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Tank Implosion due to insufficient vent diameter
4

Tank Implosion due to insufficient vent diameter

Tank Implosion due to insufficient vent diameter

(OP)
Hello,

A vessel has imploded due to having an insufficient vent diameter and I have been tasked with calculating the new appropriate diameter.

The original vent was 4", 20m long, with 3 elbows, had a fan which wasn't used in the pipe and increased in diameter to 5" after 5m.

The tank volume is 13.0 m^3 and was emptied with a flow rate of 900 l/min.

I started off by calculating the pressure drop in the pipe by using Bernoulli's equation and accounting for major and minor losses but the pressure drop I calculated was negative and insignificant (-39 Pa).

Either I have made a mistake in my calculations or my approach has been incorrect. I used incompressible flow models as the flow velocity was small (1.91 m/s) and so the Mach number < 0.1.

Is it a calculation error or is the method wrong?

Any feedback would be greatly appreciated!

RE: Tank Implosion due to insufficient vent diameter

2
Why does everyone blame every fluid flow regime on poor old Bernoulli? This is such a simple problem.

You know that the ideal gas law says P*V=n*R*T (and it is safe to use ideal gas assumptions at these pressures). So:
  • At constant temperature, as you increase the gas volume (by lowering the liquid level), at the same time you are increasing the amount of gas ("n") with inflow through the vent. If the inflow volume equals the outflow volume then pressure will be constant.
  • But pressure has to drop for there to be flow in the vent pipe. You should be able to calculate how much pressure can drop before you crush your tank (probably something like -15 kPag, but I would add a safety factor and call my target pressure -5 kPag)
  • Use the Spitzglass formula to calculate volume flow rate at actual conditions (using the -5 kPag as the downstream pressure). If the answer is less than 0.9 m^3/min then you have "n" decreasing slower than "V" and the tank will crush.
Your vent pipe is really long and kind of small. If some of those elbows you talk about can create a water trap then you can be certain of crushing a tank the first time you try to pump from a full tank on a cold day.

I use Sptzglass for this because the rate of change of gas density below atmospheric pressure is very rapid and all of the incompressible flow equations are invalid at rapidly changing density. That goes double for Bernoulli. Spitzglass tries to represent compressible flow and does an OK job of it for reasonably small flow rates (it doesn't do great with accounting for friction drops).

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

RE: Tank Implosion due to insufficient vent diameter

You have a volume of 900 l/min leaving the tank. That means you need the equivalent amount of air to enter the tank. To calculate the minimum size vent, assume that the air flow velocity is less than sonic velocity and determine the friction loss through the vent pipe.

It appears that the 4-Inch vent has adequate size as there should only be 2-3" wg loss across the vent pipe.

Is there a chemical reaction in the tank? Are you condensing steam in the tank?

What is the design pressure of the tank?

Was the vent blocked?

RE: Tank Implosion due to insufficient vent diameter

(OP)
Thanks for the replies!

Zdas04, I've used two forms of the Spitzlgass equation and got flow rates of 19.66 m^3/ min and 43.74 m^3/min. Both seem quite high and I did them in a quickly so I'll double check the units to see if I've made an error there, but at the minute because the gas flow rate is bigger than 0.9 m^/min the vent should suffice.

Type 1 Spitzglass: Q = 0.09 (( dh * D^5)/(SL(1+ 3.6/D + 0.03D))^.5 where dh is the pressure drop in inches of water, D is the pipe internal diameter (inches), S is the specific gravity of the gas, L is the pipe length (ft), and Q is the volumetric flow rate in MM scf/D.

Type 2 Spitzlgass: Q = 3550 k (dh / L*SG)^0.5 where Q is the volumetric flow rate in cfh, dh is the pressure drop in inches of water, L is the pipe length in ft, S is the specific gravity of the gas and
k = (D^5 /(1+ 3.6/D + 0.03D))^0.5 where D is the pipe diameter (in).


