9.7 Valve Types and Duties, Air Release and Air-Vacuum Valves, Water Hammer & Surge Control
Key Takeaways
- A gate valve is an isolation device only, because holding it partly open wire-draws the seat until the valve will no longer seal.
- Air release valves vent small accumulations under pressure while air-vacuum valves exhaust air during filling and admit air during draining, and most high points need a combination valve.
- Air accumulating at unvented high points progressively reduces effective flow area and is a classic cause of a force main gradually losing capacity.
- The Joukowsky equation gives surge head as wave speed times velocity change divided by 32.2, so a 4 ft/s change at 3,600 ft/s wave speed produces about 447 feet of head.
- Check valve slam on reverse flow often produces a larger transient than the pump stop itself, which is why non-slam, spring-assisted, or dual-disc checks are used on high-head lines.
Valves do more than open and close
WPI lists valves (backflow, control valve, isolating, throttling, pressure regulation) among water treatment equipment, mains and related equipment including pressure regulating valves and air relief valves among distribution components, and pressure relief valves, air relief, and valves among collection lift station devices. Choosing the wrong valve for a duty, or operating the right valve badly, causes most of the surge damage in a water system.
Valve types and their proper duty
| Valve | Construction | Correct duty | Do not use for |
|---|---|---|---|
| Gate | Wedge or resilient seat rising across the flow | Isolation — fully open or fully closed | Throttling; a partly open gate wire-draws the seat |
| Butterfly | Disc rotating in the flow stream | Isolation and coarse throttling; low headloss when open | High differential throttling; the disc obstructs the bore |
| Globe | Plug seating against a port, flow turns twice | Throttling and control | Applications needing low headloss |
| Ball | Rotating bored ball | Quick isolation; some throttling with a characterized ball | High-cycle throttling in dirty service |
| Plug | Tapered or cylindrical plug | Isolation and throttling in sludge and grit service | Fine control |
| Check | Opens with forward flow, closes on reverse | Prevent backflow | Isolation; a check valve is not a shutoff |
| Pressure reducing (PRV) | Pilot-controlled diaphragm | Hold a constant downstream pressure | Isolation |
| Pressure sustaining / relief | Pilot-controlled diaphragm | Hold a minimum upstream pressure, or relieve surge | Flow control |
| Altitude | Pilot senses tank level | Stops flow when a tank is full | Throttling |
| Air release / air-vacuum / combination | Float-operated | Release accumulated air, admit air on drain | Anything else |
A gate valve is not a throttling valve. Partially open, the flow accelerates past the wedge edge and erodes the seat, and the valve eventually will not seal. When operators need throttling in a distribution system, the answer is a control valve, a butterfly valve within its rating, or a variable frequency drive on the pump.
Air valves, and why they matter more than they look
Three distinct devices are commonly confused:
- Air release valve. Small orifice, releases small accumulations of air during normal operation while the line is pressurized. Installed at high points on force mains and transmission mains.
- Air-vacuum (air and vacuum) valve. Large orifice. Exhausts large volumes of air while a line fills and admits air while a line drains, preventing vacuum collapse. Closes once liquid reaches it and cannot release air under pressure.
- Combination air valve. Both functions in one body. This is what most high points actually need.
Two failure consequences appear repeatedly on exams and in real damage reports. Without air release at high points, accumulated air progressively reduces the effective flow area and a force main or transmission main gradually loses capacity — the "it used to keep up" problem. Without air-vacuum capability, draining a line pulls a vacuum that can collapse a pipe or pull the gasket out of a joint, and rapid filling compresses trapped air into a violent surge.
Air valves in wastewater service foul with grease and solids and require a routine cleaning schedule; a plugged air valve on a force main is effectively no air valve.
Water hammer
Water hammer is a pressure transient created when the velocity of a moving column of water changes suddenly. The physics is worth carrying: stopping a moving mass instantly converts its momentum into pressure. The Joukowsky equation gives the pressure rise for an instantaneous stop:
ΔH = (a x ΔV) / g
where a is the pressure wave speed (roughly 3,000 to 4,500 ft/s in typical water pipe, lower in flexible plastic), ΔV is the velocity change in ft/s, and g is 32.2 ft/s².
Worked example. A pump discharge line carries water at 5 ft/s. A valve closes rapidly and the wave speed is 3,500 ft/s.
- ΔH = (3,500 x 5) ÷ 32.2 = 17,500 ÷ 32.2 = 543 feet of head
- In psi: 543 ÷ 2.31 = 235 psi above the operating pressure
That is why a fast-closing valve on a 100 psi system can burst a pipe rated for 200 psi. The critical time for a pipeline is 2L/a, twice the length divided by wave speed; a closure faster than that produces the full Joukowsky rise.
Causes and controls
| Cause | Control |
|---|---|
| Rapid valve closure (including a slammed hydrant) | Close valves slowly; operator training is the cheapest control |
| Pump start against an empty or partially empty line | Fill slowly; use pump control valves that open and close on a timed ramp |
| Power failure stopping pumps instantly | Surge tanks, hydropneumatic surge vessels, surge anticipator valves, flywheels, soft-stop VFDs |
| Column separation at high points | Combination air valves properly sized and maintained |
| Check valve slam on reverse flow | Slow-closing, spring-assisted, or damped check valves rather than swing checks on high-head lines |
| Rapid demand change (fire flow start or stop) | Zone design, pressure control |
Check valve slam deserves its own note: when a pump stops, flow reverses before a plain swing check closes, and the disc slams onto the seat against reverse velocity. The resulting transient is often larger than the one from the pump stopping. Non-slam, spring-assisted, dual-disc, or tilting-disc checks close before appreciable reverse velocity develops.
Valve exercising and records
Every valve in a distribution or collection system should be operated on a program — commonly annually for critical valves and on a multi-year cycle for the remainder. Records capture valve ID, location, size, type, direction to open, total turns, turns to close, condition, and any leakage. Two practical points:
- Record the number of turns. A valve that used to take 26 turns and now closes in 18 has a broken stem or a dropped wedge.
- Return the valve to its correct position and verify it. The most common cause of a mysterious low-pressure or water-quality complaint is a valve left partly or fully closed after a repair, or a zone boundary valve left open.
A pump discharge line carries water at 4 ft/s when a valve closes rapidly. With a pressure wave speed of 3,600 ft/s, what is the approximate pressure surge in feet of head?
Which valve type is appropriate for isolation only and will be damaged if used for throttling?
A force main has gradually lost capacity over several years with no change in pump condition or wet well operation. What should be investigated first?