CAU2011-PPT-TreyWalters JimWilcox.pptx

7/28/2019 CAU2011-PPT-TreyWalters JimWilcox.pptx

http://slidepdf.com/reader/full/cau2011-ppt-treywalters-jimwilcoxpptx 1/34

Transient Hydraulic Analysis for

CAESAR II Evaluation



What is Waterhammer?

Waterhammer is…

sometimes called Hydraulic Transients

a transient phenomenon that occurs in a liquid piping

system when some event causes a departure fromsteady state

A similar phenomenon happens in vapor lines

Usually the lower density means it is of lesser importance

the process the piping system experiences as itadjusts to the new conditions

a single or series of coupled pressure/velocity waves

that travel at close to the speed of sound through the

piping system



Waterhammer Causes

Waterhammer can be caused by many events

including

Valve closure or opening (in full or in part)

Pump speed change Trip or startup

Relief valve cracking open

Rapid tank pressurization

Periodic pressure or flow conditions



Waterhammer and Force

Imbalances

Waterhammer causes transient force

imbalances in piping systems

This is a result of fast moving pressure waves which

can create temporary force imbalances Elbow pairs are especially susceptible to force

imbalances due to the change in flow direction

Structural supports need to be able to handle

these forces



Waterhammer Video



Waterhammer Software

Waterhammer is a sufficiently complicated

process such that modeling software is usually

required

AFT Impulse™ is a leading waterhammer software

AFT Impulse has been commercially available since

1996

It has been used to model thousands of piping system

transients



Waterhammer Software (2)

Typically the issue of primary interest to the

engineering analyst is understanding transient

pressure extremes

This allows selection of pipe strength and design for equipment protection and general safety



Calculating Force Imbalances

Waterhammer software like AFT Impulse

calculates transient pressures and flows

This information can be used to predict transient

force imbalances



Traditional Force Calculation

Traditional force calculation uses only pressure

differences in the force imbalance

With hydropressure effects on pressure subtracted




(2)

This works best when flow fully stops quickly,

with no in-line components

dP = − ρ c V

Where “c” is wavespeed also known as celerityOften this is referred to as “a” which is synonymous




(3)

Complexities of real systems quickly render

hand-calculations useless.

How do pressures upstream & downstream of inline

components change and add or subtract? What if a valve only partially closes?

What about other forms of energy transmission?

Friction losses

Momentum changes Area changes




(4)

Complexities of real systems quickly render

hand-calculations useless.



Traditional Force Calculation:

Example

Consider the 1080 ft (330 m) system below

where the valve at J5 closes 90% over 2

seconds



Traditional Force Calculation:

Results

For the initial and final steady-state conditions

the force imbalance should be zero

Ignoring friction leads to non-zero steady-state results



Including Friction

Including all forces including fitting pressure

losses, friction & momentum improves force

calculations

A = πD2/4

Force = PB x AForce = P A x A

Friction & pressure loss forces Other forces + PAA + PBA = 0

Momentum = m A ΔVX,A Momentum = mB ΔVX,B

ΣFfriction + PAA + PBA =

mAΔVX,A - mBΔVX,B



Including Friction and

Momentum: Results

Steady-state forces initially and finally are zero



Comparing Methods

Traditional

Friction and

momentum included

Max Min

13.5 (60.2) 1.3 (5.7)

0.2 (1.0) -9.6 (-42.8)

(k-lbf/kN) (k-lbf/kN)



Comparing Methods at First

Elbow Pair

Traditional

Friction and

momentum included

Max Min

2.7 (11.9) 0.5 (2.4)

0.1 (0.3) -1.1 (-4.8)

(k-lbf/kN) (k-lbf/kN)



Traditional Method

Weaknesses

The use of traditional force imbalance

calculation methods can be highly inaccurate

Don’t know actual load magnitudes

Directionality of max loads can also be incorrect Don’t know timing of the loads

Ignores some loads



Limitations

Transient force imbalances are sensitive to the

difference in pressures at a given time

This means it is sensitive to the speed of the pressure

wave (wavespeed or celerity) When waterhammer pressures drop to vapor

pressure then transient vaporization can occur

This changes the wavespeed

This is difficult to model using modern methods

and hence force generation under cavitating

conditions is not reliable



AFT Impulse and CAESAR II

AFT Impulse can output force imbalance data for

direct import into CAESAR II

AFT Impulse does not have 3-D coordinates and

user must match up AFT Impulse forces withCAESAR II model



AFT Impulse and CAESAR II (2)



3 Ways to Analyze this with

CAESAR II

Static Equivalent

Time-History Analysis

Spectral Analysis (CAESAR II – AFT Impulse

method assumes this)



AFT Impulse Import

Tools > External Interfaces > AFT Impulse



AFT Impulse Import (2)





Open Dynamic Input, Input is almost complete




Force Sets




Spectrum Load Cases




Static/Dynamic Combinations for Stress




Review/Set Control Parameters & Run



3 Ways to Analyze this with

CAESAR II – Here’s how…

Static Equivalent

Time-History Analysis

Spectral Analysis (CAESAR II – AFT Impulse

method assumes this)



Conclusions

It is important to model waterhammer events for

proper system design and operation

AFT Impulse can generate transient forces

which can be easily imported into CAESAR II Traditional force estimation techniques which

rely on pressure differences can be highly

inaccurate

Force imbalances in systems with transient

cavitation cannot be reliably predicted because

wavespeeds change

CAU2011-PPT-TreyWalters JimWilcox.pptx

Documents

Transcript of CAU2011-PPT-TreyWalters JimWilcox.pptx