Expansion Tank, Relief Valve, and Glycol Diagnosis on a Closed Loop
A pressurized closed-loop glycol system serves an 80-gallon solar storage tank. The collector array holds 9 gallons, the piping holds 6 gallons, and a 4.4-gallon bladder expansion tank is precharged to 12 psi on the pump suction. Cold static pressure is 12 psi at 60°F. On a sunny morning the loop pressure climbs to 28 psi and the 30-psi relief valve weeps. The glycol is 30% propylene glycol by volume. The collector sensor reads 210°F, the tank sensor reads 130°F, and the pump is running. The expansion tank is 3 ft below the top of the collector loop and connected with a 1/2-inch line. The homeowner reports the relief valve has wept three times this month. A check valve sits between the expansion tank connection and the collector return. The design low for the site is 10°F. The pump is a fixed-speed 1/16 hp circulator.
1. Diagnose why the relief valve is weeping and identify the most likely expansion-tank fault. What tests would you run?
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The pressure rise from 12 to 28 psi is close to the 30-psi relief setting, so the weeping is a symptom. The most likely expansion-tank fault is a failed bladder or a waterlogged tank, but the check valve is also suspicious. The tank is connected at the pump suction, yet a check valve sits between the tank connection and the collector return. If that check valve isolates the tank from the collector loop when the pump is off, thermal expansion in the collectors cannot be absorbed; pressure spikes and the relief valve opens. Test sequence: isolate the expansion tank, drain it, and check precharge at the Schrader valve; if water comes out, the bladder has failed. Verify the check valve location and orientation; the expansion tank must connect to the loop without a check valve between it and the heated water. Check the relief valve opening pressure and replace it if it weeps below 30 psi. Also verify the system cold fill pressure and that the tank precharge is set to the cold fill pressure.
2. Calculate whether the 4.4-gallon expansion tank has enough acceptance volume for this loop, and state the correction.
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Loop volume = 9 + 6 = 15 gal. For 30% propylene glycol from 60°F to 210°F, use an expansion factor of about 5% (0.05): expansion = 15 × 0.05 = 0.75 gal. Acceptance volume needed: V_a = V_s × (v2/v1 − 1) ÷ [1 − (P_i/P_max)]. Using P_i = 12 + 14.7 = 26.7 psia and P_max = 30 + 14.7 = 44.7 psia, the denominator is 1 − 0.597 = 0.403. V_a = 0.75 ÷ 0.403 ≈ 1.86 gal. A 4.4-gal bladder tank usually has about 2.5 gal acceptance, so it is adequately sized if the bladder is intact and the tank is properly connected. Therefore the size is not the main problem; the check valve or a failed bladder is. Correction: remove or relocate the check valve so the tank sees the loop, replace the tank if waterlogged, set precharge to 12 psi, and confirm the relief valve is piped to a safe drain.
3. Evaluate the glycol concentration and stagnation risk. What would you check and what would you recommend?
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30% propylene glycol is marginal for a 10°F design low; its freeze point is near 8°F, leaving little margin, and burst protection is not the same as freeze protection. Use a refractometer to verify concentration; for 10°F design, 40–50% is safer. Check pH and inhibitor condition; glycol degrades faster at stagnation temperatures above 300°F. The collector at 210°F with the pump running is not stagnation, but a power failure or pump stop on a sunny day can push the absorber much higher. Recommend a stagnation strategy: drainback, a heat-dump loop, or a controller function that runs the pump to dissipate heat. Confirm the relief valve discharge is piped to a safe location and the expansion tank is rated for the glycol. Label the glycol type and concentration at the tank.