Cantilever Vent Line Vibration - Mass Addition_ - Topic

7/27/2019 Cantilever Vent Line Vibration - Mass Addition_ - Topic

1/9

6/26/13 cantilever vent line vibration - mass addition? - Topic

maintenanceforums.com/eve/forums/a/tpc/f/3751089011/m/701106666/p/1

Page 12

SKF

Become a better predictive maintenance leader in just 3 days Join or

Manage Your Profile Posting Boards Asset Condition Management

Posts About vibration/alignment/balance cantilever vent line vibration -mass addition?

Go New Find Notify Tools Reply

cantilever vent line

vibration - mass addition?

TweetTweet 0

Like 0

Login/Join

electricpete posted 05 May 2010 04:13 PM

We have a 1" vent line that extends straight up from a LARGE pipe

in a cantilever fashion.

We see varying high vibration on the pipe up to 4 ips at a

subsynchronous frequency approx 1500 cpm (machine operates at

5200). I think it is flow noise. Other similar sister machine pipes

vibrate at 0.5 ips or less.

We added 5-7 pounds weight and magnitude came down as well asfrequency. Details attached.

I think we are just lowering the resonant frequency below the range

where the flow-induced excitation is highest. I realize lowering

resonant frequency is a more logical strategy for fixed-frequency

excitation rather than broadband, but it seems to have worked in

thise case.

Does it make sense to permatize weights similar to this?

SGFP23_VentlLineSmall.ppt (958 Kb, 91 downloads)

Posts: 5545 | Location: Texas Gulf Coast | Registered: 20February 2005

Ron

Brookposted 05 May 2010 04:28 PM Hide Post

EP,

Go ahead and add more weight and test it. You are adding inertia

which will lower the displacement, but it would appear that there isalso a natural frequency involved that you are moving away from.

Ron

Posts: 916 | Location: Philadelphia,PA | Registered: 18 July2006
http://maintenanceforums.com/eve/forums/a/ga/ul/801106666/SGFP23_VentlLineSmall.ppthttp://maintenanceforums.com/eve/loginhttp://maintenanceforums.com/eve/forums/a/frm/f/3751089011http://maintenanceforums.com/evehttp://maintenanceforums.com/evehttp://maintenanceforums.com/evehttp://www.reliabilityweb.com/cgi-script/csBanner/csBanner.cgi?command=click&g=709&id=905http://www.reliabilityweb.com/cgi-script/csBanner/csBanner.cgi?command=click&g=709&id=905http://www.reliabilityweb.com/cgi-script/csBanner/csBanner.cgi?command=click&g=709&id=905http://www.reliabilityweb.com/cgi-script/csBanner/csBanner.cgi?command=click&g=709&id=905http://maintenanceforums.com/eve/forums?a=userposts&sortType=1&u=6211085252http://maintenanceforums.com/eve/forums/a/tpc/f/3751089011/m/701106666?r=865106666#865106666http://void%280%29/http://maintenanceforums.com/eve/forums?a=userposts&sortType=1&u=5301028011http://maintenanceforums.com/eve/forums/a/ga/ul/801106666/SGFP23_VentlLineSmall.ppthttp://maintenanceforums.com/eve/forums/a/tpc/f/3751089011/m/701106666?r=701106666#701106666http://void%280%29/http://maintenanceforums.com/eve/loginhttp://maintenanceforums.com/eve/forums/a/frm/f/3751089011http://maintenanceforums.com/eve/forums/a/cfrm/f/998103451http://maintenanceforums.com/eve/forumshttp://maintenanceforums.com/evehttp://maintenanceconference.com/cbm13/http://maintenanceforums.com/evehttp://www.reliabilityweb.com/cgi-script/csBanner/csBanner.cgi?command=click&g=709&id=905http://www.reliabilityweb.com/cgi-script/csBanner/csBanner.cgi?command=click&g=709&id=905http://twitter.com/search?q=http%3A%2F%2Fmaintenanceforums.com%2Feve%2Fforums%2Fa%2Ftpc%2Ff%2F3751089011%2Fm%2F701106666https://twitter.com/intent/tweet?original_referer=http%3A%2F%2Fmaintenanceforums.com%2Feve%2Fforums%2Fa%2Ftpc%2Ff%2F3751089011%2Fm%2F701106666%2Fp%2F1&text=cantilever%20vent%20line%20vibration%20-%20mass%20addition%3F%20-%20Topic&tw_p=tweetbutton&url=http%3A%2F%2Fmaintenanceforums.com%2Feve%2Fforums%2Fa%2Ftpc%2Ff%2F3751089011%2Fm%2F701106666http://maintenanceforums.com/eve/forums/a/tpc/f/3751089011/m/701106666/p/2


