Dairy Establishment Inspection Manual – Chapter 17 - Higher Heat Shorter Time (H.H.S.T.) Processing and Extended Shelf-Life (ESL) Dairy Products
1.17.12 Pressure Differential Recorder Controllers (P.D.C.-recorder)

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Proper pressure relationships must exist across all media to prevent contamination of the pasteurized product by raw product, heating medium and cooling medium. Pressure relationships under the following conditions must be considered:

  1. Forward flow
  2. Divert flow
  3. Shutdown

In H.H.S.T. processing systems, failure to maintain a 14 kPa (2 psi) differential between the raw side of the regenerator and the pasteurized side will cause the FDD to assume the divert position. The feed pump operates in both forward flow and diverted flow, as long as the F.C.D. is allowed to run. This is because the pasteurized side of the regenerator and cooling section are always full, since the FDD is located after the cooling section. H.H.S.T. systems require a pressure differential recorder to monitor and record pressures to ensure that proper pressure differential has been maintained in the regenerator.

This task will assess the actual pressure devices used. The appropriate pressure differential is assessed under Regeneration ( and Cooling Section (

Tests are performed upon installation and at least once every 6 months thereafter. Appropriate records must be kept to show proper testing has occurred. General Conditions

The sensors of pressure differential controllers must be clean and in good mechanical condition. The design should allow easy dismantling of the sensors for inspection, and the indicating / recording unit must be housed in an appropriate moisture proof enclosure.

The pressure differential recorder controller shall be inter-wired with FDD such that divert occurs when the pasteurized product pressure in the regenerator is not exceeding, by 14 kPa (2 psi) or greater, the pressure on the raw side of the regenerator. It is considered acceptable to use a legal PLC to control the pressure differential in lieu of a pressure differential controller as long as the same control conditions are respected such as interwiring with FDD, pressure indicating and recording capabilities, set-point indication.

In milk-to-heat transfer medium-to-milk type regenerators, in the pasteurized milk section the heat transfer medium must be under lower pressure by at least 14 kPa (2 psi) than the pasteurized product side at all times.

Pressure gauges may be used to verify the pressure display for the pressure differential recorder controller. Gauges shall be clean and in good condition. Location

Two types of regeneration are used in H.H.S.T. systems, product-to-product regenerators, and product-to-heat transfer medium-to-product regeneration systems. The latter system is often preferred for some products, because it allows more even heat transfer and prevents burn-on.

Product-to-product regenerators shall have the raw product sensor between the feed pump and the raw product inlet to the regenerator. The pasteurized product sensor shall be installed at, or downstream from, the pasteurized product outlet of the regenerator.

Product-to-heat transfer medium-to-product regenerators shall have the raw side pressure sensor in the water loop after the water pump (location of highest media pressure in the loop). The pasteurized side pressure sensor shall be in the product line at the pasteurized side outlet of the regenerator (location of lowest pasteurized product pressure). Specifications

A circular chart shall make one revolution in not more than 12 hours and shall be graduated for a maximum record of 12 hours. Two charts shall be used if operations extend beyond.

The chart positive drive mechanism shall be equipped with a system to prevent slippage or manual rotation (e.g. pin to puncture the chart paper). The chart used shall correspond with the chart number displayed on the identification plate of the pressure differential recorder controller.

The pressure recording unit shall have chart scale divisions not to exceed 14 kPa (2 psi) on the working scale of not more than 140 kPa (20 psi) per 25 mm (1 inch). Pens may be used to record the raw side pressure and the pasteurized side pressure or the pressure differential.

Electronic data collection, storage and reporting of pressure differentials, with or without hard copy printouts, may be acceptable provided the electronically generated records are readily available at the establishment for review by the regulatory agency and meet minimum criteria required for S.T.L.R. charts. Calibration / Records

The accuracy of the pressure display and recorder, and the differential controller divert function, shall be validated at least every six months, and whenever the controller is adjusted or repaired. Pressure gauges, if used, must be checked for accuracy at least once per year.

Records shall be easily available, and must indicate testing results and satisfactory corrective action. Tests must be completed according to the required methods. Sealed

The pressure differential recorder controller/legal panel adjustments must be sealed.

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