4 replies to this topic

[Cabbarbeybaba]

Hello everyone, I have used this forum to get answers about chemical engineering problems. This is the first time I am posting here; therefore, I apologize in advance for any misunderstanding.

 

I use API 2000 to calculate the free vent size of a firewater storage tank design with respect to API 650. Firstly, I define the required flow rate according to tank dimensions, inlet/outlet flow rates, and other specifications. Secondly, I calculate the required free area with respect to API 2000 equation 25 by taking a discharge coefficient of 0.6. The result shows that 11.3 inches in diameter is required, but I choose a 14-inch-diameter free vent. This part is clear for me.

 

​Since this is a water tank, insects may enter the tank. Therefore, I wanted to put a bug screen on the free vent in order to prevent this situation. This is where my question begins.

 

1) Firstly, API 650 section 5.8.5.5 defines bird screen requirements with a minimum 19 mm opening size. This size is applicable for the specific location I am from with respect to local regulations; however, adding a bird screen is actually reducing the outlet area and increasing pressure drop. I have calculated that area reduction is not important for 14" pipe since 11.3" free area is required. However, I could not calculate the pressure drop increase because of the screen. Can you guys suggest any relations or references?

 

2) Secondly, I investigated bug screen mesh sizes. I came to the conclusion that 20x20 mesh would be sufficient. However, this would result in 50% less open area for the free vent. So I have two questions about bug screens:

 

2a) Should I increase the free vent diameter so that a 20x20 mesh would have 11.3 inches of free area?

 

2b) Should I decrease the discharge coefficient of 0.6, since there is a flow restriction at the mesh? I could not find a relation between mesh sizes and discharge coefficients in my hydraulic calculation books.

 

3) Lastly, I tried to look up vendor documents. They put a bug screen/bird screen as an option for PSVs/PVVs/free vents. However, I have not come across a sizing criterion that changes according to this screen requirement. 

 

3a) As a result, I think vendor designs do not require specific design changes according to bird/bug screens. Is this perspective applicable?

 

3b) As a result, I think vendors may include bird/bug screens in their overpressure scenarios, meaning that they may indicate 50% overpressure because of bird/bug screens. Is this point of view applicable?

 

Thanks in advance.

[Pilesar]

If you need a screen, why not have a larger surface area than the pipe diameter? Screens can come in many shapes besides flat circles.

[Cabbarbeybaba]

Thank you for your answer.

You are correct; I could use a conical or funnel-like screen. This would increase the surface area since the screen will be placed inwards of the free vent. However, the change would still result in a pressure drop across the vent, and I could not find any hydraulic calculation method for this kind of calculation.

[breizh]

Hi,

Consider this calculator:

Pressure Drop Online-Calculator

 

Breizh

[Cabbarbeybaba]

Hello again,

Thank you for reminding me about this calculator; I did not notice that there is a perforated plate calculation. I also learned that there is a calculation method suggested in Perry's Handbook 6-19. I will investigate both.

I will add the meshed flow loss to the discharge loss and calculate the total pressure drop accordingly. Thanks for all the help; much appreciated.