Quick answer: Fit a case drain valve only when water pools inside: washdown or rain ingress, a soak-drain need, or a wash routine needing an outlet.
Table of contents
- A drain valve moves liquid, not pressure
- Three ways water gets in
- Valve type and bore size
- What the extra hole costs
- FAQ
1. A drain valve moves liquid, not pressure
Latch the lid and the cavity stops exchanging air, but sealed is not dry. Wash water creeps through the lid joint, rain climbs along the latch bosses, and the day-night swing condenses vapour on the inner wall. All of it settles in the lowest corner.
A case drain valve is the outlet for that layer. It sits at the low point of the wall, opens with a turn and seals with the same turn.
The distinction matters because this part is often confused with a relief valve. One moves liquid by hand, the other moves gas pressure on its own, and a case can need both while the criteria stay separate.
Gravity does the work, so the port must sit where water collects. The bore centre stays 3 mm above the inner floor to clear sand and swarf, which leaves a 3 mm film behind; tipping the case 15° toward the port carries it out.
Volume is easy to estimate. On a 520 mm by 420 mm floor, 1 mm of standing water is roughly 0.22 L and 5 mm is 1.1 L.
2. Three ways water gets in
Washdown comes first. A 0.3 MPa lance played over the outside pushes water past the lid joint, the latch pivots and the handle bosses, and 30 seconds puts 200 mL to 400 mL inside.
Rain comes second. Under IPX5 at 12.5 L/min for 3 minutes, a case with a healthy gasket takes on almost nothing. Once the gasket has taken a permanent set, the same rain delivers over 50 mL.
Condensation comes third, and it is the route most often left out of the count.
Across a 35 °C day and a 15 °C night, vapour condenses on the inner wall and leaves 5 mL to 15 mL per night. Two weeks without opening the lid produces a film that feels wet but measures under 100 mL in a beaker.
| Use case | Water per event | Drain valve justified |
|---|---|---|
| Pressure-lance washdown | 200–400 mL | Yes |
| Open rain, IPX5 | 0–50 mL | Depends on gasket |
| Nightly condensation | 5–15 mL | Long sealed storage only |
| Short immersion | 800 mL and up | Yes, and fast |
The three routes differ by an order of magnitude, yet the decision collapses into one line: if water can sit inside and the only way out is tipping the case open, an outlet is missing.
3. Valve type and bore size
The interface has settled on one thread, M16×1.5, cut into a boss on the wall. A 2.5 mm cord O-ring seals against the boss face, and torque is specified at 1.8 N·m: below 1.2 N·m the ring never reaches 25 % compression and water weeps along the thread, while above 2.6 N·m it flattens and loses rebound.
Bore size sets the emptying rate. A φ8 mm bore clears 1.2 L in 45 s, about 1.6 L/min. Under the same head, φ6 mm needs 80 s and φ10 mm needs 28 s.
The three common types differ in how they open, not in how they seal.
| Valve type | Actuation | 1.2 L drain time | Usual bore |
|---|---|---|---|
| Screw plug | 3 turns by hand | 45 s | φ8 mm |
| Flap | Press and lift | 38 s | φ8–10 mm |
| Quick-dump | One lever throw | 25 s | φ10 mm |
The body stands 6 mm proud of the wall, which bites on site. Run the case over a roller conveyor and the boss catches a roller edge, pulling it half a metre off line; stack two cases wall to wall and the bosses hold them 6 mm apart, so the third layer rocks.
A recessed body pulls that 6 mm inside the shell, at the cost of a 24 mm flat on the inner wall for the seat.
| Bore | Matching type | 1.2 L drain time | Silt risk |
|---|---|---|---|
| φ6 mm | Miniature screw plug | 80 s | High |
| φ8 mm | Screw plug, flap | 45 s | Medium |
| φ10 mm | Flap, quick-dump | 28 s | Low |
4. What the extra hole costs
Every hole adds a sealing interface. Before the valve, one gasket around the rim held water out; after it, the thread and O-ring become a second path, sitting at the low point of the wall where pressure peaks.
Before dispatch the case goes through IP67 immersion at 1 m for 30 minutes. With the valve closed there must be no continuous bubble stream, and when it is opened afterwards no more than 0.5 mL should escape from the thread.
The O-ring is a consumable. Nitrile hardens after 12 months at 60 °C and stops sealing once compression set passes 20 %; silicone takes 200 °C but swells within 3 months of oil contact. Replace by cord diameter 2.5 mm, and never substitute thread tape.
Then there is the cost line. A valve runs 8 to 25 CNY, the mould for the boss starts at 3000 CNY, and each unit gains 20 s of tapping. A case stored indoors, holding only dry equipment and opened every 8 hours, collects under 20 mL and needs no hole. So: none of the three true, do not fit; one true, fit it, and design the boss into the first mould.
Conclusion
A drain valve is not standard equipment; it exists for the case that can hold water. If washdown, rain or condensation puts 0.8 L to 1.2 L in the base and tipping the case open will not clear it, a φ8 mm valve empties it in 45 s. If none applies, the extra hole only adds a leak path.
Related Reading
- When a pressure relief valve is mandatory
- Pressure equalisation valve versus drain valve
- Choosing a pressure valve for outdoor cases
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