I usually use PV elite to determine the maximum vacuum that the tanks can take but this tank is a wooden fermenter used for beer production and PV elite doesn't deal with wooden vessels. I might resort to Roarks for a flat beam - the wooden slat that broke is along the top where the vent pipe goes into the tank.


bimr, the tank is used in beer production and is cleaning with hot water at 80 degrees C then allowed to cool naturally. I have done similar vent calculations on stainless tanks where the tank is rinsed with 5 degree water immediately after the 80 degree rinse but this is not the case this time. Yes the vent had a fan in it which was not used and probably acted as an obstruction.




RE: Tank Implosion due to insufficient vent diameter

How do you "know" that the failure was due to insufficient diameter?

A fermenter should have some sort of bubbler or water trap on the overflow, is there not also a trap on the vent (3 elbows?) I was under the impression that it shouldn't be exposed to open air. Even if it wasn't designed that way, maybe an employee thought the same and dumped water in the U-trap (3 elbows?) of the vent.

RE: Tank Implosion due to insufficient vent diameter

tmengineer,
With 5 kPa I get 20 SCM/min. Since the tank is wooden, I would probably move the limit down to 0.5 kPa which works out to 7 SCM/min (I've seen that equation off by 20% but never by an order of magnitude). That makes me think that the pipe is big enough and I would start looking for a physical restriction.

The P-Trap from 3 elbows is a real possibility. So is restriction through the fan (you are only dealing with 500 Pa after all). I think I'd start the fan and measure the dP from the tank to the fan suction to see if there was a hidden gremlin in the 4-inch.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

RE: Tank Implosion due to insufficient vent diameter

(OP)
Hi 1gibson,

I'm pretty sure that the vent is the problem. There is 8 fermenters, 4 with 4" -> 5" vents, and 4 with 6" vents (they were installed in 2 phases) and the 4 with 6" vents do not show as much sign of damage as the 4" -> 5" lines do.

The fermenters have "switchers" in the gas space which stop the foam from going up into the vent and overflow but I don't think that there is a U bend / bubble trap in the vent line and I think that it is open to the atmosphere.



RE: Tank Implosion due to insufficient vent diameter

Given your operating conditions, I would think that simply adding cold water to a warm tank would be a far worse case than pumping out at 900 l/min.

It is also unsanitary to be sucking air into a fermentation tank through a long vent pipe that cannot be cleaned regularly. The fan was almost certainly on your pipe to draw-off the CO2 from fermentation, but with an air gap between the tank and the pipe. The air gap prevents sucking air through a dirty pipe and removes the restriction of the pipe when the tank needs to vent in or out quickly. But the fan should pick-up normal CO2 vent rates. Although it should be a blower and not a fan. A fan would create almost no flow through 20 meters of 4" pipe.

RE: Tank Implosion due to insufficient vent diameter

(OP)
Compositepro,

I think I mislead you slightly - the tank cools naturally without cold water addition, it is in a similar tank that cold water is added, and yes this causes a greater pressure drop than the outflow of 900 l/min.

I have made a spreadsheet for the rapid cooling scenario which I am quite happy with but my hand calcs I was using to model the vacuum inside the fermenter (which is not subject to rapid cooling) were not giving a large enough vacuum to cause the damage which occurred.

That is a good point about the cleanliness - the tank is at a customer site and I work for a company tasked with the repairs so I'm not sure what the cleaning regime is. I have only been tasked with finding out what size the vent needs to be.

Yes you are right - the fan is for CO2 extraction and not for venting and I suspect that it is the main obstruction that has lead to the vacuum. When the new vent pipe is added there will not be a fan in the line. I had been going down the route of calculating the minor loss caused by the fan as an obstruction (from Bernoulli's equation with frictional corrections) but I was guessing at the loss coefficient as I couldn't find a tabulated value for an unused fan.

Thanks again for your replies!

RE: Tank Implosion due to insufficient vent diameter

Is the 900 l/m pumped or is it draining by gravity?

