Vent Sizing for a Sump? 3

[polyroly]

We have 3 water lines flowing intermittently into a collection sump and then one large pipe gravity draining out of the sump and into a large containment pit.

I need to size an air vent on the collection sump big enough so that it won't collapse. Any help would be great!!!

Thanks,
Nathan

 

[bimr]

The maximum possible air velocity through the vent pipe is the sonic velocity of air:

Vs = (kgRT)^0.5 = (kg144PV)^0.5

Where,
vs = Sonic velocity of air, ft/sec
k = Ratio of specific heats for air
g = acceleration of gravity, 32.2 ft/sec2
P = Absolute pressure, psia
V = Specific volume of air, ft3/lb

From Cranes Technical Paper #410

The sonic velocity has been determined to be approximately 1,129 ft/sec.

Flow input into or output from the tank is Q in cu. ft./sec. That would be the worst case scenario or a total of your 3 water lines entering or the large pipe draining. Assume that they are operating at the worst case scenario which is the maximum flow into or out of the tank.

The minimum area of your vent pipe would be Q/1,129 ft/sec = sq. ft.

The recommended pipe area is at least one pipe size larger than the minimum.

http://www.chem.mtu.edu/~crowl/cm4310/Chapter4b.pdf

If your vent pipe is more than a few feet long, then you will have to account for the pressure drop in the vent pipe. The pressure rating of the tank must be greater than the pressure drop in the vent pipe. Otherwise, increase the size of the vent pipe to decrease the pressure drop in the vent pipe.

Pressure drop in the vent pipe can be estimated from Cranes Technical Paper #410, Page B-15 flow of air though steel pipe.

 

[PEDARRIN2]

Before this gets more technical than it needs to be (using Crane), what is the expected flow into the pit and what is the size of the gravity drainage pipe and is the containment pit open to atmosphere?

In any case, the flow in cannot exceed the flow out - or you will get water up your vent pipe.

If the flow into the pit is not extreme, you can use "plumbing venting techniques" - i.e. the vent would be at least 1/2 the size of the drainage pipe. Some of the air venting from the sump would go up the vent and some would go in the upper part of the drainage pipe (if the pit is open to atmosphere).

If the flow into the pit is extreme, you might have to upsize the vent a bit.

If you think you still need to use Crane, go ahead - it will give you a good answer. Just think you might try a simpler approach if the situation warrants it.

 

[bimr]

The most significant increase in pressure occurs when filling a tank or sump. As fluid is transferred into the tank, the pressure tends to increase unless the vent system has the capability to relieve the pressure. The worst-case scenario would probably be the overfilling of the tank and the vent would have to be able to pass the entire filling fluid at the filling rate.

When liquid is withdrawn from the tank, an equal volume of outside air must enter the tank through a vent system or a partial vacuum will be created inside the tank that could result in cracking or collapse of the tank.

Vent sizing must be consider the pressure/vacuum rating for the tank and incorporate the maximum anticipated venting requirements which are dependent upon: fluid flow rates; pipe size, length and configuration; and tank pressure/vacuum rating.

The use of a larger diameter drain line, longer vent line or vent line tied into a scrubber, etc. will require an even larger size vent line size.