Vent size - vacuum relief

[mcecasf]

Hello all,

I've seen a few similar-but-not-quite-the-same threads so here goes.

I'm trying to confirm size of a vent in a ~70 m3 atmospheric tank which sees a range of temperatures and is also cleaned at 80 degC followed by a cold rinse. (There's direct steam injection too but I'm trying to design that out so can assume it has "gone"). Tank is indoors so no bother with rain etc.

My approach was:
- tank full of air at 80 degC, atmospheric
- assume cold water going in at known flowrate, heat transferred directly from air to water, to an equilibrium temperature based on the heat capacities of both (OK so it won't be as efficient a heat transfer as that, even with the cold water spraying in, but it will give me a very bad case number to use?)
- Determine the change in volume which that cooling would cause in the air
- Take that change as the flowrate of fresh air in that is required to maintain atmospheric
- use that flowrate to determine pressure drop thru the existing vent and see if it is sensibly low....?

Kinds struggling a but to see if there is a better way of looking at it! I'm in a small firm with very little in-house knowledge and while I'm 99.9% sure the existing arrangement is OK, mainly because the tank has not collapsed yet!, I'd like some kind of maths to back it up!

Any hints / tips welcome....

Cheers

 

[MJCronin]

Any properly designed tank of this size should have AT LEAST two manways.....one on the shell and one on top.

Any competent maintenance department should have both open during cleaning activities

The minimum size for manyways considering the current crop of aging, overweight staff....is 22"

 

[mcecasf]

cheers for that... but you can't have a manway open when you are high pressure sprayball cleaning the inside with 80 degC caustic solution...! we'd soon have a 10m spray of caustic across the roof of the factory! ;o)

 

[786392]

Dear mcecasf Hello/Good Afternoon,

In your 'OP' probably there was no mention of "high pressure, caustic" ,could you recheck/confirm please!

Accordingly MJC's comments are valid/correct, I understand!

Best Regards
Qalander(Chem)

 

[mcecasf]

Thanks guys for taking the time to view the thread.

Some additional info I did not include in the original query as I thought it muddied the issue, I guess being from a food / pharma background I forgot that other industries don't necessarily have the same problems/situations and my wording could have been a lot better! To give a fuller picture:

The tanks are un use in food manufacture in a microbiological manufacturing process. Cleanliness requirements / contamination risk from the external environment is high. We have an automated cleaning cycle for the tanks, usually comprising
1. Cold water desoiling rinse
2. 80-90 degC 1%-2% sodium hydroxide wash
3. Cold water / cold disinfectant rinse (depending on risk level)

CIP is completed by spray jets inside the vessel, eg toftejorg TJ20G or TZ-74 depending on tank size. If manways are open during cleaning we will jet cleaning solution all over the factory through the manways...... Also, the more open area we have, the higher risk of micro contamination of the tank, hence all are closed during cleaning. Obviously we have a vent (whose size I am trying to validate) into a clean space and I expect we will draw air in through this, however the supply air quality can be controlled within the space where we have an open vent but cannot be controlled round the manways, which are only used for internal inspection / maintenance and not during the processing stages.

This is very similar to some pharmaceutical and many other food applications. I've seen a few ways of trying to limit the ingress of contaminants through the vents (e.g. high levels of filtration on the vent, coupled with bursting disk as a just in case) and was told at the time by the senior technologist, "that's OK because its how we did it last time", and thought little of it. But to properly design such a system one surely needs to consider the required venting rate! Docs I have looked at e.g. API2000 all have "get-outs" of "requires an engineering review" where headspace is at a higher temperature, or steam is involved. Other examples / answers I have seen concentrate on the risk from rain on the outside of the tank during steam-out - not the situation we have.

In summary:
- tanks are at atmospheric and are not pressure or vacuum rated
- I can't leave the manways open, but I have one straightforward open swan-necked vent
- I have ~80 degC (moist) air in the tank
- I spray in cold water, and expect cooling of the air, hence draw air in through my only vent
- I need to determine the required size of this vent (or rather, I want to validate that the existing one is sensible vs. lucky)
- struggling to determine how can I sensibly do that?


Any comments welcome!

 

[786392]

Dear Hello/Good Night,

then probably this link below should be the place to discuss the pharmaceutical engineering related issue!

http://www.eng-tips.com/threadminder.cfm?pid=121

Having said that,
the actual details shown on diagrammatic sketch, insulation and possibility/extent of heat exchange should be shared so as to produce vacuum conditions in what stipulated time.

All will have to carefully worked out,calculated considering total streams

material and heat balance(s) with inflow and out flow basis.

Hopefully this proves/helpful guidance for you

Best of luck in correct way forward for your problem's resolution.

Best Regards
Qalander(Chem)