Pipe Supports near Recip Compression 1

[scotty36]

I have two questions:

1. I am trying to find a supplier of pipe clamps for vibration. There are many clamps on the market that are simply a bent plate. The clamps I am seeking have long lugs for the bolts either side so that they are much stiffer. I have seen these in recommedations by Beta Machinery. It would be good to find an off-the-shelf supply, but if not, we will fabricate ourselves.

2. When modelling these clamp supports, is it reasonable to allow axial movement? We have some relatively long lengths (22metres) of straight pipe (4" ANSI 1500# system) at the compressor discharge. These lengths have clamps at 3m spacings for vibration. Modelling them as anchors is far too conservative and gives high piping stress. Then again, we cannot model them free either. Could someone provide some quidance on this or refer to a publication?

 

[hacksaw]

if you are having to use clamps on your piping to hold it down, then you have a more serious problem than you realize.

what kind of loads are you placing on you compressor outlet?

 

[bvi]

To prevent vibration but permit thermal movement, you should consider using friction type restraints. These typically hold the pipe against a structure with a preload that results in sufficient friction to prevent movement due to vibration, but permit movement due to thermal expansion without creating undue stress. I have not heard of conventional pipe clamps being used in this context.

 

[Montemayor]

scotty36 (Mechanical):

To answer your 2 specific questions I would emphasize the helpful and important comments made by the previous respondents - hacksaw & cb4. They are not, I am sure skirting your first question; rather, they are correctly pointing to other, more basic issues that can lead to the result you are experiencing.

It is inherent for a recip to cause vibration in the connected piping. HOWEVER, this vibration is tolerable if caused by gas pulsation and minor turning moments. Here, I am assuming a 100% proper installation, an adequate foundation, operation at the designed conditions, and proper maintenance. Excessive vibration causes undue stresses on the piping and the machine. But, as cb4 correctly points out: so does pipe expansion caused by the adiabatic heat of compression that the machine puts out. Pipe clamps normally arrest the expected pulsation vibration with the aid of pulsation drums or dampners.

If you are getting dynamic pipe movement ("vibration&quot to the point that you have to try to anchor down the piping, I would seriously look at the machine's operation as suffering from bad bearings, unbalanced flywheels, loose anchor bolts, loose mechanical components (in the crosshead, pistons, etc.). I would consult (& bring in) the manufacturer's engineering representative(s) to witness the effect if I could not find mechanical or installation flaws. As hacksaw infers, I would not "hunker down" the piping with beef-up clamps or other devices.

Make sure you have allowance for thermal expansion; I usually install metal flexible hoses (up to 2-3" diam.) on my discharge piping where these are applicable, in order to isolate the minor expansion and vibration tendencies.

After installing and operating many recips, I can assure you that you should not have to live with re-inforced, beefed-up pipe clamps and visible vibration from these machines if they are from reputable manufacturers (I-R, Clark, Cooper, Joy, etc.) and they are installed correctly and operated by the book. I always subjected all the recips that I ran and those that I inspected (in other plants) by the following method that was taught to me by my original engineering mentor and boss: I would place a buffalo nickel with its flat edge vertically on a flat surface on the head end of the machine while it was running. If the nickel stayed vertical and didn't fall, the machine passed my test. All the machines in my plants passed that test. Not many outside passed. This shows that it is possible to have a recip with minimal (& non-detectable vibration). I think this is what the goal should be: curing the vibration, not going after the effects.

I hope this experience helps.


Art Montemayor
Spring, TX

 

[ccfowler]

Montemayor,

Well said!

I've seen some dancing recips that were simply frightening to be near and a precious few that ran as smoothly as all of them should have. It is no surprise that the "dancing" compressors (and associated piping) had plenty of problems while the smooth running ones provided long, trouble-free service.

 

[LSThill]

Team Member and Mr. ccfowler (Mechanical)

It would seam like the Piping Engineer did not do the correct Analysis or did not have the Engineering Tool's to do the Analysis

Please Tank a good hard look at Fluids Analysis:

BOS Fluids is the engineering software package that analyzes fluid transients in pipe systems and relates this information back to the mechanical piping system transferring the fluid.

For years piping engineers have labored with simplifying hand methods, cumbersome analog computers, or user-unfriendly software products when needing basic steady state and transient fluid analysis capability. BOS Fluids was written specifically to address the need of the piping engineer for fluid reaction forces, and to provide a system whereby the fluid simulation results can be easily integrated back into the piping system design and analysis.

BOS Fluids is an interactive computer simulation package that models steady state and transient flow in liquid or gas carrying piping systems. The procedure is easy to use and interfaces with most pipe stress programs. The package contains the elements required to model most common unsteady flow conditions. The elements included in the simulation package are pipes, valves, pressure relief valves, vacuum breaker, air valves, pumps, equipment, surge vessels, inlets, outlets, and orifices. BOS Fluids makes fluid simulation simple and easily accessible and yet gives the analyst pressure transients and dynamic force results with an engineering accuracy.

