I have a problem with vibration and

[WilliamTz]

I have a problem with vibration and pipes shaking in a boiler room. 3 x boilers 4mW, 5mW and 2 mW. Pipes range from 200mm to 450mm headers. Water temps at 138degC 6 bar pressure min. Primary pumps next to the boiler all on a header . The 2mW boiler was added recently. When running after a while the entire pipe network in the plant room rumbles and shakes. maximum velocity in the pipe 2m/s....Very apparent to the eye. Maybe 1/4-1/8 inch movement over and back..Even the support steel shakes. More steel supports were added to help but no good. In the middle of the shaking yesterday, I switched off the boiler burner and let the pump on and water continue through the boiler and the shaking continued. This kind off excludes the burner being the cause of the vibrations . Not until the boiler valve was fully closed did the pipes stop shaking. Is it a hydraulic piping set-up problem.. is it the heat form the boiler ... is it residual heat form the boiler when the burner is off and flow continues...the new boiler and pump is set up in an array identical to the existing boilers and pumps. Spare valves installed for future use 15 years ago were used to connect up the new boiler and pump.
I know this is a long shot could it be anything to do with resonance?

Any ideas...?
Thaanks!

 

[crshears]

Any possibility of cavitation / flashing developing in the pump suction lines following the piping modifications? A quick look at steam tables suggests there would be little likelihood of any degree of two-phase mixture developing, and it appears your system is a high-temperature high-pressure hot water system with no generation of steam intended.

Not until the boiler valve was fully closed did the pipes stop shaking.

Which "boiler valve" were you operating? If it were up to me I'd try experimentally throttling the boiler outlet valve or valves to see if this might mitigate or eliminate the vibrations.

Can you provide more information? Pictures? System diagrams with piping dimensions, heat release rates, and design temperatures?

More information yields better answers...

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[HTURKAK]

I have extracted the available data ; Pipe dia's range from 200mm to 450mm , Water temps at 138degC 6 bar pressure, maximum velocity in the pipe 2m/s ,shaking continued when the boiler burner switched off and let the pump on and water continue through the boiler ,Spare valves installed for future use 15 years ago..

The pressure 6 bars is suitable for 138deg C . I looked to moody diagram, the flow is fully turbulent. The piping is 15 yeras old, that is , relative roughness may be tripled, the temp 138 degr. that is , dyn. viscosity of hot water is around 20 % of 20 degr. water. Reynolds no. in the range of 2 E 6.

The alternatives to solve the problem with flow; reduce the velocity and reduce the friction..

The options i am thinking; change the header piping with new one (which may be costly) or add new supports to control the vibration.

If you provide the existing piping iso. with supports, system diag. you may get better answers.

 

[LittleInch]

You say you have three boilers but only talk about one. Is this the new boiler?

Assuming it is then it looks like you're creating a fluctuating force somewhere in the loop.

These valves you talk about. Were they checked to see if they were full bore or as a dead end are they full of scale?

What does the pipework look like? Give us a sketch, photo or isometric and we might be able to spot something.

Do you have enough flow going through this new boiler? How do you know? Where are temperature controls? On each boiler or on the header?

Could the new boiler be working harder than the others?

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

 

[WilliamTz]

Hello
Thanks for the feedback
I have attached a sketch in schematic format and some pictures of the primary pumps header
We usually need the 4+5 mW boiler ON or the 5+2mW or 4+2mW boiler ON together. Occasionally mid winter we need all 3 x boiler ON
Number of primary pumps running = number of boilers ON + 1 (n+1)
It works perfectly except for the fact when the 5mW and the 2mW work together, if pump 1 is ON, it causes the entire header pipework to shake. Any other combination of 3 pumps is fine.
Now here is a little history on the Pump 1: Recently installed at the end of the header where the header actually reduces from 400 to 300mm. We did a hot tap onto it which was probably 175mm but ample for 30 L/s. There is also a butterfly valve at this extension, fully open

If pump 1 is running while the 4+5 mW is ON there is no issues. If pump 1 is running while the 4+5+2mW is ON there is no issue
Each boiler is fitted with a mechanical automatic balancing valve (FLOWCON valve) which gives the exact flow requirements of the boiler and give automatic balancing as the number of boilers and pumps varies. That appears to work well.

Q: Is there turbulence entering the system at pump 1 point of entry with the hot tap (slightly reduced tie-in 175mm) and the valve and the reducer piece causing something that is not in the other pumps where they connect in and it becomes apparent when the small boiler, probably with its higher DP with small pipes etc, just pushing back against this "weak" point....?? just a thought? I,m going to beef up the support steel anyway and tie it into the building steel to stop the whole frame shaking and swaying

B

 

[WilliamTz]

PIC 1

 

[WilliamTz]

pic2

 

[WilliamTz]

Pic 3

 

[r6155]

Something may be wrong in the structure. I don´t like the gusset plate for truss connections. Column base plate seem thin.
May be Header should be 450 mm instead of 400 mm.
I don´t like 2 elbows near the pump discharge.

Regards

 

[Compositepro]

mW = milliWatt

MW = megaWatt

 

[HTURKAK]

Dear WilliamTz (Mechanical)(OP); i just looked to sketches and pictures. I could not see the ait vents at the hump points of the outlet pipings of ea. pump.

What is the characteristics of the new pump ?

IMHO, air is trapped at hump level of piping and the brand new pump characteristics does not match with the old ones. I will suggest a three steps improvement ;
i= provide air vent at each hump ( the vents will be connected to a common small bore pipe) and an expansion tank at header pipe..
ii= regulate the flow of new pump with partially closing the CV at outlet side,
iii= change the stub -in connection with standard unequal tee ..

The last improvement, beef up the supporting steel with adding bracings.

 

[LittleInch]

Pump details please. Also details of these flow valves. There are many variations.

What is the flow going into the different boilers?

How do you balance the pumps?

Where are the check valves?

Is the location of the boiler line accurate in your sketch? If so it looks like pump 1 will take more flow and start cavitating/ excess flow and cause all manner of vibration.


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

 

[LittleInch]

The simple answer is don't use pump 1!

Is that butterfly valve at the end of the header fully open?

Can you check current / power going into the pumps during this operation?

Variance in flow in parallel pumping is common unless you can balance them or the pump curves are quite steep at the operating point.

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