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Free Vent for API 650 Tank - Bug Screen / Bird Screen Requirement 2

C

Cabbarbeybaba

Chemical
Joined
Jul 3, 2025
Messages
2
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. :)
 
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Cab,

In the past, I have gone with option 2A you presented - increase opening size until actual free area >=required free area. I don't know of any available correlations for pressure drop through a bird mesh, but your number of 50% area reduction seems reasonable. I seem to remember the mesh I dealt with in the past had a ~40% coverage, ~60% free ratio.
 
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The problem with any screen is that it can get easily clogged with bugs, spiders, rust, debris like leaves or plastic bags etc.

Hence you are well advised to also include a specific vacuum relief valve and vent valve to prevent tank collapse, especially when the fire pump kicks into life. API 650 tanks ( a bit extreme for a FW tank...) are very susceptible to vacuum collapse at very low differential heads of only a few inches of water column.

I would question the need for a bug screen. Bird cage yes to prevent nesting etc, but bugs in a non potable water system? Most of them will drown.
 
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To LI's point, I have specified bird screens in the past, with ~1-2" diamond mesh screens. Going small diameter for a "bug" mesh (<0.25"?) does seem like a good recipe to get clogged with dirt or even just rust.
 
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I am a fan of API 650, but for water tanks perhaps these might help:

NFPA-22 "NFPA 22, Standard for Water Tanks for Private Fire Protection"
AWWA D-100 "Welded Carbon Steel Tanks for Water Storage"

From a fairly old edition of NFPA-22:
4.14.2 A vent pipe shall have a cross-sectional area equal to a minimum of one-half the area of the discharge pipe(s) or fill pipe, whichever is the
larger.
4.14.3 A corrosion-resistant screen or perforated plate with in. (9.5 mm) holes, to exclude birds or other animals, shall be provided and have a net
area at least equal to the vent line.
4.14.4 In the case of a screen, this requires a gross area at least one and one-half times the cross-sectional area of the discharge pipe(s) or fill pipe,
whichever is larger.
4.14.5 The screen or perforated plate shall be protected against the accumulation of sleet.
4.14.6 The weather hood above the perforated plate or screen, or its equivalent, shall be readily removable.
4.14.7 The overflow pipe shall not be included as vent area.
4.14.8 The vent shall be permitted to be combined with the roof finial.
4.14.9 Equivalent venting shall be permitted to be used, provided the area cannot be obstructed by sleet, and the ingress of birds or other animals shall
be prevented.
4.14.10 Where dual service is specified and where local health departments require screening vents against insects, a nonmetallic screen or special
fail-safe vent shall be provided to minimize the risk in the event that the insect screens frost over.
4.14.11 A roof vent attached to a flanged neck shall be installed on steel tanks prior to entering the tank.
4.14.11.1 The flanged neck shall be designed to accommodate an adequately sized exhaust fan.

Again from an old version, this time AWWA D-100:
5.5.2 Screening. The vent shall be designed and constructed to prevent the entrance of birds or animals. When the vent is provided with screening against insects, a pressure-vacuum vent or a separate pressure-vacuum relief mechanism shall be provided that will operate in the event that the sccreens frost over of becomes clogged. The screens or relief mechanisms shall not be damaged by the occurrrence and shall return automatically to operating position after the blockage is cleared.
In older versions, this section included language for regular inspections, at least in the spring and fall.

There are several vendors that make AL or SS vents with such mechanisms.
 
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Thanks for all the replies; I am very happy that this community shares great information with this much detail.

The clogging of the screen is a serious problem. I agree that regular maintenance operations and/or vacuum relief valves shall be implemented for tanks with small screens such as bug screens.

I understand that NFPA-22 (2018) & AWWA D-100 should be investigated since this is a firewater tank. NFPA 22 suggests 1.5 times the inlet or outlet pipe diameter (whichever is greater) for the vent, and AWWA recommends an additional pressure-vacuum vent for screen use at the free vents. These have an impact on vent sizing; therefore, I will talk with the tank design team with respect to these standards.

Thanks for informing me about the pressure drop online calculator & Perry's handbook. I used an online calculator for a quick estimation for the firewater tank, but I will also check Perry's equations in order to increase my understanding about hydraulics.

I have also talked with our tank designer, who indicated that API 650 tanks are designed for -2.5 mbarg vacuum. If we use smaller vacuum pressures, Annex V is used, which covers between -2.5 mbarg and -69 mbarg. Using a vacuum design value between -2.5 mbarg and -69 mbarg resulted in thicker shell plates in our firewater tank design according to Annex V; therefore, I tried to size the vent with respect to -2.5 mbarg.

I think I will do the following for vent sizing:

1) I will recheck if a bug screen is really required since this is a non-potable water system and bugs may not cause an issue for the design, as you suggested. However, I will definitely add a bird screen even if a bug screen is not selected.

2) If a bug screen is required, I will add the meshed flow loss to the discharge loss and calculate the total pressure drop of the vent accordingly. The vent will be sized so that the maximum pressure differential is less than 2.5 mbarg.

3) I will check the calculated vent size with respect to NFPA 22, meaning that if it is smaller than 1.5 times the inlet/outlet piping. If it is smaller, then I will talk to the design team about increasing vent size with respect to NFPA 22.

4) I will talk to the design team about clogging issues and the pressure vacuum vent requirement. If decided, I will add an additional pressure vacuum vent to the tank. I will also add screen maintenance to "Startup, Shutdown & Maintenance Philosophy" document.

Again, thanks for all the information, suggestions, and experienced answers.
 
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