RE: Tank Implosion due to insufficient vent diameter

(OP)
It's pumped away. Why do you ask?

RE: Tank Implosion due to insufficient vent diameter

The problem is not in the information you have given us. The pressure drop at 900 l/min through the pipe you have described would be close to the 39 Pa that you calculated. You can use the incompressible Darcy-Weisbach formula very safely when the pressure drop is so small. I find it easier to put these small pressure drops into perspective if they are expressed in mm of water gauge - around 4 mm in this case. bimr has calculate 2-3" of WG, but I disagree with that.

The problem is much more likely something along the lines suggested by bimr and 1gibson. If there is a low point in the vent piping it WILL fill up with water if you are working with foaming beer and cleaning with 80 deg C water. If the fan is not working take it out. It will only be a further restriction or will catch plastic bags, rags and pieces of paper and cause a blockage.

I have seen tanks implode too often when hot cleaning is followed by a deliberate or accidental infill of cold water. Don't expect the operating staff to own up to that. If you don't have a pressure or temperature recorder you will never know what happened.

If this were a design problem you would be imploding tanks on a regular basis. You have had a vent blockage or an inadvertent vacuum generation through rapid temperature change.

Katmar Software - AioFlo Pipe Hydraulics
http://katmarsoftware.com

"An undefined problem has an infinite number of solutions"

RE: Tank Implosion due to insufficient vent diameter

Looks like this is a multipurpose vent line ( normal operations outbreathing, inbreathing plus emergency underpressure / overpressure break line).

Others have identified several weaknesses in the design of the vent line you have now.

Would this implosion on this wooden beer fermenter have occured if you had kept this line dedicated to normal operating venting operations, and installed a backup PVSV for emergencies on the tank?

RE: Tank Implosion due to insufficient vent diameter

If the tank was draining from a valve into an open sump, you would get less of vacuum pulled.

The vacuum would be higher if you were pumping out of the tank, or if the discharge pipe was long enough to pull a siphon.

Katmar, sorry that you disagree with my headloss estimate. In cases like this, I usually have a more worst case estimate.

RE: Tank Implosion due to insufficient vent diameter

(OP)
Zdas,

Sorry I didn't see your second comment until I was re-reading the thread this morning. That's good - if the pipe calculations suggest that an unrestricted pipe would provide an adequate flow rate then that's a useful result.

I'll focus on the possibility of an obstruction in the pipe, as Katmar, bimr and 1gibson suggested. The pipe diagram I have doesn't have a U bend or a P trap in it but I'm going to double check this with the production manager who is also working on the problem. I've had it before where a vent grill at the exit is clogged with junk, so there are a few possibilities as to how it could have been blocked.

I think I've got enough information to say that the pipe diameter and length is probably not the problem and that it is more likely to have been blocked - I'll continue on this approach. Thanks for all of your replies! There aren't any other chemical engineers working for my company and it's really useful for me to be able to discuss this problem with other engineers.

RE: Tank Implosion due to insufficient vent diameter

It would seem prudent to think about installing a vacuum relief valve right on the tank itself set at a few inches of water column. Venting I can see you don't want in an enclosed space as everyone would probably die, but inbreathing would seem to be OK direct.

Many tank companies supply these.

I was also interested in your comment that " the 4 with 6" vents do not show as much sign of damage "

What do you mean by this? It seems these vessels are incapable of any real negative pressure? Your requirement / ability to withstand negative pressure needs to be your first design consideration and everything else flows from there.

Good luck and let us know how it goes - pictures are always good!

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Tank Implosion due to insufficient vent diameter

(OP)
I'm sorry to change tack but the production manager told me that the tank failed due to the overpressure caused by the hot rinsing rather a vacuum from draining.

The vent was probably still blocked as discussed above but now I'm looking at it from a different approach. I want to know what the maximum overpressure for each vent size is. It'll either be 5", 6" or 8" and I'm told the new vent will not have a U bend or fan in it.