2/9



WaltStrong

posted 05 May 2010 05:28 PM Hide Post

El'Pete,

That was a good start. A true rigid weight of 10-12 lbs might give a

frequency redct ion over 20% (now 17%) and reduce vibration levels

further. More weight would be needed if it was installed closer to

valve body, so keep elevation high on stub pipe, as possible.

Walt

Posts: 2147 | Location: Massachusetts | Registered: 27 April2005

electricpete posted 05 May 2010 09:14 PM Hide Post

Thanks. We are going to clamp weight onto the pipe cap using

device shown attached. The clamp weight around 6 pounds with

provisions for additional weight in 3 pound increments, following

Walt's comment. Our machinist says he will figure out the exact

dimensions needed to get good clamping action on the pipe cap.

We didn't try stiffening. My thought is that the initial directionwould be increase in vibration with small amount of stiffening

(based on weight addition results), but certainly could turn back

down as we get stiffen farther. There is not a lot to brace off of as

you can see it sits in the air. The adjacent blue structure isremoved (during outages), so it would not be good to clamp to. If I

had more time, I might investigate that further but we're far enough

into weight addition that I'm going to stick with it (unless there are

some obvious problems with this design anyone can think of).

One the thing that makes me a little uneasy is the idea of tuning

away from broadband flow noise. We are now a little less than full

flow. Would frequency of flow noise be expected to change withchange in flow? How about change in fluid temperature?

Another interesting thing is that the magnitude of the vibration

wanders up and down. Would this type magnitude variation be atypical expected behavior for a resonance excited by broadband

flow noise?

This message has been edited. Last edited by: electricpete,

05 May 2010 09:24 PM

Clamp4a.ppt (1,136 Kb, 64 downloads)


Ron

Brookposted 06 May 2010 06:58 AM Hide Post

EP,The response of a component at resonance is always dependent on

the amount of energy in the forcing function. The amplification factor

doesn't change.

If the 'broadband' energy you describe in this pipe doesn't have a

defined flow resonance, then the amplitude of the response with

change with % of flow.

My guess is that there is a fluid resonance and the amount of energy
http://maintenanceforums.com/eve/forums/a/tpc/f/3751089011/m/701106666?r=144104766#144104766http://void%280%29/http://maintenanceforums.com/eve/forums?a=userposts&sortType=1&u=5301028011http://maintenanceforums.com/eve/forums/a/ga/ul/793109666/Clamp4a.ppthttp://maintenanceforums.com/eve/personal?x_myspace_page=profile&u=5301028011http://maintenanceforums.com/eve/forums/a/tpc/f/3751089011/m/701106666?r=693109666#693109666http://void%280%29/http://maintenanceforums.com/eve/forums?a=userposts&sortType=1&u=7141004131http://maintenanceforums.com/eve/forums/a/tpc/f/3751089011/m/701106666?r=158107666#158107666http://void%280%29/


3/9



at the resonance could be affected by small changes in flow day to

day.

I like the design of the weights.

Posts: 916 | Location: Philadelphia,PA | Registered: 18 July2006

Walt

Strongposted 06 May 2010 08:58 AM Hide Post

El'Pete,

An increase in flow velocity generally raises turbulence peak frequency

and amplitude. Temperature change would probably have insignificant

effect on structual natural frequency. I think that the add-on weight

will be as good or better at higher flow/turbulence. The only negative

about the weight design is that the additional weights are attached

by bolts in shear, and this is not the strongest clamping direction. If

you do add one or more plates to get to "magic weight" status, then

weld the plates together and to one of the clamp halves. This would

prevent loosening over time and reduce risk of losing parts should it be

removed for any reason.

