13 replies to this topic

Hi all,

I had problem about N2 blanketing design for Methanol storage tank we set blanketing pressure at 0.4 bar g and depad pressure at 0.5 bar g then PVRV set point around 0.55 bar g

the methad that we set that because Methanol is very high vapor pressure then the set pressure for it have to more than it vapor pressure @ 40 degree C so our set point is too high but How can we know it will exceed design pressure for tank or not???

How can we determine ATM tank design pressure, in case of we don't know about it?

[fallah]

Is the mentioned tank an existed one or under design?

What is the standard governing the design/construction of the tank?

[proinwv]

This issue comes up regularly. To determine the atm tank mawp and mawv requires reading the manufacturers label, as well as inspecting the tank to be sure it is capable. However, if it is an atm tank then by definition these values are "zero" and the tank is not to be sealed and pressurized.

You have a combustible fluid, and if you pressurize an atm tank to 0.55 bar then this is a tragedy in waiting.

Further, you really don't need these pressures, but then, your tank cannot be pressurized anyway.

[Qalander (Chem)]

Dears,

If I am not inaccurate it is limited to only few inches of water column pressure
Regards
Qalander

[ankur2061]

Asuka,

The vapor pressure of methanol at 40 and 50 deg C are as follows:

40 deg C: 0.35 bar(abs)

50 deg C: 0.55 bar (abs)

Please note that the vapor pressure units are in absolute units and not in gauge units.

The NBP of pure methanol is 64.7 deg C.

To my understanding if a nitrogen blanketing is to be provided a pressure of 75-100 mmWC (gauge) or in other terms 0.0075-0.01 bar(g) (3-4" WC(gauge)) should suffice. How on earth did you arrive at 0.4 bar(g) as your blanketing pressure.

Has it been that most common mistakes of confusion of absolute and gauge units.

Well, don't worry, even after 23 years of experience I have fallen in the same trap many a times.

Just correct your blanketing settings to the above mentioned values. However, iif you have already bought a PVRV with a factory calibrated setpoint of 0.55 bar(g), it is a far more serious issue and maybe needs immediate consultation with the PVRV manufacturer/supplier to rectify the problem. Probably needs a replacement with a new PVRV.

Hope I have been of some help.

Regards,
Ankur.

[Art Montemayor]

I agree with Ankur's statement, "if a nitrogen blanketing is to be provided a pressure of 75-100 mmWC (gauge) or in other terms 0.0075-0.01 bar(g) (3-4" WC(gauge)) should suffice".

However, that is not the impending problem that Paul is alerting us all to heed: If you don't have the tank's pressure rating firmly stamped on the tank or have the official, checked and approved rating calculations specifically identifying the MAWP, then you have TWO PROBLEMS. The first and most important problem is simply the unknown nature of the tank's MAWP - which you need to set the Pressure and Vacuum limits on the relief devices! Without this documented information, you can't logically and morally set limits on the tank by simply assuming or guessing. Of course, if you decide to take a chance and guess at the MAWP and MAWV and subsequently a tragedy or accident happens due to the tank leaking or rupturing, then YOU, as a professional engineer entrusted in protecting the well-being and safety of the workers and equipment, are 100% guilty of causing the accident and you should be prosecuted according to local laws. That's why I would seriously give attention to what Paul has alerted us all to.

The second problem is that even if you succeed in applying the 4" WC pressure that Ankur is inferring as being "OK" to employ on an undocumented tank (& we are not even focusing on the more dangerous VACUUM case), you don't have a reasonable DEAD BAND of pressure in order to have conventional instruments detect and operate Nitrogen input and venting within the tank. In other words, 4" WC is just too narrow a band to operate the makeup nitrogen feed and the pressure/vacuum relief. There just isn't enough dead band for instruments to work within.

The basic question raised by Asuka was: "How can we determine ATM tank design pressure, in case of we don't know about it?" My answer to this basic question is: Obtain the services of a professional mechanical engineer experienced and recognized in rating storage tanks and have him/her confirm in writing (and backed up by checked calculations) that the tank can take sufficient pressure and vacuum so as to allow a reasonable instrument dead band. If you put the tank into pressurized service without this type of professional evaluation and confirmation, you are taking on the total legal and moral responsibility of the tank's operation. If anything bad happens as a result of that decision (God Forbid), then you are the responsible party. I don't know how other engineers would react to this, but no one can pay me enough money to take that kind of risk.

[Qalander (Chem)]

Dear Respected Art.

Thanks for once again painting a true picture of a possible catastrophe promptly;
the forum contains a thorough discussion related to tanks. OP 'asuka' should go through using help where you posted certain images of considerable damage consequences.