I have done an energy balance of the heat from the hot water coming in and heating the gas inside the tank to determine the temperature profile during heating. For this I've assumed that there is instantaneous heat transfer between the hot liquid and the gas and inspected 0.4s increments, and plotted the equilibrium temperature for each 0.4s increment.

I've used PV = nRT and assumed that the pressure is constant to determine the increase in volume for each 0.4s increment and turned that into a required volumetric flow rate through the vent pipe.

I've then used the "Adiabatic Frictionless nozzle flow" section of Perry's Chemical Engineers Handbook 8th Edition to determine the flow through the pipe caused by the overpressure and plotted the difference between relief required and relief available. Summing the difference for each time increment gives the accumulating volume, which I've turned into the over pressure using PV = nRT.

I have two concerns about this method: 1) It seems a little contrived and I'm not sure how to validate the assumptions / determine if they are even valid
2) The method from Perry's for the adiabatic frictionless nozzle flow relies on a graph which is difficult to interpolate accurately and small changes in values determined from interpolation result in large changes in the relief available, and so the end result is strongly vulnerable to interpolation errors.

Hmmm....

Is there a simpler method / correlation that relates increasing gas pressure to volumetric flow rate through a pipe? I'm quite stuck when I look at my two concerns above - I think they are significant problems.

Data for question: tank gas volume when empty = 87m^3, volumetric flow rate of hot liquid = 15 m^3/hr, Hot liquid temperature = 80 degrees C, vessel air temperature before cleaning = 25 degrees C.



RE: Tank Implosion due to insufficient vent diameter

Unless the hot rinsing is using steam in some manner, the relief calculation consists of a simple mass balance. Inflow - outflow equals relief volume required. Worst case scenario is maximum inlet with no outlet flow from tank.

RE: Tank Implosion due to insufficient vent diameter

(OP)
Will the incoming hot water not heat up the air inside the tank causing it to expand and cause an overpressure?

RE: Tank Implosion due to insufficient vent diameter

The evaporation and condensation water in a closed container are proceeding at the same rate, so there is no net change, the system is in equilibrium. In the closed container, the pressure due to the water vapor reaches a maximum value (for a given temperature) called the vapor pressure.

The vapor pressure of water at 80 degrees C is 355.1 mm Hg. So that would be the maximum pressure on the tank if the water stayed at 80 degrees C and the tank was sealed off from the atmosphere.

That would be the worst case because there would be some cooling as the tank was filled.

If the tank was only partially filled with 80 degrees C water, the final equilibrium pressure would be less because the water would cool (with evaporation) somewhat prior to equilibrium being reached. In that case, the tank pressure would be the water vapor pressure at final water temperature.

RE: Tank Implosion due to insufficient vent diameter

If a closed tank was heated somehow from 20 degrees C to 80 degrees C, the tank pressure would increase 155.6 mm Hg above atmospheric.

RE: Tank Implosion due to insufficient vent diameter

It is these rapid heating or cooling events that destroy tanks. The vent sizes required are large because the heat transfer from a spray of hot ot cold water to the surrounding vapor space is very rapid. The API 2000 standard covers tank vents and the sizes it recommends will be very much bigger than the vents you have - which seem to be matched to infill and outflow rates only.

Katmar Software - AioFlo Pipe Hydraulics
http://katmarsoftware.com

"An undefined problem has an infinite number of solutions"

RE: Tank Implosion due to insufficient vent diameter

(OP)
bimr,

Yes when using the ideal gas law for a constant volume (closed system) and dT from 20 degrees C to 80 degrees C the overpressure is 207.4 mbar (155.6 mm Hg).

And also, when modelling the tank as a closed system with a pure water at 80 degrees C pool at the bottom the vapour pressure is 478 mbar (355.1 mm Hg).

Which of these do you think is more accurate? 478 mbar is obviously the worse case.

