chemical engineering

Sunday, July 26, 2009

Heat Radiation For Pain & Blistering Threshold

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Flare is commonly installed in oil and gas process plant to burn hydrocarbon and/or toxic gas to avoid formation of combustible mixture, to minimize green house effect (GHE), to minimize health hazards to personnel on site, etc. Flaring hydrocarbon gas may generate carbon dioxide & water for complete combustion and soot (contribute to smokeless level) & unburnt components (contributes to toxic environment) for incomplete combustion. Besides, heat and noise are generated and radiated and transmitted around the flare tip.

Heat radiated from flare may transmitted in sphere form around the flare tip. Along the transmission, the energy is distributed in sphere form and this lead to reduction in heat radiation level (heat flux, kW/m2). Personnel or equipment along the transmission path will expose to this heat radiation. Personnel or equipment closer to flare tip will experience higher heat radiation level.

With studies conducted by Stoll and Greene (1958), following graph relate heat radiation versus time for Pain threshold and Blister threshold. With heat radiation of 6.3 kW/m2 (2 000 Btu/h·ft2), the pain threshold is reached in 8 s and blistering occurs in 20 s.

The following equation derived from above graph and can be used to relates heat radiation with time for pain and blister thresholds.

Pain threshold :

q = 25.544 x t ^-0.6742

Blister threshold :

q = 75.691 x t ^-0.8399

where :
q = heat radiation (kW/m2)
t = time (seconds)

Ref :
(i) A. M. STOLL and L. C. GREEN, The Production of Burns by Thermal Radiation of Medium Intensity, Paper Number 58-A-219, American Society of Mechanical Engineers, New York, 1958

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Saturday, July 25, 2009

Pneumatic Test Failure in Mississippi Pipeline Project

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An explosion caused by pneumatic test failure has occurred in Shanghai LNG Terminal on Feb 06 2009. This accident has resulted one worker killed and fifteen others injured.

Again it was heard that another pneumatic test failure has occurred in Mississippi in July 2009 and lead to at least one fatality and three others injured during a pneumatic test failure. The following photos show how severe is the destruction of pneumatic test failure.

Overall scene

Air-lifted pipeline

Twisted metal pipe

For those who intended to conduct pneumatic test, really have to ensure that proper procedure is followed. Ensure personnel who are not directly involved in the testing process are removed from the area and not allowed in the area for any reason. If you not involved directly in pneumatic test, please be alert and move away from the testing location.

Following are collection of article that helps you to educate yourself or your operators related to Pneumatic test :

Pneumatic Test During Pre-commissioning
This article has brief introduction about pneumatic test related procedure, safety concern, test pressure level, etc

Pneumatic Test Accident in Singapore
On May 22, 2002, a fatal accident occurred in Singapore involving the failure of a refrigerant receiver during a pneumatic test. In light of this incident, it is appropriate to again remind our readers of the hazards involved in pneumatic tests and to review the precautions that must be taken in conducting such tests.

Hazards of Trapped Pressure and Vacuum
A leak test on a heat exchanger was being conducted using low pressure gas when the tube bundle was ejected with great force striking two employees. One of them died on massive internal injuries...

Pipeline fails under air pressure test - Kills worker
Two workers had completed laying a 30 metre length of 300 mm diameter PVC pipe, in order to connect it to an existing steel pipe, along a suburban roadside. The pipe was then to be pneumatically tested up to a pressure of 690 kPa (100 psi)...

Pneumatic Test Operation Maintenance
This tank is intended for use vented to atmosphere. For outdoor applications, install a weatherproof vent hood or cap on the vent riser pipe and on the interstitial space vent of double wall tanks.

Pneumatic Test - Incident in Brazil
Incident happened in a non-ExxonMobil facility in Brazil during a pneumatic test of the tank associated piping. A blind was NOT installed to isolate the ...
Pdf

Pneumatic Test - IncidentASTM A1047 / A1047M - 05
ASTM A1047 / A1047M - 05 Standard Test Method for Pneumatic Leak Testing of Tubing

Read others incident...Click HERE

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Sunday, July 19, 2009

FREE Chemical Engineering Digital Issue for July 2009

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FREE Chemical Engineering Digital Issue for July 2009 has been released !

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

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Interesting articles for this month :

FAYF - Flowmeter Selection
This one-page guide details important facts for selecting a flowmeter

Revamps - Strategies for A Smooth Turnaround
Tie-in opportunities are few and far between. These rules of thumb will help make sure everything and everyone line up in time
Removal of Fouling Deposits on Heat Transfer Surfaces in Coal-Fired Process Heaters and Boilers
When conventional soot blowers are inadequate, an automated shot-blasting system offers a powerful solution
* If you encounter problem in reading this article, you may check this article via this link (for FREE subscriber only).

Disperse Difficult Solids
Recent advances in mixing technology offer increased efficiency in dispersing powdered additives into liquids for both low- and high-viscosity applications

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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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Saturday, July 18, 2009

Energy Input or E-method In Assessing AIV

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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 process plant

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.

There are several methods have been discussed earlier in assessing AIV problem. There are "D-method" and "D/t-method". These two methods are widely used by many engineering contractor and consultants in many previous projects. There is another method, the Energy Input method or "E-method" which has been introduced in mid 1990.

