14 replies to this topic

[Afshin445]

Dear Experts,

 

I have a small capacity diesel storage tank with 3m (L) x 1.8m (w) x 2m (H) dimension.

 

This new desiel tank provided to supply diesel fuel to existing Emergency Diesel Generator(EDG)

by gravity flow for this reason tank will be installed in elevation higher than EDG (about 2m higher).

 

The storage tank itself also can be feeded by gravity with using a tote tank which currently installed in elevation of 

3.5m above new storage tank.

 

The inlet and outlet lines to and from tank is 2".Diesel tank is open to atmosphere through an open vent including a flame arrestor.

 

I try to size vent line but below questions cames in my mind:

 

1- How I can estimate inbreathing/outhbreathing flowrate  through vent line, is need to consider API-2000

    method for such a small tank or assuming air flowrate is equal incoming/outgoing diesel flowrate?

 

2- In vent pressure drop calculation I need to consider flame arrestor is in clean or dirty condition?

 

3- How I can assign tank maximum/minimum(vacuum) design pressure?

 

I appriciated if somebody clear me about above questions.

 

 

Thank you in advance

 

[fallah]

Afshin,

 

Lack of adequate info and a sketch of the system, especially not specified if any facility other than free vent ended with FA has been considered on the tank. Anyway my general answers to your question are in red color as follows:

 

 

I try to size vent line but below questions cames in my mind:

 

1- How I can estimate inbreathing/outhbreathing flowrate  through vent line, is need to consider API-2000

    method for such a small tank or assuming air flowrate is equal incoming/outgoing diesel flowrate? Normally no, but if it is followed could be useful for vent sizing...

 

2- In vent pressure drop calculation I need to consider flame arrestor is in clean or dirty condition? As per the dirty condition would be better and conservative...

 

3- How I can assign tank maximum/minimum(vacuum) design pressure? For such assigning maximum operational pressure and vacuum by which the tank might be subject to, should be specified...

 

[proinwv]

i would add that you correctly point out that an arrestor can reduce flow rate as it accumulates dirt. You should consider using appropriate pressure and vacuum protection should this occur.

 

Your tank appears not to be round. Flat sided tanks can be very weak in resisting pressure or vacuum. Be careful.

[Afshin445]

Fallah,

 

Thanks you for prompt responce.Sorry as my description was not so clear. I attached a simple schematic of the system which may clarifed your question, as shown in sketch vent line is just including FA and no any other facility.

 

In this regards, I need some clarification in your responces in blue colour as per below:

 

1- How I can estimate inbreathing/outhbreathing flowrate through vent line, is need to consider API-2000

method for such a small tank or assuming air flowrate is equal incoming/outgoing diesel flowrate? Normally no, but if it is followed could be useful for vent sizing...

What you mean for such a tank I can assume air inbreathing/outbreathing flowrate is equal to disel incoming/outgoing flowrate?

 

2- In vent pressure drop calculation I need to consider flame arrestor is in clean or dirty condition? As per the dirty condition would be better and conservative...

Do you have any estimation of FA pressure drop in clean condition? Also how we can estimate maximum pressure drop across FA when it's in dirty condition, FA can be totlay cloged if not  maitain properly.

 

3- How I can assign tank maximum/minimum(vacuum) design pressure? For such assigning maximum operational pressure and vacuum by which the tank might be subject to, should be specified...

As I understand maximum/minimum operating presure of storage tank is dependent on vent size, in this case I don't have vent size yet, so  how I can estimate this parameters without knowing vent size?

 

 

 

Attached Files

•  Diesel System Skectch.xls   505.5KB   140 downloads

Edited by Afshin, 04 April 2014 - 10:34 PM.

[Afshin445]

Paul,

 

Thank you for your fast reply.Appriciated if you describe more detail about protection devices, do you think for open vent to atm. we need to using protection devices like rupture pin or rupture disk?

 

Also I need your advice about my responce to Fallah about item no. 2 which is about how I can estimate maximum pressure drop across FA in clean and dirty condition.

[fallah]

Afshin,

 

Again my comments in red color are as follows:

 

 

 

1- How I can estimate inbreathing/outhbreathing flowrate through vent line, is need to consider API-2000

method for such a small tank or assuming air flowrate is equal incoming/outgoing diesel flowrate? Normally no, but if it is followed could be useful for vent sizing...

What you mean for such a tank I can assume air inbreathing/outbreathing flowrate is equal to disel incoming/outgoing flowrate?

I meant because you already decided to use open vent plus FA without using API 2000 as starting point for tank protection, as a conservative approach you may use large size open vent(s) plus FA along with emergency pressure/vacuum vents to ignore using API 2000. Obviously, it is better if you basically use API 2000 to specify venting devices and their capacities...

 

2- In vent pressure drop calculation I need to consider flame arrestor is in clean or dirty condition? As per the dirty condition would be better and conservative...

Do you have any estimation of FA pressure drop in clean condition? Also how we can estimate maximum pressure drop across FA when it's in dirty condition, FA can be totlay cloged if not  maitain properly.

 Normally vendor would submit a graph indicating the pressure drop vs flow passing for each FA type and size but in clean conditions. In dirty condition, a conservative approch is totally closed situation for which emergency prressure /vacuum vents should be considered...

3- How I can assign tank maximum/minimum(vacuum) design pressure? For such assigning maximum operational pressure and vacuum by which the tank might be subject to, should be specified...

