Vent openings for irrigation reservoir 1

[bnard]

We have this 7,350 cu.m underground reservoir with a height of 8.80m wherein 6.0m are underground and 3.8m is above ground. The layout is somewhat trapezoidal in shape and a two chamber system is proposed for considering the maintenance. Is there a guidelines in locating and designing the vent opening seize for this type of reservoir? Any advise would be very helpful. Thanks

 

[gerhardl]

bnard,

I do not know if there exists any (or local in your country) particular design guidelines. You could however have a look at this parallell to the consideration of a pipeline with air pockets when filling (air out), and air intake by bursting pipeline (to avoid vacuum collapse).

In brief: vent size must be large enough to accomodate sufficient air escaping when filling (air amount out equals water amount in) and vice versa: air amount in equals water out.

Forgetting the fine details, you could calculate by setting air speed at a 'reasonable rate' to determine cross sections, for instance range 30-40m/s.

You should also consider use of air venting valves, and in anyway of course protection against insects and animals and other potential pollution, even if it is only for irrigation.

 

[BarryEng]

I agree with gerhardl, in sizing vents for either inflow (pressurising the tank) or outflow (collapsing the tank). This would probably be the reason for a vent on a 7,000 m3 tank.

If the tank is for hydrocarbons, I think that API 650 (American Petroleum Institute standard for tanks) has a requirement for venting (to keep loss of condensate to a minimum).

It depends on what you want the vents for. Are you after general parameters such as ventilation, or is there a specific condition that you are trying to reduce?

I have been associated with trying to develop a method of design for vents for much larger reservoirs (300,000 m3), but the basic parameters could be similar. If the reason for the vents are water quality (ensuring that the water is not contaminated by bacterial growth in high temperatures), the parameter that we have adopted for large reservoirs, are vents with a throat area of 0.2% of the roof area. This is an empirical value based on what has been acceptable (& appears to work) in past years. Large reservoirs like this one, subjected to an average of 8 hours of sun exposure per day, can produce a large amount of water that condenses onto the underside of the roof, purlins etc. On a cold morning with no wind, it almost 'rains' under the roof.

I have contacted various HAV (heating, airconditioning & ventilation) organisations for a method of design, but if you do not have a specific thermal load, thermal gradient & a specific air flow requirement, it is very difficult to design a system without the use of mechanical ventilators.

One recent project was to ventilate a building which houses ore tipping equipment. Trains of 2 km long with 25,000 tonne of iron ore, exposed to a shade temperature of over 45deg C, create unworkable conditions for staff. It was virtually impossible to design a ventilation system without the use of forced ventilation. But the HAV people said that they could design this, because they now had a thermal load that had to be dissipated in a specific period.

When we settled on 0.2% of roof area, the HAV people told us that an approximate intake area should be about double the vent area. This resulted in a large area of screens that were installed in the (2m high) walls around the reservoir. Now we had another water quality problem - dust could be blown in thru the wall vents. This region is an industrial area & there could be contaminants in the soils around the site.

For a 7,000 m3 tank, 'gerhardl' may have the answer.

I would be interested in any guidance & experience in the design of vents for potable water storages.

BarryEng

 

[cvg]

30 - 40 m/s seems a bit high to me. Unless I am mistaken, that corresponds to a wind speed through the vent of between 60 - 90 mph - hurricane force wind.

 

[JStephen]

For atmospherica potable water reservoirs, the vents will typically be about the same size as the largest inlet or outlet.

Venting flow rates can be approximated by using duct flow data in the ASHRAE handbooks, or by checking vent flow rates on commercially available "free vents" produced by Varec, Shand & Jurs, and others. Basing vent sizes on these calculated rates will often lead to very small vents.