chemical engineering: Flare

Showing posts with label Flare. Show all posts

Saturday, October 10, 2009

Interesting Relationship Between Carbon number and LFL & UFL

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A mixture is combustible / flammable if and only if the hydrocarbon composition is within mixture LFL/LEL and UFL/UEL as discussed in "Estimate Mixture Flammability & Explosivity At Reference P & T. As the operating pressure (P) and temperature (T) change (from reference P & T, the mixture LFL/LEL and UFL/UEL at P & T will change accordingly. In recent works, found in literature an interesting relationship between number of Carbon (in paraffin hydrocarbon) with LFL/LEL and UFL/UEL. This relationship is pretty useful especially when you have no information on hand.

A paraffin hydrocarbon with N_C of Carbon (C), the Upper Explosive Limit (UFL) and Lower Explosive Limit (LFL) can be established with following equations :

Example
A Methane (CH4) contains One (1) Carbon. From literature, the UFL = 15 vol% and LFL = 5%.
From above equations,

UFL = 1 / (0.01337 x 1 + 0.05151)
UFL = 5.6% (compare to 5%)

LFL = 1 / (0.1347 x 1 + 0.04343)
LFL = 15.4% (compare to 15%)

A Propane (C3H8) contains Three (3) Carbon. From literature, the UFL = 10.1 vol% and LFL = 2.1%. From above equations,

UFL = 1 / (0.01337 x 3 + 0.05151)
UFL = 10.9% (compare to 10.1%)

LFL = 1 / (0.1347 x 3 + 0.04343)
LFL = 2.2% (compare to 2.1%)

A Hexane (C6H14) contains Six (6) Carbon. From literature, the UFL = 7.0 vol% and LFL = 1.25%. From above equations,

UFL = 1 / (0.01337 x 6 + 0.05151)
UFL = 7.6% (compare to 7%)

LFL = 1 / (0.1347 x 6 + 0.04343)
LFL = 1.2% (compare to 1.25%)

Above equation is just equations for quick estimation. It may provide some idea of UFL and LFL when no information is available. The error could be large for certain component i.e. Octane. For design and practical use, an in depth method shall be employed.

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Saturday, August 29, 2009

Minimum Oxygen Concentration (MOC) for Flare Purge

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A fire is formed or a flame can sustains and propagate, three main elements shall present as defined in well known FIRE triangle. There are combustible material (or fuel), oxygen (O2) and heat. See following image. One additional characteristic shall also present is the potential of chain reaction to maintain continuous combustion before any of these three elements is removed. This speed of chain reactions define if a mixture is combustible (slow) or explosive (fast).

Minimum Oxygen Concentration (MOC)
Oxygen is common obtained from atmosphere. Standard air at Mean Sea Level (MSL) contains 20.95 vol% Oxygen (O2), 78.08 vol% Nitrogen (N2), 0.038 vol% Carbin Dioxide (CO2) and others inert gas i.e. Neon, Xenon, etc. Although Oxygen is the major component in generating fire, there is still a minimum oxygen concentration (MOC) required present in combustible mixture so that a fire can be initiated and propagated. Below this limit, a fire will not form.

Following MOC for some hydrocarbon common found in oil and gas plant.

Component	MOC (Vol% O2)
Hydrogen	4.0
Acetylene	6.2
Methane (C1)	12.0
Ethane (C2)	11.2
Ethylene (C2=)	9.9
Propane (C3)	11.6
Propylene (C3=)	11.5
Butane (C4)	12.3
1-Butene (C4=)	11.0
Pentane (C5)	11.8
Hexane (C6)	11.8
Benzene (Bz)	11.5
Carbon Disulfide	5.0

Base on this principle, a flare header is purged with hydrocarbon (i.e. fuel gas ) to evacuate air that ingressed via flare tip and stack in order to ensure quantity of oxygen level in the flare system is always below minimum oxygen concentration (MOC).

MOC For Flare Purge
From above table, you may noticed that MOC for majority of components are equal to or more than 10 vol% except Hydrogen (H2), Acethylene and Carbon Disulfide (CS2). It is recommended MOC of 6 vol% for flare purging design with 4 vol% as design margin. One shall remember plant releasing large amount of Hydrogen shall use lower MOC with margin i.e. 2 vol%. One of the example is Hydrogenation unit in Refinery plant.

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Saturday, August 22, 2009

Inert Gas or Fuel Gas For Flare Purge ?

