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Setting Dead Band In Engineering Stage


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#1 fallah

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Posted 01 October 2009 - 05:03 AM

Dears,

I have some question regarding tank blanketing system:

1-In engineering stage while we have no information about MAWP/MAWV from tank vendor,what is the best procedure for setting the dead bands and specifying the relevant PCV,PSV,Emergency vent,....in order to supply them?How we can estimate MAWP/MAWV values such that we would sure not crossing these limitation?Are any tank vendor obliged to submit MAWP/MAWV for the tanks which being ordered?

2-Basically,specifying the tank blanketing system and relevant devices,PCV...,in procurement point of view is in the scope of engineering (process/instrument) contractor.Could the responsibility of mentioned specifying be transferred to tank vendor?If so,under which conditions?

Regards

#2 proinwv

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Posted 01 October 2009 - 07:54 PM

Answering your question #1

I cannot imagine how you could proceed with pressure settings without knowledge of the MAWP and MAWV. I also would not know how to estimate (i.e. guess) what they would be.

I am not sure what you mean in question #2 but I will try to respond.

The party responsible for the system must be one who was qualified to design the system for the intended use and willing to take responsibility for its proper functioning.

#3 proinwv

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Posted 01 October 2009 - 07:59 PM

One more thing. I suggest that you perform a search of this forum. These subjects have been addressed in detail in the past. Read some of these postings.

#4 fallah

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Posted 02 October 2009 - 01:25 PM

Answering your question #1

I cannot imagine how you could proceed with pressure settings without knowledge of the MAWP and MAWV. I also would not know how to estimate (i.e. guess) what they would be.

I am not sure what you mean in question #2 but I will try to respond.

The party responsible for the system must be one who was qualified to design the system for the intended use and willing to take responsibility for its proper functioning.


Thanks for your reply,

But i didn't get my answer at all.Also i performed a search in this forum and didn't find proper answer to my questions.

Actually i am involved in design of blanketing system for some storage tanks which their manufacturing would be performed by a vendor in the field,and i haven't any data regarding MAWP/MAWV to go ahead for design of such systems.

Do you mean i have to wait getting mentioned data (MAWP/MAWV) from vendor and then going to design such systems?

Might some individuals being involved in the same situation as above can submit better assistance.

#5 djack77494

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Posted 07 October 2009 - 06:33 PM

The one in the best position to determine the MAWP/MAWV is the tank fabricator. He can do this during the design of the tank, though it may involve a bit more effort and should be accompanied with some documentation. It is fair for him to expect a small "add on" for this additional work, and you should readily agree to any reasonable extra for this most valuable part of the overall design.

Lacking that, you can refer to any datasheets that were sent to the fabricator. If you're lucky, the datasheet will list a DESIGN pressure and a design vacuum. (If those fields say "Atmospheric", then never use that engineering company again, and shame on them.) The MAWP/MAWV should equal or exceed the design pressure/vacuum, so you can use the design values as conservative values.

If you're not so lucky, then you will need to hire a competent mechanical engineer to rate your tank. Unless new, he will probably need to take various measurements on the tank, and a process that would have cost pennies if done at the start will now cost substantial time and money. Hope you fit into the "lucky" catagory.

#6 fallah

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Posted 08 October 2009 - 02:51 AM

The one in the best position to determine the MAWP/MAWV is the tank fabricator. He can do this during the design of the tank, though it may involve a bit more effort and should be accompanied with some documentation. It is fair for him to expect a small "add on" for this additional work, and you should readily agree to any reasonable extra for this most valuable part of the overall design.

Lacking that, you can refer to any datasheets that were sent to the fabricator. If you're lucky, the datasheet will list a DESIGN pressure and a design vacuum. (If those fields say "Atmospheric", then never use that engineering company again, and shame on them.) The MAWP/MAWV should equal or exceed the design pressure/vacuum, so you can use the design values as conservative values.

If you're not so lucky, then you will need to hire a competent mechanical engineer to rate your tank. Unless new, he will probably need to take various measurements on the tank, and a process that would have cost pennies if done at the start will now cost substantial time and money. Hope you fit into the "lucky" catagory.


What is your prior suggestion (based on your experience) to solve my problem as per my previous posts in this thread?

