Dairy Establishment Inspection Manual – Chapter 19 Appendices
Appendix 4 Meter Based Timing System

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The magnetic flow meter is used to measure the flow rate in HTST, HHST, and Aseptic Processing and Packaging systems. It is essentially a short piece of tubing (approximately 10 inches (25.4 cm) long) surrounded by a housing, inside of which are located coils which generate a magnetic field. Two stainless steel electrodes about a quarter inch in diameter protrude approximately one-inch through the teflon lined tube and provide the contact points for the fluid (a conductor) passing through the magnetic fluid. These electrodes pick up a signal from the moving fluid and activate a transmitter. The signal is sent to other instrumentation where it is recorded and acted upon.

Industries have found magnetic flow meters advantageous over other types of flow control devices since they:

  1. Provide no obstruction to the fluid
  2. Contain no moving parts
  3. Remain unaffected by changes in conductivity, viscosity, density or temperature
  4. Produce negligible pressure loss
  5. Do not require purging
  6. May be installed at nearly any point along a pipeline

Magnetic flow meters do nothing but measure flow. Other components regulate the flow.

Only those meter based timing meter components that have been reviewed and found acceptable by the FDA's Milk Safety Branch and Food Engineering Branch or are fit for purpose may be used as a replacement for a flow control device on critical process systems (HTST, HHST, APPS). Systems not reviewed and found acceptable by FDA's Milk Safety Branch and Food Engineering Branch will be assessed on a case by case basis.

To be considered satisfactory as a flow control device within a critical process system the Meter Based Timing System must consist of the following components:

  1. Centrifugal or positive displacement timing pump
  2. Magnetic flow meter
  3. Product check valve or normally closed air operated valve
  4. Flow recorder with event pen
  5. High flow alarm
  6. Low flow/loss of signal alarm
  7. Flow control (with operator interfaced flow controller): (a) control valve or (b) AC variable frequency drive

Operation, Installation and Design Requirements Of A Meter Based Timing System

AC Variable frequency drive system

The operation, installation and design requirements of a Meter Based Timing System with an AC variable frequency drive system shall comply with the following:

