3 replies to this topic

[Mr Quinn]

I am familiar with API 650 and that the design pressure limit on these tanks is 2.5 psig.  Please, do not simply state that fact in your answer; it will not be helpful.

 

My question is this:  how do you choose the design pressure and design vacuum?  They must be specified on the tank data sheet so that the calculations can be done and documented.  If I try and use a go-by from a previous job, I can look on several different storage tanks for the same service liquid, and I could find that all have different design pressures and vacuums.  One might say: 10 in. H2O(g) and -2 in. H2O(g).   Another engineer might have specified 5 in. H2O(g) and -5 in. H2O(g).  Or they might have chosen 0.5 psig (13.85 in. H2O(g)) and -1 in. H2O (g).

 

The point I am trying to make is that NOBODY I ask has a clear, concise, definitive answer for how the process engineer should select the design pressure and design vacuum values for the tank data sheet of an API 650 tank (i.e. atmospheric to low pressure less than 2.5 psig).  It probably should be based on the vapor pressure of the stored liquid and also whether or not the tank is blanketed with nitrogen or another blanket gas.  Are there guidelines that I am simply not aware of?  I would implore anybody's help on this matter.  I've asked this question for nearly 10 years and I always get blank stares or answers that are completely different from each person I ask.

 

There has got to be some reference for choosing design pressure and design vacuum values when designing an API 650 storage tank.

[bmsharad]

when one has to choose the data, there cannot be any standard answer! The API are only guidelines defining the limits in that sense!

[fallah]

Mr Quinn,

 

Attached you can find my viewpoint on your query...

Attached Files

•  Mr Quinn.docx   11.38KB   551 downloads

[Art Montemayor]

Quinn:

 

You are entirely correct in your opinion as to what continues to be an area of concern – especially with the environmental and safety codes that are being applied to API 650 type of storage tanks.  However, you are preaching to the choir.  For many years I have been voicing my concern and frustration regarding the scant and little knowledge that design engineers have demonstrated in past storage tank installations.  This is particularly true in the “oil patch” – where recent environmental and VOC-related mandates are being enforced more and more.  If you use our SEARCH engine you can find threads on this specific issue all the way back to the previous century – most with comments and recommendations from me.  Some of our more knowledgeable members in this subject are Fallah and ProinWV (Paul Ostand).

 

To address your specific question: As Fallah has noted, the basis of a safe, operable, and dependable storage tank for liquids is the basic data that should be stated in the Scope of Work for the specific tank in question.  A storage tank data sheet is prepared to issue the specific data required for the tank’s design and fabrication.  However, it has been a growing and frustrating concern to find to this date the majority of all storage tanks out in industry  have little – or no – detailed, documented calculations (or even nameplates) stating the tank MAWP and MAWV.  This fact goes against all common sense and rational engineering thinking because it leaves the Instrumentation engineer out in the cold with regards to selecting and specifying the appropriate measuring and safety relief devices that must be installed on the tank.  This is a particularly bad situation in the case of hydrocarbon liquids because these tanks are routinely “steamed out” in order to allow for safe and proper human confined space entry – such as in inspections and repairs.  I have seen more than one steamed out tank totally “sucked in” (collapsed) due to vacuum created by an undersized vent nozzle.  The need for nitrogen blanketing and vapor recovery units present more basic data that needs to be addressed and resolved so the tank can operate within a safe, controllable pressure dead band.

 

It is the owner’s responsibility (or his project engineer) to clearly spell out the tank’s scope of work in detail and document it in a data sheet.  As has been pointed out, API only recommends and suggests their best and safest practices and learnings.  That does not mean that these recommendations fit or apply to your scope of work.  That decision falls on your shoulders.  You can, of course, delegate the design to the tank fabricator.  Some tank fabricators will do all the necessary and documented calculations based on your specifications and needs – subject to your approval and payment.  Please take the time to review some of the experienced material I am attaching here, which represent some of the work I have been involved in the past.  I currently am the bearer of “bad news” for one of my clients who has built a series of field storage tanks from 500 to 1,000 bbls.  As a consultant I am often called upon to review and comment on this issue – and it is an unpleasant task to have to return a design engineer’s specifications as incomplete and insufficient to install and operate.

 

It is indeed unfortunate that this is a continuing and recurring topic in the process industries.  I believe the lack of attention and design given to “atmospheric” storage tanks is due to the culture of yester-year when we dealt with them as necessary “garbage cans”, dumpsters, a wide spot in the line, or as token vessels that yielded no direct return on the investment.  Needless to say, that has always been a wrong business decision and approach.  As owners of these type of tanks we are responsible and liable to society for their safe operation and detailed knowledge should be obtained with regards – for example – to the type of pressure relief devices that are installed on them.  Not many engineers – except some in our forums – know that the “Enardo” and “Protectoseal” type of relief valves depend on weighed pallets and do not yield an instant, required relief capacity upon initial opening.  Therefore, this requires the need for specifying the pressure (or vacuum) accumulation within the tank before arriving at the design relief capacity.  These type of relief devices require a clear specification as to size, capacity, and dead band requirements in order to allow them to function within the tank’s scope of work. 

 

I hope this experience helps you and answered your rightful concern in identifying the correct design MAWP and MAWV.  The attachments are examples of how I design and specify a storage tank.  They are not meant to be accurate (they haven’t been peer checked) and serve only as illustrations of what I mean by API 650 tank design.

   API 650 Tank Design Ver 1.xlsx   111.16KB   704 downloads

   Storage Tank Padding Design.xlsx   26.18KB   521 downloads

 Tank Pressure Control_1.xlsx   298.27KB   502 downloads