18 replies to this topic

[contai]

I figured out the inbreathing and outbreathing MSCFH needed for my tank by following API 2000. However, to get the correct vent, the vendors request a tank design pressure. My tank is an atmospheric tank. I'm not sure what their asking for. How would I figure out the tank pressure? thank you, Cro

[proinwv]

They want the MAWP and MAWV and since your tank is atmospheric, the answers are zero, zero and then they likely will tell you that they can't sell you a vent because your tank cannot be pressurized.

[contai]

Thank you, Mr. Ostand.

[fallah]

contai,

Although, your tank is atmospheric, but if you have to pressurize it due to blanketing system requirement, you can calculate MAWP/design pressure of the tank (limited to 18 kpag) as per API 650 (if it is the standard governs the tank design) and report the value to the vendors for further action.

Fallah

[proinwv]

Fallah, should you not add that to do so requires detailed knowledge of the tank construction? My experience has been that little information is known of these tanks by the user.

[fallah]

Fallah, should you not add that to do so requires detailed knowledge of the tank construction? My experience has been that little information is known of these tanks by the user.

Paul, then if the user would want to use blanketing sysyem in his tank what should he do?

I think the user should be able to get necessary information from tank drsigner/manufacturer if he wants to do so.

[proinwv]

Fallah, I agree that he "should" be able to obtain the information, as you stated.

But it has been my experience that the information is often not available on the atmospheric tanks. We often see that the tank is old, and may not have a nameplate. While the user may not like to hear this but if he cannot verify the integrity of a tank then it should not be pressurized. Further, should it be an older tank, then a structural exam might be indicated.

Even if the tank is blanketed to a very low pressure, the addition of P-V vents and an emergency vent and accomodating the the operating range of each along with a dead band in between can still result in a signifcant tank pressure requirement (for an atmospheric tank).

I do realize that these tanks are blanketed; but that does not mean that all can be and it also requires careful thought into what is being done.

[fallah]

Paul,

I got your points, hope the OP would has been gotten as well. OP is also requested to shed some lights on dark part of his issue for our information.

Fallah

[mohds23]

Mr Paul / Mr Fallah,

I have indicated the requirement for MAWP and MAWV for atmospheric tanks in datasheets but the tank fabricator has denied to provide these details. His justification is that the tank is atmospheric.
Since these tanks are going to be fabricated, should the vendor not supply these data?

Kindly provide your inputs.

Mohd

[fallah]

mohds23,

Which standard governs the tank design? API 650,...If so, he can specify the tank's MAWP/MAWV after its fabrication.

Vendor justification isn't acceptable, unless specifying the MAWP/MAWV based on engineering documents and actual manufacturing conditions had already been out of his scope of work.

Fallah

[proinwv]

If the tanks are not yet made, then you should be the specifier of the desired MAWP and MAWV and have them built to the appropriate specification to satisfy your requirements.

[mohds23]

Mr Paul / Mr Fallah,

Thanks for your prompt reply.
Yes tanks are Atmospheric and designed as per API 650.

If the tanks are not yet made, then you should be the specifier of the desired MAWP and MAWV and have them built to the appropriate specification to satisfy your requirements.

I didn't understand above statement. Should i specify that the tanks should be capable of handling 18 kpag pressure and 1 inch WC vacuum?
I thought all API 650 atmospheric tanks are cable to handle above mentioned pressure and vacuum conditions.

I just wanted to get this MAWP and MAWV stamped on tanks, so that owner can provide blanketing arrangement (if required) in future, keeping these pressure /vacuum rating in mind.
Kindly guide me.
Thanks.

Mohd

[proinwv]

I am not a tank expert but I understand that these tanks can be designed, using the appropriate design method as shown in the standard, for pressures above atmospheric within certain limitations.

So yes, you need to tell the builder what pressure rating the application requires and have the tank designed, constructed and marked accordingly. If the contractor will not do this, then you need to question why!

As the engineer, it is your responsibility to assure the proper equipment is provided.

[fallah]

Yes tanks are Atmospheric and designed as per API 650.
I didn't understand above statement. Should i specify that the tanks should be capable of handling 18 kpag pressure and 1 inch WC vacuum?
I thought all API 650 atmospheric tanks are cable to handle above mentioned pressure and vacuum conditions.

I just wanted to get this MAWP and MAWV stamped on tanks, so that owner can provide blanketing arrangement (if required) in future, keeping these pressure /vacuum rating in mind.

mohd,

You should specify the tank's design pressure/vacuum in the tank data sheet.Then the fabricator, just after preparing tank's materials of construction and determination of manufacturing procedure, would be able to calculate MAWP/MAWV based on actual used plates (thickness,...) and manufacturing conditions (welding,...) and should do that if client requests.

Hence, there is no reason all API 650 atmospheric tanks are cable to handle 18 kpag pressure and 1 inch WC vacuum and these are just limitation values on which you can rely.

