10 replies to this topic

[chemks2012]

Hello all,

We have a atmospheric storage tank with design pressure 56mbarg and a height of 5m. If we perform hydraulic test by filling it up, with the height of 5m, it will exert pressure of about 1.5bara at the tank bottom, while the tank design pressure is only 1.069bara. Am I missing anything here?

Thanks
Regards,
KS

[Art Montemayor]

Yes. You have omitted the most important mechancial detail that describes the mechanical ability of the tank: the Maximum Allowable Working Pressure (MAWP) - the one important value that ASME labels as the data by which to set the pressure relieving device on that tank.

It is one thing to DESIGN the tank (and incorporate a "design" pressure); it is another thing to FABRICATE the tank and generate an actual, resulting MAWP.

[Art Montemayor]

Also:

If your design pressure is 0.56 mbarG, then the absolute design pressure is 1.56 barA - assuming the tank is at sea level. That means your calculated water pressure of 1.5 barA at the bottom is OK - as far as the design of the tank. It probably will take more than that when you consider the MAWP.

I am assuming you calculated the water static pressure OK.

[kkala]

My understanding on the design pressure is as follows, assuming that water of about ambient temperature shall be the normal content of the (cone roof) tank, filled up to 5 m from bottom.
1. Stated design pressure of +56 mbarg (1069 mbara) concerns the tank space over the HHLL of 5 m; relevant upper steel peripheral plates, as well as weldings etc (between HHLL and roof), shall have this design pressure.
2. Bottom and lowest peripheral plates (plus weldings etc) touching the bottom shall have a design pressure of +56 +5/10/1.01972*1000 = 546.33 mbarg = 1560 mbara.
3. Intermediate plates shall have an intermediate design pressure accordingly.
4. MAWP of steel plates shall be higher (at least not lower) than above values, till hydrostatic test. After the hydrostatic test, what is certified counts.
5. It is noted that design pressure, as specified by process, concerns the gaseous phase of the tank, that is the air above stored water in the specific case.
6. No problem is anticipated for the hydrostatic test, according to above; not all tank plates (weldings, etc) will have a design pressure of + 56 mbarg.
7. In atmospheric (cone roof) tanks design pressure as above has to be defined, but also vacuum; see http://www.cheresources.com/invision/topic/9281-api-650-tank-design-pressure.

Edited by kkala, 14 May 2012 - 02:54 PM.

[chemks2012]

Thanks Art
Is there any thumb rule relationship between MAWP and design pressure?

Thanks kkala,
Your reply clarifies my doubts. However, if various parts/section of thank have different design pressure, why only 56mbarg has been specified?
PS: Agree that design vacuum should also be specified.

[kkala]

By convention, design pressure refers to the tank area above the liquid level. This is the design pressure specified by Process, applied here and elsewhere. Vendor calculates max design pressure for each plate by adding the hydrostatic pressure to this (see also post No 4, para 5).

[chemks2012]

Thanks kkala for your prompt reply!

[Art Montemayor]

KS:

There is no rule of thumb. The fabricator back calculates what the ultimate MAWP of the tank will be AFTER he/she decides what materials & thickness will be used in the fabrication of the finished tank.

In other words, reverting back to the basic definition of the maximum allowable working pressure, you will realize that if the calculated wall thickness (as an example) turns out to be 0.1704 inches, the fabricator is probably going to use 0.2500 (1/4) inches because that is the next standard, available plate thickness. He also is going to look at what is the weakest member in the finished fabrication and that will set the "maximum allowable working pressure" that can be imposed on the tank for safety reasons. That is the "bottom line" and what this business of building a tank to specifications is all about. The fabricator will have to guarantee that the tank meets your Data Sheet specifications - AS A MINIMUM. That is why he has to stay on the conservative side of the specifications - and that should explain in common sense why the MAWP always is bigger than the design pressure. It has to be that way; by common sense it can't be the other way around. There can't be a "rule of thumb" because each design is specific and - as ridiculous as it may seem, I've been in situations where the fabricator justified the use of a plate that was TWICE the thickness required simply because he had it in stock, had previously bought it at a reduced price, and he could capture the bid by quoting an early delivery. As I've said so many times, engineering is so practical and common sensical that it drives the pure scientists into crazy asylums trying to understand it.