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Flare collection header is normally "NO flow" as most (if not all) devices connected to flare header are non-discharging fluid into flare system. Among all are pressure relief valve (PRV), blowdown valve (BDV), overpressure dump valve (PCV), etc. All these devices are kept as close position during normal plant operation and will only open in the event of overpressure, emergency situation i.e. fire, runaway reaction, plant shutdown/blowdown for maintenance.

On the flare tip end, it is open to atmosphere. it is very likely that atmosphere air contain oxygen ingress and stay into flare collection header. PRV/BDV/PCV passing and open on demand will discharge large quantity of hydrocarbon gas into flare collector header filled with air and create combustible mixture, as this combustible mixture travel along flare header and reach flare tip which equipped with flare pilot, combustible mixture will be ignited and potentially created flash back to the flare header and flare knock-out drum (KOD). Subject to flare header capacity and mechanical integrity, large flash back lead to severe internal pressure act on the piping & vessel and vapor wave results severe vibration and movement of structure, this potential lead to catastrophe failure of flare collection system. Therefore a flare header is sweep or purge with fuel gas or inert gas i.e. Nitrogen.

Advantages using inert gas compare to fuel gas as purge gas

i) Environment & Green House Effect (GHE)
IG : Inert gas has NO impact to environment
FG : Burn fuel gas in atmosphere generate Carbon Dioxide (CO2) which contributes to increase of Co2 content in atmosphere and increases Green House Effect (GHE)

ii) Burn back damage flare tip - reduce life span of flare tip
IG : Inert gas do not burn. NO burn back and potential damage of flare tip.
FG : Potential FG burn back damage flare and shorten flare tip life span.

iii) High OPEX avoid Burn back
IG : NO burn back. Minimum purge rate and low OPEX.
FG : Potential burn back lead to high purge rate (potential 10 times higher than purge rate of IG) and high OPEX

iv) Visible Flame
IG : Inert gas do not burn. No flame present.
FG : FG continuous burn and continuous visible flare at flare tip. Potential create uneasy situation in environment sensitive area.

v) Smoke Flaring
IG : Inert gas do not burn. No smoke flaring issue.
FG : Burning heavy FG lead to smoke flaring. Potential create uneasy situation in environment sensitive area. Increase likelihood of unburnt component and impact on environment.

vi) Steam injection for smokeless flaring
IG : Inert gas do not burn. No smoke flaring issue.
FG : Burning heavy FG lead to smoke flaring. Steam injection to reduce/eliminate smoke flaring. This increases CAPEX (additional steam injection facilities) and OPEX (steam loss).

vii) Radiation
IG : Inert gas do not burn. No additional radiation.
FG : Fuel gas burn lead to increase of radiation level (on top of solar radiation) to personnel working near flare stack.

Disadvantages using inert gas compare to fuel gas as purge gas

a) Fuel Gas Readily Available in Plant
IG : Required Nitrogen generator or use of Liquid Nitrogen and evaporator. Additional CAPEX and OPEX.
FG : Fuel gas readily available in plant. Minimum CAPEX and OPEX. Some plant generate hydrocarbon gas which shall be disposed off. This gas is readily serve as purge gas and inccur NO cost.

b) Inert Gas Cloud
IG : Flare system purge with inert gas, entire flare system is filled with IG gas (which potential heavier than air). Once any PSV/BDV open and release large amount of gas into flare header, it will "push" IG release through the flare tip. Heavy IG (compare to air) will sink create a IG gas cloud near plant. This is potential fatal thread (suffocation) to personnel on site.
FG : Continuous flaring lead to no or nearly no potential of gas present in atmosphere

c) Unburnt hydrocarbon gas emission
IG : PRV/BDV/PCV leak or passing lead to low heating value mixure (less than 200 btu/ft3) which is non-combustible. Release of hydrocarbon gas into atmosphere directly has more GHE impact than burning it. For example 1 mol of methane create 1 mol of CO2 if it is burnt. 1 mol of methane create 20-21 mol of equivalent CO2 if it unburnt.
FG : Continuous flaring lead to no or nearly no unburnt gas in atmosphere

d) Combustible Cloud lead to Instant Ignition
IG : Slowly hydrocarbon gas emission to atmosphere and built-up of combustible mixture in the plant, once the heating value for auto-ignition is reached, the combustible mixture potentially ignited. Its impact is just like a explosion and potential thread to personnel and surrounding facilities.
FG : Continuous flaring lead to no or nearly no unburnt gas in atmosphere

Concluding remark
Inert gas purging is normally understood as clean, low CAPEX, low OPEX, etc and regards as most likely candidate for flare purging. However, the associated SAFETY related issue may needs additional attention and focus. All...use inert gas wisely...

