How much can be done in Six days ?

What you can do with Six days ? Many things... Have you ever thought of a 15-story building erected within Six days ?
 

It took six days to build a level 9 Earthquake-resistant, sound-proofed, thermal-insulated 15-story hotel in Changsha, complete with everything, from the cabling to three-pane windows. This of course excludes the prefabrication of components and modules, earthing and foundation preparation. But believe it is sufficiently impress many peoples. This is the Ark Hotel in Changsha, HuNan county, China. May check Changsha using Google map.

Following video clip (Youtube) shows how a building is erected within 6 days.

This believe is another record... Nevertheless many out there still questioning on safety of this building. What do you think ?

The way Chinese do things, in particular this case, possibly may trigger another revolution in business...

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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.




Related Topic

• Relate LFL to MOC
• Estimate Mixture Flammability & Explosivity At Operating P & T
• Estimate Mixture Flammability & Explosivity At Reference P & T
• Inert Gas or Fuel Gas For Flare Purge ?
• Nitrogen Purging - What are the factors you need to consider ?
• Fire related topics...
• Flare related topics...

Relate LFL to MOC

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Minimum oxygen concentration (MOC) is minimum quantity of oxygen required present in hydrocarbon mixture so that a fire can be initiated and propagated. Below this limit, a fire will not form. More discussion in "Minimum Oxygen Concentration (MOC) for Flare Purge". 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 One shall remember, 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. This post will discuss the relationship between MOC with LFL/LEL.

LFL Relate to MOC
LFL is minimum hydrocarbon (HC) concentration in air which a mixture will burn when an ignition source is present. LFL can be written as follow

MOC is minimum oxygen present in hydrcarbon mixture which a mixture will burn. MOC can be written as follow



Combining [1] & [2],




Combustion of Hydrocarbon



Above equation may be used to estimate MOC if you know the LFL of hydrocarbon.

Example
1) A Ethane (C₂) having LFL of 3.0 vol% (Refer to "Estimate Mixture Flammability & Explosivity At Reference P & T". Estimate MOC of Ethane.

Combustion of C₂H₆,

C₂H₆ + d.O₂ ==> 2CO₂ + 3H₂O

a = 2
b = 6
c = 0
d = 2 + 6 /4 - 0/2 = 3.5

MOC = 3.5 x 3.0 = 10.5 Vol%, close to 11.2 vol% in literature.


2) A n-butane (nC₄) having LFL of 1.86 vol% (Refer to "Estimate Mixture Flammability & Explosivity At Reference P & T". Estimate MOC of n-butane.

Combustion of C₄H₁₀,

C₄H₁₀ + d.O₂ ==> 4CO₂ + 5H₂O

a = 4
b = 10
c = 0
d = 4 + 10 /4 - 0/2 = 6.5

MOC = 6.5 x 1.86 = 12.1 Vol%. Close to 12.3 vol% in literature.

Related Topic

• Estimate Mixture Flammability & Explosivity At Operating P & T
• Estimate Mixture Flammability & Explosivity At Reference P & T
• Inert Gas or Fuel Gas For Flare Purge ?
• Nitrogen Purging - What are the factors you need to consider ?
• Fire related topics...
• Flare related topics...

Estimate Mixture Flammability & Explosivity At Operating P & T

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Earlier post "Estimate Mixture Flammability & Explosivity At Reference P & T" discussed about the Lower flammable limit (LFL) or Lower Explosive Limit (LEL) and Upper flammable limit (UFL) or Upper Explosive Limit (UEL) for single component fluid and mixture at reference pressure (P_ref) and temperature (T_ref). This post will discuss the way to correlate the LFL/LEL and UFL/UEL at P_ref and T_ref and operating pressure (P) and temperature (T) .

Temperature & Pressure Corrected LFL/LEL & UFL/UEL
Below are two equations may be used to correlate the LFL/LEL and UFL/UEL at P_ref and T_ref and operating pressure (P) and temperature (T).