This basic, common sense stuff is important because - as I've said so many times - it often goes to the heart of a design: for example, you are mandated by ASME to set the relief pressure on a vessel relative to the MAWP - not the design pressure. And if you stop and think about it real hard, it starts to make good horse sense: when religiously and strictly defined, it is the MAWP that counts - not the design pressure. Who cares what the design pressure was 10 years ago, before the vessel wall thickness corroded 1/8 of an inch??? And you should then respect the fact that, as owner, you are responsible for ACCURATELY IDENTIFYING AND KNOWING the ACTUAL EXISTING vessel wall thickness and the resulting MAWP at ALL times - not just when the vessel was fabricated! What all this says is that ASME was smart when they dictated that the MAWP is used to set the PSV relief pressure - not the design pressure. Of course, initially at the start of the vessel's life, the design pressure will be a conservative value; however, as time goes by, it starts to become a relatively dangerous value to rely on. Forcing owners to identify the MAWP is the same as forcing them to routinely inspect and test their pressure vessels. Now, wouldn't that just be ideal and smart engineering?

[chemks2012]

Hello Art,

Thanks very much for you reply and explanation.

I really liked this one: I've been in situations where the fabricator justified the use of a plate that was TWICE the thickness required simply because he had it in stock, had previously bought it at a reduced price, and he could capture the bid by quoting an early delivery.

It clarifies my doubts between MAWP and design pressure but now I have following few queries to ask.
1) For an example if design pressure of a tank is 2barg and MAWP is 2.5barg when it is fabricated. There could be the situation in the future [due to corrosion, rust etc] let's say 10years in time that the MAWP could be 1.8barg [i.e. below design pressure].
Therefore, as per ASME if we wish to install a relief valve [after 10years], the set pressure of the the relief valve should be 1.8barg. However, for an example again after few years if MAWP get reduced further, we need to change the set pressure of the relief valve to a new reduced value of MAWP [?].

Is my above understanding is correct?

2) If I have a bursting disc and relief valve installed in series [i.e. RV is installed just after BD], what set pressure should we select both of these devices?

3) If I have a BD and RV installed independently,what set pressure should we select both of these devices?

4) I came across a situation wherein MAWP of the vessel is 6barg [vessel tested a year ago] having 10" BD and 2"RV installed independently and set pressure of BD is 2.75barg while that of RV is 2.4barg. I have considered two phase flow relief case for the water ingress and found that BD is insufficient while RV is no where nearer. My queries for this situation are

4.1 Why the set pressure of the BD [and even RV] is very low while MAWP is 6barg?

I believe lower BD set pressure will result into situation wherein BD will lift frequently which obviously unnecessary.

4.2 If I purpose to increase the set pressure of BD to MAWP [6barg], the existing 10" BD seems sufficient - Can I suggest this option? In fact, I have already suggested this option but client is scared for some reason and I don't know how to convince him?

Your input will be highly appreciated.
Regards,
KS

Edited by chemks2012, 15 May 2012 - 02:23 PM.

[TS1979]

If this is not a pressure vessel, depending on the commodity inside the vessel, you probably don't need a pressure test.

It seems that your vessel design pressure is too low. Hydrotest test pressure a few times over design pressure may not necessary collapse the vessel but there will be a good chance the collapse happens.

[TS1979]

For pressure vessel, always use the design pressure as MAWP. If the vessel has been corroded, you need to rerate the pressure rating of the vessel based on the new situation, that means, to reduce the MAWP.

Pressure relief valve setting pressure: based on the code requirement, the maximu set pressure is MAWP. However, if you want to set lower pressure, that's your decision and is code allowable.

The setting pressure for rupture disc and PSV in series is no difference as single relief valve.