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Emergency Tank Venting - Api 2000 7Th Ed, 2014

emergency relief valve sizing absolute pressure ratio molecular mass diesel ratio of specific heats

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#1 Hanro Rousseau

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Posted 31 March 2016 - 04:15 AM

Good day,

 

I have spent some time going through the forum topics and resources. I apologize if I am repeating a previously discussed and resolved topic.

 

I have a fuel depot project that includes tank API inspections. Part of the work scope is to provide emergency venting to six 83cu.m vertical pencil tanks. The tanks are not API tanks and do not satisfy API 650 5.8.5.3, regarding a frangible tank to roof joint.

 

API 2000 7th Ed, 2014 has been followed to determine the normal venting required. The emergency vent sizing has provided some queries though.

 

The tank details are -

Dia:             2.87m

Hgt:            14.4m

Product:      Diesel (AGO 50)

 

Due to the tank being a diesel tank, and will remain in diesel service, the normal venting requirement and the emergency venting requirement will be met with a single free vent.

 

The required effective discharge area has been calculated from eqn D.27 in Appedix D of API 2000 -

 

Aeff = qf/(125.15*pi*√((1/M*Tb*Z)*k/(k-1)*((po/pi)2/k-(po/pi)(k+1)/k))

 

Calculating qf from eqn 14 section 3.3.3.3.2 in API 2000 -

 

qf = 906.6*(QF/L)*(Tb/M)^0.5

 

Some of the variables here are -

 

M  = 170g/mol (diesel) or 29g/mol (air)

k   = 1.4 (air), diesel not known

pi  = 123.84kPa

po = 101.325kPa

L   = 250 000 J/kg (diesel)

 

The queries are as follows:

 

1. Considering the product is diesel, should the relative molar mass (M) of air be used in determining the venting requirement?

 

2. In determining the absolute pressure ratio -

 

    i. API 620 9.2.2 states that a supplemental pressure-relieving device shall be capable of preventing the pressure from rising more than 20% above the maximum allowable working pressure.

   ii. API 620 5.3.1.1 states that the working pressure shall not exceed 103.2kPa

   

   Based on API 620 is it reasonable to size the emergency vent on an absolute pressure at device inlet (pi) of 20% higher than the maximum allowable working pressure, giving pi = 123.84kPa? Bearing in mind that the tank is not an API tank.

 

Thank you in advance for any assistance on these queries. The chereresources.com forums and resources are live and provide excellent support based on the reading I have done thus far.


Edited by Hanro Rousseau, 31 March 2016 - 08:42 AM.


#2 ryn376

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Posted 10 May 2016 - 07:20 AM

1. Yes, use API 2000 equation (14) q = 906.6*Q*F/L*(T/M)^0.5

 

2. Maximum design pressure per API 620 is 15 psig, but most tanks built to API 620 CANNOT withstand 15 psig!. The MAWP is calculated. Without an MAWP/design pressure (note that MAWP is equal to or greater than design pressure), you cannot size an emergency vent.

   Since it is not built to API 620, I would use a more conservative basis such as API 650 (Welded Oil Storage Tanks up to 2.5 psig per Section 5.2c). Using API 650 limits the maximum pressure in any scenario to less than the MAWP. This means that the device must be set below the MAWP.

 

Additionally, there is no need to calculate the orifice area, as with PSVs, since manufacturers do not usually sell their devices by orifice size (though you could probably obtain that info, it isn't necessary). Calculate the venting requirements per Section 3.3.3 then obtain vendor curves to determine your maximum pressure during relief.


Edited by ryn376, 11 May 2016 - 12:43 PM.


#3 AlertO

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Posted 07 June 2016 - 09:16 PM

Hi Hanro

1. The equation is for your fluid in case your fluid charateristic is far from Hexane which API shows in Table 7 - Emergency Venting Required for Fire Exposure vs Wetted Surface Area.

2. It is important that you should have the actual MAWP of your tank, not refer to the number from the standard. It will be referred for identifying emergency vent set point as well as % overpressure of the device.

As ryn376 mentioned, you don't need to calculate the area. vendor will select the relief device matching with your flow rate and %overpressure you specify.

#4 S J

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Posted 08 June 2016 - 03:11 AM

Hanro Rousseau

 

Because emergency venting fluid is diesel vapor, molar mass shall be that of diesel.

And regarding set pressure and MAWP, you need to discuss with emergency vent vendor.

After checking overpressure , set pressure and MAWP could be confirmed.



#5 duongbq

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Posted 16 June 2016 - 09:18 PM

Dear all

 

In my project, we have one bitumen tanks (appropriate 7000 m3) and now i need select vents for tank. After read API 2000, I have determined gas flow rate in normal inbreathing/outbreathing flow rate and emergency venting flow rate when external fire case but i don't know solution to determine gas flow rate goes out the tank when this tank is fired.

 

Please help me

 

Thanks,



#6 Pan Nata

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Posted 17 June 2016 - 02:11 AM

Dear all

 

In my project, we have one bitumen tanks (appropriate 7000 m3) and now i need select vents for tank. After read API 2000, I have determined gas flow rate in normal inbreathing/outbreathing flow rate and emergency venting flow rate when external fire case but i don't know solution to determine gas flow rate goes out the tank when this tank is fired.

 

Please help me

 

Thanks,

 

Dear Duong,

 

If the fluid similar with hexane, you can use table 5 and 6 presented in API 2000. Otherwise, the method to determine emergency venting rate more and less it is same like to determine relief load for PSV with fire wetted case. You will require data like wetted area (storage tank), latent heat of the liquid inside the tank (at relieving condition), relieving temperature, molecular weight of the fluid (vapor phase) at its bubble point and environmental factor as per table 9 of API 2000.

 

good luck






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