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

[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.

 

[TiCl4]

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.

 

[LittleInch]

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.

 

[pierreick]

Pressure Drop Online-Calculator

www.pressure-drop.com

Consider the resource above to calculate the pressure drop of your mesh.
Pierre

 

[TiCl4]

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.

 

[IFRs]

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.

 

[georgeverghese]

Discussion on screen pressure drop estimate may be found in Perry Chem Eng Handbook 7th edn on page 6-19 - subsection on Screens.