Chemical & Process Technology

A talk on "Energy Aspect In Industrial Drying", organized by Chemical Engineering Technical Division, IEM has been scheduled.

• Practicing Engineering & Route To Professional Engineer in MALAYSIA
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A talk on "Industrial Drying", organized by Chemical Engineering Technical Division, IEM has been scheduled.

• Practicing Engineering & Route To Professional Engineer in MALAYSIA
• R&D engineer, Academician and Student...Don't miss this !

A young engineer asked…He is conducting a flare network studies using ASFA / FLARENET in conceptual stage. He need to allocate pressure drop across flare stack and tip. What is the quickest way ?

Flare tip pressure drop generally subject to flare tip design and only the tip vendor would be able to provide. Quickest way is to obtain vendor advice on similar kind of tip. However, in the absence of vendor information, the flare tip may be estimated according to discussion in earlier posts "Estimate Subsonic Flare Tip Pressure Drop With Graph Derived Correlation" and "Quick Estimate Flare Tip Pressure Drop".

Flare stack is made of vertical pipe in onshore plant and incline pipe (some called it boom) in offshore platform. The height is primarily govern by personnel allowable heat radiation at sterile area. As it is a pipe in nature, gas flow through flare stack during emergency and normal operation will result velocity and pressure drop across the flare stack. Thus, the flare stack diameter is primarily determine by Mach no and pressure drop. The recommended Mach no for flare stack is 0.5 during emergency relief (design capacity) and 0.2 during normal flow. Normal flow could be gas discharge from those pressure control valve (PCV) used to avoid pressure spike in process system.

Mach no here will be the Mach number at downstream of flare stack. One shall remember, the pressure drop across flare tip may be taken into account and it potentially reduce the Mach no and hence the flare stack diameter. The flare tip pressure drop could be estimated according discussion in earlier posts "Estimate Subsonic Flare Tip Pressure Drop With Graph Derived Correlation" and "Quick Estimate Flare Tip Pressure Drop".

Select a diameter, D which large or equal to minimum diameter (d) calculated above. Once the D is selected, the pressure drop across the flare stack may be estimated using Isothermal flow equation.

Related Topic

• Estimate Subsonic Flare Tip Pressure Drop With Graph Derived Correlation
• Quick Estimate Flare Tip Pressure Drop
• Several Criteria and Constraints for Flare Network - Process
• Several Criteria and Constraints for Flare Network - Piping
• Provide More than One Flare KOD in SERIES
• Flare Tip Quick Selection Chart
• Flare Smokeless Ringlemann Chart

Earth Hour, an annual international event created by the WWF (World Wide Fund for Nature/World Wildlife Fund) is coming back on March 28, 2009 to create awareness on global warming and the need to take action on climate change. The event has no boundary and is extended from individual to large organization, from households to business. Every living can participate...

We in Chemical & Process, Oil and Gas industries known as one of the major "contributor" in global warming. Why not take this opportunity to do something ?

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Earlier post "Quick Estimate Flare Tip Pressure Drop" has presented some clues in obtaining flare tip pressure drop for sonic and subsonic tip. Particularly for subsonic flare tip, a simple pressure drop versus flow curve was presented. This graph may be used for quick estimation during conceptual stage so that it minimize unnecessary time lost. However, the graph has limited to 0.5 - 40 kPa with correspondent minimum and maximum flow. Some conceptual studies may have flaring capacity resultant pressure drop more than 40 kPa, the graph may not be useful. In this post, a simple formula will be presented in order to estimate pressure drop for any flow rate for dedicated flare tip.

Note 1 : The standard condition may be different from project to project (read more in "Avoid Confusion In "Standard" Flow Definition". Present standard definition is at 101.325 kPa abs and 15 degC.

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Previous post "Several Criteria and Constraints for Flare Network - Process" has discussed several major important criteria and constraints related flare system. In particular, back pressure is critical to performance of pressure relief valve (PRV) from capacity and stability aspect. Back pressure at PRV is typically back calculated from flare tip, main header, sub-header and finally tail pipe. Isothermal equation may be used for flare pipe pressure drop calculation for conservatism. AFSA or FLARENET is commonly used for flare network modeling. In calculating back pressure at the PRV, flare tip pressure drop is required. However, the pressure drop of flare is subject to Flare tip vendor design. How shall engineer determine the pressure drop of flare tip without vendor information especially during conceptual design ?

Some simple clues for pressure drop estimation across a sonic flare tip and subsonic flare tip will be discussed. It may be used as first estimate and shall be used for detailed design. As flare tip pressure drop is subject to flare tip design, pressure drop provided by vendor shall always be used during detailed design.

