Sealed Immersion Tank Overpressure — Relief Valve Trip

Cut hash load immediately. Power off every miner submerged in the tank from the PDU — not the dashboard. Letting the tank cool with miners off lets the fluid contract and pressure drop, which buys you time to engineer the right fix instead of cracking a fitting on a hot pressurised system. Do not touch the tank lid, sight-glass, or any fitting until fluid temperature is within `5 °C` of room ambient. Pressurised hot dielectric ejects with force and burns.

Document the tank exterior, lid, gasket seam, sight-glass, and any visible overpressure protection (relief valve, expansion tank, vent line) with photographs and notes. Record pressure-gauge readings if a gauge is present at hot, warm, and cold states. Photograph any manufacturer labels, code stamps, or CRN registration plates. Warranty, insurance, and engineering review all want this evidence before you start changing the system.

Inventory the existing overpressure protection. Walk the tank lid and adjacent plumbing manifold and list every fitting: relief valve (manufacturer, set pressure, flow rating), expansion tank (volume, pre-charge), vent line (size, route, terminus), pressure gauge (range, location, last calibration). Most failed-overpressure cases have none of these. The inventory tells you what's missing; what's missing is the design gap to close.

After the tank has cooled to ambient, crack the highest bleed point on the cold tank to relieve any residual pressure to atmosphere through a clean catch container. Confirm the gauge drops to `0 PSI` before continuing further work. Trapped pressure can persist after fluid contraction if a check valve, plugged vent, or chemistry-driven gas generation is active.

Inspect the lid gasket for extrusion, splitting, compression set, or hardening. Replace with the OEM gasket spec or a Viton / fluorocarbon equivalent rated for your specific dielectric and operating-temperature range. Mineral oil generally tolerates nitrile (Buna-N); hydrocarbon and synthetic esters typically need Viton, EPDM, or PTFE — verify against the fluid manufacturer's compatibility matrix before installation.

Sight-glass inspection — any crazing, cracking, radial fractures, or hazing of borosilicate glass means immediate replacement before re-pressurising or refilling. Sight-glass burst under pressure is a personal-injury hazard. Source the replacement from the original tank manufacturer or a borosilicate-grade industrial sight-glass supplier rated for the fluid temperature and the planned operating pressure (with margin).

Bleed all high points of the loop after refilling. Trapped gas pockets at unions, elbows, and instrument tees act as hidden compressible volumes that can mask developing pressure problems and cavitate the circulation pump. Open every high-point bleed valve in sequence with the pump running at low speed until steady fluid runs without bubbles or gas bursts.

Compute the thermal-expansion volumetric demand for the tank. Formula: fluid volume (L) × coefficient of thermal expansion (≈ `8 × 10⁻⁴` per `°C` for most hydrocarbon dielectrics — verify against your fluid sheet) × maximum operating temperature swing (cold-soak ambient to hot-load steady state, typically `25-35 °C` for indoor builds, `40-50 °C` for outdoor / unconditioned-room builds). The result in litres sizes both your expansion tank and your relief valve flow rating.

Bench-test any existing relief valve before assuming it works. Isolate the valve, plumb to a hand-pumped hydrotest rig with a calibrated pressure gauge, slowly raise pressure, and record the cracking pressure (first pass of fluid through the valve) and full-flow pressure. Healthy valves crack within `±10 %` of the spec sheet number. Drift, sticking, or leaking-by below set pressure means valve replacement. Boiler-grade relief valves are typically `5-10 year` service-life parts on clean dielectric; older than that and unserviced, replace on principle.

Install a Watts 530C, Bell & Gossett SuperFlo, or Wilkins XL boiler-grade calibrated pressure relief valve set at `0.5-2 PSI` for atmospheric-build pleb tanks. Pipe the discharge to a clean catch container outside the equipment room or vent to atmosphere through a debris screen. The relief valve is a safety device, not an operating-pressure regulator — it should never lift in normal service. Set pressure must be at least `20 %` below the lowest-rated component on the tank (usually the sight-glass or lid gasket).