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Sunday, August 9, 2009

Relate Heat Radiation Level and Temperature

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Earlier post "Heat Radiation For Pain & Blistering Threshold" and "Personnel Exposure Time For Heat Radiation" have discussed the heat radiation level lead to plant personnel pain and blistering threshold within an exposure time. Heat radiation design criteria for personnel exposure has been proposed for continuous and emergency exposure. Flare radiation run using software i.e. FLARESIM or calculation using Brzustowski and Sommer method, heat radiation level will be estimated and presented. For equipment, instrument, piping, painting, steel structure, etc expose to flare radiation will experienced increase temperature.

How shall a heat radiation level relates to temperature ?

Heat level (Et) at recepting location can be related to the sum of radiation heat (Er) plus convection heat (Ec).

Et = Er + Ec

For unit area (m2),

qt = qr + qc

where
qt = total heat flux at receptor (kW/m2)
qr = radiation heat flux at receptor (kW/m2)
qc = convection heat flux at receptor (kW/m2)

Radiation heat flux,

qr = sigma x (T⁴ - Ta⁴)

Convection heat flux

qc = (h / e) x (T - Ta)

where
sigma = stefan-Boltzman constant = 5.67 e -11 (kW/m2K)
h = convective heat transfer co-efficient = 0.007 (kW/m2K) at zero wind speed
e = Emissivity (0.8-0.9 for copper or rusted CS pipe, 0.1-0.2 for polished SS pipe)
T = Surface temperature (K)
Ta = Ambient temperature (K)

Therefore

qt = sigma x (T⁴ - Ta⁴) + (h / e) x (T - Ta)

If a heat radiation level is known at particular location, the corresponding temperature may be estimated using above equation.

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Monday, August 3, 2009

Personnel Exposure Time For Heat Radiation

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Earlier post "Heat Radiation For Pain & Blistering Threshold" has briefing discussed about the exposure time for heat radiation level which lead pain and blister threshold. Equations have been proposed in order to link between exposure time for dedicated heat radiation. Example, 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.

Maximum allowable personnel exposure time to particular heat radiation level proposed in API RP 521 (previous revision - 1997) and API Std 521 (latest revision - May 2008) are marginally different. Detail list out as follow

Heat Radiation (kw/m2)	API Rp 521 (March 1997)	API Std 521 (May 2008)
9.45	Exposure must be limited to a few (approx. six) seconds, sufficient for escape only. May consider tower or structure provide some degree of shielding.	Required urgent emergency action. Radiation shielding and/or special protective apparel (e.g. a fire approach suit) required
6.31	Emergency actions lasting up to 1 minutes without shielding but with appropriate clothing.	Emergency actions lasting up to 30s without shielding but with appropriate clothing.
4.73	Emergency actions lasting up to several minutes without shielding but with appropriate clothing.	Emergency actions lasting 2 min to 3 min without shielding but with appropriate clothing.
1.58	personnel with appropriate clothing can be continuously exposed	personnel with appropriate clothing can be continuously exposed

Above listing shows that latest API Std 521 has more stringent requirement compare to previous revision (1997), specifically for 6.31 and 4.73 kW/m2. The maximum allowable exposure time limit have been reduced. Continuous exposure limit remain unchanged.

Emergency Exposure
Maximum allowable exposure heat radiation of 6.31 and 4.73 kW/m2 (during emergency scenario) have been used as criteria in determining sterile area, an area around flare stack which no personnel shall be around without any personnel protective apparel. For conservative design and/or expect potential increase in flaring rate in future, a limit of 4.73 kW/m2 may be used. For common design, heat radiation limit of 6.31 kW/m2 is widely used. In general, maximum allowable heat radiation of 9.46 kW/m2 will be limited at the flare stack base where personnel may access to sterile area with proper personnel protective apparel.

Continuous Exposure
For continuous exposure, operator is assumed wearing appropriate clothing in a plant. Heat radiation at any location shall be limited to 1.58 kW/m2.

