Re: [PipingDesign] Nozzle loads on Rotating Equipments

From: <George>
Date: Thu Jul 07 2005 - 13:01:00 EDT

Hello Pankaj (and Tanmoy)
I may have given the wrong impression on my piping situation. The complete case is that the piping had long straight horizontal runs to and from the compressor, with the piping buried underground, and a "sleeper" support close to the compressor. At that point, it elbowed upward with a 45 degree elbow, a short open run, and another 45 elbow to return to horizontal at the compressor flanges. So there was some degree of vertical and horizontal restraint and dampening. But there was no significant restraint to pipe longitudinal expansion until you got out 100 feet or so. End result, when the pipe was pressurized up from atmospheric to 1000 Psig, it stretched, with enough force to shift the compressor skid on its base.

But the point of my statement is that without proper design and anchoring, piping can exert sufficient forces to distort compressor cases, create misalignment and definitely promote vibration due to the misalignment. And, with varying effects due to temperature and/or pressure, it can be very specific to operating condition.

George McKinney

   Goerge,

   Like I said in my first mail, one very important aspect in rotating machinery is what is called "Global stability analysis" wherein one has to combine all the external loads imparted on the machinery, equate it with the inertia of the machine and find out if the machine would be stable on its foundation. For example, if net piping load creates a huge unbalanced vector in one direction, then the system may not be stable although individual forces may be within limit. We regularly did such checks for steam turbines. However, these are static analysis and in no way governing the dynamics of the system.

   The case in concern, 100ft (30m) of piping free run in front of a compressor is a bit, I should say, uncommon. Normally, for a compressor pipe routing, first support is an adjustable base support followed with rigid line stops as close as possible (keeping nozzle loads within limits). One word of caution: be extremely cautious if you are taking first pipe support from compressor foundation. Also ensure that eigen frequency of piping is substantially away from machine frequency (inclusive of start-up modes).

   By the way, I am interested to know the natural frequency of the 100ft free pipe run on either side.

   Regards,

   Pankaj Mandal.

   George McKinney <gmckinney@augustaeng.com> wrote:    Guys:
   Just a cautionary word on nozzle loads. Be sure you check all the loads involved, and that analysis runs at least to the closest significant piping anchor. We installed a couple of 6000 hp centrifugal compressors, with axial inlet/radial outlet piping (20", 1000 Psig rated). Had about 100 feet of unanchored piping on each side, and made up the piping with no load. When piping was pressurized, it shifted the compressor about 1/4 inch on its mounting. I assure, if it had been running, would have caused some vibration. Point in effect, look at what total loads on nozzles would be, and possible resulting misalignment. This can cause problems which are specific to a certain operating condition, as pressure or temperature on piping change force on nozzles.

   George McKinney

     Somehow this view may be too generous for rotating equipments I feel. Some
     rotating equipments are not that generous that they will permit high piping
     loads.

     My personal experience has been that whats there in the model should
     translate in the field. To find out that, we need a thorough audit. With all
     due respect to our site counterparts, we should also ensure whether these
     piping connections are freely aligned with the nozzles ? We should also run
     a case for this (for free alignment) I believe. Even the first few supports
     are also very crucial.

     Regards,
     Shreedhar.
     ----- Original Message -----
     From: "Pankaj Mandal" <man45039@yahoo.co.in>
     To: <PipingDesign@yahoogroups.com>
     Sent: Monday, July 04, 2005 12:41 PM
     Subject: RE: [PipingDesign] ->Nozzle loads on Rotating Equipments



> Rajesh,
>
> Most of machine vibrations I have encountered are not due to piping. Big
machines like turbine, big pumps etc are too massive and piping loads in itself are really trivial, considering the inertia of the machine (with foundation). Of couse exception are bad piping design, but again, these are really exceptions.
>
> I appreciate your concern regarding equipment vibration. I always believe
that the root cause for these vibration is not stand-alone piping design but the system in itself. Hence reducing static load in equipment nozzle may not be the correct solution.
>
> Pankaj.
>
> Rajesh.Balapure@ril.com wrote:
>
> Dear Pankaj,
> Thanks for your reply,
> There are lot of theories ,hypotheses,conjectures,hunches,feelings on how
> the piping loads affect the rotary equipment performance for which keeping
> the vibration levels within limits is an absolute requirement.We do not
> want to have a theoretical discussion on this issue but we are looking
> for any real experience on the effect of piping loads on machine
> performance(i.e.Vibration levels of the machine ).
> If you are having such inputs please let us know.
>
> P.S.
> "Now, regarding piping load on rotating equipment nozzles, the real
concern
> is casing distortion. Since these are precision equipment, gap between
> casing and impellers are small and a small local distortion in casing
could
> result in performance failure. Also, overall stability of the equipment
> needs to be considered".---->Your statement in a way justifies the concern
> of vibration levels increasing with machine loads.
>
>
> Regards,
> Rajesh V Balapure
>
>
>
> Pankaj Mandal
> <man45039@yahoo.c To:
PipingDesign@yahoogroups.com
> o.in> cc:
> Sent by: Subject: RE:
[PipingDesign] ->Nozzle loads on Rotating Equipments
> PipingDesign@yaho Importance: Normal Sender's OU:
Reliance |------------------|
> ogroups.com
| [ ] Confidential |
>
|------------------|
>
> 07/02/2005 02:33
> PM

> Please respond to
> PipingDesign
>
>
>
>
>
>
> Rajesh,
>
> Your statement "There are concerns that the vibration levels of the
machine
> can be increased with high piping loads"- is not fully correct. Piping
> loads on rotating equipment are mostly static in nature. Piping, in
itself,
> do not vibrate. In some cases like pump cavitation, slug flow or other
flow
> related issues causes vibration in piping. In extreme cases, due to poor
> design, eigen frequency of piping comes in proximity to the natural
> frequency of the rotating equipment (with foundation) and this causes
> vibration in the system.
>
> Now, regarding piping load on rotating equipment nozzles, the real concern
> is casing distortion. Since these are precision equipment, gap between
> casing and impellers are small and a small local distortion in casing
could
> result in performance failure. Also, overall stability of the equipment
> needs to be considered.
>
> I agree that in some cases codes, specially API610, gives nozzle
allowables
> that are hard to justify. Like pump allowable for a discharge pressure of
> 150psi is same as 1500psi. A higher pressure (discharge) pump should have
a
> stronger casing and hence should be able to sustain higher load. But code
> has no such provision.
>
> That's it in a nut shell. You are right. Lots of literature is already
> available and one should go through it for further details.
>
> Best regards,
> Pankaj Mandal.
>
>
> Rajesh.Balapure@ril.com wrote:
>
> Dear group members,
>
> A lot has been said &discussed (& also huge amount of literature available
> ) about the piping loads on the nozzles of the rotary equipment.There are
> concerns that the vibration levels of the machine can be increased with
> high piping loads.We have come across many machine vibration problems
which
> were supposed to be caused by piping loads on the nozzles but finally the
> problems were found to be else where.
> I would like to know from the team members about your "actual experience
> "on the effect of piping loads on the vibration of the rotary equipment.
>
>
>
> Regards,
> Rajesh V Balapure
>
> [Non-text portions of this message have been removed]
>
>
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[Non-text portions of this message have been removed] Received on Thu Jul 07 13:01:00 2005

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