  1. The systems shall have a suitable flow recorder capable of recording flow at the high flow alarm set point and also at least five (5) gallons (19 litres) per minute higher than the high flow alarm setting. The flow recorder shall have an event or divert pen which shall record the duration of the alarm condition.
  2. A high flow alarm with an adjustable set point shall be installed within the system which will automatically cause the flow diversion device to be moved to the divert position whenever excessive flow rate causes the product holding time to be less than the legal holding time for the pasteurization process being used. Such adjustment shall be tested and sealed.
  3. A low flow or loss of signal alarm shall be installed with the system which will automatically cause the flow diversion device to be moved to the divert position whenever there is a loss of signal from the meter or the flow rate is below 5% of the maximum signal alarm set point. See Figure 1 Logic Diagram.
  4. When the legal flow rate has been re-established following an excessive flow rate, a time delay must be instituted which will prevent the flow diversion device from assuming the forward flow position until a least a 15 second (milk) (HTST) or 25 second (frozen dessert mix) (HTST) or 1 second (APPS and HHST), continuous legal flow has been re-established. The time delay must be tested and if it is of the adjustable type it shall be sealed.
  5. A sanitary product check valve or normally closed air operated valve shall be installed in the system to prevent positive pressure in the raw milk side of the regenerator whenever a power failure or shut down occurs. The check valve or normally closed air operated valve must be placed between the magnetic flow meter and the start of the holding tube.
  6. The placement of the individual components in the system shall comply with the following conditions:
    1. The AC variable frequency controlled centrifugal or positive displacement pump shall be located downstream from the raw milk regenerator section if a regenerator is used.
    2. The magnetic flow meter shall be placed downstream from the AC variable frequency centrifugal or positive displacement pump. There shall be no intervening components in the system other than normal sanitary piping with no valves or control devices between them.
    3. Both the AC variable frequency centrifugal or positive displacement pump and the magnetic flow meter shall be located upstream from the holding tube.
    4. All other flow promoting devices such as booster pumps, stuffing pumps, separators, clarifiers and homogenizers, as well as the AC variable frequency centrifugal or positive displacement pump, shall be properly inter-wired with the flow diversion device so that they may run and produce flow through the system only when the flow diversion device is in the fully diverted or fully forward flow position when in product run mode. Separators or clarifiers which continue to run after power is shut off to them must be automatically valved out of the system with fail-safe valves so that they are incapable of producing flow during this period and when the flow diversion device is in the diverted flow position. In the case of aseptic processing systems utilizing indirect or direct heating systems, the product divert device will remain in the divert position when the flow rate is higher that the specified value in the scheduled process.
    5. There shall be no product entering or leaving the system (i.e. cream or skim milk from a separator or other product components) between the AC variable frequency centrifugal or positive displacement pump and the flow diversion device.
    6. The magnetic flow meter shall be so installed that the product has contact with both electrodes at all times when there is flow through the system. This is most easily accomplished by mounting the flow tube of the magnetic flow meter in a vertical position with the direction of flow from the bottom to the top. However, horizontal mounting is acceptable when other precautions are taken to assure that both electrodes are in contact with product. They should not be mounted on a high horizontal line which may be only partially full and thereby trap air.
    7. The magnetic flow meter shall be piped in such a manner that at least 10 pipe diameters of straight pipe exists upstream and downstream from the centre of the meter before any elbow or change of direction takes place.
  7. When a regenerator is used with these systems, it probably will be necessary to bypass the regenerator during start-up and when the flow diversion device is in the diverted flow position. Care should be taken in the design of such bypass systems to assure that a dead-end does not exist. A dead-end could allow product to remain at ambient temperature for long periods of time and allow bacterial growth in the product. Caution should also be observed with such bypass systems and any valves used in them so that raw milk product will not be trapped under pressure in the raw regenerator plates and not have free drainage back to the constant level tank when shutdown occurs.
  8. Most such systems will utilize a dual stem flow diversion device and will be using the AC variable frequency centrifugal pump during the CIP cleaning cycle. All controls required of such systems must be applicable. When switching to the CIP position, the flow diversion device must move to the divert position and must remain in the diverted flow position for at least 10 minutes of the CIP cycle regardless of temperature and the booster pump cannot run during the first 10 minutes of the CIP cycle. Once the CIP cycle has been activated the operator has 10 minutes to switch back to "process mode".
  9. When public health computers or programmable logic controllers are used with these systems, they must be installed in such a manner that no public health controls are under the computer or programmable logic controller during the product run operations except that the computer or programmable logic controller may control the speed of the AC variable frequency centrifugal or positive displacement pump provided the high flow alarm is set and sealed to provide for diversion of the flow diversion device whenever the design flow rate is exceeded.
  10. All required controls shall be tested at the recommended frequency. Where adjustment or changes can be made to these devices or controls, appropriate seals shall be applied so that changes cannot be made without detection.
  11. Upon initial installation and at the recommended frequency, all Meter Based Timing Systems shall be tested for holding times. Tests shall be performed in forward flow (above legal temperature) and diverted flow (below legal temperature). At least six (6) consecutive results shall be within a half (0.5) second range of each other. If six (6) consecutive times within a half (0.5) second range cannot be obtained in forward and diverted flow, the designer of the installation should be contacted to correct the problem before using the system. All timing tests shall be performed in accordance with the procedures outlined in CFIA's "Test Procedures For Critical Processes Equipment and Controls" manual. The system must be designed and installed so that the timing tests can be conducted in automatic mode in both forward and diverted flow. Automatic mode means that the flow through the system is under control of the magnetic flow meter, and the system controls will automatically vary the speed of the centrifugal pump to maintain a constant flow rate through the system. In automatic mode, the set point of the automatically controlled flow rate must be manually adjustable.