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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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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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Thursday, June 11, 2009

Estimate Wind Speed At Flare Tip At Different Height

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Wind speed affect flare performance
Wind speed affects flare performance at least in few ways :

Wind drag flame to downwind, lower the flame center and radiation level of downwind receptor
Wind carry unburnt component to downwind
Wind potential put-off flame on flare under low gas flow and pilot

Therefore, 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.

Wind speed change with flare height
One of characteristic of wind speed is wind speed increase with height from grade. Thus, higher the flare stack, flare tip will expose to higher wind speed. Generally wind map will be obtained from local authority or organization. The wind map may be measured at specific height i.e. 10m from grade.

In case flare stack height is different than wind speed at specific height, how shall an engineer determine wind speed flare tip ?

Corrected wind speed (without return duration correction)

Where
U_Z = Wind speed at Z m at return duration of t0 hour (ft/s)
U₀ = Wind speed at specified height of 10m at return duration of t0 hour (ft/s)
t₀ = Wind speed at return duration i.e. 60 minutes, 1 minutes, etc
Z = Flare stack height (ft)

Corrected wind speed (with return duration correction)

Where
t = Wind speed at return duration i.e. 60 minutes, 1 minutes, etc (different than t₀ )

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 (a) the return duration stay as 60 minutes (b) return duration change to 1 minute.

Solution (a)
Z = 200 m = 200 x 32.8 ft
U_Z = 10 m/s = 10 x 32.8 ft/s

U_Z = 41.7 ft/s = 12.7 m/s
Wind speed at flare tip with return duration stay as 60 minutes = 12.7 m/s

Solution (b)

u_Z,t = 44.8 ft/s = 13.66 m/s

Wind speed at flare tip with return duration at 1 minutes = 13.66 m/s

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Tuesday, May 19, 2009

Quick Estimate Ground Level Unburnt Flammable Gas For Vent Pipe or Flame-out Flare Stack

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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. Although there are many benefits in using flare for proper disposal of hydrocarbon gases, combustion efficiency is still a common concern about flare. It is commonly accepted a flare combustion efficiency can reach approximately 98% (refer API Std 521). This would still results some remaining unburnt hydrocarbon gases release into atmosphere. Thus, there are operators especially in high environment concern area, zero emission and flaring principle is implemented.

Flare is normally lit and a highly reliable pilot system is maintaining the flame at the flare tip. In an extreme condition i.e. storm, heavy rain, strong wind, long serviced pilot, etc, flare may lose the flame. This situation commonly called as flame-out condition. Any release during this period would lead to flammable gas (heavier than air) release and settle to ground level in process area. Wind blowing may disperse the flammable gas and reduce concentration of flammable gas at ground level. However, there is still possibility of flammable gas settled and form combustible mixture. Thus, it is very important to estimate maximum concentration of flammable gas at ground level.

Nowadays with sophisticated software development, software such as PHAST may be used to estimate concentration of flammable gas at ground level at specific location. However, quick estimation method may be important during conceptual phase. Following will present a simple estimation method to calculate maximum concentration of flammable gas at ground level.

Maximum Ground Level Concentration of flammable gas,

C = 0.23 x Q / (U x H'²)

where
C = Flammable gas concentration (g/m³)
Q = Mass flow of flammable gas (g/s)
U = Wind speed (m/s)
H' = Effective height (m)

Effective flare height can be determined with

H' = H_s + 3 d_s V_ex / U

where
H_s = Flare stack height (m)
d_s = Flare tip diameter (m)
V_ex = Exit velocity (m/s)
U = Wind velocity (m/s)

Concentration conversion g/m³ to ppm,

[C in ppm] = [C in mg/m³] x 24.45 / MW

where
[C in ppm] = Concentration in ppm
[C in g/m³] = Concentration in mg/m³
MW = Gas molecular weight

Example
Estimate the maximum ground-level concentration, C, if a flammable gas is accidentally released unburned from a flare, if the release rate to the atmosphere, Q, is 25,200 g/s, the exit
velocity is 83.8 m/s, and flare tip diameter is 0.46 m. The flare stack height is 61 m. Assume that the wind speed 3.1 m/s. The molecular weight of the gas is 54.

