Troy Valve Sewage Air/Vacuum Release Valve Application Notes

Valve sizing

1. Combination air release and vacuum valves should be sized to handle the maximum amount of air to be exhausted or admitted into the pipeline and not exceed an acceptable pressure differential across the valve discharge orifice.
2. Each high point or change in grade must be examined independently when determining valve size. Use the steepest slope for calculations.
3. Use the flow capacity charts located on Model 36 Data Sheet to assist in Valve sizing.
4. Determine the smallest valve size capable of exhausting air equal to the filling rate of the pipeline in CFS while not exceeding a pressure differential of 2 psi across the valve orifice. (Based on pump capacity.)
   
   The following formula is recommended to calculate the rate of flow in CFS for filling the pipeline:
   
   CFS = GPM / 448.83
   Where:
   CFS = Cubic feet per second
   GPM = Gallons per minute
   
   
5. Determine the smallest valve size capable of admitting air equal to the potential flow in CFS while not exceeding a pressure differential of 5 psi across the valve orifice. (Based on gravity flow.)
6. The following formula should be used to calculate the rate of flow in CFS that can occur within the pipeline under gravity flow conditions.
   
   Q = .0007872 CÖ(S D5)
   Where:
   Q = Flow of water in cubic feet per second
   C = Coefficient in Chezy's formula = 110
   S = Slope in feet per foot of length
   D = Inside pipe diameter in inches
   
   
7. If thin wall pipe is being used, the risk of pipeline collapse due to the formation of vacuum must be considered. The following formula may be used to calculate the collapsing pressure of thin walled cylindrical steel pipe using a safety factor of four:
   
   P = 16,250,000(T/D)3
   Where:
   P = Collapsing pressure in psi
   T = Thickness of pipe in inches
   D = Diameter of pipe in inches
   
   
8. For other pipe materials or thicknesses consult pipe manufacturer for pipe collapsing pressure.
9. Determine the smallest valve size capable of admitting the required air in CFS (as found in step 5) without exceeding the collapsing pressure (as found in step 6) or 5 psi, whichever is less. Do not exceed a pressure differential greater than 5 psi.
10. Finally compare the valve size determined in step 4 with the valve size determined in steps 5 or 6. If they differ, always select the larger valve size.