21 replies to this topic

[Fr3dd]

Hi everyone!

I'm working in a refinery design, I'm dealing right now with a conceptual problem which I'm completely sure you can help me to solve, or at least give me your valuable opinions.

We have a bunch of pressure vessels in different units, some of them will be subject to steam-out; we have read very interesting topics in this forum about vacuum due to steam condensation on equipment subject to this procedure and we're taking care of all the details about this matter but there's one thing that isn't clear enough for us.

Here's the thing: in most of the cases we're specifying "Full vacuum" as an alternative design case for this kind of equipment, in order to do this, we have to clearly establish the temperature at which the vacuum condition is expected to happen; it is ussually specified "full vacuum at ambient temperature".

The major problem is that, as a project basis of design, we have the following items that could be taken as "ambient temperature":
- Maximum Summer Temperature: 92°F
- Mean Annual: 81°F
- Minimum Winter Temperature: 69°F

The question is which of these conditions we have to consider for design at full vacuum conditions?.

We have dismissed the maximum summer temperature since it isn't the most critical condition for a full vacuum scenario; however we're having a hard time deciding between Mean Annual and Minimum Winter Temperature. The minimum winter temperature looks like the worst condition for a full vacuum scenario due to steam condensation, but we're not sure if we're going a little too far since 69°F is also our "Minimum Design Metal Temperature".

Your comments and recommendations about this matter will be highly appreciated. Thanks in advance.

Fr3dd

[ankur2061]

Fr3dd,

Now that you know that FV can occur due to condensation of steam subsequent to a steam-out operation that scenario is pretty clear to you and I suppose you are considering that in your design.

Other than that what could be the scenario when the vessel sees vacuum. There could be several:

1. Failure of heating medium provided to a reactor / vessel / heat exchanger could cause low temperatures and lead to condensation of the vapors.

2. Any liquid full vessel being pumped out could see vacuum and requires vacuum protection.

3. Any vessel exposed to ambient conditions and a sudden drop in ambient temeperature causes the vapors in the vapor space to condense.

If you will notice the common link in points 1 & 3 mentioned above is low temperature. Essentially it means that the conditions of the lowest temperature that a vessel can see would govern the formation of vacuum in the vessel provided the process dictates condensation in the vessel at the said temperature

Hope I have been of some help.

Regards,
Ankur.

[Fr3dd]

First of all, thanks Ankur!

I totally agree with you, a lower temperature will lead to vacuum formation in most of the cases; but establishing 69°F which is minimum design metal temperature (that is, the MINIMUM temperature the metal will face) isn't exaggerated?.
We would like to be pretty sure of this in order to design the a vessel which covers all the possible scenarios (in operational and emergency conditions) with a lower cost. However, its good to have feedback from an outsider, i'm sure its easier for you to think "outside the box"...

[ankur2061]

Fr3dd,

The minimum temperature does not necessarily mean the MDMT for the vessel. It really means the minimum temperature that the vessel can see during normal or process upset conditions. This depends on the type of process and has to be evaluated on a case-to-case basis. Specifying numerous vessel process data sheets over the years I never had a dilemma in specifying vacuum for the subject vessel. Wherever it was envisaged that the vessel could be subjected to vacuum I used to specify FV for the vessel and be done with it. As regards, the design temperature the maximum value would in most cases would govern. Special cases where the vessel required a rapid depressurizing (blowdown) and the envisaged temperatures due to the Joule-Thomson effect were below -20 deg F needed that the vessel process datasheet be provided input for the low temperature for reasons of selecting a material of construction amenable to low temperatures. Such cases were very few and far.

Regards,
Ankur.

[Fr3dd]

Thanks for your answer Ankur,

I mentioned that the M.D.M.T. corresponds to the minimum winter temperature because it is established as a design basis of the project.
Certainly, each case that we're considering for Full Vacuum design has been evaluated. Neither blowdown conditions nor auto-refrigeration are expected. Normally, we specified "FV" as a design condition but, in this case, our client has requested to specify the conditions in which vacuum is expected. Obviously, considering the minimum expected temperature will cover all the vacuum scenarios, but we're not sure if the minimum external temperature that the vessel will ever see is a good asumption or its just too much.

Thank you very much for your comments,

[kkala]

If some autorefrigeration can occur, min developped temperature should be adopted, as mentioned in previous posts. But almost all cases are understood to concern steam out conditions. In my opinion no difference is expected in this case (steam out) if ambient temperature is taken as 92, 81, 69 oF (33, 27, 21 oC), for tensile strength / permitted stress of steel is considered same. Vacuum creates a condition not familiar to me (something like buckling), but I assume all three conditions are practically same since permitted stress is same.
Looking into the matter, suppose that steam temperature is 300 oC when vessel gets isolated and progressively goes down to ambient temperature. This creates higher and higher vacuum. It is not certain that the weakest status of the vessel is at ambient temperature, when vacuum gets highest, since stress gets stronger and stronger with decreasing temperature (up to a limit of, say, 0 oC, where steel gets fragile). If I find some practices on this matter, I will let you know.