For a sonic flare tip, pressure drop may be in the range of 3 - 5 bar. Pressure drop of upto to 7 bar has also been used. Flare system with sonic flare tip may experience high back pressure, high pressure (HP) system with high set pressure PRV may discharge into flare system with sonic tip.

For a subsonic flare tip, pressure drop is generally very small (possibly lower than 1 bar). Low pressure (LP) system PRV is generally discharge into flare system with subsonic flare tip as low back pressure is expected. Following is a typical flare tip pressure drop versus flow for different flare tip diameter.





Related Topic

• Several Criteria and Constraints for Flare Network - Process
• Several Criteria and Constraints for Flare Network - Piping
• Provide More than One Flare KOD in SERIES
• Elevated Flare (SI Unit) Datasheet
• ANSI/API Std 537 / ISO 25457 2nd edition, Dec 2008 is Released
• Flare Tip Quick Selection Chart
• Flare Smokeless Ringlemann Chart

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In the first release of ANSI / API Std 537 / ISO 25457:2008 "Flare Details For General Refinery and Petrochemical Service", there are few Flare data sheets have been included. The main purpose of these data sheets is to assist plant operator, engineering contractor and flare tip manufacturer in information transfer and communication.

In earlier post "Several Criteria and Constraints for Flare Network - Process", several major important criteria and constraints have been discussed. They includes Mach number, back pressure, noise level and two phase flow pattern. This post is the continuation and futher discuss other criteria and constraints for flare networt.

Flow Induced Vibration (FIV)
In a process plant, some of pressure control valves (PCV) may be provided in the process system to release gas into flare network to avoid transient peak pressure in the process system which potentially lead to unnecessary plant shutdown. Besides, this PCV may be used to release off specification gas into flare during plant start-up. Above events will increase the operational time of the PCV and its downstream piping and increase the piping Likelihood of Failure (LOF) due to flow induced vibration (FIV). Typically these PCV would experience low frequency exitation. It is good engineering practice to ensure the

• LOF2 < 0.3 + Good engineerng practice for Small Bore Connection (SBC) or;
• 0.3 <= LOF2 < 0.5 +LOF3 < 0.4

LOF2 is Likelihood of Failure for main pipe assessment
LOF3 is Likelihood of Failure for small bore connection

Acoustic Induced Vibration (AIV)
Pressure control valve (PCV) with choking condition, pressure relief valve (PRV) and Blowdown valve with restriction orifice (BDV/RO) provided in the process and relief to flare network would cause high frequency acoustic exitation to downstream piping and potential results piping failure on Acoustic Induced Vibration (AIV). If the PWL generated by these devices is below PWL lower than 155 dB, the piping downstream of these devices are considered safe from AIV fatigue failure.

Piping downstream of these device may have different wall thickness. Piping with higher wall thickness will be more likely to resist higher PWL. Based on field experiences, table below list the maxmum limit of PWL with increasing ratio of Outside diameter (OD) to wall thickness (wt).

Limit of PWL vs OD/wt
OD / wt	PWL,max
20	174
30	173
40	172
50	170
60	169
70	168
80	167
90	165
100	164
110	163
120	162
130	160
140	159
150	158

Above is the PWL limit , some margin i.e. 3 dB may need to be added.

Maximum and Minimum Design Temperature and Thermal Shock
A flare network maximum and minimum design temperature will be typically determine from fluid discharging into the flare network. The maximum design temperature would typicall determine based on maximum possible fluid temperature with margin (plus) i.e. 10 degC whilst the minimum design temperature would typically determine from minimum fluid temperature with margin (minus) i.e. -10 degC or the fluid minimum depressured temperature with no or lower magin i.e. - 5 degC. The flare network potentially expose to high temeperature fluid during normal plant operaion and follow by cold fluid discharge from process system. Sudden large temeprature decrease in piping may cause thermal shock. Thus, flare piping stress shall analysed base on worst possible and credible temperature different and ensure sufficient expansion loops are provided and proper location of support.

Slug Hammer Elbow
For flare network piping identify potentiall expose to slugging condition, beside provision of necessary support, the slug hammering elbow is potential event and needs to further analyse the impact of the slug hammer.

Solidification, Crystillization, Polymerization, Hydrate and/or Ice Formation
Relief fluid believe to solidify, crystallize and/or polymerize, it potentially plug / block the relief tail pipe and flare network. Further analysis shall be conducted to check should these fluids be discharged in to the flare network. Possible solution is discharge to separate and dedicated disposal drum and vent.

