TXV Sensing Bulb Gas Loss — Valve Fully Closed

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Accepted Answer

Sensing bulb charge gas fully leaked — closing spring overcomes opening force. Check: Field test: detach bulb from suction line → wrap with warm hand for 30 seconds → watch low-side gauge. Normal = immediate 2–3 bar rise; no response at all = confirmed bulb gas loss. This test gives a verdict in under 1 minute What to do: Replace TXV assembly (bulb usually not replaceable alone). Recover refrigerant → remove valve (flare nut or cut at braze) → install new valve (match tonnage, refrigerant type, temperature range) → nitrogen flow during brazing → vacuum 500 micron 30 min → verify superheat 5–8°C after restart. Always replace drier at the same time Bulb poorly mounted — detached from suction line or missing insulation. Check: Check bulb mounting — 12 o'clock on suction line for ≤7/8″ pipe, 10/2 o'clock for larger. Stainless band tight (loose = wobble). Confirm 2+ wraps of rubber/latex insulation tape fully covering. Check for direct sunlight or heat source nearby What to do: Clean suction line (sandpaper, solvent) → place bulb at 12 o'clock in firm contact → tighten stainless band → wrap with insulation and seal with high-grade rubber tape, 3 cm overhang on each end. In sunny spots, add aluminum tape over insulation. After install, recheck superheat 5–8°C Capillary tube cracked or pinhole leak. Check: Inspect entire capillary visually — look for oil stains at bends, rub points, and clamp compressions (oil stain = confirmed leak). Spray leak-detect foam and watch for bubbles, or scan closely with electronic detector. Particularly vulnerable in high-vibration machine rooms What to do: Replace TXV assembly — capillary cannot be repaired alone (no way to recharge bulb). When fitting the new valve, route capillary with slack but keep bend radius ≥25 mm; protect rub/vibration points with rubber bushings/clamps. In heavy-vibration rooms, add vibration damping pads Charge gas liquefied after prolonged overcooling — pressure can't transmit. Check: Suction gauge shows 30–50% drop from normal as TXV stays stuck closed. Occurs when bulb is located in cold liquid-pooling section (bottom of suction trap). Check operation history for prolonged min-load operation What to do: Short-term: relocate bulb to a straight horizontal section without liquid pooling, above the pipe bottom. Medium-term: replace with cross-ambient (MOP) type TXV — charge is immune to ambient bulb temperature. After relocation, verify SH 5–8°C and monitor stability through load swings Bulb improperly reinstalled during prior service. Check: Visually inspect bulb, capillary, insulation — loose band, gaps in insulation, kinked capillary. Check service logbook or labels for recent work. If performance dropped right after recent service, this is a top candidate What to do: Reinstall bulb — sand suction line to a shiny finish, seat bulb firmly at 12 (or 10/2) o'clock, clamp tight with stainless band. Cover with 2+ wraps of insulation, 3 cm overhang each end. After 30 min settle → verify SH 5–8°C; if still off, consider that the bulb itself is bad and replace TXV assembly

TXV Sensing Bulb Gas Loss — Valve Fully Closed

Likely causes

Sensing bulb charge gas fully leaked — closing spring overcomes opening force

Bulb poorly mounted — detached from suction line or missing insulation

Capillary tube cracked or pinhole leak

Charge gas liquefied after prolonged overcooling — pressure can't transmit

Bulb improperly reinstalled during prior service

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