chemical engineering

Wednesday, April 29, 2009

Assess AIV with "D/t-method"

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Sound Power Level (PWL) is the acoustic energy generated by a pressure reduction device. There are several ways to assess the adequacy of the piping to resist AIV fatigue. One of the way to ensure piping downstream of pressure reducing device sufficiently strong to resist AIV fatigue is to ensure the PWL allowable limit of downstream piping higher than the PWL generated by the pressure reducing device. Refer to "Sound Power Level (PWL) Prediction" for equation in prediction PWL.

A piping will have it stiffness and resistivity against vibration. It is very much subject to piping diameter, material properties, wall thickness, distribution of masses, pipe support, etc. Allowable sound power level and/or dynamic stress of piping can be a measurement of this resistivity of piping against excitation due to AIV. Several allowable PWL curves for piping which derived from different method i.e. "D-method", "D/t-method ", "E-method", etc have been used. Besides PWL related method, there are other method i.e. "Dynamic stress method", "Likelihood of Failure (LOF) method", etc which adopting allowable stress level also been used.

"D/t-Method"

Based on data available in Carucci & Mueller (1982) studies, "No failure" and "Failure" points have been plotted in following chart with Sound Power Level (PWL) versus Pipe Diameter / Wall thickness (D/t).

Note :
1) A "failure" point (0.3m, 165dB) is below curve. It is failed on bad welding. No further failure after good welding.

2) Red point are "failure" point and Blue point are "No failure" point

3) May be used for 0.2m < D < 0.9m. Use at risk for D < 0.2m and D > 0.9m

PWL Limit Equation For "D/t-method"
Eisinger (1997) has proposed a PWL limit line (blue line).

PWL_limit = 176.6 - 125 (D/t)

In this post, a new PWL limt line has been derived and following equation representing this line (black line).

PWL_limit = 203 - 8.4 Ln (1000D/t)

where
PWL_limit = Maximum allowable PWL at Pipe diameter D meter, (dB)
D = Pipe diameter, (m)
t = Wall thickness, (mm)

This line may be used to assess potential failure of piping downstream of pressure reducing device. Any point above the line potentially fail on AIV, piping treatment or redesign required to minimise the risk of AIV failure. This will be discussed in future...

There is additional discussion on unclear area and now proposed polynomial PWL limit line. Read more in "Assess AIV with "D/t-method" with Polynomial PWL Limit Line".

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Sunday, April 26, 2009

Protect Pump for Longer Operation

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In today’s complex and frequently rugged process plant environments, liquid pumps are often over-worked and under-protected from adverse operating conditions. Many pumps run nearly non-stop 24-hours a day over multiple shifts. Poor operating conditions can reduce pump performance, require extra maintenance, shorten their lives and increase costs.

Unexpectedly losing a pump is often an expensive or even a dangerous problem in a chemical plant. The results of a pump failure potentially range from the loss of product in sensitive pharmaceutical batch control applications to damage to other nearby equipment from pressure build-up of material that isn’t moving or even to safety hazards when a pump fails to shut-down and literally burns-up in the presence of combustible materials. It really pays to protect and care properly for your pumps. Read more here

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Friday, April 24, 2009

Assess AIV with "D-method"

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High frequency acoustic excitation downstream of pressure reducing device and potential of downstream piping failure on Acoustic Induced Vibration (AIV) has raised concern in many plant design. Earlier post "Extra Attention to Common Point and Similarity on AIV Failure" has discussed the common points and similarity of AIV such as typical failure location, system experienced failure in the past, failure time & period and Mach no. An engineer assessing AIV shall pay extra attention in these factors.

Sound Power Level (PWL) is the acoustic energy generated by a pressure reduction device. There are several ways to assess the adequacy of the piping to resist AIV fatigue. One of the way to ensure piping downstream of pressure reducing device sufficiently strong to resist AIV fatigue is to ensure the PWL allowable limit of downstream piping higher than the PWL generated by the pressure reducing device. Refer to "Sound Power Level (PWL) Prediction" for equation in prediction PWL.

A piping will have it stiffness and resistivity against vibration. It is very much subject to piping diameter, material properties, wall thickness, distribution of masses, pipe support, etc. Allowable sound power level and/or dynamic stress of piping can be a measurement of this resistivity of piping against excitation due to AIV. Several allowable PWL curves for piping which derived from different method i.e. "D-method", "D/t-method ", "E-method", etc have been used. Besides PWL related method, there are other method i.e. "Dynamic stress method", "Likelihood of Failure (LOF) method", etc which adopting allowable stress level also been used.

"D-Method"
Based on data available in Carucci & Mueller (1982) studies, "No failure" and "Failure" points have been plotted in the following chart.

Note :
1) A "failure" point (0.3m, 165dB) is below curve. It is failed on bad welding. No further failure after good welding.

2) Red point are "failure" point and Blue point are "No failure" point

3) May be used for 0.2m < D < 0.9m. Use at risk for D < 0.2m and D > 0.9m

PWL Limit Equation For "D-method"
A PWL limt line has been derived from above data and following equation representing this line.

PWL_limit = 156.8 - 9.8 Ln (D)

where
PWL_limit = Maximum allowable PWL at Pipe diameter D meter, (dB)
D = Pipe diameter, (m)

This line may be used to assess potential failure of piping downstream of pressure reducing device. Any point above the line potentially fail on AIV, piping treatment or redesign required to minimise the risk of AIV failure. This will be discussed in future...

