Emergency Vent Snow Load Consideration for Pressure Setpoint 4

[az5333]

Hi,

I am working on sizing an Emergency Vent [EV] for an API650 atmospheric storage tank built using API 2000 standard. I understand that API 2000 gives guidance in terms of capacity required for the EV. However, after I am done sizing the EV (per API 2000), next step is to determine the right pressure setpoint to set the EV vent at. My question is: Do I need to consider snow loads that might hinder the EV to open properly when determining the right setpoint? The EV is exposed to atmosphere and does not have any cover over it. I would really appreciate if I can be directed to the relevant standards and practices that are followed in the storage tank design engineering community for such a scenario. Thank you.

 

[MJCronin]

Specifically, what type of roof is on the tank ?

What type of EV are you considering where a coating of snow would affect operation ? (be specific, show a drawing or sketch)

What type of liquid contents are in the tank and is there a tendency for the vapor to clog vents ?

What are the tank design pressures and temperatures ? What is the EV setpoint ?

Do you already have a properly sized nozzle on the tank roof to accept the emergency vent or do you plan to use a manway ?

(Emergency Tank vents tend to be large)

MJCronin
Sr. Process Engineer

 

[az5333]

Hi MJCronin,

Thank you very much for your response. Please see responses to your questions below. Please let me know any other info you might want to consider to provide your feedback based on your experience. Thanks you.

Specifically, what type of roof is on the tank ? Conical non-frangible roof

What type of EV are you considering where a coating of snow would affect operation ? (be specific, show a drawing or sketch)Similar to Enardo 2100 series EV. Attaching a picture of the EV we have right now. Since it is exposed to atmosphere att he top, I am not sure if we need to worry about external snow loads in this case.

What type of liquid contents are in the tank and is there a tendency for the vapor to clog vents ? Gasoline, No tendency of vapors to clog the vent

What are the tank design pressures and temperatures ? What is the EV setpoint ? Atmospheric pressure and temperatures (-20F to 100F ambient variation). Current set point is 1.5 oz/in2.

Do you already have a properly sized nozzle on the tank roof to accept the emergency vent or do you plan to use a manway ? Yes the EV is installed on a 20" manway. The size of the EV is 20" as well.

 

[LittleInch]

Interesting.

Not sure anyones considered that before now. If the activation pressure is impacted by the weight of snow on the top plate then yes - you've got an issue.

So you need to either provide a cover so that the snow doesn't land on it or use a different emergency vent.

You might want to look at pressure and vacuum vent valves like these

https://omegavalves.com/product/pressure-vacuum-vent-valves/

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

 

[JStephen]

The main purpose of the emergency vent is for cases when the tank is engulfed by fire, and it is generally not considered necessary to combine that load case and full snow load simultaneously.
That said, if you did have snow on the roof of a tank engulfed by fire, you would get product condensing on the roof instead of needing to be exhausted and also would pick up some additional overpressure resistance from the additional snow load.

 

[MJCronin]

I would call several vendors of manway-style EVs and ask if this loading case has ever been considered.. I am sure someone has run across this type of snow loading situation before and ask about the evaluation or fix

Secondly, API 2000 discusses the issues of snow and ice, but only in terms of discharge piping leading from the relief device. Because you are interested in the manway adaptation, these cautions and comments do not apply ..... in my opinion. There is no discharge piping from the manway device

Additionally, snow and ice would be a major threat to proper operation of a manway adapted device, based on the configuration .... again in my opinion.

PROTECTOSEAL is the gold standard in devices of this kind and do not mention problems with ice or snow in their literature...

https://protectoseal.com/PDF_VENTS/SPEC_SHEETS/53300_SPEC.pdf

Please keep us involved an let us know about your final decision

MJCronin
Sr. Process Engineer

 

[IFRs]

This is an interesting question, and I'm glad you brought it up.
IMHO, if for your tank you envision a scenario in which snow or ice (or sticky product condensation) would affect the ability of the vent to open when it was needed to open, you should find a way to fix it.
Obviously not an issue where freezing temperatures don't happen.
If the adjacent tank is on fire and you need this emergency vent to operate and this vent is 150 feet away on the opposite side of the tank as the one burning you might have a case for using another type vent or putting a snow shield over it.
Could melting/freezing could weld the cover shut?
Is the bulk liquid temperature and minimum fill height enough thermal mass to keep the vent clear of snow?
Do your tanks normally accumulate snow or ice?
Is heat tracing the vent neck a possibility?
Is the tank heated and/or insulated?
What is your potential snow load? It may be a small fraction of the normal set pressure...

 

[LittleInch]

These seem to be very simple lids which seal under self weight.

Looking at the picture below the weight of sat 4 " of snow on top of the middle unit could be up to 25kg - a significant increase.

I think you could build a snow shelter but the question of icing up might be relevant as well.

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

 

[az5333]

Thank you MJCronin, LittleInch, JStephen and IFRs for your thoughts and feedback on it. I will keep you guys updated about what solution we end up with in our case. I was also of the opinion that since Emergency Vents are designed for a "Fire" exposure scenario, any snow that might be on adjacent tanks would have been melted anyways, so it shouldn't be a concern. Ill talk to the EV vendors and see what they have to say as well. Thanks again!