For some flare network receiving wet fluid from high pressure system i.e. slugcatcher, theoretically there is potential of hydrate formation. However, due to high velocity, warm flare piping, intermittent service and non-sustainable discharge flow, field experience shows the likelihood of hydrate formation in flare network is low. Having said that additional measures shall be taken in to consideration. Read more in "Hydrate formed Downstream of PRV ?".

Other issue such as correct material for discharge fluid, corrosion and stress cracking, fluid compatibility studies, etc are those studies have been conducted and decided prior to flare network analysis. Thus, they are not discuss further in this post.

Concluding Remark
Previous post "Several Criteria and Constraints for Flare Network - Process" have discussed those criteria and constraints typically for process aspect whilst this post are typically from piping aspect. All criteria and constrains are listed as follow :

• Mach no
• Momentum
  
• Back pressure
• Noise
• Two phase flow pattern
• Flow Induced Vibration
• Acosutic Induced Vibration
• Thermal Shock
• Slug Hammering
• Hydrate / Ice Formation

These criteria and constraints shall be analysed in detail for flare network studies.

Related Topic

• Provide More than One Flare KOD in SERIES
• Elevated Flare (SI Unit) Datasheet
• ANSI/API Std 537 / ISO 25457 2nd edition, Dec 2008 is Released
• Flare Tip Quick Selection Chart
• Flare Smokeless Ringlemann Chart
• Guideline on Quick Determination of Flare Stack Support Type
  
• Must Read Flare Handbook

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Condensate Pump Recirculation Valve Selection
Power plant condenser receives exhaust steam from the low pressure turbine and condenses it to liquid for reuse. Condenser back pressure range from 1.0 to 4.5 Hg absolute (3.4 to 15.2 kPa) with higher pressures possible when the cooling water temperature is elevated by using an air-cooled steam condenser. Condensate is collected in the bottom of the condenser in the hot well. The condensate feed pump supplies the subcooled water to the feedwater heaters.

As with most centrifugal pumps, the condensate pump is subject to overheating and cavitation if used at a flow under a minimum value recommendation by the pump manufacturer. When the flow required by the deaerator level control loop falls below this minimum recommended value, additional flow is recirculated back to the condenser by opening a valve installed in a bypass line thus maintaining the minimum flow through the pump at all time. There are problems associate with this valve and its selection. Read more...

Flare system commonly consists of collection networks, liquid Knock-Out drum, knock-out pump and flare stack with tip. Some flare system may includes liquid seal drum, air ingress & purge reduction seal, flare recovery system, liquid heater and/or vaporiser, etc In recent posting, there are several topics related to Flare have been discussed :

• Providing More than One Flare KOD in SERIES
• Elevated Flare (SI Unit) Datasheet
• ANSI/API Std 537 / ISO 25457 2nd edition, Dec 2008 is Released
• Flare Tip Quick Selection Chart
• Flare Smokeless Ringlemann Chart
• Guideline on Quick Determination of Flare Stack Support Type
  
• Why not bury flare pipe header ?

If you are dealing with Flare, one of the technical book that you may not missed is John Zink Combustion handbook. Flare network hydraulic simulation may be conducted using Aspen Flare System Analyzer, AFSA (formerly FLARENET). Interesting and useful documents related to AFSA / FLARENET can refer to "Useful Documentation for AFSA / FLARENET...". There are several constraints, parameters and/or criteria that may be considered while carry out Flare network hydraulic studies :

Mach No.
Mach number is the ratio of fluid flowing velocity to fluid sonic velocity. Mach number equation for a fluid may refer to previous post "...Mach No. & Critical Pressure Calculation". For Pressure Relief Valve tail pipe (pipe immediate downstream of PRV), Mach no is commonly limits to 0.7 whilst for collection header, Mach no limit to 0.5. The flow for calculation for tail pipe and header are subject to PRV type. Read more in "Consider Rated flow or Required Relieving Flow ?".

One shall take note, above are common recommendation and good engineering practice. Some engineers may design flare network to Mach no of 1.0, Several concerns related to flare system design to Mach no of 1 may refer to "Is PSV tail pipe & lateral at CHOKED (Mach no = 1) Accpetable ?".

Momentum (density x velocity2)
Momentum is fluid density time fluid flowing velocity power two. For tail pipe, maximum momemtum may be limited to 150,000 Pa whilst for collection header, limited to 100,000 Pa. The flow for calculation for tail pipe and header are subject to PRV type. Read more in "Consider Rated flow or Required Relieving Flow ?". Above value may be increased (not more than 200,000 Pa) provided that the piping support and vibration analysis are healthy.

