Can any one suggest me that which method is most appropriate to do dynamic analysis of piping system with centrifugal pumps? And how the Frequency and other assumptions are to be established before start of the cAESER II input?

thanks

The theoretical answer is Harmonic analysis.But my exprerience tells me something different and I prefer to do a modal analysis.

For Harmonic analysis the exciting frequencies will be the fundamental speed and multiples thereof.The magnitude of the force and phase angles can be given as an input based on Guideline given in CAESAR II user manual where it is very well written. For that you need to know the speed,eccentricity.These informations can be gathered beforehand to do the Harmonic analysis.

I would prefer to do a modal analysis making sure that none of the natural frequencies are close to the exciting frequencies. If some of the modal frequencies are close to the exciting frequencies, observing the mode shape, I will change the supporting pattern to avoid such natural frequencies.This has given me some valuable solutions for many field problems.

Anindya Bhattacharya

Stress analyst

Bechjtel Corporation

The best Engineering Tool's is "BOS/Fluids" for the above type 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 Stephen Thill
Leonard@thill.biz
www.thill.biz