Best regards
Qalander

[proinwv]

Asuka,

I hope that you take very seriously the comments that have been made here regarding your application. They are all in agreement as to the potential serious consequences to blanketing an atm tank.

Further, in any blanketing application, when the vacuum, pressure, and emergency vent setpoints are selected, as well as the blanketing valve setpoint, you must consider the operating band each device will have. This encompasses the crack open to full open and then the drop to reclosing pressures. These must be integrated into one system to avoid interaction as well as provide a complete picture of the pressures the tank might be expected to experience.

My website might contain some additional information of use to you, www.ostand.com

Regards

[djack77494]

So my question regarding this whole business is why doesn't API or some other regulatory authority require the very simple step of stamping rating information onto tanks? We'd all be horrified at the thought of pressure vessels not having the appropriate ASME or other stamp clearly indicating design pressure and temperature. Why is it OK to omit this most important information about a tank? There's certainly plenty of potential for disasterous consequences if an error is commited. And while I know it wouldn't be free to get the rating added, surely it would not cost much. Can anyone imagine that being told to go determine the pressure holding capability of a pressure vessel whenever it was being considered for alternative service if the necessary paperwork couldn't be found? That would be totally unacceptable I think. So why is it accepted for tanks? It makes no sense at all to me.

[Art Montemayor]

Doug is absolutely correct in his logic and engineering concern for the safe design of what appears to many lay people as a benign and mundane vessel, when in reality we professional engineers know all too well that the application of a so-called “atmospheric storage tanks” is often a fatal accident in the waiting. However we may want to think about it, the application of a storage tank is certainly not a “no-brainer” – nor is it safe from potential hazardous traps and engineering mistakes. Storage tanks are all too often taken for granted and, unfortunately, the heavy price for possible mistakes is paid afterwards.

Doug’s righteous concern focuses on the following points:

1) There are presently no API “Recommended Practices” or “Standard” (to my knowledge) on generating, certifying, and documenting the specific and official Maximum Allowable Working Pressure(MAWP) and Maximum Allowable Working Vacuum(MAWV) for API type storage tanks. Why this is so is a mystery that I have failed to uncover during all my years as an engineer. The impractical, illogical, and unsafe appears to thrive in an engineering environment. This is tantamount to allowing your enemies to move in next door, without any gates or fences between you. This, in my opinion, is cause for concern.

2) Unlike the standard PSVs used to protect pressure vessels from over-pressure, storage tanks are forced to employ weighed pallets (PVSV) as the main element in determining the set, relieving pressure. What this means is that the PVSV does not behave like a standard PSV. It begins to open (or crack) at the set pressure, but it does not reach the full, rated design capacity until the pallet is well-raised and operating. This effect is important to know and to take into consideration when applying pressure and vacuum devices on tanks. This is why I never initiate the design of such devices myself. I always relegate that sizing procedure to the “expert” – the manufacturer of the device. A significant lag in attaining full, design, relief capacity can be very important, and should be taken into account when sizing the relief device. If anyone has gone through the PVSV sizing curves, they will understand what I mean. These curves are not user-friendly and take time and experience to use with confidence.

3) As indicated above, the action of the PVSV is such that the set pressure and sizing method take on special care to get it correct. However, the basic input data is often lacking, unknown, or simply a mystery: WHAT IS THE MAWP AND MAWV? Without these certified values in hand, one can’t specify - or much less – purchase and install the correct PVSV. This is not only a frustrating situation, but one that is totally illogical to an engineer. This, as Doug well points out, should not be allowed or tolerated. There should be an in-place procedure that ensures that the above critical values are made known and confirmed – at the very least for the tank when it was initially built. Once the tank is purchased, the owner (or user) is the party responsible for maintaining the documentation and identifying the actual physical condition of the tank through its useful application time. This means that the MAWP and MAWV are checked and confirmed every so often (usually every five years). This maintenance and engineering practice ensures a safe and reliable tank operation.

I believe that the “hole in the floor” (or breech) that has been created with respect to enforcing a logical and safe design of storage tanks has largely been caused due to the official mandate given to pressure vessel codes: these codes (such as ASME) have, as a primary basis, the definition of a pressure vessel well-established: a pressure vessel is one in excess of 15 psig. This definition immediately excludes any effort or work done on storage tanks that operate in the pressure zone of 0 – 15 psig. To make things worse, I know of no agency or institution that focuses on “Non-pressurized” vessels. In effect, then, storage tanks are left out in the cold with respect to outlining and specifying safe and recommended design and operating conditions. We are left to our own, specific means to deal with the situation. While most of us can deal with this situation on a case-by-case basis, it means that we are left without a central means of consolidating lessons learned, experiences shared, and recommended practices being distributed for the benefit of all. This is not the smart engineering way that we all know for solving problems .