Energy Input or E-method In Assessing AIV
The "E-method" is focus on the acoustic energy transmitted along the pipe, downstream of pressure reducing devices. The acoustic energy can be measured by Ma2.dP where Ma2 is the Mach number at downstream pipe and dP is the pressure drop across pressure reducing devices. The higher the acoustic energy or Ma2.dP, higher the risk for AIV failure.

Similar to the other two methods, based on data available in Carucci & Mueller (1982) studies, "No failure" and "Failure" points have been plotted in following chart with Ma2.dP versus Pipe Diameter / Wall thickness (D/t).

Note :
1) A "failure" point (54.8, 0.35) 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

From above chart a clear distinctive limit line can be established. 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.

*As limit line is straight or following any pattern, a representative equation is yet to be developed.

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Wednesday, June 24, 2009

FAYF - Fluid Flow

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Chemical Engineering has shared a FAYF related FLUID FLOW. This FAYF is infact has been released in previous month, however, CE share with CE FREE subscriber again this month.

Fluid Type
In this FAYF, it starts with definition of fluid type such as :
i) Newtonian
ii) Power law
iii) Bingham plastic

where

Newtonian fluid
A fluid is known to be Newtonian when shear stresses associated with flow are directly proportional to the shear rate of the fluid.

Power law fluid
A structural fluid has a structure that forms in the undeformed state, but then breaks down as shear rate increases. Such a fluid exhibits “power law” behavior at intermediate shear rates

Bingham plastic fluid
A plastic is a material that exhibits a yield stress, meaning that it behaves as a solid below the stress level and as a fluid above the stress level

This one-page fact sheet summarizes information pertinent to laminar and turbulent pipe flow for the various types of fluids commonly encountered in the CPI...

Ref. :
1. Darby, R., Take the Mystery Out of Non-Newtonian Fluids, Chem. Eng., March 2001, pp. 66–73.
2. Churchil, S. W., Friction Factor Equation Spans all Fluid- Flow Regimes, Chem. Eng., November 1997, p. 91.
3. Darby, R., and Chang, H. D., A Generalized Correlation for Friction Loss in Drag-reducing Polymer Solutions, AIChE J., 30, p. 274, 1984.
4. Darby, R., and Chang, H. D., A Friction Factor Equation for Bingham Plastics, Slurries and Suspensions for all Fluid Flow Regimes, Chem. Eng., December 28, 1981, pp. 59–61.
5. Darby, R., “Fluid Mechanics for Chemical Engineers,” Vol. 2, Marcel Dekker, New York, N.Y., 2001.

Download (Only for FREE CE Subscriber)

Note :
*This FAYF is only available FREE to Chemical Engineering Magazine registered user. Login required. Subscribe FREE CE, click here.
** Download immediately as article available FREE within short period only. Do not wait.
*** Found lost link or unable to download, may contact me...

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Monday, June 22, 2009

Comparison for LNG Production Processess

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Liquefied Natural Gas (LNG) is one of the cleanest energy among all other energy sources.LNG and Supply Chain discussed briefly entire production, transportation, receiving and distribution path of LNG. There are still many peoples who are safety, environment & security concerns, not agreeing with the LNG production as discussed in "LNG SES Issues...".

Liquefy Natural gas is a process of cooling down the natural gas to form liquid for easy storage and transportation. There are few ways to cool down natural gas. Typically are mechanical refrigeration, JT valve and expansion turbine. More discussion in "Techniques to Achieve Cryogenic Temperature". Nowadays, liquefying LNG processes generally adopting combination of two or three techniques as discussed above. Following is a tabulation of a few well-known LNG processes.

Process	C3MR	Cascade	SMR	DMR	MFC	N2 Exp
Thermal Eff.	High	High	Med.	High	High	Low
Equip. no	Med.	High	Low	Med.	Med.	Med.
Precooling HX	Kettle	Core-in -Kettle	Plate-fin	Spiral Wound	Plate-fin	Kettle
Liq. HX	Spiral Wound	Core-in -Kettle Plate-fin	Plate-fin	Spiral Wound	Spiral Wound	Plate-fin
Refrig. Storage	Large	Large	Med.	Med.	Med.	None
CAPEX	Med.	Med.	Low	Med.	Med.	High

Monday, June 15, 2009

Simplified Equation for Wind Speed Estimation At Different Height

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As discussed in earlier post "Estimate Wind Speed At Flare Tip At Different Height", correct wind speed at flare tip (elevated flare) is important in order to obtain correct estimate of radiation level and unburnt component concentration at downwind location, determination of minimum vapor flow to avoid flame-out and performance of pilots. A rather complicated equations may be used to estimate wind speed at different height.

This will present a rather simple relation to estimate the wind speed. The following equation may be considered.

Where
U_Z = Wind speed at Z m at return duration of t₀ hour (m/s)
U₀ = Wind speed at specified height of 10m at return duration of t₀ hour (m/s)
t₀ = Wind speed at return duration i.e. 60 minutes, 1 minutes, etc
Z = Flare stack height (m)
K = Field data derived parameter (may use 0.125)

Example :
A flare stack with height of 200m, expose to wind speed (at 60 minutes return duration) of 10 m/s measured at 10m from grade. Determine wind speed at flare tip if the return duration stay as 60 minutes.

Solution (a)
Z = 200 m
U₀ = 10 m/s

U_Z = 14.54 m/s
Wind speed at flare tip with return duration stay as 60 minutes = 14.54 m/s

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