Control valve system

The operation, installation and placement requirements for a Meter Based Timing System utilizing, a single speed centrifugal or positive displacement pump and control valve shall comply with the following:

  1. The systems shall have a suitable flow recorder capable of recording flow at the high flow alarm set point and also at least five (5) gallons (19 litres) per minute higher than the high flow alarm setting. The flow recorder shall have an event or divert pen which shall record the duration of the alarm condition.
  2. A high flow alarm with an adjustable set point shall be installed within the system which will automatically cause the flow diversion device to be moved to the divert position whenever excessive flow rate causes the product holding time to be less than the legal holding time for the pasteurization process being used. Such adjustment shall be tested and sealed.
  3. A low flow or loss of signal alarm shall be installed with the system which will automatically cause the flow diversion device to be moved to the divert position whenever there is a loss of signal from the meter or the flow rate is below 5% of the maximum signal alarm set point. See Figure 1 Logic Diagram.
  4. When the legal flow rate has been re-established following an excessive flow rate, a time delay must be instituted which will prevent the flow diversion device from assuming the forward flow position until a least a 15 second (milk) (HTST) or 25 second (frozen dessert mix) (HTST) or 1 second (APPS and HHST), continuous legal flow has been re-established. The time delay must be tested and if it is of the adjustable type it shall be sealed.
  5. A sanitary product check valve or normally closed air operated valve shall be installed in the system to prevent positive pressure in the raw milk side of the regenerator whenever a power failure or shut down occurs. The check valve or normally closed air operated valve must be placed between the magnetic flow meter and the start of the holding tube.
  6. The placement of the individual components in the system shall comply with the following conditions:
    1. The centrifugal or positive displacement pump shall be located downstream from the raw milk section if a regenerator is used.
    2. The magnetic flow meter shall be placed downstream from the centrifugal or positive displacement pump. There shall be no intervening components in the system other than normal sanitary piping with no valves or control devices between them.
    3. The control valve must be placed downstream from the magnetic flow meter and upstream from the start of the holding tube.
    4. The centrifugal or positive displacement pump, the magnetic flow meter and the control valve shall be located upstream from the holding tube.
    5. All other flow promoting devices such as booster pumps, stuffer pumps, separators, clarifiers and homogenizers, as well as the centrifugal or positive displacement pump, shall be properly inter-wired with the flow diversion device so that they may run and produce flow through the system only when the flow diversion device is in the fully diverted or safe forward position when in product run mode. Separators or clarifiers which continue to run after power is shut off to them must be automatically valved out of the system with fail-safe valves so that they are incapable of producing flow during this period and when the flow diversion device is in the diverted flow position. In the case of aseptic processing systems utilizing indirect or direct heating systems, the product divert device will remain in the divert position when the flow rate is higher that the specified value in the scheduled process.
    6. There shall be no product entering or leaving the system (i.e. cream or skim from a separator or other product components) between the centrifugal or positive displacement pump and the flow diversion device.
    7. The magnetic flow meter shall be so installed that the product has contact with both electrodes at all times when there is flow through the system. This is most easily accomplished by mounting the flow tube of the magnetic flow meter in a vertical position with the direction of flow from the bottom to the top. However, horizontal mounting is acceptable when other precautions are taken to assure that both electrodes are in contact with product. They should not be mounted on a high horizontal line which may be only partially full and thereby trap air.
    8. The magnetic flow meter shall be piped in such a manner that at least 10 pipe diameters of straight pipe exists upstream and downstream from the centre of the meter before any elbow or change of direction takes place.
  7. When a regenerator is used with these systems, it probably will be necessary to bypass the regenerator during start-up and when the flow diversion device is in the diverted flow position. Care should be taken in the design of such bypass systems to assure that a dead-end does not exist. A dead-end could allow product to remain at ambient temperature for long periods of time and allow bacterial growth in the product. Caution should also be observed with such bypass systems and any valves used in them so that raw milk product will not be trapped under pressure in the raw regenerator plates and not have free drainage back to the constant level tank when shutdown occurs.
  8. Most such systems will utilize a dual stem flow diversion device and will be using the AC variable frequency centrifugal pump during the C.I.P. cleaning cycle. All controls required of such systems must be applicable. When switching to the C.I.P. position, the flow diversion device must move to the divert position and must remain in the diverted flow position for at least 10 minutes of the C.I.P. cycle regardless of temperature and the booster pump cannot run during the first 10 minutes of the C.I.P. cycle. Once the C.I.P. cycle has been activated the operator has 10 minutes to switch back to "process mode".
  9. When public health computers or programmable logic controllers are used with these systems, they must be installed in such a manner that no public health controls are under the computer or programmable logic controller during the product run operations except that the computer or programmable logic controller may control the speed of the AC variable frequency centrifugal or positive displacement pump provided the high flow alarm is set and sealed to provide for diversion of the flow diversion device whenever the design flow rate is exceeded.
  10. All required controls shall be tested at the recommended frequency. Where adjustment or changes can be made to these devices or controls, appropriate seals shall be applied so that changes cannot be made without detection.
  11. Upon initial installation and at the recommended frequency, all Meter Based Timing Systems shall be tested for holding times. Tests shall be performed in forward flow (above legal temperature) and diverted flow (below legal temperature). At least six (6) consecutive results shall be within a half (0.5) second range of each other. If six (6) consecutive times within a half (0.5) second range cannot be obtained in forward and diverted flow, the designer of the installation should be contacted to correct the problem before using the system. All timing tests shall be performed in accordance with the procedures outlined in CFIA's "Test Procedures For Critical Processes Equipment and Controls" manual. The system must be designed and installed so that the timing tests can be conducted in automatic mode in both forward and diverted flow. Automatic mode means that the flow through the system is under control of the magnetic flow meter, and the system controls will automatically vary the speed of the centrifugal pump to maintain a constant flow rate through the system. In automatic mode, the set point of the automatically controlled flow rate must be manually adjustable.

