36 replies to this topic

[Shahzad Ahmad]

Dear Members,

 

I need to calculate the diameter of vent of a Water Storage tank.

 

The available design data I have at hand is..

 

Fluid in tank: Water

Tank Size (volume) : 10 m3

Inlet flow : 200 m3/hr 

Outlet flow: 200 m3/hr

outlet flow velocity: 3 m/sec

 

If I am not wrong the purpose of such vent nozzels is to maintain atmospheric pressure withing water storage tank. 

 

Appreciate your coments..

[fallah]

shahzad,

 

Refer to following link.It can help you out for your query:

 

http://www.cheresour...nk-vent-sizing/

Edited by fallah, 21 April 2015 - 10:14 AM.

[Zauberberg]

Also make sure to cover all relevant operating and other scenarios (e.g. steaming if applicable, ambient heating/cooling, etc.).

[Shahzad Ahmad]

Thank you fallah & Zauberberg

[Shahzad Ahmad]

Dear Fallah..

 

following the procedure, the minimum area requred is not reasonable, diameter value comes out as  0.0001 inch.

My calculations are:

Vs=6060 ft/sec=1847m/sec (atmospheric temper 20C=527 R).

Area = 200 /(1847*3600) as (outlet flow is 200 m3/hr)

 

What is missing, kindly guide...

[breizh]

Hi ,
You should review the terms of the equation:
Sonic velocity :sqrt (k*P*V) =sqrt(k*r*T)

T= 273.15+20 K
R=8.314 SI ; Mw : molecular weight of gas ; r=R/M
k# 1.3 to 1.4

Good luck

Breizh

Edited by breizh, 22 April 2015 - 03:44 AM.

[ankitg009]

Your calculated velocity is huge. Even huge seems small compared to this velocity. Kindly check your calculations.

 

This velocity corresponds to hypersonic region.

[Shahzad Ahmad]

Breizh 

 

Thank you for your response.

 

You should review the terms of the equation:
Sonic velocity :sqrt (k*P*V) =sqrt(k*R*T)

T= 273.15+20 K
R=8.314 SI
k# 1.3 to 1.4

 

following the same procedure, the results,

 

Vs= 58 m/sec.

Minimum Diameter of vent= 2 inch.

 

But still doesnt seem practical, because we have already a water storage tank which have the same outlet flow of 200 m3/hr, and have vent size of 8 inch, however have different capacity of 16000 m3 instead of current scenario where we have capacity of 500 m3.

 

Your suggestion plz...

 

Thanks..

[katmar]

In order to have a flow of air through the vent there has to be a pressure differential between the atmosphere and the inside of the tank.  If the vent is breathing out then the pressure inside the tank will be higher than atmospheric and vice versa.  What you need to determine is what is the maximum differential pressure your tank can tolerate for the two cases (i.e. breathing in and out) and then to size the vent for each of those cases and finally select the larger of the two calculated diameters.

 

The velocity of the air through the vent is a result of the sizing done as described above.  You cannot use the air velocity as the criterion for sizing the vent.

[curious_cat]

Is current scenario 500 m3 or 10 m3? 

[Shahzad Ahmad]

CuriousCat:

 

 Tank storage capacity is 500 m3.

 

outlet water flow rate from this tank is 200 m3/hr.

 

While the already installed system has.

Tank capacity 16000 m3.

outlet flow : 200 m3/hr

Vent size : 12 inch.

[curious_cat]

What's the 10 m3 you mention in your first post in this thread? 

 

Is that  a typo?

[fallah]

shahzad,

 

The velocity of sound in air at 20 C:

 

V_s= SQRT(1.4*287J/K.Kg*293 K)=343 m/s

 

Q= 200 m³/hr=0.055 m3/s

 

A (minimum required area for vent)=0.055/343=1.6*10^(-4) m²= 0.25 in²

 

Minimum diameter of vent pipe= 0.56 in

 

Then, vent pipe size of 1 in might be adequate if vent pipe length not to be more than, say, one meter. Per the length of vent line, to consider the effect of pressure drop, you can get vent line size of 2 in or higher...

[curious_cat]

@fallah:

 

In this sort of situation what safety system protects against vessel overfilling typically? 

 

This vent won't be sufficient in a situation where liquid overfills, correct? 

[Shahzad Ahmad]

CuriousCat:

 

typing error.. its 500 m3.

[Shahzad Ahmad]

Fallah:

 

Yes, I got. and but others in the department argue that the outlet flow of water from the tank is equal to air inlet in the tank. by this way atmospheric pressure can be maintained in the tank.

 

doing so, and applying, Q=AV, where

Q=200 m3/hr (outlet water flow)

A= Vent Area to be calculated.

V= Air velocity to be equal water outlet velocity whcih is 3m/sec.

 

By applying this formula the vent dia comes out to be 6 inch. 

 

Is there any one to endorse these calculation>... 

[fallah]

 

The case raised by OP relates to tank emptying and the vent defends against vacuum creation. Of course, in the case of tank filling, high high level alarm along with an proper sized overflow line can protect the tank from liquid overfilling...
 

[fallah]

 

 

shahzad,

 

No need the velocity of the incoming air inside the vent line to be equal to the velocity of outgoing liquid from the outlet line. The velocity of the air inside the vent line just needed to be subsonic and, in addition, to be such that the pressure drop in the vent line being lower than the pressure rating of the tank itself.

 

Of course, it's obvious that in your case 6" vent line could be acceptable provided that the above mentioned criteria would be met...

[curious_cat]

 

 

 

Can one design so that the liquid overflow line itself provides vent protection? I guess it depends where / how that line terminates?

[Shahzad Ahmad]

knowing the pressure difference, the formula Weymouth equation will applied for dia,   Am I right sir,

[Shahzad Ahmad]

Fallah:

 

Thank you sir for your prompt response.

..

[fallah]

 

 

An overflow line might play the role of a vent in tank emptying/filling operation, especially if not to be terminated to, say, a seal leg for isolation to atmosphere, but the credit can't be taken for overflow line to cover full/partial venting requirement...

[katmar]

I would not use the Weymouth Equation.  That equation is intended for long gas pipelines where there is a significant pressure drop over the line and where most of the resistance is due to wall friction.  In this case I would use the incompressible Darcy-Weisbach equation.  Yes, the one intended for liquids because you effectively have a constant density situation.  You need to take into account the entrance and exit losses, which could well be greater than the pipe friction in the vent.  This is most easily and accurately done with the D-W equation.

 

I don't understand why people are talking of sonic velocities.  You do not want sonic velocities anywhere near an atmospheric storage tank.

 

It is not the velocity of the air and water that are the same - it is the volumetric flow rates that are the same.  If you are pumping out 200 m3/h of water then you need to bring in 200 m3/h of air.  The velocity of the air will depend on the vent diameter, which in turn would depend on the allowable differential pressure.

 

Go back and re-read what Zauberberg said.  It is not the filling and emptying of tanks that causes problems.  It is the non-routine things like collapsed steam due to a rain storm (that was never planned) that destroys tanks.

[fallah]

Yes, no one wants (in fact, can't want) having sonic velocity anywhere near an atmospheric storage tank; but to be far away enough from the sonic velocity, the corresponding vent line diameter should be specified and known; then according to vent line length and relevant total friction loss, considering pressure rating of the tank the size of the vent line can be adjusted well above the corresponding size of the sonic velocity...

[Shahzad Ahmad]

Thank you all for making me able to solve the problem...