Walt


electricpete posted 06 May 2010 11:08 AM Hide Post

Thanks. Good comments.

Now a crucially important aspect I have to describe this thing on

paper. Everyone wants to call it a dynamic absorber, but that's not

what it is. We all know it is a weight added to reduce resonant

frequency. What is the short/quick catchy name for that?


Walt

Strongposted 06 May 2010 11:45 AM Hide Post

El'Pete,

"Tuning Mass"

"Detuning Mass"

"NF Modifier"

"Stubby the pacifier mass"

"Magic Mass"

Be aware that the valve and stub pipe may be acting like a "dynamic

absorber" if the large pipe has a similar natural frequency. It would be

worthwhile to inspect and/or measure vibrations at other locations in

the pipe system to be sure that no new high vibration problems werecreated.

Tip: Use thin SS shim stock to be sure there is a tight 2-point (each

clamp half) contact to stub pipe.

Walt

http://maintenanceforums.com/eve/forums?a=userposts&sortType=1&u=7141004131http://maintenanceforums.com/eve/forums/a/tpc/f/3751089011/m/701106666?r=838106766#838106766http://void%280%29/http://maintenanceforums.com/eve/forums?a=userposts&sortType=1&u=5301028011http://maintenanceforums.com/eve/forums/a/tpc/f/3751089011/m/701106666?r=346106766#346106766http://void%280%29/http://maintenanceforums.com/eve/forums?a=userposts&sortType=1&u=7141004131http://maintenanceforums.com/eve/forums/a/tpc/f/3751089011/m/701106666?r=359104766#359104766http://void%280%29/http://maintenanceforums.com/eve/forums?a=userposts&sortType=1&u=6211085252


4/9



William_C._Foiles posted 06 May 2010 12:57 PM Hide Post

Should the goal be to eliminate the branch pipe vibration or to

just follow the pipe vibration?

One can address the main pipe vibration separately.

Mass can not only de-tune; it can change the mode shape.

Regards,

Bill

Posts: 2545 | Location: Houston, TX USA | Registered:23 February 2005


The small pipe has enough access we could measure bottom, middle

and top. Levels highest at top, lowest at bottm, in the middle

between. The pattern stayed the same with our initial 5 pounds

added mass. So I think it is 1st mode shape of cantilever beam andthe main pipe is not vibrating excessively.

LOL - I would have liked to seen my bosses face if I wrote "magic

mass" in my paperwork. I like "Detuning mass" - it is very short and

descriptive, and I would've used that if I'd have thought of it or

read it in time, but I ended up using "vibration-reducing weight"(pretty lame).

Operations wouldn't let us attach anything to the pipe cap since

they have to remove it for fluid system venting during every plant

startup. So we have to clamp to the pipe below the cap rather

than to the cap. Also we have to keep enough clearance to

operate the valves. I had to get agreement from a pipe stress guy.He wanted to restrict the total weight added to 15 pounds and

ensure it was symmetric about the pipe (no moment). All of this

forced the design to evolve to what is shown in attached. It has

minimum 5 pounds (clamp) plus provisions to add additional weightin 1.8 pound increments. Our machinists built the device yesterday.

It may be installed today or over the weekend or Monday. Will let

you know results.

FinalVibrationReducingWeight.ppt (1,224 Kb, 52 downloads)


Steve

Cieslaposted 07 May 2010 02:42 PM Hide Post

At least the name you picked meets the nuclear criteria for TLA

(Three Letter Acronymn).

