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Underground Atmospheric Tank Venting


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#1 shahzadshah

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Posted 22 September 2016 - 09:36 AM

Hi,
I am in the process of replacing an underground atmospheric sump tank (gravity driven). The product is gas condensate. The tank will have 4m high vent complete with inline flame arrestor. We had a HAZOP done where it was identified a possible scenario of blocked flame arrestor while pumping out (about 4m3). The tank is designed for 5mbar vacuum. Client is asking me to find out design vacuum for the flame arrestor. Also how much vacuum 4m3 will generate (with/without blocked flame arresotor).
How do I find out vaccum generated by a particular pump out operation.
Apologies if this has been asked before.

Thanks.

#2 fallah

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Posted 23 September 2016 - 01:20 AM

shahzadshah,

 

Puming out rate should be reported within a time duration; i.e. 4 m3/hr...

 

If API 2000 can be applied to the tank you can use it to find inbreathing rate due to pumping out...



#3 shahzadshah

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Posted 26 September 2016 - 03:37 AM

Thank you Naser and my apologies for late reply; I was away.
pumping out time is 15 minutes. I have pasted the calculation below( sorry didn't know how to attach it). Kindly check if these are correct.

 
ASSUMPTIONS
1.0 Tank Data - From various emails and drawings
1.1 Below ground, vertical cylindrical tank
Diameter: 1.892m
Capacity: 7m3
Design pressure: 0.3 barg pressure and 50mbarg vacuum.  However, tank supplier advised to use .035barg and 2mbarg vaccuum respectively.
Operating pressure: ATM
 
2.0 Pump Data - From various clarification emails
2.1 Drainage from sump tank
Pressure: Unknown
Flowrate: Gravity Driven
Controlled release of product
 
2.2 sump tank transfer to XXX Line
Tag No's: T-02
Pressure: 4.97 barg
Flowrate: 66.6 litres/min (max) = 4 m3/hr
 
6.0 Calculation of Normal Venting Capacity - Inbreathing (vacuum relief)
Normal venting capacity shall be at least the sum of (1) venting requirments for liquid movement
and (2) thermal effect (API sec 4.3.2)
6.1 Venting Requirements for Liquid Movement
Max flowrate out of tank = section 2.2
Max flowrate out of tank 4.00 m3/hr
Flash point of diesel fuel is -60°C which is less than 37.8°C threshold
Therefore 'liquid movement out' factor  (API 2000, Table 1B, pg.5) = 0.94
Venting requirement for liquid movement [A] = 3.75624 m3/hr
6.2 Venting Requirements for Thermal Effect
From API 2000, Table 2B, pg.7:-
Thermal inbreathing capacity for a 10m3 tank [B] = 1.69 m3/hr
6.3 Normal Venting Capacity - Inbreathing (vacuum relief)
Normal Venting Capacity = [A] + [B]
Normal Venting Capacity = 5.44624 m3/hr
Note: The tank capacity is 7m3 hence B is conservative.
 
7.0 Calculation of Normal Venting Capacity - Outbreathing (pressure relief)
Normal venting capacity shall be at least the sum of (1) venting requirments for liquid movement
and (2) thermal effect (API sec 4.3.2)
7.1 Venting Requirements for Liquid Movement
Assuming tank gets filled in daily which is very conservative.
Max flowrate into tank =   0.29 m3/hr
Flash point of diesel fuel is -60°C which is less than 37.8°C threshold
Therefore 'liquid movement in' factor (API 2000, Table 1B, pg.5) = 2.02
Venting requirement for liquid movement [A] = 0.58782 m3/hr
7.2 Venting Requirements for Thermal Effect
From API 2000, Table 2B, pg.7:-
Thermal outbreathing capacity for a 50m3 tank using extrapolation [B] = 5.05 m3/hr
7.3 Normal Venting Capacity - Outbreathing (pressure relief)
Normal Venting Capacity = [A] + [B]
Normal Venting Capacity = 5.63782 m3/hr
 
8.0 Calculation of nozzle area required (In-breathing)
Where
Q = Theoretical Flow Rate - In-breathing (Nm3/h) 5.45 m3/hr
P1 = Pressure at device inlet (barA) 1.0133 bar
P2 = Pressure at device outlet (barA) 1.0113 bar
A = Minimum flow area of device (cm2)
K= Ratio of specific heats of test medium (for between 0 and 50 degrees celsius) 1.40
T= Temperature at inlet (K) 298 k
M = Molecular weight of test medium 28.97
Z = Compressibility factor 1
Minimum  Area = 0.91 cm2
Diameter 10.77 mm
Diameter 0.42 inches
 
Assumptions
1) P1 is atmospheric pressure which is 1.0133 bar
2) P2 is 2mbar less than atmospheric. Note tank is designed for
50mbar vacuum) see section 1.1
3) K is 1.4 for air
4) Z is unknown so a value of 1 is used as stated in API 2000
5) T assumed to be 25°C or 298K
 
9.0 Calculation of nozzle area required (Out-breathing)
Where
Q = Theoretical Flow Rate - out-breathing (Nm3/h) 5.64
P1 = Pressure at device inlet (barA) 1.0133
P2 = Pressure at device outlet (barA) 1.0133
A = Minimum flow area of device (cm2)
K= Ratio of specific heats of test medium 1.40
T= Temperature at inlet (K) 273
M = Molecular weight of test medium 28.97
Z = Compressibility factor 1
Minimum Area 5.63 cm2
Diameter 26.78 mm
Diameter 1.05 inches
 
Assumptions
1) P1 is the relief pressure which is assumed to be 3.3 mbar
below atmospheric - tank design pressure is 0.4 barg
2) P2 is atmospheric pressure which is 1.0133 bar
3) K is 1.4 for air
4) Z is unknown so a value of 1 is used as stated in API 2000
5) T is 0°C or 273K
 


#4 fallah

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Posted 26 September 2016 - 04:06 AM

shahzadshah,

 

You should also attach a sketch or drawing of the system you described. To attach any file you can use "More reply options" below the box in which you are writing the reply.


Edited by fallah, 27 September 2016 - 06:05 AM.


#5 shahzadshah

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Posted 26 September 2016 - 09:25 AM

Fallah,

Thanks again. Please see attached marked node describing the system(inlet/outlet). Also reminder about my question. How do you find the negative pressure created by pumping out assuming the vent/flame arrestor is blocked. 

 

Regards.

Attached Files






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