Calculating Vent Size for a Water Tank

[agileengineering]

Hi guys,

I have a tank design for a 30m3 water storage tank. The pump flow rate to the tank is 45 m3/hr. I currently have a tank vent of 1", a 2" overflow and a manway.

The client has requested some calculations to prove adequacy of vent size. This is something new for me.

Looking through API 2000 I can find the inbreathing and outbreathing rates, but my question is how do I then get a vent diameter from that value?

 

[elridyeng]

i saw a formula before for H2So4 Tank said that :

vent DN = suction line DN + drain line DN

For example :
The outlet DN from the tank is 2" and the drain line 1" so vent line at least 3"
but i'am not sure 100% if it is work for all fluids

 

[LittleInch]

here's one I answered before.

http://www.eng-tips.com/viewthread.cfm?qid=365884

also search vent size in the storage tank forum and you'll find more.

Basic ROT is to have a vent as short as possible and at least as big as the water pipe coming in or out.

You can calculate from the sites listed. Also the tank supplier normally has formula you can use.

The real issue is normally venting for outflow as you can collapse a tank very easily with only a small differential pressure. 30m3 is quite big so you would often have a free vent and possibly a spare.

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

 

[vt2012]

elridyeng - that rule of thumb is actually for area, not simply the pipe diameter. So for instance if you have 3" outlet, 2" inlet, and 1" drain a 4" vent would be sufficient (even though 1+2+3 !< 4 obviously, the cross sectional area of 4" is greater than the areas of 1" + 2" + 3").

 

[bimr]

The air flow rate into the tank can be calculated based upon maximum flow rates required in the tank system operation. The flow rate of air into or out of the tank will correspond to the rate that the tank is drained or filled. Using the conversion rate of 7.48 gallons per minute (gpm) equaling 1.0 cubic feet per minute (cfm) of air flow, it is then
possible to determine the actual cubic feet of air flow.

One factor that will significantly alter the sizing is the maximum temperature that the tank will be exposed to. Using the Ideal Gas laws, it is understood that volume of gas expands as it is heated and contracts as it cools. This can result in significant variations in the actual volume of air through the vent. The most critical consideration is when the system goes through a steam cycle. As the air cools from steam temperature, the volume may rapidly be reduced which results in a significant inflow into the vessel. It is essential to take into account this critical demand, if applicable.

The velocity of the air inside the vent line just needs to be subsonic and, in addition, to be such that the pressure drop in the vent line is lower than the pressure rating of the tank itself. Include a safety factor of 2 in the calculation to protect equipment.

An air vent velocity of 1,500 - 2,000 ft/min (8 - 10 m/s) will not generate noise or significant pressure drop.

 

[agileengineering]

Thanks guys

Elridyeng/ vt2012 - do you have a source for that method. It appears to make sense and is the simplest method I've seen so far.

 

[georgeverghese]

Your client may ask for a rigorous calc for this, so simple thumb rules may not go down too well with them. If so, ask a senior process engineer to assist with this and have the calcs checked and signed off - starting point is the design pressure of this water tank. Also check local jurisdiction code procedures for these if they exist.

 

[bimr]

That formula is nonsense as the bottom nozzles have a fluid that is incompressible and the upper vent nozzles have gas (compressible).

 

[bimr]

On a water tank, one can assume the headloss through the vent will be approximately the same as the headloss through an orifice.

As an example, an air velocity of 1,500 ft/min through an 8-Inch orifice will generate a headloss of 0.01 psig. As a point of reference, HVAC air ducts are typically sized for 1,500 ft/min.

http://www.tlv.com/global/TI/calculator/air-flow-rate-through-orifice.html

 

[georgeverghese]

See expression 21 on page 30 of the yr 2009 edition of API 2000 to get the theoretical value for the venting rate for a given free vent dia. But this flow has to multiplied by the coeff of discharge for the free vent - you need to check alternate sources for this (a value of 0.5 is recommended in this API for manhole type vents; my recollection is that K is about 0.6 for a circular exit).