chemical engineering: April 2009

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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TIPS
If you are subscriber, you may access previous digital releases. Learn more in "How to Access Previous Chemical Engineering Digital Issue".

If you yet to be subscriber of Chemical Engineering, requested your FREE subscription via this link (click HERE). Prior to fill-up the form, read "Tips on Succession in FREE Subscription".

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Sunday, April 12, 2009

Consider PST To Improve SIL

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Many real life experiences have shown that if valves are not work or stroke for substantial period, they may failed to work (stuck at preset position) when it is on demand. This is particular critical to safety related valves i.e. Emergency shutdown (ESD) valve, blowdown (BDV) valves, pressure dump valves, etc and potentially lead catastrophe event. Valve reliability and availability is extremely critical and important in a safety system. Functional test of these valves during operation is required to main valves reliability and availability while maintaining zero / minimum production downtime. One of the testing method is on-line Partial Stroke Testing (PST), which is particular suitable for ESD valves, as well as other critical valves. PST—supplemental testing offers a method of testing the valve by moving it, typically 15-25%, and back to the original position in a short period of time in order to confirm the valve’s ability to move (not stuck in place) and its suitability for continued safety instrumented system (SIS) service.

A PST with dedicated testing interval improve a Safety Integrity Level (SIL) level of a particular SIS system. For example, a typical one-out-of-one (1oo1) ball valve with 1oo1 solenoid (Proof test, PT interval of 1 year, the average Probability of Failure on Demand (PFD_avg) is about 2.25 x 10^-2, this lead to SIL of 1.65. With a PST, it improve the PFD_avg to 7.36 x 10^-3, equivalent to SIL of 2.13 with typical PFD_avg Reduction of 67%. Detail refer to "Valve Failure :Not an Option". This sometime is important and useful during plant revamping and modification. For example, present SIS system demand only SIL 1 . Plant revamping and modification results SIL 2 demand. Inclusion of PST would make SIS system sufficient.

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Saturday, April 11, 2009

Calculate Wetted Surface Area For Horizontal Vessel With Elliptical Head (Simplified)

Earlier post "Calculate Wetted Surface Area For Horizontal Vessel With Elliptical Head" has presented an accurate equation may be used to calculate wetted surface area for Horizontal Vessel with Elliptical Head. This post will presented a simplified equation to calculate the wetted surface area. Compare to accurate presented earlier, the difference is within 2%.

Let take the same sketch and example used in earlier post... :
A horizontal pressure vessel with radius R and length L (tan-tan) with liquid height of H, the wetted surface area can be the total wetted surface area of cylindrical section and elliptical head (2 heads).

Wetted Surface Area (Cylindrical section)
Wetted Surface Area for Cylindrical section can be calculated with following equation :

Wetted Surface Area (Elliptical head)
Wetted Surface Area for Elliptical head (one head) can be calculated with following equation :

where
R = Vessel inside radius (m)
H = Liquid height from bottom (m)
L = Vessel tangent-to-tangent length (m)

Example

An ellipsoidal heads horizontal vessel with internal diameter (D) of 3m and tangent-tangent length is 6m. Determine wetted surface area when maximum liquid level is at 1m above vessel bottom.

L = 6m
R = D / 2 = 1.5m
Awet,Cyl = 2LRxAcos[(R-H)/R]
Awet,Cyl = 2x6x1.5xAcos[(1.5-1)/1.5]= 22.16 m2
Awet,Head = [2.178 / (2 x 3.141592654)]x(2x1.5)^2 x Acos[(1.5-1)/1.5] = 3.84 m2
Total wetted surface area, S = Scyl + 2 x Shd = 29.84 m2

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Above equations have been programmed by Ankur & Kenny, share with readers of Chemical and Process Technology. You may download here.

Thanks to Ankur & Kenny
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*If you have any useful program and would like to share within our community, please send to me.

