How to size a vent for 90,000 gallons per minute 3

[Shells]

Hi there, I am new to eng-tips and pretty excited about what I see so far. I would appreciate any help with my question.

I am sizing a vent for a water tank. It has a maximum outflow of 90,000 gallons per minute.

I need the pressure differential to be less than 2 inches however, I am not sure how I get to this.

I figure it has to do with pressures but I am not sure how to get my velocities to pressures

Any help would be appreciated.

Shelly

 

[cvg]

are you sure about the 90,000 gpm? that seems a bit high to me. How big is this water tank?

 

[Shells]

Yes, I am sure about the 90,000 gallons per minute. It is a 4.2 MG reservoir.

 

[JStephen]

Typically, vents on water tanks are sized about the same size as inlets or outlets (IE, same nominal pipe size). Sizing them this way nearly always makes them substantially oversized for a given flowrate. But, water tank vents are fairly cheap, and owners don't want to see a 4" vent on a tank with 36" suctions, so that's the way it's done.

Occasionally, people come up with "what if the pipe breaks" or "what if the manhole cover pops off" type questions, and in those cases, you may need a larger vent.

One source for flow data is the ASHRAE Handbook of Fundamentals. It includes flow data for ducts that include an exhaust hood similar to a typical mushroom-style tank vent, as well as factors for screens, abrupt inlets, ells, etc. It's approximate, since you're not actually putting the fittings on a duct, but it'll be close enough for most purposes. Refer specifically to the older versions of the H.O.F. The newer ones either didn't have all the data (IE, showed exhaust but not intake factor for that hood) or perhaps put that data in other volumes. An HVAC engineer could probably point you to this info fairly easily. The flow factors give the relation between the pressure drop and the velocity, so that's how you get from one to the other.

A second source of information is published flow factors from vent manufacturers. Varec (currently made by Tyco) and Shand and Jurs both have "free vents" for which test data are available showing flow rates versus pressure (I think Varec's data is available online). I have seen Greenheck vents that show a flow rate on the drawing, but they don't show the basis for the flow rate. The Varec and Tyco Free Vents are only made in sizes up to 12" or so, but that size is liable to move more air than you need. You may also run across Groth vents; I haven't dealt with them in a while, though.

You can treat the vent as simply being a hole in the tank, and apply Bernouli's equation between the interior of the tank (pressure = 2", velocity = 0) and the vent neck (pressure = 0, velocity unknown) to get another idea of flow velocity. Of course, this doesn't allow for screen or geometry restrictions. But it'll give you a minimum area that is required.

It should be fairly simple to make up a flow-testing station for vents. However, it is even simpler to bump the vent size up a couple of sizes if anyone is concerned about it, and so I haven't seen any tests done.

FYI, I typically use 1" of water (IE, 5 PSF) for venting calculations. This is somewhat arbitrary, as the AWWA tank codes don't specify this pressure. But API-650 and API-620 both allow 1"/ 5 PSF for internal vacuum design. For venting from the tank, you could work through the API-620 design of the roof (assuming a steel tank) & anchorage to derive a higher allowable pressure (or up to the dead weight of the roof if it's concrete). Usually, large steel tanks are not good for much more pressure than the weight of the roof plate.

API tanks include allowances for sudden weather changes (cold rain on hot roof) and fire exposure (contents boiling off), but water tank flow rates are usually based on water movement only. Typically, calculations for venting do not include a safety factor, but sizing tends to be inherently conservative.

 

[25362]

For pressure conversions one may use 1 in. wg = 5.1934 psf = 0.03607 psi = 1.8651 mm Hg.

For standard air, a velocity pressure of one-inch water gage corresponds to 4005 feet per minute, and vice versa. And two-inch wg = 5664 fpm. If you wish I can give you the corresponding values at other temperatures.

Another interesting value is 69.24 ft of standard air is
equivalent to 1 in. wg.

Standard air (70F and 29.92 in. Hg) has a [ρ] = 0.075 lb_m/ft³. Good luck.

 

[BobPE]

You have the vent structure and the penetration into the tank, both cheap in my opinion, so the bigger the better. The vent is the most overlooked structure on a tank and yet, it is the most important by far. The vent itself is a difficult thing to model because there are many types of vents and they vary by manufacturer. I am usually never the advocate for pushing design work off onto anyone but the engineer, but if you work for a smaller engineering firm, have a tank manufacturer do the calcs for their vent for you to demonstrate 2 inches of water column. Depending on where the tank is located, cold weather comes into play. There are also allowances they recommend for insect screening blockage. And there are also provisions for emergency venting when the primary venting fails. This is typically by using a weighted release panel. All this leads to a difficult answer to your question, but an answer still worthy of estimating...

