Broken bolts on MYCOM 125 screw.

[ICEMANNL]

Hello,

For the second time in 2 years I found 5 broken bolts on a mycom 125 screw compressor.
The bolts are all on the same corner on the compressor, female rotor on the drive side.

Who had this problem before??

Here is some running data:
NH3, P in bar T in celcius.
Psuc: 3.6 (+2)
Tsuc: +7
Pdis: 12 (34)
Tdis: 72
Tliq: 28
Poil: 2.7
Toil:69
Tevap in: +7
Tevap out: +5
Tcond in: 17
Cap: 100%
Amp: 211A

Kind regards,
Roel

[Grizzly]

Hi Icemannl.
Have you tried having a vibration analysis test done.
I have known screw comps to resonate (Vibrate heavily) at certain RPM's / % loads.
In fact I can think of 1 instance where after vibration analysis.
The screw compressor software was trimmed.
To stop the comp sitting at certain problematic levels of loading.
I have also known of a bad batch of high grade steel bolts.
Which had not been tempered correctly so where prone to fracturing.
Although i doubt this is your issue.

If the bolt that shears is in one of the feet then maybe the casting is uneven?
You don't give an exact position.
The above was not on Mycom Comps by the way!
Grizzly

[ICEMANNL]

Grizzly,

The compressor is placed in a chemical plant.
Mounting en alignment is perfect (0.01mm).
The entire vibrations tests are being done every month and are perfect.
The last time (2 years ago) we check the casing for any kind of defects, nothing found.
The first time we thought of a bad batch of the bolts.
Lab results didn’t give any answers.

When the compressor is stopped, first the load is taken off and then it shuts down.
At start up the compressor doesn't get to full power until the oiltemp is higher than 45c. max 30%.
So that’s no problem.

Regards
ICEMANNL

[glenn1340]

We had the same problem twice in six months with tie rods on a 110kw variable speed comressor motor direct drive to the air end. It was found to be resonation at approx 1400 rpm so the manufacturer modified the software to give a skip speed band that avoided that speed. I`m not saying this is your problem but as Grizzly says it may be worth looking at. This is a regular problem on Ingersoll Rand VFD compressors, so much so they build in an skip speed band to the software that can be easily adjusted in the field.

[RANGER1]

In our company we have experienced same problem with Mycom 200VLD's .

We put it down to poor quality bolts ( made in China ).
Can only suggest advising Mycom of
problem .
In the meantime instal highest quality cap screws you can get .
Use a toque wrench on them , as they are easy to overtension being so small .
Torque is in manual i will get back to you on that .

[Grizzly]

In our company we have experienced same problem with Mycom 200VLD's .

We put it down to poor quality bolts ( made in China ).
Can only suggest advising Mycom of
problem .
In the meantime instal highest quality cap screws you can get .
Use a toque wrench on them , as they are easy to overtension being so small .
Torque is in manual i will get back to you on that .

Good sound advise Ranger.
I do believe at least 1 of the occasions I referred to above.
Was solved by a change in the Bolts. I think Higher Tensile ones were used!
Grizzly

[RANGER1]

Mycom recommended torque

M6 11.57 Ft .Ib
M8 23.15
M10 40.5
M12 62.2
M14 89.69

You could consider using hi tensile bolt with a spring washer .
Use a spring washer so bolts head does'nt foul with housing .

[Grizzly]

Grizzly,

The compressor is placed in a chemical plant.
Mounting en alignment is perfect (0.01mm).
The entire vibrations tests are being done every month and are perfect.
The last time (2 years ago) we check the casing for any kind of defects, nothing found.
The first time we thought of a bad batch of the bolts.
Lab results didn’t give any answers.

When the compressor is stopped, first the load is taken off and then it shuts down.
At start up the compressor doesn't get to full power until the oiltemp is higher than 45c. max 30%.
So that’s no problem.

Regards
ICEMANNL

By vibration tests in this instance.
I mean stepping the compressor through each % of load and observing any Resonance in the comp and pipework ECT.
Much as Glenn suggests (thanks by the way).
However Ranger may have the solution.
You seem to have covered almost all the bases Just maybe one of these suggestions may help?
Grizzly

[Sandro Baptista]

Can you send a pic of the broken bolts? How is the fracture (brittle?)? Shear stress? Normal tension stress?

Fatigue stress due to abnormal vibrations or thermal expand of the skid? Just an idea...

[Sandro Baptista]

Or maybe the external piping, that is connected to the compressor unit and is making a high strain on the unit pushing or pulling the unit. And if this is happening it may be because there is lacking of "damping" on the pipe due to bad assembly.

[RANGER1]

Or maybe the external piping, that is connected to the compressor unit and is making a high strain on the unit pushing or pulling the unit. And if this is happening it may be because there is lacking of "damping" on the pipe due to bad assembly.

Good points even a severe case of soft foot on compressor .

Loosten one bolt at a time to see if compressor foot is still touching or gap between compressor & base .

The few we have had problems with were broken in middle of thread but randomly thoughout machine , so we concluded poor bolt quality .

[ICEMANNL]

The guys from MYCOM use a pipe to torque the bolts. Exept for the bolts for the axial bearings.

[Sandro Baptista]

Doubt that (about the axial bearings) be the main cause of the problem...but everything is possible.

[Sandro Baptista]

Good points even a severe case of soft foot on compressor .

Loosten one bolt at a time to see if compressor foot is still touching or gap between compressor & base .

The few we have had problems with were broken in middle of thread but randomly thoughout machine , so we concluded poor bolt quality .

Good point also Ranger1. We had an extreme case some day where the compressor just been on the air and supported by the pipes...amazing

[Sandro Baptista]

...the floor where the compressors were just "hit" a lot...bad building constructors

[RANGER1]

ICEMANNL ,
We also use a pipe but this machine is very small and would definately use torque wrench to at least eliminate overtightening .

On bigger machines you have to lean on them considerably , but not this size .

[mad fridgie]

Is the compressor set mounted on the oil seporator, It may be that if this is the case, check when the oil sep is at running temp, as some deflection may occur between cold and hot