If we have narrow range between design pressure/design vacuum such that we have problems to cover all instruments ranges,how would your second suggestion ( Lacking that, you can refer to any datasheets that were sent to the fabricator. If you're lucky, the datasheet will list a DESIGN pressure and a design......) be executed?

Edited by fallah, 08 October 2009 - 02:58 AM.


#7 Art Montemayor

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Posted 08 October 2009 - 04:15 AM


Fallah:

I assume that you mean that you are in the Detailed Engineering stage – the stage well beyond conceptual and preparatory to completing all the detailed Data Sheets required for requisitioning all major equipment.

If that is the case, then I would expect that the process engineer doing the detailed design for the storage tank (I also have to assume this is a storage tank, as opposed to process tanks) would select and fix the design temperature and pressure needed for the tank. Since the MAWP won’t be known until the tank is actually ready for fabrication, the design pressure is the only valid value that can be used in lieu of the MAWP or MAWV - and these will be conservative values. You can always adjust the deadband after the tank arrives at site. That is what I have done in the past. There is no obligation from the tank fabricator (not vendor) to submit MAWP or MAWV information if it is not requested. At least, that has always been my experience. This may change, depending on some specific countries – but I am unaware of it. Of course, when you request the calculations and information, you will be subject to being invoiced for the cost of such information – unless you negotiate with the fabricator or if the fabricator wants to do you a good turn – which would be highly unlikely in a free economy.

I would never assign the detailed control process design to a tank fabricator (not vendor). I have never known or met a fabricator that had the acceptable and valid resources to undertake such a scope of work. Process design – and much less process control – are not in a tank designer/fabricator’s expertise. Ultimately the entity that knows more about your process is you and, in my opinion, that is where the control responsibility is best suited to be handled.


#8 fallah

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Posted 08 October 2009 - 12:30 PM

Dear Art,

As usual, you did present valuable response, but as per my investigations and submitted viewpoints in this thread by you, djack, and proinwv as experts in this field, seems if in detail engineering stage while the tank not to be fabricated (regardless of MAWP/MAWV information being requested from manufacturer or not) we have to place order for supplying the tank's blanketing system, then we must rely on design pressure/design vacuum submitted in relevant data sheet and therefore we shall forget about utilizing MAWP/MAWV in our design.
Am i right in my conclusion?

#9 Art Montemayor

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Posted 09 October 2009 - 01:02 AM


Fallah:

If, as you state, you need to design the blanketing system for a new, proposed tank that hasn't been fabricated yet, then you are - in my opinion - wise in employing the design data specified on the tank's datasheet. This is a conservative method and, as I stated before, you can always readjust the dead band later. Additionally, you have no alternative if you have to design and place the blanketing system on order while the tank is being fabricated.

However, I would recommend you take strong, written instructions and a response from the pre-selected fabricator(s) that the design pressure and partial vacuum specified in the datasheets are attainable values for a tank and that they can be easily met. That will reinforce your basis of using the design pressure for the blanketing system and your going forward without waiting for the tank calculations will be justified.

This type of situation happens often - especially in "fast track" projects. The important thing is to be prepared by good planning and be backed up in written documentation.


#10 fallah

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Posted 09 October 2009 - 05:54 AM

Additionally, you have no alternative if you have to design and place the blanketing system on order while the tank is being fabricated.

Dear Art,

Again thanks,but i think by above statement you mean:

"..........the tank is not being fabricated."

#11 djack77494

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Posted 13 October 2009 - 09:41 AM

I've seen lots of crazy things regarding atmospheric storage tanks, and I think failures of such tanks are probably among the most common equipment failures in the CEP industries. Many (most) process engineers seem quite content to merely list "atmospheric" as the design pressure for AST's. A suitable punishment should be inflicted on them all. There are others who note that code will permit a design pressure of up to 2.4999 psig before needing to move to the next more stringent code. Those folks will happily specify that a 150 foot (50 meter) diameter AST should be designed for 2.49 psig (172 millibarg). They should be given twice their annual salary and then personally fined for the cost differential between their design and a design of 10 inches of water column (10" W.C.) or 25 millibarg. Common sense seems to be in very short supply in this area, and no one seems to understand the value of experience.

More to point, there is usually a very small difference between the design pressure/vacuum and the MAWP/MAWV. Large differences mean inefficient tank designs = you won't be in business for long. Due to this, I see no problem in using design pressures in place of MAWP's. As Art has pointed out, if you really want to fine tune your system, you can do so later.




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