Install a diaphragm-type expansion tank (Amtrol Extrol, Wessels, Watts equivalent) sized at `1.25 ×` your computed thermal-expansion volume from Step 8. Pre-charge the air side per manufacturer instructions before connecting fluid. Plumb the fluid side to the immersion loop with isolation valves and mount the expansion tank above the immersion tank fluid level so it self-bleeds and self-orients. The expansion tank absorbs thermal swing without lifting the relief — relief valve as backstop, expansion tank as working capacity.

Add a calibrated `0-15 PSI` glycerin-filled pressure gauge on the tank with a needle-valve isolation. Range: `0-15 PSI` for atmospheric builds, `0-30 PSI` for engineered pressure vessels with code stamps. Log gauge readings during commissioning across at least one cold-to-hot full thermal cycle to validate the expansion-tank sizing math and to baseline future drift detection. Replace gauges every `3-5 years` or when calibration drift exceeds `5 %` of full scale.

Specify and install a vent line of adequate diameter — minimum `1/2"` NPT for tanks `< 500 L`, `3/4"` NPT for larger systems — terminating in a clean catch vessel outside the equipment room. Slope the vent line continuously downward from the tank to prevent fluid traps. Protect the vessel terminus from intrusion with a debris screen or hardware cloth. The vent provides atmospheric equilibration during normal operation; the relief valve is the backstop for fault conditions.

Hydrotest the completed relief path before re-introducing miners or hash load. Plug the tank, pressurise via hand pump on the fluid side, verify the relief cracks at set pressure within `±10 %`, verify the expansion tank absorbs the rated volume without lifting the relief, verify the vent flows freely to its terminus. Document with a pressure-vs-time chart. Save the hydrotest record with the tank build file — this is the boiler-room paperwork your insurance carrier will want to see.

Cycle the system through three controlled thermal events (full cold-soak to full hash-load and back) under monitoring before declaring the relief path validated. Real-world thermal cycling reveals undersized expansion tanks, sticking relief valves, and trapped-gas pockets that bench testing alone misses. Log pressure, top-of-tank temperature, bottom-of-tank temperature, fluid level in the reservoir, and any audible events at every cycle.

Mark the maximum cold-fill fluid line on the tank exterior and on the sight-glass with permanent etching, painted bands, or sight-glass tape. The line is the maximum cold-fill level that the expansion-tank sizing math was anchored to. Operators will top up by eye and exceed it without thinking; train against the behaviour, mark the line in three colours, and make it impossible to add fluid past it without active effort. Most lifted-lid events trace to overfill.

Stop DIY and book D-Central immersion engineering review when: the tank is a DIY conversion and you need code-compliant retrofit spec, two or more overpressure events have occurred in 12 months, the fluid is a two-phase or pressurised chemistry and you do not hold pressure-design certification, the tank exceeds `500 L` and provincial registration thresholds apply, or hashboard damage from a prior fluid-ejection event requires multi-board bench diagnostics. We carry hydrotest equipment and the engineering rigour to specify a code-conforming retrofit.

Document for code compliance on commercial-scale tanks. CSA B51 (Canada) and ASME Section VIII (U.S.) require: vessel material certification, weld procedure qualification (if welded), pressure-vessel registration with the provincial authority for vessels above threshold size, an annual or biennial inspection regime, and a written inspection log. For Canadian tanks above provincial thresholds (Alberta ABSA, Ontario TSSA, Quebec RBQ all publish their own thresholds), CRN registration is mandatory. Skipping this on commercial-scale tanks is an enforcement and insurance risk.

Specify a pressure-relief P&ID with redundancy on production tanks: primary relief valve sized for thermal expansion, secondary rupture disc as the hard backstop for catastrophic failure, dedicated atmospheric vent for normal-operation equilibration, and remote-readable pressure transducer wired to the PDU shutoff for operator notification before the relief lifts. The PDU shutoff means a controller-level alarm drops hash load before the mechanical relief has to fire — relief stays a safety device, not a daily-driver.

Establish a written relief-path inspection schedule and calendar it: monthly visual check of the relief valve, expansion tank, gauge, and vent terminus; quarterly pressure-cycle log review against expected thermal envelope; annual relief-valve bench retest or replacement; biennial vessel inspection per the relevant code (CSA B51 / ASME). Train every operator with tank access on the cold-tank-only rule: no fitting, gauge, or sight-glass is touched while the tank is at hash-load temperature, no exception. Document training. The pressure does not announce itself before it ejects.