This image shows an HTST System with Magnetic Flow Meter Using an AC Variable Speed Centrifugal Pump

  • Starting with raw milk coming in through the Raw In and into the Constant Level Tank, the material goes into the Booster Pump, past the Booster Pressure Differential Switch, through the Regenerator for initial warming, and through the Separator Feed Valve. It passes through the Separator which separates the milk into skim milk and raw cream. The raw cream exits through the raw cream outlet. The skim milk continues and goes through the Skim Back Pressure Valve, the Separator By-Pass Valve, and into the AC Drive Centrifugal Timing Pump. It then passes through the Magnetic Flow Meter. The Magnetic Flow Meter measures the flow rate for display and recording on the Flow Rate Recorder which is connected to the Flow Alarm by means of a Pneumatic Transducer.
  • After passing the flow meter, the milk flows through the Check Valve and into the homogenizer for homogenization. Milk then moves to the Heater where it is heated before entering the Holding Tube. After holding, it flows past the indicating and recording thermometers, and into the Flow Diversion Device. If the Safety Thermal Limit Recorder (STLR) records a satisfactory holding temperature, the flow will continue to the regenerator for pre-cooling, and through the cooler for final cooling. Pasteurized product goes past the vacuum breaker out through the pasteurized out line, or back through the recycle line to the constant level tank. If the milk does not meet time and temperature requirements, the flow diversion device sends it back to the constant level tank for recirculation.
  • Flow rates and temperatures are recorded for process control records.

Click on image for larger view
Image - High temperature, short-time system with magnetic flow meter using an alternating current variable speed centrifugal pump. Description follows.

Description of image - High temperature, short-time system with magnetic flow meter using an alternating current variable speed centrifugal pump

This image shows a High temperature, short-time system with magnetic flow meter using an alternating current variable speed centrifugal pump.