One shall take note that these equations are used for single component. For a mixtures, the LFL/LEL and UFL/UEL at operating pressure (P) and temperature (T) will be calculated using following equation (as discussed in "Estimate Mixture Flammability & Explosivity At Reference P & T":

Calculation Steps
2 steps in determining mixtures LFL/LEL and UFL/UEL at operating pressure (P) and temperature (T) .
(i) Estimate LFL/LEL and UFL/UEL at operating pressure (P) and temperature (T) for every component in a mixture
(ii) Estimate mixture LFL/LEL and UFL/UEL

Example
A mixture contains of Methane, Ethane and Propane with volume% of 20%, 20% and 60%. Estimate UEL at (i) 20 degC & 101.325 Pa, (ii) 70 degC & 3 MPa.

Data
Methane (C1)
UEL_C1,20C,1ATM = 15.0%, E_{C1,combustion} = 212.79 kcal/mole

Ethane (C2)
UEL_C2,20C,1ATM = 12.4%, E_{C2,combustion} = 372.81 kcal/mole

Propane (C3)
UEL_C3,20C,1ATM = 10.1%, E_{C3,combustion} = 526.74 kcal/mole

Output
(i) UEL_Mix at 20 degC & 101.325 Pa

UEL_Mix = 1 / [ 0.2/15 + 0.2 /12.4 + 0.6 / 10.1 ]

UEL_Mix = 11.25 vol% at 20 degC & 101.325 kPaA

(ii) UEL_Mix at 70 degC & 3 MPa
UEL_C1,7oC,3MPa
= 15 x [1+0.75(70-20)/212.79]
+ 20.6 x [Log10(3)+1]
= 48.07 vol%

UEL_C2,7oC,3MPa
= 12.4 x [1+0.75(70-20)/372.81]
+ 20.6 x [Log10(3)+1]
= 44.08 vol%

UEL_{C3,7oC, 3MPa}
= 10.1 x [1+0.75(70-20)/526.74]
+ 20.6 x [Log10(3)+1]
= 41.25 vol%

UEL_Mix,70C,3MPa = 1 / [ 0.2/48.07 + 0.2 /44.08 + 0.6 / 41.25 ]

UEL_Mix,70C,3MPa = 43.02 vol% at 70 degC & 3 MPaA

Related Topic

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Estimate Mixture Flammability & Explosivity At Reference P & T

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Lower flammable limit (LFL) or Lower Explosive Limit (LEL) is minimum vapor concentration in air which a mixture will burn when an ignition source is present. Upper flammable limit (UFL) or Upper Explosive Limit (UEL) is maximum vapor concentration in air which a mixture will burn when an ignition source is present. Concentration of mixture of vapor in air below LFL/LEL (too lean) or above UFL/UEL (too rich), mixture will not burn even an ignition source is present. Therefore, flammable range or explosive range is concentrations between LFL/UFL and UFL/UEL.

Component LEL & UEL
LFL/LEL and UFL/UEL for some common gases are indicated in table below. Some of the gases are commonly used as fuel in combustion processes.

Fuel Gas	(LFL/LEL) (%)	(UEL/UFL) (%)
Acetaldehyde	4	60
Acetone	2.6	12.8
Acetylene	2.5	81
Ammonia	15	28
Arsine	5.1	78
Benzene	1.35	6.65
n-Butane	1.86	8.41
iso-Butane	1.80	8.44
iso-Butene	1.8	9.0
Butylene	1.98	9.65
Carbon Disulfide	1.3	50
Carbon Monoxide	12	75
Cyclohexane	1.3	8
Cyclopropane	2.4	10.4
Dimethyl Ether	3.4	27
Diethyl Ether	1.9	36
Ethane	3	12.4
Ethylene	2.75	28.6
Ethylene Oxide	3.6	100
Ethyl Alcohol	3.3	19
Ethyl Chloride	3.8	15.4
Fuel Oil No.1	0.7	5
Hydrogen	4	75
Isobutane	1.8	9.6
Isopropyl Alcohol	2	12
Gasoline	1.4	7.6
Kerosine	0.7	5
Methane	5	15
Methyl Alcohol	6.7	36
Methyl Chloride	10.7	17.4
Methyl Ethyl Ketone	1.8	10
Naphthalene	0.9	5.9
n-Heptane	1.0	6.0
n-Hexane	1.25	7.0
n-Pentene	1.65	7.7
Neopentane	1.38	7.22
Neohexane	1.19	7.58
n-Octane	0.95	3.20
iso-Octane	0.79	5.94
n-Pentane	1.4	7.8
iso-Pentane	1.32	9.16
Propane	2.1	10.1
Propylene	2.0	11.1
Silane	1.5	98
Styrene	1.1	6.1
Toluene	1.27	6.75
Triptane	1.08	6.69
p-Xylene	1.0	6.0

Note : The limits indicated are for component and air at 20^oC and atmospheric pressure.

