10 replies to this topic

Please find the attached excel file.

I have one old tank which has over flow line on it's vent line.

Tank is desigend for 14" W.C internal pressure and 4 OZ for external pressure.

Old recommendation from relief valve studya says that

"Tank will not be protected against liquid overflow because liquid head of about 5 feet will cause more than 2 psig pressure. Overflow on the side at seam level is recommended"

I do not understand this above statment.

What i undestand is that the tank is designed for about 0.5 psig and if tank is liquid full it will excert lot more head pressure on tank.

Your thoughts on this topic will be helpful.

Thanks,

Amit Joshi.

Attached Files

•  Liquid_Overflow.xls   17KB   140 downloads

[mishra.anand72@gmail.com]

SI units please.

Excel file is not opening.

[djack77494]

SI units please.

mishra,
This forum has members from around the world and we must deal with a variety of units. Unfortunately, not all parts of the world have adopted the SI system, and even among those which supposedly have, there are still differences. We don't seem to have a problem with that. We either are familiar with the units as presented or we are able to make any needed conversions. If neither situation applies, there is no need to comment.

[Technocrat]

Hi Josh,

Please confirm opertaing temperature of stored water. If the water level is 32 ft then the pressure exerted at the tank base becomes 32 x 12 = 384 inch WC, then I think the tank can not take the load and will rupture . Can experts throw more light on this?

Regards.

[Art Montemayor]

The normal method of designating the operating pressure of an API storage tank (which I presume this one is) is to measure (or define) the pressure as existing in the vapor space of the storage tank - which is the vapor pressure above the liquid, not at the floor of the tank (which would involve the static head of the liquid).

All storage tanks should be hydrotested (as I hope this one was). If that is the case, then the minimum pressure the tank has had to contain is the static head of water it was filled with during that test. Whatever the existing case may be, the tank should have back-up mechanical calculations regarding its safe MAWP as well as the MAWV. If these don't exist or have been thrown away, new ones should be generated. This is THE ONLY SAFE WAY TO OPERATE AND MAINTAIN A STORAGE TANK. When you operate a tank in this manner you always have access to the exact definition of what the MAWP and MAWV are for the tank - and where and how they are measured.

This is the most practical and safest way of operating a tank that I know of. I wouldn't do it any other way and I hope other engineers follow suit.

[Technocrat]

Thanks Art,

It means the tank must be designed for full water level plus vapor pressure of the contained liquid (water, in this case) at the tank design temperature, isn't it?

Regards.

[Art Montemayor]

Yes, it means the tank must be designed for full water level plus vapor pressure of the contained liquid (water, in this case) at the tank design temperature PLUS any other vapor pressure imposed on the liquid contents - such as inert gas blanketing, flashing vapors, or any blow-through of other vapors or gases. Additionally a corrosion allowance is added to the plate thickness.

Always remember that storage tank technology is basically a technology of common, rational sense. When you submit a specification to a bidder-fabricator, a calculation of the minimum required shell and roof plate thicknesses will more often than not yield a value that is NOT PRACTICAL. What I mean is that there are only standard steel plate thicknesses in the marketplace and no one in his right mind is going to insist that the tank walls be built of exactly 5/32" thick plate. No such thickness exists. Fabricators, if forced to guarantee the operating results (and who wouldn't demand a guarantee?) will always select a standard plate thickness that is greater than the theoretical calculated value AND one that is readily available, in stock, or in excess stock (the fabricator needs to get rid of it in order to rotate his stock or simply to lessen his stock. The plate thickness has to be easily weldable without warpage of deformation due to weld heat (some shops do not weld anything less than 3/16" thick. This a practical and common sense engineering solution. The end result is a tank that is usually able to exceed the specifications and expectations of the user. BUT if the user does not agree to pay for the tank calculations, the user will never know what the tank's limitations are - unless the user generates them.

It has been my experience that the majority of present-day storage tank owners have no reasonable idea what the ACTUAL MAWP and MAWV are for all of their storage tanks. This is a sad and totally wasteful situation - not even addressing the SAFETY aspects of such situations. My personal advice is to always have the mechanical design calculations of the tank in hand and always up to date with respect to tank inspections and physical conditions (at least once every 3 years) in order to be able to rate the existing corrosion state and the actual MAWP and MAWV together with the appropriate relief devices on the tank(s).

P.S. -- One more, important item I would bring to everyone who is reading this thread: DO NOT employ terminology that has very little or no meaning as to the pressure relief requirements of a vessel (especially a storage tank). What I refer to is the terminology being used in this thread such as "Tank is desigend for 14" W.C internal pressure and 4 OZ for external pressure." This does not tell us what the MAWP or the MAWV are. In fact it doesn't tell us the limitations of the tank. If the tank can safely withstand 2 psig, of course it is designed for 14" W.C.. The total story is that it can safely withstand more pressure. The important point is that the MAWP and MAWV should ALWAYS be identified as they exist at the time of writing (as per the last inspection).

[JEBradley]

With reference to preferred units -
I wish everyone would work in SI - the world would be a better place - except for miles, we should keep those.

Anyway What's an OZ? (pressure term referred to) I assumed its something to do with ounces (Hmmm Imperial or Avoirdupois though???)