Back Pressure
Increase (or reduction) in PRV tail pipe or flare header size may affect Mach no. It also decrease (or increase) back pressure to PRV. A conventional Spring loaded pressure relief valve, maximum allowable back pressure (MABP) is typically limited to 10% of PRV set pressure. A balanced bellow (or piston) type pressure relief valve, MABP is typically limited to 30% -50% of PRV set pressure. For pilot operated PRV, MABP of more than 50% of PRV set pressure may be allowed (some previous experience may reach 80% of set pressure). Above are typical value base on Good Engineering practice. Detail and exact MABP is subject to actual PRV and guaranteed by PRV vendor.

Above are typically related to performance (relief capability) and stability of PRV (as discussed in "Several Impact of Backpressure on Conventional PRV". One shall take note that there is Maximum Allowable Backpressure due to mechanical limitation which subject to temperature. Detail may refer to API Std 526.

When discussed about PRV back pressure, correct definition of "back pressure" shall be used in communicating information to PRV vendor. Discussion on confusion about "back pressure" may refer to "PRD Backpressure".

Noise Level
As fluid passing through the PRV (and tail pipe & header), significant noise would be generated and transmitted along the tail pipe and header. The noise may also emitted to atmosphere. One of the common safety requirement is limit the noise level to 115 dBA (Noise level with A-weighted) during intermittent emergency relief scenario. Besides intermittent relief from PRV, some Pressure control valve (PCV) may discharge (continuous or frequent) fluid into flare network. The noise level may limit to 85 dBA for continuous scenario. One shall remember, this noise level should be the mix of noise from device and back ground noise i.e. pump compressor, etc. Acoustic insulation may be considered to minimise noise emission from PRV, tail pipe and headers.

Two Phase Flow Pattern
During common mode relief scenario i.e. total plant power failure, total cooling water failure, etc may leads to multiple PRVs relieve. JT cooling due to pressure reduction, hot fluid mix with cold fluid and composition change may results two phase flow in the tail pipe and header. The flow pattern of this two phase flow shall be analysed and avoid slugging flow pattern as much as possible. Typically may consider to use Taitel-Dukler map to determin flow pattern. Flare network exposing to two phase flow, piping support designer shall make aware and provide sufficient support for piping with two phase flow. If necessary, may consider additional intermediate knock out drum to remove liquid (as discussed in "Provide More than One Flare KOD in SERIES".

[More disucssion about AIV, FIV, Thermal shock, Slug hammering, etc in "Several Criteria and Constraints for Flare Network - Piping"]

Related Topic

• Provide More than One Flare KOD in SERIES
• Elevated Flare (SI Unit) Datasheet
• ANSI/API Std 537 / ISO 25457 2nd edition, Dec 2008 is Released
• Flare Tip Quick Selection Chart
• Flare Smokeless Ringlemann Chart
• Guideline on Quick Determination of Flare Stack Support Type
  
• Must Read Flare Handbook

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The petroleum characterization method in Aspen HYSYS converts laboratory analysis of condensates, crude oils, petroleum cuts, and coal-tar liquids into a series of discrete hypothetical components. These petroleum hypo components provide the basis for the property package to predict the remaining thermodynamic and transport properties necessary for fluid modeling. Aspen HYSYS produces a complete set of physical and critical properties for the petroleum hypo components with a minimal amount of information. However, the more information you supply about the fluid, the more accurate these properties will be, and the better Aspen HYSYS will predict the fluid's actual behavior.

In this example, the Oil Characterization option in Aspen HYSYS is used to model a reservoir fluid. The fluid is a combined gas and oil stream. Read more in Oil Characterization.

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• Dynamic Modeling - Process Control and Design Applications
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• Useful Documentation for HYSYS ...

(i) Vapor & Liquid Recover to Source
In a plant, there may consist several production trains with common flare system. This is one of the strategy to minimize capital investment cost. Typical plant with multiple trains and common flare is Liquefied Natural Gas (LNG) and gas processing plant. Example Qatar gas LNG plant has at least 7 trains, Australia North West Shelf has at least 5 trains, etc. Although all trains are located in same place sharing common flare system, however the owner of these train may be different. Probably Train 1, 2 & 3 are owned by company ABC, train 4 & 5 own by company MMM and train 6 & 7 own by company XYZ. Vapor and liquid hydrocarbon leaks or relief from a train, this valuable hydrocarbon may be recovered back to its train process system by same owner. Thus, providing a dedicated KOD for trains belonging to dedicated owner may serve above purpose.