Edited by kkala, 11 November 2010 - 04:40 PM.

[fallah]

if ambient temperature is taken as 92, 81, 69 oF (33, 27, 21 oC), for tensile strength / permitted stress of steel is considered same. Vacuum creates a condition not familiar to me (something like buckling), but I assume all three conditions are practically same since permitted stress is same.

Seems permitted stress couldn't be same for all three conditions because of higher allowable stress in lower temperature.

[kkala]

Following post of 11 Nov 2010, a widely adopted practice (for steam out) is to define vacuum of -0.5 Atm (i.e. internal press=0.5 Atm a) at max operating steam temperature as design conditions. So for the mentioned example of 300 oC steam, design condition for steam out would be -0.51 Barg at 300 oC. This in addition to the design conditions caused by the fluid stored in the vessel.
Concerning Fallah's query, let us read found allowable carbon steel stresses: 1290 kp/cm2 to 100 oF, 1230 kp/cm2 at 200 oF, 960 kp/cm2 at 650 oF. Any difference of allowable stress in case of 69 or 92 oF is understood not to be recognized by the code. Besides ASME would use 960 kp/cm2 for any temperature below 650 oF.
Of course as common carbon steel goes below (say) 0 oC, it gets fragile and usually unsuitable for service (change of material is required), but this does not concern this case.

Edited by kkala, 12 November 2010 - 02:34 PM.

[Himanshu Sharma]

Hmm getting down to fundamentals the pressure in the vessel if the steam is allowed to condense inside the vessel will be the vapor pressure of water at that particular temperature.

Now while establishing design conditions , the worst case scenario is that when metal wall reaches MDMT(assuming you blocked steam in the chilliest winter day in the season )

This is a fairly valid design basis and normally you get FV conditions at MDMT.


@kkala
Sir, your post invoked yet another thought ,i myself specify these numbers i.e 0.5kg/cm2 steam and max operating temperature of steam(normally MP steam) for steam out conditions.

Is there any basis for this ?? from where do these widely accepted conditions originate???

Any information on this will be of great help

[kkala]

your post invoked yet another thought ,i myself specify these numbers i.e 0.5kg/cm2 steam and max operating temperature of steam(normally MP steam) for steam out conditions.
Is there any basis for this ?? from where do these widely accepted conditions originate???
Any information on this will be of great help

Sharma, your writing indeed indicates wide applicability of the design conditions rule for steam out! I have read it, but not found an explanatory basis; post of Nov 11th included my own thoughts, yet other arguments may be:
1. Vacuum of -1.0 Barg can be hardly created in a vessel from steam out (valves are leaking a bit), so -0.5 Barg is specified. Once we searched for no leaking valves facing full vacuum, but did not find them in the market.
2. Thickness (and generally construction) requirements for -0.5 Barg may be close to -1.0 Barg, so we accept -0.5 Barg for this case.

Edited by kkala, 16 November 2010 - 09:25 AM.

[kkala]

For steam out design conditions, it seems that half or full vacuum at (max) steam operating temperature is widely specified, as can be concluded from http://www.eng-tips.....cfm?qid=208133.
So design temperature should be higher than 69 or 92 oF.

Edited by kkala, 16 November 2010 - 05:16 PM.

[Himanshu Sharma]

Sir,

Thanks for the plausible explanations.

Argument 1 holds up to certain extent but in case of Point 2 the thickness of the vessel will increase to great extent more than factor of 2.

I have not seen till date full vacuum conditions being specified by an EPC contractor !!!

[kkala]

Argument 1 holds up to certain extent but in case of Point 2 the thickness of the vessel will increase to great extent more than factor of 2.
I have not seen till date full vacuum conditions being specified by an EPC contractor !!!

I do not have expertise on "point 2", having come from the advice "specify full vacuum in case of (any) vacuum" (so there is not much difference).
Searching the matter a bit, Coulson & Richardson's Vol 6 (2002), "Chemical Engineering Design", Mechanical Design of Process Equipment, indicates for long cylindrical vessels requiring thickness t to withstand external pressure difference P (e.g. vauum) that t is proportional to D*cubicroot(P) (D=diameter). So ratio of required t for full vacuum / half vacuum is 1.26 (for same D). Noticed ratio > 2 probably concerned a special case.
I have not seen full vacuum specified for steam out,either; yet many say so in the mentioned site (doct9960, SnTMan, vesselfab).

Edited by kkala, 17 November 2010 - 01:59 PM.

[Art Montemayor]

Fr3dd:2

I am coming into this thread late in the discussions; I have been absent from the Forums for about 2 weeks, so I’m just catching up.