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Thursday, April 9, 2009

Calculate Wetted Surface Area For Horizontal Vessel With Elliptical Head

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Fire relief estimation for two phase gas liquid in horizontal pressure vessel, wetted surface area is required to estimate total heat input into the horizontal pressure vessel. A horizontal pressure vessel with radius R and length L (tan-tan) with liquid height of H, the wetted surface area can be the total wetted surface area of cylindrical section and elliptical head (2 heads).

Wetted Surface Area (Cylindrical section)
Wetted Surface Area for Cylindrical section can be calculated with following equation :

Wetted Surface Area (Elliptical head)
Wetted Surface Area for Elliptical head (one head) can be calculated with following equation :

where
R = Vessel inside radius (m)
H = Liquid height from bottom (m)
L = Vessel tangent-to-tangent length (m)
e (epsilon) = Eccentricity (0.866 for 2:1 Ellipsoidal head)
F = Fraction Liquid Level, = H / 2R

Example

An ellipsoidal heads horizontal vessel with internal diameter (D) of 3m and tangent-tangent length is 6m. Determine wetted surface area when maximum liquid level is at 1m above vessel bottom.

L = 6m
R = D / 2 = 1.5m
F = H / 2R = 1 / (2 x 1.5) = 0.333
A = F - 0.5 = 0.333 - 0.5 = -0.167
B = SQRT[1+12A^2] = SQRT[1+12x(-0.167)^2] = 1.1547
Awet,Cyl = 2LRxAcos[(R-H)/R]
Awet,Cyl = 2x6x1.5xAcos[(1.5-1)/1.5]= 22.16 m2
Awet,Head = (PIxR^2/2)x[AxB+1+1/(4e)xLn[((4exA)+B)/(2-3^0.5)]=3.64 m2
Total wetted surface area, S = Scyl + 2 x Shd = 29.43 m2

Ref : "Accurate Wetted Areas for Partially Filled Vessels", by Richard C. Doane, "Chemical Engineering", December 2007

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Above equations have been programmed by Ankur, a experience Chemical Engineer, share with readers of Chemical and Process Technology. You may download here.

Thanks to Ankur
Download

*If you have any useful program and would like to share within our community, please send to me.

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Wednesday, April 8, 2009

Chemical Engineer Average Salary From 2005 - 2009

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In recent world economy crisis, oil & gas and Chemical Processing industries have partly affected by this crisis. So far Chemical engineer is categorized as one of high-income-level position among all top engineering profession. How recent world economy crisis has affected the income as Chemical engineer ? You may also wonder how annual income of a chemical engineer change from year to year to 2009 ?

ChemicalProcessing.com has conducted annual salary survey from 2005, to 2009. Survey show that average salaries have steadily increased from 2005 to 2009. Average salaries started with $85,234 in 2005, increased to $89,690 in 2006, $90,038 in 2007, $95,231 in 2008 and lately $107,804 in 2009. However, the average pay raise was down slightly, 4.23% compared to the 5.06% reported in 2008. Details of these surveys may refer to :

Above surveys indicated that our industry still remain strong in present economy downturn.

Thanks to Ken Schnepf, Managing Editor of Chemical Processing.

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HSE in projects - Focusing on design

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Although International and National Oil and Gas companies are outsourcing more and more services to contractors, they still have the responsibility for formally demonstrating that HSE is being managed satisfactorily in all of their assets. It is becoming a mandatory requirement by governmental bodies globally that O&G companies submit this demonstration (i.e., HSE Case) for approval to conduct the asset operation. In Malaysia, for example, these take the form of Environmental Impact Assessment (EIA) reports by DOE or Safety Reports by DOSH as relevant.

The objective of this talk is to provide an overview of the phases of a project along with the project management controls necessary for the successful management of HSE as applicable to E&P type projects. It will describe the specific applications of Hazards and Risk Management tools at key stages of the project and will review how the ALARP target factors in on major project conceptual and detailed design decisions and direction.