 

[LittleInch]

There's a whole lot of difference between snow "would have" or "might have" or "should have" melted and it didn't...

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

 

[az5333]

Very true LittleInch..This is the reason I am seeking out other expert opinions on this forum as I don't want to go by my assumptions...

 

[Geoff13]

I've designed tanks for Canada for 35+ years, and never thought about this. Interesting question, but I'm also not going to lose any sleep about my existing tanks.

Not asking you to answer any of this, but this would be my thought process.

Determine how much the set pressure is raised by the snow accumulation. Fire is an extreme load, so don't combine it with full roof design snow load. Perhaps 25% of the roof design snow load, but it's a judgment call. Since API allows the emergency relieving pressure to be calculated with no wind (clause 5.8.5.3.a) they accept not combining other design loads with the emergency fire case.

Next look at the vent flow capacity curve to see if the higher set point pressure will still allow the vent(s) to reach the relieving pressure and thus the required API 2000 venting rate. Since the flow rate typically jumps very quickly above the set pressure I would hope this would be the case, unless you have a very tight range between your set and relieving pressures and the extra snow uses this up.

This is getting to the extreme case, but if the snow seems to reduce your venting capacity too much, do you add another vent or could you sleep at night with slightly exceeding the relieving pressure? I know some will shoot me for suggesting option two, but the permitted relieving pressure isn't the failure pressure of the tank. I could sleep at 1% or 2% over but not at 25%, and I don't know where my cutoff would be in between.

I've only repaired one fire damaged tank. It was a large tank, and the fire damage on the roof was basically confined to the area where the fire was directly below outside. I would not count on snow melt.

Good luck.
Geoff

 

[JStephen]

Just to point out more clearly- in the event that you do have a bunch of snow on a tank engulfed in an inferno, the effective setpoint of a lifting-cover vent would be increased, but the pressure rating of the tank would also be temporarily increased by that same weight of snow across the roof area.

 

[LittleInch]

From my fairly basic calcs it looks like the pressure could be double - so maybe 3 oz/in^2.

Maybe you just need an operating instruction to clear the snow off the emergency vents when the operators go on a daily walk around on top of the tank?

Or a simple snow cover.

Or do nothing and live with the increase in max pressure.

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

 

[IFRs]

The emergency vent may also be needed if an adjacent tank is burning and the heat input to your tank is high enough to cause an increase in pressure.
You always want the vent (or frangible joint) to relieve before the corner weld fails.
Emergency vents often are set to open at the weight of the roof plate.
If the vent opens at a pressure that is increased by even a foot of water column because of snow or ice on top, the effect is to add a foot of liquid to the hydtostatic pressure at the corner weld. I rate this increase as typically insignificant.

 

[HTURKAK]

This is an interesting subject and this time , i read the thread and posts carefully rather than screening them..



It could be a concern depending upon existing lay out of the tanks and the scenario . Consider two or more tanks located in a common dike and for some reason , one of the tanks ruptured and a nearby ground fire started.

Suppose that , this is the worst probable scenario with the ambient temp is -30 degr and the roof has snow . And suppose that fire protection does not include tank cooling system due to -30 degr. If the tank exposed to nearby ground fire, the roof plate temp. will not increase above 0 degr. as long as all the snow melts and flows to ground. During this period ,the vapour pressure could be lower than the set point since some of the vapour will condensate . Then the vapour pressure may increase rapidly .

I would like to remind that , emergency venting is a requirement to release the excessive vapour pressure when tank is exposed to fire. Two fire scenarios could be the adjacent tank is on fire or nearby ground fire ..

Having long years of cold region experience ,based on one of the probable scenarios , my concerns are;

- The condensation rate of the gasoline vapour due to cold roof could be lower than evaporation rate and may not keep the pressure below set point. In this case, the set point should be determined lower than 1.5 oz /in2

- I did not count the type of snows ( may be more than hundred..) , assume that during freeze thaw period or wet snow accumulated around manhole nozzle and frozen. That is, literally the cover will be locked to nozzle as long as ice does not melt..

- Assume that the tank wall experience fire while the tank roof plate at ambient temp .( say -20 degr). The shell portion btw liquid level and roof will experience sudden temperature rise with thermal shock and may be tearing of roof to shell junction.

My suggestions will be;

- The probable scenarios and set point should be studied by a process engineer having cold climate experience and local knowledge..

- Conduct an assessment study for applicable fire fighting and cooling options

- Conduct winterization study and consider heat tracing of roof manhole, valves , critical piping to keep operational..

- You may consider to post similar thread to process development forum .

You may provide more detail ( size of the tank, no of tanks and lay out etc..) to get better responds..

Edit: The following doc. is ( free ) is useful to get the winterization concept.

https://rules.dnv.com/docs/pdf/DNV/OS/2015-07/DNVGL-OS-A201.pdf