Primary/Secondary Pump Decoupler Reverse Flow — Bypass Piping Design Error

Question

Accepted Answer

Decoupler straight pipe length under 6D causes reverse flow. Check: Measure decoupler pipe length — verify ≥6× pipe diameter (6D) of straight pipe. Install temporary thermometers at both ends of decoupler → if return-side temp is 0.5°C+ higher than supply side, reverse flow confirmed What to do: Secure straight pipe ≥6D — extend pipe if existing is too short. If immediate fix not possible, add one spring check valve to decoupler → increase primary pump speed so primary flow (LPM) ≥ secondary flow Primary flow < secondary flow — excess reverses through decoupler. Check: Measure primary and secondary flow separately (ultrasonic flow meter or BMS display). If primary flow < secondary flow, reverse flow is occurring. Supply temperature rise (chilled water 7°C → 8.5°C+) suspects warm return water mixing due to reverse flow What to do: Increase primary pump speed (raise inverter frequency 3~5 Hz) → recheck flow measurement. Re-adjust secondary loop balancing valves — bring each loop flow variance within ±10%. If no flow meter, use clamp-type ultrasonic meter for temporary measurement then plan permanent installation Poor secondary loop balancing — flow variance between loops exceeds 30%. Check: Check AHU or FCU inlet/outlet temperature difference — larger ΔT variance indicates flow imbalance. If no pressure-independent control valves (PICV), determine current balancing valve opening status on each loop What to do: Sequentially adjust each loop balancing valve — use the farthest loop as reference (100% open) and proportionally adjust remaining loops. After adjustment, re-measure entire system to confirm no total flow reduction. Long-term: recommend replacing with PICV (pressure-independent control valves)

Primary/Secondary Pump Decoupler Reverse Flow — Bypass Piping Design Error

Likely causes

Decoupler straight pipe length under 6D causes reverse flow

Primary flow < secondary flow — excess reverses through decoupler

Poor secondary loop balancing — flow variance between loops exceeds 30%

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