3 replies to this topic

[ahmetdecoder]

Hello friends,

 

I am a mechanical engineer working for a tank-farm project located in a CCGT project, in Turkey.

 

Venting of the storage tanks is a task that I came across unplanned. I took over this task after realizing that the designer did nothing but "copied without questioning" the vent design that came with original specifications provided by the customer. 

 

So lately I was all over the internet and API 650 & 2000 to determine the required venting for our tanks. 

 

Our tanks are water tanks, cargo varying from demineralized water to running water.

 

 

Below summarizes the reasoning I conducted:

 

Since the tanks store water, I considered only the normal venting case since the tank cargos are unflammable (I MAY BE WRONG).

 

The tanks are built as per API 650, so I took the MAWP and MAWV as the roof weight and 1 inch of water (249 Pa), respectively.

 

To calculate the required inbreathing and outbreathing flowrates, I added the values obtained from Table 1 and Table 2 of API 2000 (1998 issue). I picked the data from Tables as per the boiling point of water, however, Im doubtful about this, since API 2000 is targeted on petroleum products.

 

To obtain the required vent area, I used the formula given at Clause 4.6.1.2, applying the Coefficient of Discharge, K, as well.

 

The calculation sequence is roughly the same as in   13_Venting_requirements.pdf   19.18KB   390 downloads which I got from http://www.eng-tips.....cfm?qid=311070. I copied the sequence in this spreadsheet except for the MAWV value, which I took as 1 inch of water.

 

 

 

 

So, I have these questions to ask to verify myself,

 

1. Does the fact that the stored products being water make any difference for the vent size requirement ? If so, in which direction ?

 

2. Is "inch of water" a safe assumption for MAWV ? Is there any way to determine it ?

 

3. All these calculations are valid for simple open vent areas. And our vent nozzles are equipped with goose necks, CO2 absorbers or pressure-vacuum relief valves. Should I insert this factor to the calculation, if so, how to determine the factor ? Would it have a significant effect over the estimated pressure difference ?

 

4. Should I also consider the emergency venting requirement ?

 

 

I really need your help because we have limited time to make extra venting modifications, and since it is a matter of safety.

 

Kind regards,

Umut

Edited by ahmetdecoder, 15 June 2013 - 07:51 AM.

[fallah]

Hi,

 

1-No significant difference, just you should consider blanketing system requirement and possibility of tank engulfing in a pool fire,

2-Almost is a good estimation when the actual value isn't available,

3-Could be valid for open vent sizing,

4-EV might be required if the fire case is a credible scenario for the tank.

[Art Montemayor]

In addition to what Fallah has contributed, I offer the following experience:

 

An "inch of water" is a safe assumption for MAWV but it will result in excessively large venting equipment.  Besides, it isn’t rational that some fabricator built an API tank to only one inch of water pressure as the MAWV.  What the fabricator probably did was employ the best plate steel he had at his/her disposal at the time.  If you don’t know the certified MAWP, go to the fabricator and ask for it and the backup calculations.  If you can’t do that, then as a mechanical engineer you should re-rate the tank.  The need to know the MAWP (as well as the Maximum Allowable Working Vacuum) BEFORE designing and purchasing your relief equipment is vital because you don’t have a leg to stand on should local authorities inspect or audit your installation.  I would not go any further before identifying and documenting both figures (for each tank).  A water tank collapsing under vacuum or pressure is just as deadly as any hydrocarbon.  Don’t forget, water is one of the heaviest liquids God put on this earth.

 

This topic has been – and continues – to be a top item of discussion in our Forums and much information and uploads (such as spreadsheets for calculations and design) has been posted here.  Use the SEARCH engine in our Forums and you will find this information.

[ahmetdecoder]

Fallah, Art

 

Thank you for your attention. 

 

I will try to obtain the specific MAWV for the tanks, I guess I will have to look into API 650-related sources.

I am going to send the calculation report to the Customer, as well. I think this may provide a good verification and I guess I already should have done this as per the "legislations"

 

Meanwhile I must ask:

 

1. Am I correct at obtaining the required in/outbreathing values as per the the first message? Please note that I followed the 1998 edition of API 650.

 

2. What can you say for the friction coefficient of a goose-neck vent compared to a circular opening (which I presume is what I get the area of as the result of calculations) ? Below is one of the designs of our goose-neck-type open vents. I will also be looking for this, I ask because you are experienced in this area.

 

3. Should I insert a safety factor to the calculated min. vent area, or do API 2000 values already involve a safety factor ?