Posts: 442 | Location: Southern California | Registered: 23February 2005


lol - good point Steve.
http://maintenanceforums.com/eve/forums/a/tpc/f/3751089011/m/701106666?r=109104076#109104076http://void%280%29/http://maintenanceforums.com/eve/forums?a=userposts&sortType=1&u=1501014111http://maintenanceforums.com/eve/forums/a/tpc/f/3751089011/m/701106666?r=411109866#411109866http://void%280%29/http://maintenanceforums.com/eve/forums?a=userposts&sortType=1&u=5301028011http://maintenanceforums.com/eve/forums/a/ga/ul/359107866/FinalVibrationReducingWeight.ppthttp://maintenanceforums.com/eve/forums/a/tpc/f/3751089011/m/701106666?r=259107866#259107866http://void%280%29/http://maintenanceforums.com/eve/forums?a=userposts&sortType=1&u=3911027111http://maintenanceforums.com/eve/forums/a/tpc/f/3751089011/m/701106666?r=433107766#433107766http://void%280%29/


5/9



The "VRW" was installed today. I was not there, but the report is

that the full 14.5 pounds was installed all at once (didnt try any

intermediate mass level) . The vibration reduced to 0.20 0.23 ips

on top of the pipe in both directions at a frequency of 1145 cpm.

Vibration remained approx 0.20 at bottom of the pipe. I have no

report of vibration level in the middle of the pipe (will check Monday

to try to get an idea of the shape).

Fitting this data against SDOF model:f = C / sqrt(M0+Madded)

Madded = 0 pounds => we had 1555 cpmMadded = 5.5 pounds => we had 1290 cpm.

From above first two datapoints we solve C=5418 cpm*lbm^0.5 ,

M0 = 12.14 lbm

From the above we would predict:

Madded = 14.5 pounds => 1050 cpm resonant freq (SDOF

prediction)

But we actually had

Madded = 14.5 pounds => 1145 cpm resonant freq (actualmeasured)

So a SDOF model does not fit very well accross this range of

attached masses.


08 May 2010 09:28 PM


William_C._Foiles posted 09 May 2010 05:08 PM Hide Post

Often one can brace the smaller pipe to the larger one. Could

this not be done?

When the frequency changes a great deal and when the

added mass is a considerable fraction of the total mass it ispossible that the mode shape has changed. The frequency

reduction for this mode could be close to reaching a limit, i.e.

close to the point where it would be with infinite mass

(probably some room to go further if desired).

Regards,

Bill


roughrider posted 09 May 2010 05:44 PM Hide Post

Reminds me of using a stabilizer on a bow, the longer the stabilizer

the less weight required to prevent torque.

Posts: 12 | Location: alaska | Registered: 01 March 2010

Walt

Strongposted 09 May 2010 07:44 PM Hide Post

El'Pete,
http://maintenanceforums.com/eve/forums/a/tpc/f/3751089011/m/701106666?r=935105176#935105176http://void%280%29/http://maintenanceforums.com/eve/forums?a=userposts&sortType=1&u=399106595http://maintenanceforums.com/eve/forums/a/tpc/f/3751089011/m/701106666?r=858104176#858104176http://void%280%29/http://maintenanceforums.com/eve/forums?a=userposts&sortType=1&u=3911027111http://maintenanceforums.com/eve/forums/a/tpc/f/3751089011/m/701106666?r=416104176#416104176http://void%280%29/http://maintenanceforums.com/eve/forums?a=userposts&sortType=1&u=5301028011http://maintenanceforums.com/eve/personal?x_myspace_page=profile&u=5301028011


6/9



4 ips down to 0.2 ips is fantastic! The deviation from SDOF may be

nonlinear effect of clamping the weight to the pipe as opposed to

welding it. I think you had the right solution with the weight instead of

bracing to the main pipe, and an infinite mass was not needed either.

Paint it and file the results of the Case History"!

Walt

Posts: 2147 | Location: Massachusetts | Registered: 27 April

2005

William_C._Foiles posted 10 May 2010 07:58 AM Hide Post

If you got a similar reduction in acceleration, did the force at

the tip from inertia go down, remain the same, or go up? How

about the dynamic moment at the intersection with the big

pipe?

Regards,

Bill



We did get a chance to take a closer look today. Vibration at

bottom/middle/top in both direction was no more than 0.23 ips and

no less than 0.17 ips (0.2 0.23 ips at top and 0.17 0.2 at

bottom). We dont have a 2-channel analyser to take phase, but

even if we did, Im not sure we would get a stable phase reading

given the flow excitation.

