Why air gets in (and out) of force mains
Three sources put air into a pressurized water/wastewater pipeline:
1. Dissolved air coming out of solution as pressure drops along the pipe profile. 2. Suction-side air entrained at the pump intake (poor submergence, vortex, false air-lift). 3. Initial-fill air trapped during pipe commissioning or after maintenance shutdowns.
Air collects at high points along the profile (gravity wants water to settle, air to rise). At any fluid velocity below ~3 ft/s, air pockets stay put — they don't carry along with the flow. Above 3 ft/s, air *might* be swept along, but design shouldn't rely on that.
The presence of air in a force main causes:
- Reduced effective pipe diameter — air pocket displaces water, friction loss climbs, pump duty point shifts left
- Increased surge potential — air slugs trapped during pump trip create transient hammer events at the slug boundaries
- Corrosion at the air-water interface — internal pipe surface alternates wet-dry, accelerating loss
- Failed metering — flow meters mis-read air-laden flow
The fix: air valves at every high point.
The three types of air valves
1. Air-vacuum valves (large orifice)
Open when pressure drops below atmospheric. Admit large volumes of air to prevent pipe collapse during draining or pump trip. Close when water level reaches the float.
- Orifice typically 2-12 inches diameter (large)
- Function: prevent vacuum collapse during draining
- Don't release small air pockets at full operating pressure
- Standard at pipe high points + at low points where vacuum can occur during draindown
2. Air-release valves (small orifice)
Release accumulated air pockets at full operating pressure. Have a small orifice (<1/2 inch) that releases small amounts of air continuously.
- Function: bleed off accumulated dissolved-air pockets
- Don't admit large volumes during draining
- Standard at pipe high points where small pockets accumulate
3. Combination air valves (both functions)
Combine the large air-vacuum orifice + small air-release orifice in a single body.
- Function: both
- Slightly more expensive than either alone, but cheaper than two separate valves + fittings
- Standard at most major high points in modern design
For 90% of new installations, spec combination air valves at high points and air-vacuum valves at draindown low points.
Where to install them
Per AWWA M51 *Air Valves: Air Release, Air-Vacuum, and Combination,* and 10-States Standards §43:
Air valves required at:
1. Every high point along the profile (any local maximum) 2. Long ascending runs (>2,500 ft) — even without a clear high point, install one every 1,500-2,500 ft 3. Long descending runs — install combination valves to handle drain-down 4. Just downstream of every pump — protects against startup transients 5. At the upstream end of any horizontal run >1,000 ft that crosses a saddle profile
Air-vacuum (large) valves at:
1. High points where the upstream profile descends rapidly — protects against column-separation collapse 2. Long horizontal runs (>1 mile) at the midpoint 3. Just upstream of any pump (vacuum protection during pump trip) 4. Near the discharge of any large lift station
Avoid air-valve placement:
- In wet wells / sumps (debris fouls them)
- Below grade in flooded vaults (won't work — air pressure can't escape)
- Within 10 pipe diameters of fittings (turbulent flow makes them unreliable)
- In pressure zones above the valve's pressure rating (causes seat failure)
Sizing them
Air-vacuum orifice sizing per AWWA M51, simplified for water/wastewater service:
Air-vacuum orifice diameter (inches) = 0.45 × √(pipe diameter in inches)For a 12-inch force main:
d_orifice = 0.45 × √12 = 1.56 inches → spec 2-inch valveLarger pipes, larger valves. AWWA M51 has tables for fast lookup.
Air-release orifice is smaller and less critical — most manufacturers spec a single small orifice (3/16" - 3/8") that handles any reasonable air-release rate.
The vault discussion
Air valves require a vault for access. The vault must:
- Drain freely (air valves discharge water occasionally; a flooded vault is a dead air valve)
- Have positive ventilation (to release water and gases)
- Allow service access (lid, ladder, lighting)
- Not flood from groundwater (perimeter drainage + waterproof construction)
A common failure mode is a vault that's intentionally drainless to keep groundwater out — and over time, the air valve dribbles + the vault floods + the valve quits working. The fix: separate air-valve discharge piping (carry to a curb-side outlet) instead of relying on vault drainage.
Maintenance — the part nobody does
Air valves need annual inspection:
- Float free? (Test by lifting cover and pressing down on the float)
- Orifice clear? (Inspect for sediment, mineral buildup, biofilm)
- Seat sealed? (Listen for hissing in service)
- Discharge piping clear? (Verify drainage path)
A force main with 12 air valves needs maybe 1-2 hours per year of inspection. Skipping it means the air valves fail in service and the engineer learns about it from a corroded pipe or a water-hammer event.
Vacuum-relief on long descending lines
A pipeline that descends rapidly from a pump can experience column separation when the pump trips. The fluid coast continues briefly; pressure on the pump-side of the descent drops to vapor pressure; the column separates.
The standard mitigation: large air-vacuum valves at the top of the descent. Air rushes in, prevents column separation, and air-cushions the eventual hydraulic re-collision.
Without these valves, the only mitigation is more dramatic: surge tanks, pressurized air vessels, or rapid-closing check valves — all 10-100× more expensive than air valves.
A common error: spec'ing the wrong valve type
A force main goes into operation with air-release-only (small orifice) valves at every high point. During the first pump trip, vacuum collapses 200 ft of 12-inch HDPE pipe between two air-release valves — because no air-vacuum valve was installed. The small orifice can't admit air fast enough to prevent collapse.
Replacement and repair: $300,000.
The fix would have been: replace each air-release valve with a combination valve at the original installation. Cost difference at install: $1,500.
How to think about it
Air valves are insurance. The premium is small (a few thousand dollars per high point), the deductible is huge (column collapse, surge damage, cathodic corrosion). For any force main with significant length, elevation change, or pump capacity:
1. Design the pipe profile. 2. Identify every high point (local max in elevation). 3. Place a combination air valve at each. 4. Identify long flat or descending runs. 5. Add air-vacuum valves for vacuum protection. 6. Spec a vault that drains. 7. Plan annual inspection.
How the calculator handles it
The Headloss Calculator's pipe-profile panel doesn't currently flag missing air valves — that's a layout decision the engineer makes during the routing phase. But the system curve calculation correctly accounts for the friction headloss including K-factors for air valves (typically K ≈ 0.5 for full-open service), so once you've placed them in your model, the headloss is right.
References
- AWWA. *Manual M51 — Air Valves: Air Release, Air-Vacuum, and Combination Air Valves.*
- AWWA. *Manual M11 — Steel Pipe: A Guide for Design and Installation* (chapters on air valves and column separation).
- 10-States Standards (Recommended Standards for Wastewater Facilities) — §43 Force Mains.
- Wylie, E. B., and Streeter, V. L. *Fluid Transients in Systems* — column separation chapters.