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Wednesday, April 22, 2009

Earth Day - Minimize Paper Consumption...

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April 22, 2009... Annual EARTH DAY... An effort to inspire appreciation for the earth's environment and awareness of issues that threatening Earth - One and only one where we are now... preserving is responsibility of everyone of us...

Earth Day in March 25, 2009 has called to turn off all non-essential electrical appliance for 60 minutes. Chemical Process Technology would call to minimize paper consumption (Check out world paper consumption). Whenever you just about to print some documents or article in hardcopy... Ask yourself the following questions :

Why not print and keep in softcopy ?
Don't print (hardcopy) if it is not absolutely required
Think twice if you really want to print...
Continue to ask similar question other than Earth day...

Come on... let minimize paper consumption...

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Tuesday, April 21, 2009

Extra Attention to Common Point and Similarity on AIV Failure

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High frequency acoustic excitation downstream of pressure reducing device and potential of downstream piping failure on Acoustic Induced Vibration (AIV) has raised concern in many plant design. Recent incident such as Insalleh Gas plant shown that the piping failure caused by Vibration (acoustically induced, or flow) is typically occur in flare piping with thin wall, high sound power level and high velocity (Closed or equal to Mach 1). It is also indicated that acoustically or flow induced may occur in INTERMITTENT service. A quick analysis on the previous failure cases conducted and following discusses the common points among all the failure cases.

Typical Failure Location
Earlier discussion "Piping Excitation When Expose to Acoustic Energy" has briefly discussed piping segment may be excited in circumferential and longitudinal mode and images showing piping circumferential and longitudinal excitation patterns in different nodal arrangement. Although entire piping system including pipe, welded part, support,tee, etc, previous experiences have shown that typically (not necessarily only) failure occur at the following locations :

Small bore connection (SBC) i.e. instrument tap, bypass line, etc
Fabricated Tee
Welded Pipe support
Internals i.e. attemporator & inline silencer

Above has based on data available in Carucci & Mueller (1982) studies and some private communications. Special attention shall be paid in above locations when carry out critical line analysis. Above area typically a location within a piping system with DISCONTINUITY. Those area with discontinuity shall pay extra attention. Having said that, welded piping may still remain a concerns.

Typical Failure System
Based on analysis of data available in Carucci & Mueller (1982) studies and some private communications, following are typical system exprienced failure :

Compressore anti-surge recycle or Capacity control
Main Gas Letdown station
Gas letdown to Flare
Steam desuperheater
Small bore connection at large pipe

Thus, special attention shall be paid in above systems when carry out critical line analysis.

High Velocity (Mach no)
Another common syntom among failures is piping experiencing sonic flow or choked flow. Whenever sonic flow or choked flow occured, it is expected that flow in the piping has reached it limit. Extremely high turbulence level impacting the piping and results irregular movement of piping. This potentially lead to high frequency of peak-to-peak stress.

Sonic flow with flow limitation will results pressure upstream of choking location increase. As pressure increases, it generate higher density and lead to breakage of sonic flow at choking location. Higher flow is passing the choking location. As the flow is increases, sonic flow form immediately and flow is decreases. Above will results in a "pulsating flow" in the choking location. This "pulsating flow" will occur in very short period, induce additional stress to the piping and increase likelihood of failure. Read more advices in piping related to Mach no.

Although High mach no is one of the main common point among all failure, previous failure cases includes piping with low mach no (Mach no less than 0.3)). Avoiding high velocity is one of the recommendation however it is not ultimate factor in determining failure of piping due to AIV.

Failure Time & Period
One of the common points is the failure occured during start-up and the failure to occur period is range from 5-10 hours to 2-3 months subject to the sound power level and piping condition. Thus, special attention shall be paid during start-up period.

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Monday, April 20, 2009

Spin Battery - Store Energy in Magnet ?

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“spin battery”, a battery that is "charged" by applying a large magnetic field to nano-magnets in magnetic tunnel junction (MTJ). This technology is basically store energy in magnets and potentially lead to creation of computer hard drives with no moving parts and new battery for power cars. A team lead by Physicist Stewart E. Barnes and and his collaborators.

The secret behind this technology is the use of nano-magnets to induce an electromotive force. It uses the same principles as those in a conventional battery, except in a more direct fashion. The energy stored in a battery, be it in an iPod or an electric car, is in the form of chemical energy. When something is turned "on" there is a chemical reaction which occurs and produces an electric current. The new technology converts the magnetic energy directly into electrical energy, without a chemical reaction. The electrical current made in this process is called a spin polarized current and finds use in a new technology called "spintronics.”

Tuesday, April 14, 2009

FREE Chemical Engineering Digital Issue for April 2009

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FREE Chemical Engineering Digital Issue for April 2009 has just been released !

Chemical Engineering has just released April 2009 issue. If you are subscriber of Chemical Engineering, you should have received similar notification.

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Interesting articles for this month :

Achema Defies Economic Odds
Even in a recession year, over 180,000 visitors from 100 countries are anticipated at this triennial event

Fingertips Energy Efficiency In Steam Systems
This one-page guide explains the importance of efficient condensate handling in steam systems

Optimizing Pumping Systems
Often overlooked, pump system efficiency makes a dramatic impact on process performance

Calculate Capital Costs Quickly
Estimating capital costs early can prevent wasting money on dead-end projects

Understanding Bends In Pneumatic Conveying Systems
Despite their apparent simplicity, bends are often poorly understood and unless properly designed, they are potentially problematic

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