Katmar,

Thank you! I had always assumed that the heat transfer between the incoming fluid and vessel gas was instantaneous but I wasn't sure if that was a valid assumption. I have API 2000 5th edition from 1998 (it's the only one that was free to download) is there a particular section that you think is relevant? Just to save me from reading the whole thing :)

The spreadsheet that I made gives a maximum overpressure for a 5" vent of 84 mbar (63 mm Hg), for a 6" vent 54 mbar (40 mm Hg). Do these seem reasonable? The spreadsheet removes the closed system assumption by accounting for the outflow of gas due to the overpressure.

I still haven't figured out a way to determine what is the maximum overpressure that the tank can take. Zdas04 suggested 0.5 kPa (5mbar) but this is far below the pressures that my spreadsheet is giving out.

RE: Tank Implosion due to insufficient vent diameter

I'm not sufficiently familiar with API 2000 to be able to recommend which bits to read and which to ignore. I seem to remember a discussion on Eng-Tips somewhere regarding a recent revision that made it even more conservative than it was before. A search here for discussions regarding API 2000 should be useful.

Any calculations of the overpressure will have to make some assumptions regarding the rate of heat transfer. I think that is the real value of API 2000 - they have done the research to know what works and what doesn't.

Katmar Software - AioFlo Pipe Hydraulics
http://katmarsoftware.com

"An undefined problem has an infinite number of solutions"

RE: Tank Implosion due to insufficient vent diameter

I think you should read the entire API 2000 - not necessarily because some paragraphs are less (or more) related to your particular case, but rather to get a full insight of inbreathing/outbreathing requirements and all the factors that come into play for determining the vent size. Once you get a complete picture of all the processes which may occur during transient conditions, doing the calculations afterwards is the easier part.

As other contributors to this topic have said already, there are multiple factors which contribute to the venting requirements - apart from liquid inflow/outflow. Heat transfer can be a complete game changer, and this is evident in your particular case. API 2000 provides the best and the most detailed analysis of all possible scenarios. I think you won't regret reading it entirely, since it will come back to you on many occasions during your professional life.

I am attaching a handy API 2000 spreadsheet to this post, developed by one of the veteran members at ChE Forums and available for download at the parent website.

Dejan IVANOVIC
Process Engineer, MSChE

RE: Tank Implosion due to insufficient vent diameter

Although API 2000 is a great publication, I an not sure that API 2000 is really relevant to a scenario with a wooden tank that is just 2-3 m diameter and 3 meter tall. API deals with weather, flashing, conflagration, petroleum, etc., none of which are present here.

The OP recently stated that "that the tank failed due to the overpressure caused by the hot rinsing rather than a vacuum from draining" but neglected to say at what flow rate (or volume) the high rinsing is done. There is a temperature of 80 Degrees C.

To resolve this problem, the flow rate and volume of rinsing needs to be understood.

Since the purpose for the rinsing is disinfection, the temperature of the tank is probably above 60 Degrees C. Is the tank full at this point?

Based on everything the OP has stated to this point, the pressure drop for air flow through a 4-Inch vent is very low and adequate.

It also seems that the OP should also have some concern for condensation in the vent pipe since it likely to fall downward into the product. One would think that was the purpose of the fan. To prevent condensation from forming in the vent pipe.

RE: Tank Implosion due to insufficient vent diameter

The other factor to consider here is that many of the equations and systems used are designed for steady state and over the long term things settle down. The real situation though is a rapid transient change and the instantaneous flow rates and over pressure can be 4 or 5 times what the steady state flow is, but even if this lasts for only 2 or 3 seconds, the damage is done. Although low, the mass of air in the vent has an incremental effect when being asked to accelerate from nothing to several m/sec in 1 or 2 seconds.

tmengineer - you really need to see one of these operations in action to see how they do it, what sort of start flow of hot water they use, the start temperature of the vessel, the temperature and pressure of the hot water, the amount of free space, the orientation of the vent pipes etc before you can start any serious transient assessment.

They may be able to solve it simply by introducing a small flow of hot water for 1 or 2 minutes before turning it on flat out.