[djack77494]

Amit,
I do not like the "overflow protection" system on your tank. I'm also confused by you're showing of a device in the vent line. What is this device and what sort of pressure differential is required to move gas through it? Is it designed/able to handle liquid? It WILL see liquid flow in the event of an overflow (duh). Beyond this, it would be far better to install a side mounted overflow nozzle for the tank. If required, seal the overflow pipe. The design pressure of the tank refers to the pressure at the top of the tank. As I understand your problem, you will have 5 feet = 60 inch of level above the top of the tank. As this exceeds the tank design pressure of 14 inch, the tank would be overpressured. Thus, it looks to me as if you do NOT have a safe/acceptable design. Please stress the importance and urgency of correcting this unsafe situation.
Doug

[Art Montemayor]

With reference to preferred units -
I wish everyone would work in SI - the world would be a better place - except for miles, we should keep those.

Anyway What's an OZ? (pressure term referred to) I assumed its something to do with ounces (Hmmm Imperial or Avoirdupois though???)

JE & all those demanding use of only SI units:

If all your teachers were as knowledgeable and cooperative as mine were, you would have been taught where the US engineering units came from (& why) - from the UK - as well as where the SI units came from. You would also have been taught that the main and sole reason the UK adopted the SI was to maintain commercial trade with the rest of Europe - especially France.

I know the SI system (& all its political shortfalls) very well. I worked under it and with it for over 10 years - in countries outside the USA. The USA has no incentives (like the UK did) to make the SI system (note that I don't yield to French intimidation and spell the words in French) mandatory. Until it does, we are as independent as we have always been - and hope that other people make the decisions they have to make on their own volition. The USA has never been known to give in to ANYBODY - especially those entities that set themselves up as self-serving dictators of their preferences. No one has democratically elected the SI system in the USA. Until we do, we will continue to do what we have so far done quite successfully - without aid or improvement from the SI system and those that "rule" it. We talk and walk SI just as well as anyone else. Why can't you do the same with US engineering units?

The term "Oz" was handed over to us by our ancestors here in the USA. I think they came from Europe where the term was invented. If the term is odious, bad, politically incorrect, or not to be used, we have not had one european come here to the USA to tell us that - yet. When one does, we have an engineering reply similar to the one we shouted to the Nazis at Bastogne on Dec 1944: "Nuts"! I think you will find we USA citizens to be very compliant and understanding about European preferences and wishes - as long as they don't "push" on us or imagine that we (and everyone else) can be forced to act, think, and respond like europeans. We are Americans. We'll always be american. And we will act in accordance with our ways, as we have always done. Americans have not - and do not believe in - made it mandatory or a requirement for europeans to adopt the USA units. We didn't do this after every effort we have made to help out our european "cousins". And we would expect the same, reciprocal action if the europeans ever assist us in the same manner.

The SI system is a good basic system. I believe it needs some common sense injected into it - something that we Americans would probably immediately proceed to do if we adopted the system. The French spelling (& "ownership") would be one of the first things to go. No one nation or culture should exercise a dictatorship over what is called an "international" system. Next would be some of the nonsensical units created simply to burn incense to some "national" science heros.

I hope I have correctly expressed American feelings over engineering units used world-wide. And I continue to underscore that we don't force our system on anyone. Everyone is free to use the system they prefer and love to snuggle up with. We have ours. Others have theirs. We believe in freedom of choice and we fight to keep it that way for everyone - not just ourselves.

P.S. - By the way, why do non-francophones use the term "SI"? Why not use the correct and common sensical English term "IS" (International System)?

[JoeWong]

Amit,
I do not like the "overflow protection" system on your tank. I'm also confused by you're showing of a device in the vent line. What is this device and what sort of pressure differential is required to move gas through it? Is it designed/able to handle liquid? It WILL see liquid flow in the event of an overflow (duh). Beyond this, it would be far better to install a side mounted overflow nozzle for the tank. If required, seal the overflow pipe. The design pressure of the tank refers to the pressure at the top of the tank. As I understand your problem, you will have 5 feet = 60 inch of level above the top of the tank. As this exceeds the tank design pressure of 14 inch, the tank would be overpressured. Thus, it looks to me as if you do NOT have a safe/acceptable design. Please stress the importance and urgency of correcting this unsafe situation.
Doug

There are many missing informations and confusing...

"Tank is desigend for 14" W.C internal pressure and 4 OZ for external pressure."

Is the tank designed for 14" W.C Internal pressure only or 14" W.C Internal pressure plus Full LIQUID ?

14" W.C. ? No way...Doug has clarified...

Is the LIQUID DENSITY higher than WATER ? If so, Static head caused by LIQUID shall be used.

The sketch shown the overflow is from tank top. What is maximum height the liquid can "climb" ? It made a big impact to the tank design pressure. Without this data, no way we can define correctly the design pressure of liquid.

The "small" thing on the vent lookslike a flame arrester. Is there any rupture disc across this "small" thing ? Is the fluid vaporised and solidified / crystallized when temperature drop and potentially block the flame arrester ?

What's the pump-out rate ?
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It is difficult to provides advices without proper understanding the situation.