Mixture LFL/LEL & UFL/UEL

A mixture is combustible / flammable within mixture LFL/LEL and UFL/UEL. Common units for both limits is mole (or volume) percent fuel in air [moles fuel/(moles fuel + moles air)]. A mixture LFL/LEL and UFL/UEL limits can be calculated using the equations first proposed by Le Chatelier in 1891 :

Example
A vapor contains of 20 vol% of Methane (C1), 20 vol% of Ethane (C2) and 60 vol% of Propane (C3). Find LEL of this mixture at 20 degC and Atmospheric pressure (101325 kPaA).

LEL_C1 = 5 vol% at 20 degC & 101.325 kPaA
LEL_C2 = 3 vol% at 20 degC & 101.325 kPaA
LEL_C3 = 2.1 vol% at 20 degC & 101.325 kPaA

LEL_Mix = 1 / [ 0.2/5 + 0.2 / 3 + 0.6 / 2.1 ]

LEL_Mix = 2.55 vol% at 20 degC & 101.325 kPaA

Above LEL may be linked to MOC as discussed in "Minimum Oxygen Concentration (MOC) for Flare Purge". Vapor mixture flammability & explosivity at Operating P & T discussed in this post.

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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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Noise Level Across Pressure Reducing Device For Different Pipe Wall Thickness

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Earlier post "Quick Estimation of Noise Level Across Pressure Reducing Device", discussion has been focused on estimation of Noise level across pressure reducing device i.e. control valve, pressure relief valve, restriction orifice, etc based on generated internal acoustic energy and transmission losses. The table presented in this post is typical wall thickness for Schedule STD. Increases in wall thickness (e) will results higher transmission loss and lower noise level is expected. Besides, magnitude of transmission loss is also depends to line diameter (D). How to relates transmission loss with wall thickness for different diameter ?

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Noise Level Estimation
Noise level at 1 meter from a pressure reducing device can be estimated from Sound Power Level (PWL) as discussed in "Sound Power Level (PWL) Prediction from AIV Aspect". The Sound Power Level will be transmitted across the pipe wall and emitted to atmosphere. There will be noise correction when Sound power is transmitted across pipe wall (metal).

Noise level at 1 meter from a pressure reducing device,

L_1m = PWL - L_A

where
PWL = Sound Power Level (from Sound Power Level (PWL) Prediction)
L_A = Noise correction (dB)

Noise Correction
The noise correction is subject to wall thickness and pipe size. Thicker wall will result higher noise correction. Following are sets of noise correction equation for different pipe size and wall thickness.

For Nominal Diameter equal to 750 (30 inches) and smaller,

L_A = C3.e³ + C2.e² + C1.e + C0 ......[1]

For Nominal Diameter 900 (36 inches) and above,

L_A = C1.Ln (e) + C0 .....[2]

Where
e = Pipe wall thickness (mm)
C0, C1, C2, C3 = Parameters

Parameter for Noise Correction
Nom.Dia. (Inch)	Eq.	C3	C2	C1	C0
25	1	0.0580	-1.2556	11.6751	26.0942
50	1	0.0319	-0.8039	8.8155	25.7428
100	1	0.0070	-0.3107	5.6726	25.7582
150	1	0.0046	-0.2351	4.8724	24.2083
200	1	-0.0002	-0.0376	2.3897	31.0074
250	1	0.0023	-0.1395	3.1106	28.5213
300	1	0.0013	-0.0949	2.5482	29.9195
350	1	0.0012	-0.0858	2.4017	29.2649
450	1	0.0008	-0.0660	2.1185	30.5846
600	1	0.0003	-0.0371	1.5550	32.7016
750	1	0.0007	-0.0523	1.7118	31.1754
900	2	-	-	6.9521	27.3305
1050	2	-	-	10.4282	18.4957

Example

A pressure control valve (PV) passing 100,000 kg/h of gas with molecular weight (MW) of 22. The inlet condition is 87 barg and 50 degC and downstream pressure is about 7 barg. The pipe diameter is 18 inch with wall thickness of 9.53mm, estimate noise level at 1 m from PV.

