plan view dumping the two chemicals to be mixed (one is labelled filtered water)? Any idea on make and model of the pumps, sizes?

Are these pumps pumping from a float or level switch/sensor, and is slosh in the tank a possibility (enough for the pumps to suck air)? Air bubbles slugging their way thru static mixers will definitely cause some shaking.

How well are the mixing elements secured in the piping? Is the vibration a high freqency, maybe due to the elements twisting/flexing and banging on the pipe walls?

Or is it a lower frequency, with the whole string of pipe rattling and moving between mounts? This latter would point more towards surging/slugging and/or air aspiration from the pumps.

 

[Twoballcane]

"Each time the submersible pumps starts the stainless steel pipe vibrates."

This sounds like water hammering. When you start or stop a flow, I think the back pressure(?) or velocity head(?) or something like that continues after starting and stopping fluid in pipes. There is a device not sure what it is called, but it has a diaphragm that compensates for this and will stop the pipes from vibrating.

Sorry to be so vague, but I hope it helps

Tobalcane
"If you avoid failure, you also avoid success."

 

[JJPellin]

Since your description is lacking many critical details, we all end up making educated guesses. I have a different guess than the others. I assume that the vibration is high the entire time the pump is running, not just at start-up. I assume that the vibration that concerns you is in the horizontal run of pipe in the pump discharge where the static mixers are mounted. I assume that when you remove these static mixers, you are replacing them with an equivalent length of pipe. Or perhaps they can be removed separately, leaving the original pipe in place. (I know nothing about static mixers) In either case, it sounds to me like a piping resonance in that horizontal run. If so, all you have to do is de-tune it. The piping has a natural frequency that corresponds to some frequency coming from the pumps. This could be exited at pump run speed, vane pass frequency or a multiple of either. Removal of the static mixers reduces the mass of that line, which increases the natural frequency of the pipe. This theory can be easily tested. Take a heavy bag of sand and lay it across the pipe at the location with the highest vibration. If the vibration goes away almost completely, then it could be resonance. If so, any permanent modification that significantly changes the stiffness or mass can detune the vibration response and “eliminate” it. If you add a pipe support at mid-span, this could do the trick. If my theory is correct, this would not transfer the vibration to another part of the pipe. By changing the natural frequency of that section of pipe, you would stop the resonant response. A further sign that this is piping resonance would be to compare the piping vibration in two directions. For example, if the piping vibration is very high up and down, but very low side to side, this would suggest a resonance. You can check for this with a coin. Hold the coin in your hand and run it along the pipe. You will feel the coin chattering against the vibrating pipe where the amplitude is highest. This will smooth out at locations (or directions) where the amplitude is lower. A resonating pipe will take on a mode shape which is usually a simply arc. The vibration would be highest in the middle of the span and have nodes at each end with little or no vibration.

If I am wrong about the location of the vibration or the conditions under which it exists, please correct me.


Johnny Pellin

 

[4Pipes]

Could vapour be collecting in the 2 upward loops adjacent to the check valves. Vent valves are not shown. Could the trapped vapour be acting as a spring and causing the liquid downstream to of the check valves to oscillate as liquid tries to get underneath.
One pump on its own might be operating with enough flow in its own leg to clear the vapour. When operating together the flow may drop in each leg to a value that prevents the vapour being forced out.
I suspect you might need vent valves on both sides of each check valve.