I note that this thread is getting rather long for what I gather – from your initial posting – is a very straight-forward resolution. You state that you are designing pressure vessels subject to steam-out type of cleaning and that your client requires you to establish the temperature at which vacuum conditions are expected to happen. From what you have explained, I don’t believe you need to apply full vacuum specifications to a pressure vessel solely because you plan to apply steam-out cleaning in the future. I have specified pressure vessels as well as storage tanks – all subjected to steam-out cleaning – and never had to specify vacuum – or full vacuum – design specifications because of the clean out method used. Granted, some of the pressure vessels I have specified also employ a full vacuum design – but that has always been for other, credible process requirements and not because of steam-out cleaning methods. Let me explain myself on this subject a bit further.

I have worked out in the field for some years and have personally witnessed the “suck-in” (physical collapse) of at least 2 storage tanks due to a sudden partial vacuum condition built up during steam out procedures. However, this does not imply nor - does this mean - that designing the tanks for full vacuum was the right answer. I found out what was the correct answer and was instrumental in applying the corrective methods to salvage other storage tanks from the same misfortune.

When a steam-out cleaning method is applied it is done with vessel vent openings – usually top nozzle vents being removed and allowing excess steam vapors to exit that way. Hot steam contacting slime, oil, sludge, and other contaminants in the walls, roof, and floor of the tank is condensed and washes down by gravity to the floor of the tank. It is in the floor that the hot condensate (at max. 200 oF) is collected and drained, carrying out all the contaminants. While this process is being carried out, there is a possibility of the tank being exposed to a cold rain shower or storm. Should this happen – as was the case with me – the steam inside the tank (that has displaced all the original air) is subjected to a sudden cooling effect and is condensed rapidly – sometimes at a rate faster than it is being introduced. If this happens, then the only way to prevent a partial vacuum from taking place inside the tank is to have the vents sized in such a way that their cross-sectional area allows atmospheric air to immediately enter the tank and “break” a potential partial vacuum.

Therefore, if you size your steam outlet vents (which are sometimes process vents during normal use) such that they can allow sufficient air to ingress during a potential partial vacuum scenario, you should have no problem and no need to consider a vacuum design as necessary for applying a steam-out cleaning method. I have applied this design philosophy in the past and have had no problems with it or any opposition from other design engineers in the field. Of course, you must establish strict and clear operating steam-out procedures in order to ensure that your vessels are kept protected. I found that instituting sign-out procedures before initiating steam-out procedures with our maintenance crews insured that the integrity and the safety of the operation was respected. This also means, of course, that you must establish, generate, and document clear and accurate nozzle calculations for all tanks meant for steam-out procedures. And you should also clearly tag and identify those nozzles that are calculated and intended for steam-out service.

So, as you can probably see, there is no logical need to specify vacuum conditions for the sake of applying steam-out procedures. That is how I would resolve your dilemma. There is more than one way to skin a cat.

I hope this experience is of help to you.

[fallah]

I have specified pressure vessels as well as storage tanks – all subjected to steam-out cleaning – and never had to specify vacuum – or full vacuum – design specifications because of the clean out method used.

Dear Art,

1-May i have the major difference between your clean out method and conventional one will result in no need to full vacuum conditions?

2-Would you please submit the size of the steam out vent line,you had designed for vessel/storage tank in order to no need to full vacuum,with respect to vessel/storage tank diameter?

Regards

[Art Montemayor]

Fallah:

The steam-out method I described is what I have always considered to be the "conventional" method for cleaning out a contaminated tank or vessel with steam. Therefore, there is no difference between my clean out method and a conventional one.

The size of the steam-out vent line I designed for a storage tank (in the application I refer to) in order to avoid a partial vacuum was, as I remember, 12" nozzles - and I used two of them per tank.

[chimistul]

Hi!

I guess your critical temperature is cooling water temperature. The higher value is in mid summer condition. Few degree of water temperature can affect almost with 50 mm Hg the vacuum.

[djack77494]

Seems to me that the original posting has been fully resolved and that we'd normally be expecting the topic to be concluded. One item that appeared in mid-stream, however, concerned me and I did not want to let it go without comment.

The MDMT is too low! Don't be trying to specify too high an MDMT. I know Venezuela has a warm climate, but the vessel may be fabricated elesewhere. It may then be pressure tested elsewhere. Even in a warm climate, cold water in mid winter may be used for hydrotesting. The effect of using a slightly lower temperature if you're not below freezing should be very minimal, so I'd recommend that you don't skimp on picking an MDMT.

[ogpprocessing]

The proposed method by Mr.Art Montemayor (to consider steam out vent nozzle instead of designing for FV condition) made some doubt in my mind.

As far as I know steam out is used to make internal space of the pressure vessel "air free". So considering this steam out vent nozzle is in contrast with steam out application.

