A ball valve holds liquid in its body cavity, the pocket around the ball between the two seats, when it sits fully closed and, in many designs, fully open. When that trapped liquid warms, ball valve cavity pressure climbs fast: in a rigid cavity, water gains about 2 bar per °C near 10 °C and 12 to 14 bar per °C at 60 to 80 °C. When it freezes, the ice needs about 9% more room. The fixes are a defined relief path on liquid lines that warm while isolated, and draining with the ball at about 45° before cold weather.
Where the liquid gets trapped
The seats seal against the ball, so the ball, seats and body form a closed pocket. Fully closed, the bore is part of that pocket. Fully open, many designs still close the pocket off from the flow path. At a part-open position of about 45°, the pocket connects to the pipe and can drain.
ASME B16.34, the flanged, threaded and welding-end valve standard, warns that liquid trapped in the center cavity of a double-seated valve can overpressure the valve when it is heated, and it leaves prevention to the user. Small threaded and solder-end ball valves often follow MSS SP-110 instead; the physics is the same. Cavity relief is a design choice you specify, not a feature every ball valve has.
Worked example: chilled water isolated over a weekend
A ball valve on a chilled water branch is closed at 7 °C and about 3 bar(g) for a weekend shutdown, and the water in its cavity warms to the 25 °C mechanical room.
- Assume a rigid, liquid-full cavity: the mass and volume of water are fixed, so its density stays at the 7 °C value of about 1,000.05 kg/m³.
- Find the pressure at which 25 °C water has that density. From the IAPWS-95 formulation, it is about 67 bar(g).
- The rise is about 64 bar, or roughly 930 psi.
- For scale: under ASME B16.34, a Class 150 WCB industrial valve body is rated 19.6 bar (285 psig) from -29 to 38 °C, and a Class 300 WCB body about 51 bar (740 psig).
A real valve usually stops short of 64 bar because the seats deflect, leak or take a set first. That is the field damage: PTFE seats that pass when closed, a stem seal or body joint that weeps after the warm-up, and a handle or actuator that suddenly needs far more torque.
| Water temperature | Pressure rise per °C | Pressure rise per °F |
|---|---|---|
| 10 °C (50 °F) | 1.9 bar (27 psi) | 15 psi |
| 20 °C (68 °F) | 4.5 bar (66 psi) | 36 psi |
| 40 °C (104 °F) | 8.7 bar (127 psi) | 70 psi |
| 60 °C (140 °F) | 11.8 bar (171 psi) | 95 psi |
| 80 °C (176 °F) | 13.9 bar (202 psi) | 112 psi |
Trapped water in a rigid cavity at about 3 bar(g), calculated from IAPWS-95.
A rule of thumb quoted in trade articles is about 100 psi per °F. That fits hot water but overstates the rise for chilled water, because water barely expands just above 4 °C. Over the 18 °C warm-up above, the rise still totals 64 bar.
Freeze damage in Canadian winters
Water expands by about 9% in volume when it freezes, and a drained line can still hold water in a valve left closed or fully open. Ontario’s SB-1 climatic data put the January 2.5% design temperature at -18 °C for Toronto City Hall and -20 °C for Etobicoke, North York and Scarborough, so ball valves in unheated parking levels, loading docks or louvred mechanical rooms must be kept warm or left truly empty.
- Seasonal drain-down (condenser water, outdoor coils, hose lines): drain, turn each ball valve to about 45° and leave it there for the winter, as Colorado State University Extension advises for backflow assembly ball valves. Close it before refilling and open it slowly.
- Glycol loops: glycol at the right concentration stops freezing, but trapped glycol still expands when it warms.
- Sprinkler systems: water-filled piping must stay at or above 4 °C (40 °F), and colder spaces are typically protected by dry pipe, preaction or antifreeze systems. NFPA 25 has dry-system auxiliary drains operated after every system operation, before freezing weather and as needed through the winter (see the NFSA’s winter guidance).
Cavity relief options compared
| Option | How it relieves | Direction | Check on the submittal |
|---|---|---|---|
| Vented ball (upstream relief hole) | Connects the cavity to the upstream side when closed | Hole toward the pressure source; treat as one-way | Hole stated as upstream (vents when closed), flow arrow on the body, orientation on the drawings |
| Self-relieving seats, floating ball | A seat flexes and bleeds excess cavity pressure back into the line | Two-way | Seat stated as self-relieving, seat material, pressure-temperature limits |
| Single piston effect (SPE) seats, trunnion ball | Cavity pressure pushes the seat off the ball and relieves into the line | Two-way | SPE or DPE stated for each seat |
| Double piston effect (DPE) seats, trunnion ball | Seats hold against cavity pressure, so an external relief valve is needed | Set by the relief valve piping | Relief valve set pressure and discharge point |
Some balls also have a stem-slot hole that vents the cavity only while the valve is open; it is sometimes called a vent too, so ask which one the datasheet means. For LNG, see our BS 6364 cryogenic valve guide; for design trade-offs, floating vs trunnion ball valves.
Field checklist
- List ball valves that can sit closed on a liquid-full line while it warms: chilled water isolation, equipment out for service, coils, heat exchangers, process liquids.
- Write the relief arrangement into the valve schedule (upstream relief hole and direction, self-relieving seats, or DPE seats with an external relief valve) and check that the submittal datasheet states it.
- Install vented valves with the hole toward the pressure source, and mark the direction on the drawings and valve tag.
- Treat pipe isolated between two closed valves the same way: keep one side open to the expansion tank or fit a thermal relief valve.
- Before freezing weather, drain unheated sections and leave their ball valves at about 45°.
Planning a ball valve submittal?
ValveAtlas supplies HVAC flanged ball valves, brass ball valves and floating and trunnion ball valves for projects in Canada, the US, the UK and EMEA; datasheets for the industrial ranges are on our valve datasheets page. Send your valve schedule through our contact page or to info@valve-atlas.com and flag the lines that need cavity relief.
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