H' = H_s + 3 d_s V_ex / U
H' = 61 + 3 (0.46) (83.8 / 3.1)
H' = 98.3 m

C = 0.23 x Q / (U x H'²)
C = 0.23 x 25200 / (3.1 x 98.3²)
C = 0.193 g/m³

[C in ppm] = [C in mg/m³] x 24.45 / MW
[C in ppm] = 0.193 x 1000 x 24.45 / 54
[C in ppm] = 87.6 ppm

Ref : Section 15.11, Handbook of Chemical Engineering Calculations, 3rd Edition, Nicholas P. Chopey

***********************************

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Tuesday, May 5, 2009

Model Fix Pressure Device in FLARENET

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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. 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.

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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.

Sunday, March 29, 2009

Quick Estimate Flare Stack Diameter

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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".

Flare stack diameter,

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.

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Friday, March 27, 2009

Acoustic Induced Vibration (AIV) Fatigue

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An oil & gas processing and production facilities may consist of several level of pressure let down station. Typical examples are choke valve between christmas tree and flowline/production header, slugcatcher control valve, onshore pressure reduction station, steam control valve, desuperheating station, etc. Above valves are normally in continuous operation. Besides, there are other valves such as compressor surge / capacity control valve, overpressure dumping control valve, blowdown valve with restriction orifice, pressure relief valve, etc will experience large pressure drop and they are operated intermittently.

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. If the downstream piping system is not properly designed, effect of this acoustic excitation would lead to fatigue failure. Above acoustic excitation phenomena is also known as Acoustic Induced Vibration Fatigue. Generally high frequency (more than 1000Hz) acoustic energy contribute to AIV fatigue and once piping expose to AIV, piping can fail in very short period (possibly in several minutes to hours).

Fluid Phase
Generally Acoustic Induced Vibration (AIV) fatigue occur in gaseous system and normally AIV does not occur in liquid system. For two phase gas liquid flow system with high gas flow (i.e. more than 50% gas flow), AIV may starts to be problem and need to be investigated. As two phase flow is complicated in estimating the Acoustic energy, it is always conservative to consider 100% gas flow.

Short & Long Term Operation
AIV fatigue failure of piping downstream of piping is subject to operation time i.e. number of fatigue cycle. A piping in continuous (long term) operation, a design fatigue limit of 80MPa is normally used based on ASME fatigue design limits with suitable safety factors for long term sevice life. A fatigue limit of 185MPa (10^7 cycles) has been used for short-term operation based on published fatigue life data for carbon steel and stainless steel. This fatigue limit represents the maximum acceptable level of stress for 10^7 cycles (12-24 hours of operation) without any safety margin. Those pressure reduction devices has total accumulated service hour within the plant life less than 12 hours, one may consider AIV fatigue may not occur. Pressure relief valve is one of those devices potentially drop in this category.

Experiences has shown that there is high frequency of operation of pressure relief device during plant black start-up and restart-up, AIV fatigue may occur in very short period (minutes to hours), PRV and other pressure reducing device may connect on same downstream pipe, etc, in many event AIV studies for PRV downstream piping to be conducted. Those PRV relief to ATM has very high potential drop in the short term category.

Sound Power Level (PWL)
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 storng to resist AIV fatigue is to ensure the PWL allowable limit of downstream piping higher than the PWL generated by the pressure reducing device. This will be disucssed in coming post.

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Monday, March 23, 2009

Estimate Subsonic Flare Tip Pressure Drop With Graph Derived Correlation

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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.

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.

Q = 30,000 x 1000 / 3600 =8333.3 dm3/s @ Std

(i) A DN250 tip,
dP = 10 ^ ( 1.772663 x Log Q - 5.287376)
dP = 10 ^ ( 1.772663 x Log 8333.3 - 5.287376)
dP = 46 kPa (outside curve)

(ii) A DN400 tip,
dP = 10 ^ ( 1.92372 x Log Q - 6.671917)
dP = 10 ^ ( 1.92372 x Log 8333.3 - 6.671917)
dP = 7.4 kPa

(iii) A DN450 tip,
dP = 10 ^ ( 1.948852 x Log Q - 7.034009)
dP = 10 ^ ( 1.948852 x Log 8333.3 - 7.034009)
dP = 4.0 kPa

(iv) A DN500 tip,
dP = 10 ^ ( 1.945342 x Log Q - 7.329128)
dP = 10 ^ ( 1.945342 x Log 8333.3 - 7.329128)
dP = 2.0 kPa

You may compare results with chart in "Quick Estimate Flare Tip Pressure Drop".

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