  • The Raw milk starts at the Raw in and goes into the Constant Level Tank. From here material passes through the Booster Pump, passes the Booster Pressure Differential Switch and through the Regenerator. It flows through the Separator Feed Valve, and then goes into the clean in-place-Type Separation.
  • From here it can go one of two ways. The first way is past the Cream Return Valve and it comes out through the Raw Cream out.
  • The second route is to pass the Skim Back Pressure Valve, the Separator By-Pass Valve, and to go through the alternating current Drive Centrifugul Timing Pump. It then flows through the Magnetic Flow Meter, which consists of the Flow Rate Recorder, the Flow Alarm and the Pneumatic Transducer. The material then passes through the Check Valve, the Homogenizer, the Recirculation Line and into the Heater. It passes through the Flow Transmitter and through the Holding Tube, passing the Recorder Controller.
  • From the Flow Diversion Device, the material can go one of two ways. It either goes through the Regenerator, the Cooler and then it either goes out the Pasteurized Out, or down the Recycle Line to the Constant Level Tank.
  • The second route is for the material to flow through the Divert Line, and into the Constant Level Tank.

References

  • Grade "A" Pasteurized Milk Ordinance 1993
  • U.S. Department of Health and Human Services
  • Public Health Service, Food and Drug Administration

This image shows an HTST System with Magnetic Flow Meter Using a Constant Speed Centrifugal Pump and a Control Valve

  • Starting with raw milk coming in through the Raw In and into the Constant Level Tank, the material goes into the Booster Pump, past the Booster Pressure Differential Switch, through the Regenerator for initial warming, and through the Separator Feed Pump and through the separator feed valve to the separator. The Separator separates the milk into skim milk and raw cream. The raw cream exits through the raw cream outlet. The skim milk continues and goes through the Skim Back Pressure Valve, the Separator By-Pass Valve, and into the Centrifugal Timing Pump. It then passes through the Magnetic Flow Meter. The Magnetic Flow Meter measures the flow rate for display and recording on the Flow Rate Recorder which is connected to the Flow Alarm by means of a Pneumatic Transducer.
  • After passing the flow meter, the milk flows through the Flow Control Valve, to the Heater where it is heated before entering the Holding Tube. After holding, it flows past the indicating and recording thermometers, and into the Flow Diversion Device. If the Safety Thermal Limit Recorder (STLR) records a satisfactory holding temperature, the flow will continue to the regenerator and past the vacuum breaker out through the pasteurized out line, or back through the recycle line to the constant level tank. If the milk does not meet temperature requirements, the flow diversion device sends it back to the constant level tank for recirculation.
  • Flow rates and temperatures are recorded for process control records.

Click on image for larger view
Image - High temperature, short-time system with magnetic flow meter using a constant speed centrifugal pump and a control valve. Description follows.

Description of image - High temperature, short-time system with magnetic flow meter using a constant speed centrifugal pump and a control valve

This image shows an high temperature, short-time System with Magnetic Flow Meter Using a Constant Speed Centrifugal Pump and a Control Valve.

  • Staring with raw milk coming in through the Raw In and into the Constant Level Tank, the material goes into the Booster Pump, past the Booster Pressure Differential Switch, through the Regenerator, and through the Separator Feed Pump. It passes through clean in-place-Type Separation and then goes one of two ways. One way is to go through the Cream Return Valve and to exit through the Raw Cream out.
  • The second route is for the material to go through the Skim Back Pressure Valve, the Separator By-Pass Valve, and the Centrifugal Timing Pump. It then goes through the Magnetic Flow Meter, which consists of a Pneumatic Transducer, a Flow Rate Recorder, and a Flow Alarm. From there, it flows through the Flow Control Valve, through the Heather, the Flow Transmitter and the Holding Tube. It then flows past the Safety Thermal Limit Recorder, through the Flow Diversion Device and from here it can go one of two ways. It can either flow through the Regenerator. From here, material either goes through the Vacuum Breaker, or through the Back Pressure Controller and Valve and out the Pasteurized Out. Material can also flow through the Recycle Line to the Constant Level Tank.
  • The other route from the Flow Diversion Device is for material to flow through the Divert Line and into the Constant Level Tank.

References

  • Grade "A" Pasteurized Milk Ordinance 1993
  • U.S. Department of Health and Human Services
  • Public Health Service, Food and Drug Administration
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