TL;DR
A holding tube temperature drop triggers the FDV and stops product release. Check flow rate first, then heater output and temperature sensor accuracy.
What you might see
- temperature at the holding tube exit falling below the lethal temperature setpoint
- FDV diverting product to drain automatically
- holding tube insulation damaged or wet
- flow rate exceeding the validated holding tube design
Likely causes
Flow rate above the design rate reducing holding time below the minimum
Heater section (steam or hot water) not maintaining output temperature
Temperature sensor drift causing false reading at the FDV setpoint
Heat loss from damaged holding tube insulation on cold days
Required tools
- Calibrated reference thermometer
- Steam pressure gauge
- Insulation inspection tools
Safety first
- The holding tube temperature is a regulatory critical control point under the Pasteurized Milk Ordinance (PMO) or equivalent regulation. Any unexplained temperature drop requires a product hold and regulatory notification procedure.
- Do not run product through the pasteurizer while the holding tube temperature fault is unresolved. FDV diversion protects consumers.
Procedure
- 1
Check the current flow rate against the design specification. Higher flow reduces residence time in the tube below the minimum holding time.[1]
- 2
Check the temperature at the heater section exit and at the holding tube entry. A drop between these points indicates heater insufficient output.[1]
- 3
Calibrate the holding tube exit temperature sensor against a reference thermometer. Sensor accuracy is regulatory critical on an HTST.
- 4
Inspect the holding tube insulation for damage, compression, or moisture saturation. Replace wet or damaged insulation.
- 5
Review the steam or hot water supply to the heating section. Verify supply pressure and temperature are at setpoint.
- 6
Do not adjust the FDV temperature setpoint as a workaround. The setpoint is a regulatory value from the pasteurizer permit.
Warning: The holding tube temperature setpoint is a regulatory critical control point. Changes require regulatory approval and equipment re-permitting. - 7
Document all holding tube temperature excursions and corrective actions in the pasteurizer logbook.
Sources
Tetra Pak Tetra Pak A3 / Flex Pasteurizer / HTST System general technical documentation, Tetra Pak
Tetra Pak A3 / Flex HTST pasteurizer general holding tube temperature control and regulatory procedures (general)
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