The talk will focus, on various project stages and the relevant HSE objectives, tools and techniques that typically are employed to achieve the desired objectives and the deliverables expected at the end of each stage. It will also provide pointers to more detailed references and guidance for their application. It is not the intention of this talk to provide a complete specification of the HSE activities to be undertaken at each stage of a project. Nonetheless, it will provide guidance on key activities that need to be considered by those responsible for project design.

A talk on "HSE in projects - Focusing on design", organized by Oil, Gas & Mining Technical Division, IEM has been scheduled.

Date : 13 April 2009 (Monday)
Time : 5.30 pm to 7.00 pm
CPD : 2 Hours (approved by BEM)
Venue : Wisma IEM (New Building), Petaling Jaya
Speaker : Mr. Christopher James Bernard (CJB

*Any queries, please contact sec@iem.org.my.

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Monday, April 6, 2009

Sound Power Level (PWL) Prediction from AIV Aspect

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Earlier discussion in "Acoustic Induced Vibration (AIV) Fatigue" has discussed the generation of high frequency acoustic excitation downstream of pressure reducing device and potential of downstream piping failure due to Acoustic Induced Vibration (AIV). Acoustic energy can lead to circumferential and longitudinal excitation. Read more in "Piping Excitation When Expose to Acoustic Energy" to see how piping is excited by acoustic energy. Piping downstream of pressure reducing device shall be designed sufficiently strong to resist AIV fatigue. One of the common acceptable criteria to indicate piping resistivity to AIV fatigue is ensuring Sound Power Level (PWL) allowable limit of downstream piping higher than the PWL generated by the pressure reducing device.

Sound Power Level Generated by Pressure Reducing Device
Acoustic energy generated by a pressure reducing device may range from 0 to 10 MW. It is more convinient to express this energy in another term called Sound Power Level (PWL) in logarithmic scale with reference to a most common acceptable reference power of 10^(-12) watts (W). Thus, PWL is 10 times the logarithm to the base 10 of the ratio of acoustic enerygy to reference power of 10^(-12) watts. A common acceptable method to predict Sound Power Level (PWL in dB) is as follow :

Example
A Depressuring valve open to pass 100,000 kg/h of gas with molecular weight (MW) of 22. The inlet condition is 87 barg and 50 degC and estimated backpressure is about 7 barg.

PWL = 10 x Log [((87-7) / (87+1.01325))^3.6
x (100,000 / 3600)^2
x ((50+273.15)/22)^1.2]
+ 126.1

PWL = 167.5 dB

Allowable Sound Power Level (PWL) of Piping
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. This may be discussed in future post.

Ref :
i) "Designing Piping Systems Against Acoustically Induced Structural Fatigue", E.L. Eisinger, Journal of Pressure Vessel Technology, Aug 1997.

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Saturday, April 4, 2009

Piping Excitation When Expose to Acoustic Energy

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Earlier discussion in "Acoustic Induced Vibration (AIV) Fatigue" has discussed the generation of high frequency acoustic excitation downstream of pressure reducing device and potential of downstream piping failure due to Acoustic Induced Vibration (AIV). Whenever there is pressure drop with mass passing through the valve, internal acoustic energy is generated and transmitted to downstream piping and potentially lead to severe piping excitation, vibration and stresses on downstream piping, in particular at discontinuity section i.e fabricated Tee, small bore connection, welded pipe and pipe support, etc. This acoustic excitation phenomena is generally involve high frequency (more than 1000Hz) acoustic energy. When high frequency acoustic energy is matches with mechanical natural frequency of piping and its component, excitation amplitude is at maximum and lead to increased stress level.

A piping segment may be excited in circumferential and longitudinal mode. Following image shows piping circumferential and longitudinal excitation patterns in different nodal arrangement.

The following image display acoustic excitation in 3D.

The following video clip shown a piping excitation in circumferential and lead to severe stress level at branch.