BobPE

 

[Shells]

Great help thank you very much.

Yes, normally I would just size the vent according to the piping and I use a Greenheck vent (which in their literature gives me the pressure convegence for their vents) I have an Owner who wants just "basically" a screen put up. So I was stuck on the pressure convergence and how much area to privide for the sceen.

Again thanks

 

[JStephen]

If this is actually a Greenheck vent, you might just contact them and ask directly. I'm sure they get many similar questions.

 

[JedClampett]

The owner might just want a screened opening, but you need to follow AWWA D100-96. Section 5.7.2 requires that screened vent openings operate even if the screen is clogged with ice or "foreign material". The tank manufacturer has details to accomplish this. It involves a hinged screen contraption that moves either due to oeverpressure or vacuum, even if clogged. It also closes automatically whenever the vacuum/overpressure is relieved.
I'm not sure a manufactured Greenheck vent will be acceptable.

 

[JStephen]

In reality, the majority of ground storage tanks do not have these "failsafe" type vents, though. I believe Greenheck does have some sort of frost-resistant vent, but not sure of the details.

The problem with the "failsafe" type vent is that the icing condition that could cause a screen to clog is apparently very very uncommon, and AWWA D100 does not specify what kind of condition the vent must operate in. This makes it impossible to show that the pallet itself won't freeze shut under the same conditions that freeze the screen over. The most common frostfree vents I've seen use annular shaped pallets which make for maximum perimeter with a minimum of activating load, which would seem to be exactly opposite of how you'd want to design the things- compare them to the Varec P/V vents, for example.

 

[BobPE]

JStephen:

I would have to disagree with you about screen freezing/blockage. It is all too comon a problem in colder climates, it is just not a very well understood problem. I observe damage from vacuum loading on just about every tank I have had the opportunity to observe. AWWA puts forward recommendations that engineers use to design things. I used to use a vent system made by PDM (now CBI) on tank retrofits, but they no longer make just the vent available for sale as I understand. I have designed something similar and now use that. The probelm with the secondary vent system is that once it operated, it usually doesn't seat well again. Since very few tank owners have regular tank inspections programmed in their O&M. Venting of ground level tanks is probably the worst offender. The large roof surface area makes the vacuum loading quite extreme.

I find that tanks are very obvious and usually taken for granted since they appear so simple on the surface. It is the details that make a good tank great.

BobPE

 

[JStephen]

Seems like the story that I have heard was that one of the two (CBI or PDM) always used the big frost-free vent, and the other had actually tried to induce freeze-up of a screen and hadn't been able to accomplish that on their standard screens (and had video demonstrating that). Now that they're one entity, I'd be curious to know their approach. Probably the big vent detail, which costs more.

I would say, too, that if you are observing vacuum damage on every roof you inspect, then evidently the fail-safe vents that are already being used must not get the job done. What kind of damage are you seeing, by the way?

 

[JedClampett]

Without getting into the merits of the fail-safe vents vs. a simple screened vent, I would advise that failure to follow the code (AWWA D100-96) is done at your own risk.
We put in the fail-safe vent design in Phoenix and Las Vegas where freezing is not a possibility because we don't want to be paying to fix/replace steel tanks. If the tank fails for any reason and you didn't follow the code, get your deposition clothes ready.

 

[BobPE]

I would agree with you JedClampett, but I would add AWWA is NOT a code, nor should anyone consider it as such. It is a set of reccomended guidelines to follow made by engineers for engineers....

Jstephen: The damage I got to see was associated with vent structures that did not fail safe design vents...just the old j vent or worse the older ball fenial. Saving a few bucks on a vent just isn't worth it...

The damage was mostly roof support defelections and sags associated with the deflections. Then follows the corrosion damage associated with the deflections and sagging. Some of the more severe damage was to the top ring where permenant deformations would allow gaps in the roof joint to develop...I guess the scariest part was that it could be 5 (or worse more than 5) years before these defects can be observed though intermittent inspections, leaving breaches and corrosion to run their course....

BobPE