In the cases you have not MAWP/MAWV in hand you have to limit your blanketing system's operating range between tank's design pressure/vacuum.

Fallah

Edited by fallah, 12 March 2012 - 08:21 AM.

[mohds23]

Thanks for supports.
Regards,
Mohd

[Art Montemayor]

Before this thread closes, I want to add some comments based on personal experience in specifying, rating, blanketing, installing, and modifying API storage tanks (particularly API 650 type). Additionally, I want to take the opportunity to personally thank both Paul Ostand and Fallah for some succinct and valuable engineering insights into the problem surrounding the engineering design (from a process standpoint) required of storage tanks. Both Paul and Fallah have hit on some very important and salient points that should seriously be taken into consideration and applied every time the service or operating conditions for “atmospheric” storage tanks are discussed or brought up.

I experienced for some years the taxing problem and unfortunate duty to advise owners of storage tanks that their operation, maintenance, and application was not only wrong (engineering and legally), but they were on the precarious edge of causing a major catastrophe. This is not an easy message to convey to someone who is not prepared or willing to hear the truth. And my clients were chemical processing “heavyweights” in the USA industry. It was my unfortunate duty to tell them they just couldn’t convert a non-stamped, non-rated, and non-documented storage tank into a nitrogen-blanketed vessel without taking the appropriate and legal steps of clearly and accurately identifying the design basis and actual conditions of the subject tank(s). What we wound up having to do was to hire an experienced and proven mechanical engineer who rated the tanks and performed the necessary tank inspections and calculations in order to justify the operating conditions that we proposed to impose on the tanks. This took time, man-hours, resources, and money. But it was necessary and well-worth it for the safety and peace of mind of all involved. Through this experience, I learned how the tank owners got into such a sad and sorry state of affairs in the first place. My research showed that most – if not all storage tanks had been viewed and treated as if they were not of any process importance since they were an expense (not an investment) that was necessary and didn’t add any value to the sold product. When looked upon this way, they were not deemed worthy of “wasting” any process design time and specifications on them. They were ordered for purchase as merely “atmospheric tanks” – without even taking into consideration the density (and consequently the hydrostatic head at the base plate) of the stored fluid. When the time came to confront environmental concerns the need for nitrogen blanketing was recognized and applied – without regards to identifying the MAWP and MAWV. And this is where I got involved and started raising holy hell.

At one time, during the early years of our Forums, I uploaded a compressed file that contained information on the importance of setting pressure and vacuum setting on API tanks. I am attaching a copy of this file in case it can be of any use to the OP, Mohd.

Additonally, I would add some personal observations and advice for Mohd:

If you fail to specify your pressure and vacuum requirements to a tank fabricator, you will not receive any design information on the tank. I strongly recommend that you specify and require a full set of tank calculations – complete with fabrication drawings of the tank and a welded stainless steel name plate that clearly states in peened letters, the MAWP and MAWV. This information is always required when setting and calibrating the relief devices for the tank in the future. You should insist that your Purchasing Department follow your instructions on the specific Purchase Order, with regards to this need.

The reason your fabricator has not furnished the MAWP and MAWV is because of one of two reasons:

• He has no tank design capability and would have to contract out the required and necessary calculations. This would cost money that would have to be added to the quoted tank price and it would possibly place his bid out of the running.
• He has the design capability but fears that you won’t want to pay for the engineering design manhours needed to generate the calculations.

When you specify in your Purchase Order that you want and need a set of calculations and the MAWP and MAWV, you will “weed out” any fabricator that doesn’t technically qualify. I am more than sure that you will find that there are serious and capable fabricators in the UAE that can furnish you with exactly what you need and specify. You merely have to specify it. It is simply a question of money. Please look at the tank Specification Data Sheet that I give in the attached workbook.

Also read the valuable information I include from Paul's website (with his prior approval, of course). You will find a lot of experience in his writings.

I hope this experience on this very important topic helps you out.

Attached Files

•  Art\'s Tank Relief & N2 Blanket.zip   1013.49KB   257 downloads

[proinwv]

Art, it is good to hear from you again. Thanks for the nice background and summary of this issue which repeats itself with some regularity.

I might add that I have even had a tank owner blatantly lie to me about what the tank capabilities were when it was obvious from previous conversations that it was an atmospheric tank. I would offer this to the thread and add that there is no job worth the safety of others, the risk to property, or my reputation that I would ignore the reality of what MAWP and MAWV are.

[mohds23]

Greetings!
It is great to hear from such experienced personalities. Thanks again gentlemen.

Most of the things are clear now except one small doubt.
When I see the attachment (API 650 tank design), I found negative design pressure mentioned as 4 oz/in2 (equal to 6.92 in WC).
I feel we can allow vacuum upto 1 in WC and this is typo error.

Kindly advise.

Regards,
Mohd

[fallah]

mohds23,

In API 650 Eleventh Ed. 2007 (page 43), design external pressure is mentionrd equal to 1.0 inch of water and there is no typo error. You may refer to older version.

Fallah