I summarized comparison of 4 configurations in attachment:

Slide 1 original configuration 1555 cpm (1 4 ips)Slide 2 5.5 pounds weight (clamps added) 1290 cpm (0.6 1.4

ips)

Slide 3 [NEW] 14.5 pounds permanent weight 1145 cpm (0.17

0.23 ips)

Slide 4 [NEW] Sister units with different (smaller) valve style

2300 to 2700 cpm frequency. 0.2 0.45 ips among 5 sister units

with smaller valve (we only have one with the larger valves... the

one that originally had the 1-4 ips vibration)

So, I conclude the sister valves likely have 1st resonance above

the excitation frequency, and that the flow excitation falls of

rapidly outside of a narrow frequency band somewhere in theneighborhood of 1550 cpm to somewhere below 2300 cpm. (I would

have thunk it would be a wider range).

I tend to agree with Walt that the only thing left to do is the

painting. But certainly willing to discuss and learn what we can from

this.

Bill as I mentioned earlier there is nothing handy to easily stiffen

against. Certainly would be more work to try to build something like

that. What advantage would it have been to stiffen instead of

adding weight?
http://maintenanceforums.com/eve/forums/a/tpc/f/3751089011/m/701106666?r=240102276#240102276http://void%280%29/http://maintenanceforums.com/eve/forums?a=userposts&sortType=1&u=3911027111http://maintenanceforums.com/eve/forums/a/tpc/f/3751089011/m/701106666?r=142101276#142101276http://void%280%29/http://maintenanceforums.com/eve/forums?a=userposts&sortType=1&u=7141004131


7/9



I dont think stresses from shear forces will be relevant since this

resembles a long thin beam and bending stresses (from moment) are

typically much greater than shear stresses (from shear force) in a

long thin beam. It was our objective to reduce bending stresses

from moment near the bottom (as Bill mentioned) where this vent

line is welded to the main pipe which should be much larger. I tend

to think we have accomplished that objective simply because we

tuned obvious resonance away from the excitation, although I dont

have means to calculate any stresses to provide proof. In the initialconfiguration we had what I believe is excitation near first resonant

frequency where 1st mode shape is cantilever beam 1st modeshapewith highest moment and bending stresses at the base. Im not

positive what the final mode shape looks like after weight addition,

but my suspicion is we cannot get anywhere near the 2nd resonant

frequency based on trying some preliminary calcs. The piping is 1

schedule 160, i.e. 0.815 id and 1.35 od. The length from big pipe

to tip of vent line is 23. The 1st valve is attached 8 from main

pipe, 2nd valve is attached 19.5 from main pipe, and permanent

14.5 weight is attached 22 from main pipe. The valves I estimated

in the neighborhood 10-30 pounds. Thats a big range

(uncertaintly), but anything within that range still gives us apredicted 1st resonant frequency in the very rough ballpark of our

measurements, and predicts 2nd resonant frequency way up in theneighborhood above 6000cpm. If in fac t the 2nd mode is that high

in frequency, then we must still be seeing a first mode shape whose

shape is not tremendously different than the original and I dont

think there can be enough increase in curvature to overcome the

dramatic decrease in peak vibration magnitude (4.0 ips down to

0.23 ips). My preliminary calcs considered the main pipe stationary.

Will try again sometime with main pipe movement allowed (forcing

function)... I imagine that will have some effect on the conclusion

of how high the 2nd resonant frequency is.

Always open to further discussion and comments.

This message has been edited. Last edited by: electricpete,10 May 2010 11:13 AM

PhotoComparison.ppt (3,934 Kb, 33 downloads)



One thing you guys may have noticed... there is a gap between the

clamp halves. We did not shim as Walt had suggested. It certainlydoesn't look right to the eye. My first thought is to wonder whether

it will increase susceptibility of the bolts to fatigue (thinking aboutbolt joint diagram, where compression of the joint plays an

important role). My 2nd thought, maybe the stretch in the clamp

halves plays a similar role for this device as joint compression does

for a typical bolted joint. What do you guys think?