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

For 18 inches, equation [1] will be used.

L_A = C3.e³ + C2.e² + C1.e + C0
L_A = 0.0008 x9.53³ -0.0660x9.53² + 2.1185 x 9.53 + 30.5846
L_A = 45.5 dB


Noise level at 1m,
L_1m = PWL - L_A
L_1m = 167.5 - 45.5
L_1m = 122 dBA

Related Topic

• Quick Estimation of Noise Level Across Pressure Reducing Device
• Assess AIV with "D/t-method"
• Extra Attention to Common Point and Similarity on AIV Failure
• Piping Excitation When Expose to Acoustic Energy
• Acoustic Induced Vibration (AIV) Fatigue
• Sound Power Level (PWL) Prediction from AIV Aspect
• Several Criteria and Constraints for Flare Network - Piping
• Flow Element (FE) Upstream or Downstream of Control Valve (CV) ?

Quick Estimation of Noise Level Across Pressure Reducing Device

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Pressure reducing device such as control valve, pressure relief valve, restriction orifice, etc, there will be pressure drop and mass passing through these device, internal acoustic energy is generated and transmitted to downstream piping and potentially lead to severe piping excitation, vibration and stresses on downstream piping and potentially lead to fatigue failure. Internal acoustic energy transmitted along the pipe may also transmitted to through the pipe and emitted as Noise.

One of the common safety requirement is limit the noise level to 85 dBA (Noise level with A-weighted) in continuous exposure and 115 dBA during intermittent exposure. In earlier conceptual or Front End Engineering Design (FEED) stage, the noise level across pressure reducing device may be estimated to determine overall noise control philosophy. The following will present a simple method to estimate noise level generated at 1 meter from a pressure reducing device.

Noise Level Estimation
Noise level at 1 meter from a pressure reducing device can be estimated from Sound Power Level (PWL) as discussed in "Sound Power Level (PWL) Prediction from AIV Aspect". The Sound Power Level will be transmitted across the pipe wall and emitted to atmosphere. There will be noise correction when Sound power is transmitted across pipe wall (metal). The noise correction is subject to wall thickness. Thicker wall will result higher noise correction. Following are typical noise correction for pipe size and wall thickness.

Noise Correction
Nominal Dia. (Inch)	Wall thickness (mm)	Noise Correction (dB)
25	3.38	54
50	3.91	50
100	6.02	50
150	7.12	49
200	8.18	48
250	9.3	47
300	9.53	47
350	9.53	46
450	9.53	46
600	9.53	45
750	9.53	43
900	9.53	43
1050	9.53	42

Noise level at 1 meter from a pressure reducing device,

L_1m = PWL - L_A

where
PWL = Sound Power Level (from Sound Power Level (PWL) Prediction)
L_A = Noise correction from above table

Example

A pressure control valve (PV) passing 100,000 kg/h of gas with molecular weight (MW) of 22. The inlet condition is 87 barg and 50 degC and downstream pressure is about 7 barg. The pipe diameter is 18 inch with wall thickness of 9.53mm, estimate noise level at 1 m from PV.

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

Noise level at 1m,
L_1m = PWL - L_A
L_1m = 167.5 - 46
L_1m = 121.5 dBA

As the noise level at 1m (normal trim) is 121.5 dBA, this is far too big from normal requirement. A low noise trim control valve may be considered and/or acoustic insulation to be provided.


Related Topic

• Assess AIV with "D/t-method"
• Extra Attention to Common Point and Similarity on AIV Failure
• Piping Excitation When Expose to Acoustic Energy
• Acoustic Induced Vibration (AIV) Fatigue
• Sound Power Level (PWL) Prediction from AIV Aspect
• Several Criteria and Constraints for Flare Network - Piping
• Flow Element (FE) Upstream or Downstream of Control Valve (CV) ?

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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A Story about BOG area...

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Qatar LNG production train presently is the world no.1 in term of production rate per train (about 7.8 MTPA compare to "nowadays normal train production of 4 MTPA). With it 7 trains (3 small and 4 large) LNG production in Qatar gas and another few trains in Rasgas have pushed Qatar as world no.1 in term of production.