API 650 does include the mechanical design of storage tanks and, consequently, an experienced and professional mechanical engineer calculates the proper MAWP and MAWV of new tanks. However, this does not apply to older tanks or tanks in various services – which, as Doug has pointed out, is often the case. Additionally, without a mandatory ruling on creating certified MAWP and MAWV calculations and documentation, we are forced to remember (& specify) that the new tank fabricator has to be instructed to furnish the MAWP and MAWV documented calculations and the API “Stamp”. This, as Doug infers, is an additional cost and service that normally is not furnished unless specified by the purchaser. Without the API recommending or local laws enforcing the mandatory issuance of the above documents and stamp, I’m afraid we are at a standstill. Nothing will be done, and things will continue as they historically have. This situation is one that I fear will not change until a horrid disaster occurs in the industry and API, ASME, DIN, and other agencies are alarmed to the point of doing something positive about it. In the meantime, persons such as Doug and I will continue to complain, whine, criticize, and generally make pests of ourselves pointing to this ridiculous situation.

What most of us can do, for now, is to demand that our clients and employers specify the supply of MAWP and MAWV documentation and stamp on all NEW tanks. They should also employ and follow a strict maintenance policy on storage tanks that includes the confirmation of the mechanical integrity of these tanks.

[Qalander (Chem)]

This issue is very rightly raised by Doug/Seconded by Art.

Considering that the professional skill levels of various ASME and API Committees must be sufficient with respect to the knowledge pooled.
As well as the resources already available.

They must be perfectly/beutifully posed to suggest/recommend or permit stamping on tanks for an optimum value each for 'MAWP' and 'MAWV'.

Hopefully, some of our learned/influential colleagues might be on these board or at least have connection with these people to pursue this 'GOOD ACTIVITY' for everyone's sake.

Best regards
Qalander

[proinwv]

This has certainly becoming a very interesting thread of good information. Our thanks to all for that.

I have thought that one reason for the floor on ASME VIII at 15 psig was that if you do the calculations for lesser pressures you might find that the wall thickness requirements become very small. So thin that the tank would be very vulnerable to physical damage from external forces such as handling or fork lifts etc.

In my practice I am more exposed to existing tanks and this is a real delicate issue to deal with with a client. Sometimes you must simply walk away. But I suspect that many will still go ahead with their project.

Art's comment on re-inspection and certification of tanks is on target. Art, what are your feelings on what percentage of atmospheric tanks receive this attention?

[Qalander (Chem)]

Dear Paul, Hello

If I recall this correctly the minimum steel sheet thickness is considered 3/16" for above ground storage tank(Safeguard against lightening strike from piercing-in).

Above and beyond this value; many factors of corrosion allowance, hydraulic loading related & code guided factors provide added thickness values for various structural members; i.e., Shell, Bottom, and Roof etc.

Now, Considering an optimum value of thickness and in a way the withholding strength may guide us to a workable basis to determine an estimated value for 'MAWP' and also possibly 'MAWV'.

May be this is conceptually supporting help for your thoughts.
Regards
Qalander

[Art Montemayor]

All Forum Members:

Paul Ostand has been kind enough to alert me of a news release on a recent (November 2008) tank accident that eerily resembles a lot of the hazards and engineering decisions we have been discussing in this thread.

Please go to: http://www.csb.gov/n...nals12.8.08.pdf

Although I have repeatedly recommended all engineers - especially students - to always monitor the CSB webpage for reading and studying the latest accident reports in order to strengthen and develop a strong sense of safety-related experience, I have been delinquent in doing so in the recent past due to my extended vacation with my grandchildren. This is another example of what can happen when we least expect it.

Note the age of the subject tank(s) - 1929. Also note the type of construction - rivets. This should not come as a surprise to any experienced engineer (I'm sure that Doug can match this type of experience) who has had to deal with bulk liquid storage. As we have noted, the call to employ old tanks or tanks that have been in prior service (or in different applications) is always around the corner. And the majority of these tanks have no documentation.

I note that the State of New Jersey has enacted new and strong laws after the disasterous event of the Motiva oil refinery sulfuric acid tanks there. I am very interested in finding out if New Jersey has attacked the various weaknesses in the existing "system" that we have discussed in this thread. If any one has access or copy of the New Jersey tank laws, please let me know or send me a copy.

Thank you, Paul.