As I understand maximum/minimum operating presure of storage tank is dependent on vent size, in this case I don't have vent size yet, so  how I can estimate this parameters without knowing vent size?

The tank appears to be rectangular (non circular) and such tanks are to be manufactured as per their own code & standard. Obviously, the design pressure/vacuum of this tank are to be specified based its manufacturing code and following that the relevant maximum/minimum operating pressure can be set...

 

 

[yazdanpanahimehrdad]

Dear Afshin;

for part 3 of your question normally design pressure/temperature are determined based on the criteria of the project you are working in. from project to project criteria may differs a little. but as far as i have seen in the projects for the tanks which are connected to the open atmosphere design pressure would be hydraulic pressure of the fluid. for your tank it would be:

P=density of gas oilX g Xh(m),h=(30.1-26.5+d(portable tank hight))

in some projects conservatively use 3.5 barg as minimum design pressure.

vacuum condition does not probably happened in your system because it is connected to open vent and overflow line.

generally this fluid is in ambient temperature. so, design temperature will not be so high. in most specification:

design temperature=maximum operating temperature +25 C or boiling point of the liquid in the operating pressue whichever is higher. Of course, if vessel is exposed to sunshine, design temperature will be decided based on 1-sunshine temperature and 2-above mentioned criteria, whichever is higher.

Edited by yazdanpanahimehrdad, 05 April 2014 - 06:26 AM.

[proinwv]

The open vent is useless if the flame arrestor is blocked. To me that is the ultimate pressure drop of a blocked FA.

 

Pressure/Vacuum vents for tanks are available from many manufacturers. You may have a problem finding one small enough for your vessel.

[Afshin445]

Fallah and Paul first thanks for your valuable comments.I just sized vent line based on API 2000 normal venting and result shown 3" vent size is sufficent.In this regards, I am wondering is it necessary to check this size based on considering tank is exposed to external fire?

 

Tank elevation from main deck is about 3.5 m which apperantly is below than API recommendation of 25 ft for considering of fire case.

 

Appriciated if you clear me about that.

Edited by Afshin, 16 May 2014 - 10:13 PM.

[fallah]

Afshin,

 

You should size a dedicated emergency vent for emergency flow capacity due to fire case. If you can take full credit for the flow capacity provided for normal venting then you should provide emergency venting facility with considering normal vent as a part of emergency vent can handle part of required emergency flow capacity.

[Afshin445]

Afshin,

 

You should size a dedicated emergency vent for emergency flow capacity due to fire case. If you can take full credit for the flow capacity provided for normal venting then you should provide emergency venting facility with considering normal vent as a part of emergency vent can handle part of required emergency flow capacity.

 

Fallah,

 

Tank will design based on API 650 code, and only provided by manway and vent nozzle. Is it API 650 requirement to provide dedicated emergency venting for fire case? Could you provide me your reference?

 

In this regards, what should be wrong if normal venting nozzle also will use for emergency venting (e.g. fire)? 

Edited by Afshin, 16 May 2014 - 10:34 PM.

[fallah]

 

Tank will design based on API 650 code, and only provided by manway and vent nozzle. Is it API 650 requirement to provide dedicated emergency venting for fire case? Could you provide me your reference?

 

In this regards, what should be wrong if normal venting nozzle also will use for emergency venting (e.g. fire)? 

 

 

Afshin,

 

Attached you will find a page of API 650 (sec. 5.8.5.1) in which tank requirement to emergency vent due to fire case is mentioned.

 

Normal venting would normally require much lower relieving area than that of the fire case, then cannot handle the relief load of fire scenario. Indeed, as i mentioned, in all conditions you cannot take credit for the flow capacity of normal venting to be used as part of required relieving area for fire case because it might during a fire occurrence the exposed tank wouldn't be isolated and normal venting facility handles its own relief load then cannot take part in relieving the load of the fire case.

Attached Files

•  API 650 Emergency Vent Requirement.docx   287.15KB   108 downloads

Edited by fallah, 17 May 2014 - 01:34 AM.

[Afshin445]

Fallah,

 

Thank you for your prompt reply again.I just went through attached page from API 650 which mentioned emergency venting requirements are satisfied if the tank is equipped with pressure relief devices.My concern is as tank already is open to atm. by vent line can we consider vent line as emergency venting device also.I never saw a tank connected to atm. and also have separate relief valve device.Please advice.

 

In other hand tank aslo provided by an oveflw line including liqiuid seal which can be designed so during emergency case liquid seal can break (e.g. fire case) , what is your idea about using overflow line as emergency case outlet?

[fallah]

Afshin,

 

Normal venting with limited relieving cross sectional area, even if opens to atmosphere, may not handle the relief capacity of emergency conditions such as exposing to an external fire. Then it's not uncommon having emergency vent facility beside the free vent for the cases in which the fire scenario is credible.

 

Even though the overflow line might take part in relieving the overpressure due to an emergency condition such as fire case, it isn't recommended at all to be used as a venting facility for emergency condition. 

[proinwv]

Possibly I missed something, but let me add that with the presence of an overflow line, its outlet location should be such that should emergency venting be directed through it (by its presence) then that outlet must not be such that it would present a hazard in itself.

 

This is not to suggest that the overflow should be used for venting; just to point out that the presence and location must be considered. Vented gasses due to a fire will exit the line with considerable velocity as compared to overflowing liquid.