LNG is normally stored in atmorpsheric tank at LNG liquid temperature of about -160 degC. More information about LNG in "LNG and Supply Chain", safety related in "LNG SES Issues..." and several ways to achieve LNG storage temperature of -160 degC in "Techniques to Achieve Cryogenic Temperature".

Recently received a batch of photos from reader of this blog. They are story related to Qatar Raslaffan LNG storage Boil-off gas (BOG) area...

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Vacuum Collapse...

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Vaccum hazard is potentially a hazard and damaging factor to many pressure vessel. Vacuum should be addressed and extra caution to be taken during design, construction, operation and maintenance to avoid catastrophic failure.

In previous posts, "Vacuum Hazard - Another Catastrophic Factor..." discussed about the damage of vacuum, main factors resulting Vacuum and several scenario may lead to Vacuum Hazard. "4-steps Approach to Combat Vacuum Hazard" discuss about approach may be taken to minimize and avoid Vacuum Hazard. "How powerful is ATMOSPHERIC ?" also has presented a video clip demonstrates how powerful is ATMOSPHERIC pressure and how quick a "vessel" collapse.

This post will share a screen shot of rail car collapsed due to vacuum condition. The rail car had been steamed out and remained hot inside when it started to rain. Quick cooling due to rain running on the rail car and lead to generation of vacuum.

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Why Restriction Orifice is some distance from Blowdown valve ? - Clarif #01

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A reader MT (abbre.) has read "Why Restriction Orifice is some distance from Blowdown valve ?" and drop me a note. The question are more-or-less as follow :

i) For cases where huge pressure drop is not foreseen or low temperature is not expected downstream of restriction orifice (RO), is it still necessary to provide the minimum distance ?

ii) If the Flare header is Stainless steel (SS), and the process piping is Carbon steel (CS), the Spec break between CS and SS will be shown at the blowdown valve (BDV), i.e. BDV is designed for the most conservative material - SS. In this case, i do not seen the reason of the minimum distance.

Original Intention

The intention of the proposed arrangement in "Why Restriction Orifice is some distance from Blowdown valve ?" is to avoid the BDV stem stuck at position in case the depressured fluid temperature is dropped to sub-zero (below zero degree Celcious).

Responses

i) In case low pressure drop and the depressured temperature is still higher than sub-zero, this requirement is NOT necessary. As long as there is NO risk of fluid temperature drop below zero degree Celcious, then this arrangement is not required.

ii) The process piping upto BDV (excluded BDV) is CS and from BDV onwards is SS. This is a good arrangement with good spec break. The 600mm requirement is mainly related to frozen of moisture content (below zero degree Celcious), it has no/less relationship with the material. Thus, as long as the depressured temperature can drop below zero degree Celcious, regardless what the material of construction (MOC) is, the 600 mm arrangement still applicable.

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Pyrophoric Fire

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"Pyrophoric" material is any material can be ignited or burned spontaneously in air when it is scratched, or struck, cracked, etc. This type of fire is called pyrophoric fire. Typical component is Sulfur in Carbon steel vessel. Ferum (Fe) in carbon steel vessel reacts with Sulfur present in fluid and formed iron sulfide (FeS) which is a pyrophoric material that oxidizes exothermically when exposed to air. Refinery, LNG production and gas treatment plant may expose to sulfur (in the form of H2S). Iron sulfide scale may appeared in Carbon steel vessel. It is typically a conversion of iron oxide, Fe2O3 (rust) to iron sulfide (FeS) in an oxygen-free atmosphere where hydrogen sulfide gas is present.

Reaction

In Oxygen-free environment (inside CS vessel during normal operation) :
Fe2O3 + 3H2S ==> 2FeS + 3H2O + S

When expose to Oxygen (opening of vessel with air ingress during maintenance) :
FeS + 3O2 ==> 2Fe2O3 + 4S + Heat
FeS + 7O2 ==> 2Fe2O3 + 4SO2 + Heat

Heat is dissipated quickly and white smoke of SO2 gas released and followed by pyrophoric fires. This process is quick and exothermic oxidation.

Typically a CS vessel exposed to H2S would be either steam-out followed by water wash OR chemical cleaning prior to opening of the vessel.

Read more here

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