Note - if this device should fall off, there is nothing fragile below it

and also it's not a place where people would be.


10 May 2010 07:43 PM

vrw.ppt (446 Kb, 26 downloads)
http://maintenanceforums.com/eve/forums/a/ga/ul/997105276/vrw.ppthttp://maintenanceforums.com/eve/personal?x_myspace_page=profile&u=5301028011http://maintenanceforums.com/eve/forums/a/tpc/f/3751089011/m/701106666?r=897105276#897105276http://void%280%29/http://maintenanceforums.com/eve/forums?a=userposts&sortType=1&u=5301028011http://maintenanceforums.com/eve/forums/a/ga/ul/340102276/PhotoComparison.ppthttp://maintenanceforums.com/eve/personal?x_myspace_page=profile&u=5301028011


8/9



Reply



Heres a related quest ion. Would you consider this fix as

permanent ?

We require 2 barriers for the high-temperature water in the main

pipe. The pipe cap counts as one barrier while installed. The pipecap is only removed for system venting when the system is cool

during plant outage/startup. So we only need one of those two

valves. We could remove one of the vent valves during the next

plant outage (we cant do it on-line because the system is hot andwe wouldnt have isolation to cut out the valve out, short of using

freeze seal).

As I said, the consequence of weight falling off is not a problem.

The consequence of vent pipe breaking at the bottom weld from

vibration fatigue would be big problem.

Any opinions on whether this weight should be considered

permanent, or whether we should go to the expense/effort ofcutting out the extra valve (and updating drawings) instead?



Thanks Sam. What are your reasons?

fwiw - I tend to agree. There is of course one selfish reason - If

the thing works fine forever, no-one will ever think about the

weight or my involvement in it again. But if something goes wrong

many years from now (lets say the year before I retire!), they willlook up the paperwork and come straight back to me.

If I were to champion a request for design change and work order

to cut the valve off, I would need a little more justification.

I could say the flow excitation might change over time which could

eventually make the weight ineffective... but if that's a concern

than it applies to both solutions (the weight and cutting the valve).

Although I do tend to think we would have a lot more margin above

the existing flow excitation frequency band before we get to

resonance with one valve cut off than we have below the existing

flow excitation band before we get to resonance with the weight

installed.


Powered by Social Strata Page 1 2

Become a better predictive maintenance leader in just 3 days Join orManage Your Profile Posting Boards Asset Condition Management

Posts About vibration/alignment/balance cantilever vent line vibration -

mass addition?
http://maintenanceforums.com/eve/forums/a/frm/f/3751089011http://maintenanceforums.com/eve/forums/a/cfrm/f/998103451http://maintenanceforums.com/eve/forumshttp://maintenanceforums.com/evehttp://maintenanceconference.com/cbm13/http://maintenanceforums.com/eve/forums/a/tpc/f/3751089011/m/701106666/p/2http://socialstrata.com/landing/goto.php?a=evehttp://maintenanceforums.com/eve/forums?a=userposts&sortType=1&u=5301028011http://maintenanceforums.com/eve/forums/a/tpc/f/3751089011/m/701106666?r=969101576#969101576http://void%280%29/http://maintenanceforums.com/eve/forums?a=userposts&sortType=1&u=5301028011http://maintenanceforums.com/eve/forums/a/tpc/f/3751089011/m/701106666?r=763101476#763101476http://void%280%29/http://maintenanceforums.com/eve/forums?a=userposts&sortType=1&u=5301028011


9/9


maintenanceforums com/eve/forums/a/tpc/f/3751089011/m/701106666/p/1

Contact Us | Become a better predictive maintenance leader in just 3 days | Privacy Statement | Terms of

Service

Copyright 2004-2013 Reliabilityweb.com All rights reserved.
http://www.maintenanceforums.com/privacy_policy.htmhttp://maintenanceconference.com/cbm13/mailto:[email protected]

Cantilever Vent Line Vibration - Mass Addition_ - Topic

Documents

Transcript of Cantilever Vent Line Vibration - Mass Addition_ - Topic