Could you please clarify this?

Edited by ogpprocessing, 23 September 2011 - 03:53 PM.

[kkala]

You mean that air inside the vessel is displaced by passing steam through it (inlet-exit nozzle); then you isolate the vessel by closing all open valves and probably placing blinds. In this case vacuum will be created as vessel temperature falls. However my understanding on local refinery practices suggests following.
1. Nitrogen gas is used to make the vessel internal volume air free. This does not create risk of vacuum; its operating temperature is ambient temperature, or a bit less (and no condensation).
2.1 Purpose of steam out is to clean the vessel internally. Vessel is first emptied, drained, degassed through vents, accepts nitrogen flow to displace flammable gases (if required). Just before "steam out", vessel contains already air, but mentioned procedure protects it; air is also introduced after "steam out", through the steam out vent line (which must be free) as long as vessel is cooled.
2.2 Note: Vacuum will be created in above procedure in case that all valves around vessel is inadvertedly closed. If you can assure that mentioned steam out vent line remains free during cooling, no need to design the vessel for vacuum (due to steam out). A lot of vessels seem to have been designed on this base. Requirement to resist vacuum (full, half, etc), when "steam out " occurs, seems to be a rather recent development.
3. If steam is used to make the vessel air free, afterwards vacuum creates some risk of air ingression through leaking valves or blinds.
Probably you have seen steam used for this service only in an imergency (or lack of N2 gas)?

Hoping above is useful; comments or other opinions are gladly welcomed.

Edited by kkala, 24 September 2011 - 06:25 AM.

[ankur2061]

The term steam-out in the chemcial industry means introducing steam into a vessel / tank for the purpose of cleaning / draining out the residual material remaining in the tank /vessel which could not be removed during the normal draining of the tank / vessel including any pressure-assisted draining.

What Art Montemayor has mentioned is perfectly logical and there can be vessels designed for steam-out without vacuum specified as a design condition for the vessel / tank. This requires that administrative procedures are in place to ensure that the vessel / tank is protected by air entering the vessel / tank during a steam-out operation from the steam-out vents and thus preventing any vacuum formation.

Fortunately one of my earliest mentors in engineering design gave me one of my first valuable lessons in engineering design that try to avoid taking credit of any administrative measures for safe design of any equipment or plant. In other words, make your design idiot-proof. Consider that human errors are part and parcel of any operation and with this in mind design any equipment / plant. This ensures a safe design despite those unforeseen human errors.

The idea of the above explanation was that while designing a vessel / tank with steam-out conditions specified, I will not consider the administrative measures for steam-out operation and provide vacuum (i.e Full Vacuum) design conditions for the vessel / tank in consideration. I will only take an exception to this if the client provides me in writing that they want to take credit for steam-out vents and the vessel / tank need not be designed for vacuum.

Hope I have been able to provide some insight on why vessels / tanks with steam-out conditions need to be specified for vacuum design.

Regards,
Ankur.

[kkala]

Fortunately one of my earliest mentors in engineering design gave me one of my first valuable lessons in engineering design that try to avoid taking credit of any administrative measures for safe design of any equipment or plant. In other words, make your design idiot-proof. Consider that human errors are part and parcel of any operation and with this in mind design any equipment / plant. This ensures a safe design despite those unforeseen human errors. The idea of the above explanation was that while designing a vessel / tank with steam-out conditions specified, I will not consider the administrative measures for steam-out operation and provide vacuum (i.e Full Vacuum) design conditions for the vessel / tank in consideration.

Principle expressed is much appreciated. May all young engineers receive similar lessons in the beginning of their career. The late Director of my first job (in a fertilizer factory) pointed out safety as first role of Operations Engineer in the welcome meeting.
I would add that "administrative measures" cannot be totally avoided, yet permissible extent of them undergoes development with time. I suppose in the past a vessel with steam-out provision and without proved resistance to vacuum represented the usual case, and care was paid during steam-out. Today this is still acceptable, but a lot of engineering practices require proved resistance to vacuum as an additional safety. Safety level tends to be enhanced in this point, as natural.
Applicable safety levels are often a compromise between absolute safety (if we can conceive it) and cost, expressed by codes and practices of each age. Technology progress, understanding, mentality, even accidents, hopefully make these applicable levels better and better. This is also a need, as plants get more and more complex.
It is a challenge for the Engineer to apply the proper technical measures, after having understood enough of the complex status of today safety and environmental requirements. In this way administrative measures can be limited, or replaced by simpler. They cannot be totally eliminated, plant is too complex for it.
In parallel, alert for safety should be always vivid to make measures effective and locate the safety gaps. Covering a risk at one point and almost neglecting risks (even lower) at other points is of little use. A clear mind would try to "properly" limit all possible accidents, including those from human errors and lack of training, making an appreciable percentage in total accidents.