Based on a number of realistic assumptions a simplified form of the time dependent conservation (Navier-Stokes) equations are solved for the internal channel flow. The assumptions made are:

1. Fluid behavior in pipes is one dimensional i.e. similarity of cross sectional distribution of properties does exist.
2. Fluid transport velocity is small compared to wave speed.
3. Wave fronts remain plane while propagating.
4. Gas simulations assume that flow velocities are below sonic, and that pressure drops through the system are less than 30%.

Based on these approximations friction effects are lumped. The present friction model used is Colebrook-White. The Darcy-Weisbach flow model is used for steady state pressure drop calculations and the basic theory applied in BOS Fluids can be found in Wylie & Streeter's "Fluid Transients" published by FEB Press. BOS Fluids is capable of simulating both the steady and transient behavior of liquid carrying closed conduit systems of pipes, valves, pumps and surge relief devices. The following special features are available:

1. Pipe stress models from either CAESAR or PipePak can be downloaded for fluid analysis.
2. The analyst can pick different fluids from a database or add their own fluids to the database
3. Two different models are available to simulate column separation: Concentrated Air Pocket (CAP) model and the Vapor Cavity Model (VCM).
4. Various pipe materials can be applied-both isotropic (Metals) and Orthotropic (FRP) materials are included.
5. Based on geometry typical pump properties are generated automatically.
6. Buried and above ground systems can be simulated.
7. Simultaneously the transient response of multiple sources: Pump starts, Pump Failures, Valve Operations can be simulated.
8. Harmonic option allows an analysis of the occurrence of standing waves.
9. Maximum and minimum pressures and velocities occurring during transient and/or harmonics are traced.
10. The force processor allows an analyst to survey the time history of the unbalanced forces on pipe sections and preprocesses the force time histories to be used in the dynamical module of the pipe stress program.
11. A spectrum breakdown of force time histories is available. The analyst can see the natural frequencies of the fluid response that tend to excite the piping system.
12. Both Metric (SI) and English Units can be selected.

Typical analyses using BOS Fluids include: water transmission and distribution systems, main cooling water systems for chemical plants, sewage water systems, combined power and drinking water cycle power stations, oil product transport lines, tanker loading and unloading systems and dynamical behavior of chemical liquid transport lines. Acoustic analyses for compressors and pumps.

Leonard@thill.biz

http://www.thill.biz

 

[scotty36]

Thank you all for your responses.

CB4 (or others), do you have a reference for me to follow up on "friction type restraints"?

I should explain that the question pertains to a Natural Gas Pipeline project in its design stages. The compressor is on order and the preliminary layout for the station is prepared. The piping layout is being analysed for stresses, nozzle loads etc by a formal Caesar II analysis. Also, the support spacings are being selected with natural frequency away from the excitation frequency. In a few weeks, the results from a detailed vibration study will be available. Until then, we are doing everything we can to refine the layout. The restraint details will be used to assist in the obvious trade off between flexibility and vibration suppression.

 

[FreeWheel]

Sounds like marketing got in a hurry and and inked a deal on a napkin over a bottle of wine and did not envolve engineering until the deal was made. Am I close?

 

[hiliter]

I believe that you are looking for what is called a "Pipe and Bottle Clamp" with or without belting material. I was involved in a project in which Southwest Reasearch Institute did an acoustic "analog" analysis of a piping system on a reciprocating compressor. (10,000 HP). They made a number of recommendations as to location and type of supports. They have standard designs for Pipe Clamps, Bottle Clamps, X-Strong Pipe Clamps, Thermal Guides, Wedges in sizes from 1/2" up to 60". These clamps are not stock items, but came be made in any fab shop.

I believe that Engineering Dynamics Inc. offers the same service, but they do not use an analog model.

Both are located in San Antonio. It has been a number of years since I have spoken to either company, but I beleive they cn help you out.

Bos Fluids is a great program, but it does not design a clamp for you.

 

[zeven]

We have been involved in solving vibrating piping systems, due to fluid dynamic problems.
Where the the problem is not too severe, and there is a very flexible piping system, then the best solution is to restrain the pipe. There are several types of support from plain boxing in with steel and u bolts through to snubbers and dampers. Snubbers and dampers are soft and there should be no fatigue or fretting problems, they also allow thermal growth in the direction restraint. They are expensive for good long life ones.
Where the support is rigid (hard), such as rigid strut and clamp configuation, then fretting and fatigue can occur.
I have used rubber inserts between the clamp and the pipes to stop the fretting, and the feedback from most sites is that this works. Offshore project uses inserts like Tyco, from James Walker, but there are many alternatives. We have also successfully used silicon rubber strips on higher temp lines.