(ii) Design limitation of Main Flare On Operational Non-Smoking Requirement
Common main flare shall be designed for largest load from all trains in any relieving scenario. Common relieve scenario contributes to large relief load are cooling water failure, total power failure, total plant blowdown, etc. This possibly lead to common main flare with large capacity. However, it is also common requirement to have non-smoking flaring during normal operation with low flow. As common main flare with large capacity may have limited turndown and exceeded the minimum normal operational flow, thus a dedicated operational flare may be provided for train(s) with same owner. Providing a dedicated KOD and operational flare for trains belonging to dedicated owner may serve above purpose.

iii) Mixing of Product may not be recoverable in any plant
Some plants with common flare system but relieve different of product. There is potential the mixture of the products may not be recoverable by any of the plant. Thus, a dedicated KOD for dedicated plants are provided so that the product relief from dedicated plant is recoverable in the plant which relieved the fluid.

(iv) Mixing of Fluid Cause Slugging
Common main flare system for plant with hot fluid and cold fluid may lead to condensation and results slugging flow in the flare header. Slugging in flare header potentially results severe erosion, noise and vibration. Providing a dedicated KOD will remove liquid from the relieve fluids and minimise the potential of slugging in the common header. This may only minimise, but not totally avoid as hot vapor may still mix with cold vapor in the common header and condensation/slugging flow may still possibly present. However, providing of dedicated KOD will reduce the slugging flow potential. In the event, severe slugging still possibly present and results problem to flare support, it is always advisable to provide separate flare system.

(v) Reduce Common Header Size
Providing dedicated KOD would possibly reduce relief flow (liquid) to the common header during common relief scenario. Reduction in relief flow will reduce the main flare header size.

Related Topic

• Elevated Flare (SI Unit) Datasheet
• ANSI/API Std 537 / ISO 25457 2nd edition, Dec 2008 is Released
• Flare Tip Quick Selection Chart
• Flare Smokeless Ringlemann Chart
• Guideline on Quick Determination of Flare Stack Support Type
  
• Must Read Flare Handbook
• Why not bury flare pipe header ?

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In the first release of ANSI / API Std 537 / ISO 25457:2008 "Flare Details For General Refinery and Petrochemical Service", there are few Flare data sheets have been included. The main purpose of these data sheets is to assist plant operator, engineering contractor and flare tip manufacturer in information transfer and communication.

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An explosion occurred in Shanghai LNG Terminal when the construction worker conducting pneumatic test of the equipment on Feb 06 2009. ONE worker was killed and 15 injured in an explosion on a construction site at Shanghai's Yangshan Deep Water Port. The accident happened at a Shanghai LNG Co Ltd work site on Ximentang Isle, north of the Yangshan Deep Water Port, an international shipping center about 45 kilometers from Pudong International Airport. This LNG terminal is expected to receive 3 million tons of the fuel annually after the first phase becomes operational this year. When the facility is online, LNG shipped by sea from Malaysia will be transformed into a gaseous state and sent to downtown Shanghai through pipes.

(Click to view larger image)

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The man who died was pierced by a flying steel rod while lying in his dormitory bed. He was pronounced dead at the scene, his co-workers told reporters. The explosion occurred during a pressure test of the equipment, according to the city government media office. Workers were pumping air into a gasifier when some 500 meters of the piping network burst into fragments, buckling cement crossbeams.




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

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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• Vacuum Hazard - Another Catastrophic Factor...
• Extra Caution When Eliminating Overpressure by Fire Attacks

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Aspen Flare System Analyzer, AFSA (formerly FLARENET) enables engineers to perform steady-state design, rating, or debottlenecking of single or multiple flare and vent systems. AFSA may calculate minimum sizes for new flare systems or evaluate alternatives to remove bottlenecks in existing relief networks and can be used to identify potentially dangerous relief scenarios during design phase or current operational scenarios.


Similar to "Useful Documentation for HYSYS ...", following are compilation of documents related to AFSA or FLARENET. If you found any documents related to AFSA or FLARENET and/or available FREE for all, you are encourage to share within our community. You may drop a note via email or comment. Please include your nickname.

• FLARENET 2004 - Getting Started Guide
• FLARENET 2006 - Getting Started Guide
• AFSA V7 - Getting Started Guide
• AFSA V7 - Reference Manual
  
• Depressuring - Save Some Time in HYSYS - FLARENET Iteration
• How to Merge Several FLARENET Models into Single Model ?

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• Useful Documentation for UNISIM...
• How to Merge Several FLARENET Models into Single Model ?
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• ProMax from BR&E - Renowed Process Simulator in Gas Treating Industry