As someone said on a different thread - don't trust what an operator says he does, do you think they're going to admit they did something bad? or maybe no one has told them what the correct procedure is and why.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Tank Implosion due to insufficient vent diameter

If at some time you intend on going for a backup PVSV, there are a few things to note - there must be sufficient head room on design overpressure and underpressure to allow for

a) control (+/-) pressure range ( ie. normal operating press range that gives you room to carry out your normal operations) - these should be within the capacity limits of a well designed normal operating vent line

and

b)the setpoint of the vacuum and overpressure PVSV - industry practice on low pressure tanks is setpoint is at 80% of tank design pressure ( for either (+) or (-) settings, with typical accumulation of 20%.

Others have mentioned the prudence in using industry wide guidelines for deriving the capacities for these PVSVs. If the vapor space in these tank is fouling, 2x100% PVSV units with a slide valve / plate for each set may be better if you want to minimise downtime for cleaning and recalibrating these offline.

It may be a good idea to check if the newer SS tanks meet these requirements.

RE: Tank Implosion due to insufficient vent diameter

(OP)
Hello!

Sorry for the late reply - I haven't forgotten about this I've just been busy doing some other work for the last few days. I'll get back to you properly next week once I've had a chance to look at this again.

Thank you so much for your contributions so far!

RE: Tank Implosion due to insufficient vent diameter

I'll bet a full round of drinks that this was a collapsing vapor event. When steam collapses back to water, it happens so fast that you will outrun the Conservation Vent capacity by enough that the roof or shell will be sucked in. And when the engineer shows up to investigate, all the operators swear that they didn't purge that tank out with steam. Have hat one shell 'beer can; collapse , and about 6 sucked in roofs in the past decade. All these small tanks had functioning Cons Vents -- 3NPS on 15-ft tanks. Only mechanism with enough energy, that fast, is a steam collapse/condensation.

And none of the [union] operators did Anything Wrong; Honest! After the tank cools off, there is no latent evidence, except the massive energy it took to happen while vented.

RE: Tank Implosion due to insufficient vent diameter

(OP)
Duwe6,

At 80 degrees C and 1 atm the saturated vapour pressure of water is 481 mbar (calculated with the Buck equation - similar to Antoine but simpler), and the water vapour pressure is 478 mbar.

The temperature which the saturated vapour pressure is 478 mbar is 79.82 degrees C (again, calculated using the Buck equation). Does this mean that as the tank cools from 80 to 79.8 deg C there will be no decrease in pressure, but then as the T decreases from there the pressure will drop due to condensing vapour?

Or is it the case that the difference in the two pressures is due to errors in the Buck equation and I've just calculated the saturated vapour pressure by two different methods?

This is different from the failure mode I had been looking at (overpressure) - but it seems plausible.

LittleInch,

Yes I completely agree. This is a transient problem that cannot be assessed with steady state equations. I've tried not to fall into that trap but it's possible that I have.

Bimr,

Volumetric flow rate of hot rinse is 15 m^3/hr, tank is empty with a gas volume of 86.4 m^3

RE: Tank Implosion due to insufficient vent diameter

A lot of good things covered here. Utilizing API 2000 is a great one. The major thing that stands out for me in addition to what is discussed here is the 20m length of vent pipe (in addition to your other pressure drop cause by extra elbows and fan). Vents on tanks should be short as possible. A Vent on a tank should be short, oversized and with minimal elbows to prevent rainwater entering. The overall purpose is to avoid collapse during draining or lifting the roof off it - which happens from over-pressure.

Conor

Bellows Manufacturing and Research, Inc.
https://bellowsmfg.com

RE: Tank Implosion due to insufficient vent diameter

I'm with Duwe6 on the possible cause. Vent calculation for collapsing vapor has been addressed in thread391-329825: vent sizing Vent Sizing

RE: Tank Implosion due to insufficient vent diameter

API 2000 was updated in the last few years and I believe that the tank venting calculations were updated. I would suggest that you make the effort to get the latest version.

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