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Emergency Vent In Storage Atmosferic Water Tanks


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#1 Marcos1986

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Posted 16 May 2018 - 05:51 AM

HI,

 

I´m Mechanical Engineer and i´m doing a project of a water storage tank.

 

I´m sizing my gooseneck vent (open) and i´m concern about the calculations for emergengy venting requirements.

 

This is a change in the volume of an existing tank of 191 m3 that now will be 238 m3. The existing open vent is 12''.

 

The pump rates in and out is about 500 m3/h.

 

I have to be aware for emergency vent? because with emergency requirements my calculations give a 20'' vent and without emergency requirements a vent with 6''.

 

The slope is 20º so i think that doens´t meet the requirements for frangible roof...

 

Thanks for your precisous time,

 

Best Regards

 

Marcos



#2 Art Montemayor

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Posted 16 May 2018 - 09:52 AM

Marcos:

 

Two basic things can occur when operating your water tank: you can over-fill it and rupture it if your vent is not enough to relieve the 500+ m3/hr fill rate; or you can collapse the tank due to vacuum created by an excessive drainage or pump-out rate.  Either of these incidents is possible - depending on your specific tank and piping design as well as construction.  Water as a hazardous chemical is not a problem - but it could create a hazard depending on where it is located and its surroundings.  The rupture or collapse of the tank is certainly a dangerous hazard depending on the location and surroundings.  I would design for preventing both incidents even though your tank is relatively not large at approximately 7 meters x 7 meters.

 

We can’t comment on your sizing of the goose-neck vent since we don’t have access to your calculations.  Bear in mind that if your pump-in or pump-out pumps are centrifugal (which I’m almost sure they are) they have a performance curve that if you investigate it, you will find that it is probably very possible for the pump out or the pump-in flow rates to be well in excess of the design 500 m3/hr.  Any centrifugal pump is going to be throttled at the discharge to control the pumping rate - which means that if the discharge throttle valve is left open (or a connecting hose ruptures), the pump’s flow rate will instantly increase.  You have a tank that will fill from empty to full in ½ hour.  That, to me is a very fast rate when going into a storage tank.  At that flow rate you don't have much time to react to a level alarm.  You should have pump shutdown instrumentation conforming to high, high-high as well as low levels on the tank.

 

In order to go deeper into specifics, our members need specific basic data - such as a detailed sketch - or better yet, a Piping and Instrumentation Diagram, plus your calculations for the pressure and vacuum vent relief, as well as pump curves.



#3 Marcos1986

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Posted 17 May 2018 - 05:49 AM

Hi Art Montemayor,

 

Thanks for your time and response.

 

So if I understand well i have to size my vent in function of my filling rate and my drainage / pump out put rate.

 

For this case is not necessary to consider de emergency requiremet rates refer in the API2000?

 

Thanks for your time.

 

Best Regards,

 

Marcos Vilela



#4 Art Montemayor

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Posted 17 May 2018 - 11:59 AM

Marcos:

 

In my response to your post, I did not mean to imply that your application does not merit any emergency design consideration.  The API Standard 2000 states:

 

“This standard covers the normal and emergency vapor venting requirements for above-ground liquid petroleum storage tanks and aboveground and underground refrigerated storage tanks designed for operation at pressures from vacuum through 15 psig (1.034 barg).  Discussed in this standard are the causes of overpressure or vacuum; determination of venting requirements; means of venting; selection, installation and maintenance of venting devices; and testing and marking of relief devices.”

 

Under Special Notes, it further states: “API standards are published to facilitate the broad availability of proven, sound engineering and operating practices.  These standards are not intended to obviate the need for applying sound engineering judgment regarding when and where these standards should be utilized.  The formulation and publication of API standards is not intended in any way to inhibit anyone from using any other practices.”

 

You are not storing a hydrocarbon, but instead are dealing with plain water.  It is perfectly admissible to apply API 2000 to your project - even though you are not dealing with relatively higher vapor pressures that are flammable and possibly explosive.  The term “emergency” in the API standards is normally related to the ignition, spillage, over-pressure, over-filling, or release of potentially dangerous hydrocarbon fluids.

 

In your case you are dealing with a relatively benign fluid.  But, nevertheless, the storage and handling of even water can cause some unforeseen hazards - and these, in my opinion, merit that they always be at least considered before implementing a project to completion.  For example, have you taken into consideration:

  • The sudden rupture or disconnection of a hose while filling or pumping-out?  What would be the possible vacuum caused in the tank and can your vent handle the possible sonic velocity in the vent nozzle?
  • Do you have credible, safe protection against a tank over-fill?  If your vent cannot safely handle a potential overfill, the tank might be over-pressured and rupture.
  • Your tank will go from empty to full in approximately 30 minutes as designed.  Are your operations manual or automatic?  Can your local operators react fast enough to a potential overfill or sudden excessive pump-out?

Depending on where your tank is located, these could be defined as emergency situations.

I hope I have cleared up my comments on a safe storage tank design.  The spillage or leakage of water may not be considered hazardous.  But the sudden rupture or collapse of a storage tank certainly poses a potential danger to humans and its surroundings.

 

Cumprimentos.



#5 Marcos1986

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Posted 18 May 2018 - 02:56 AM

Hi Art Montemayor,

 

Thanks for your answers.

 

I understand perfectly what you are saying.

 

The nominal operation for filling is approx. 500 m3/h and for pumping out is the same. But i check the maximum possible flow for the pumps and i considerar 850 m3/h for filling in and 850 m3/h for pumping out.

 

The Tank is 15 m (Height) x 4,5 m (diameter) and is not insulated, storage of water is about 20º C (ambient temperature).

 

Do you think that i should considerer emergency condition a external fire? Because with API 2000 for emergency venting the venting required is 7870 m3/h. much more than 850 m3/h that i talked above.

 

Thanks for your time.

 

Best Regards,

 

Marcos






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