Aspen product including HYSYS and FLARENET used to have English interface. Nevertheless, engineer and operators from certain countries are normally not used to English language and this lead to many error in understanding and operation of Aspen products. Typical countries are China, Japan, Korean, Brazil, Latin American countries, etc.
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. In some flare system, i.e. Refinery flare system, a water seal is provided at the bottom of stack to minimum air ingress into main flare system. This water seal (subject to design) may induce a rather fix pressure drop to the back pressure estimation, in particular to very low pressure system i.e. sour gas flare.
How shall engineer create a constant pressure drop in AFSA / FLARENET model ?
One of the way is to use the "Flow Bleed" component. "Flow Bleed" is normally use for fix flow splitter with a fix pressure drop. In this case, introduce a "Flow Bleed" component with following setting
Offtake Multiplier set to zero (0)
Offtake Offset set to zero (0)
Pressure drop set to intended pressure drop
Above image shown the use of "Flow Bleed" as fix pressure drop device. A fix pressure drop of 0.1 bar has been introduced in the flare network.
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 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.
d = [3.23 x 10-5 ( W / ( P2 x Ma2)) x (z T / MW)^0.5] ^ 0.5
where W = Mass flow (kg/h) P2 = Outlet pressure (kPa abs) Ma2 = Outlet Mach no z = compressibility factor T = Temperature (K) MW = Molecular weight
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.
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.
dP = 10 ^ (a x Log Q + b)
where dP = tip pressure drop, kPa Q = Flow at standard condition, dm3/s (note 1) a & b = coefficient correspondent to tip size as list in following table
Tip Nom. Dia. (mm)
a
b
250
1.772663
-5.287376
400
1.92372
-6.671917
450
1.948852
-7.034009
500
1.945342
-7.329128
600
1.954552
-7.791866
750
1.968898
-8.24243
900
1.89152
-8.14936
1050
1.722666
-7.781574
1200
1.532001
-7.181798
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.
Example Estimate pressure drop of a subsonic flare tip with flow of 30,000 Sm3/h.
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.
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.
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 :
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".
Flare system commonly consists of collection networks, liquid Knock-Out drum (with liquid retention capability), 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. Sometime, two or more flare knock-out drums (KOD) are installed in parallel to reduce KOD size on large flare design flow. However, in some design, two or more flare KODs are installed in series within the flare collection system. What are the common reasons behind providing KODs in series ?
(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.
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.
Nowadays Oil & gas, LNG, Petrochemical, Refinery plant capacity are large and some time a few design centers are working together to manage the project during design phase. The project may splits into several large units and each design center will design their own units. The design include flare system modeling using FLARENET.
Several design center will generate their own FLARENET models and upto one point of time, all models may required to merge in order to check the overall flare system performance. Some steps presented in thmay be used to merge several FLARENET models into single model using built in utility in FLARENET.
