Case study: critical factors in process control

Assumed case study: a milk pasteurisation and packaging section of a dairy plant (HTST process: raw milk tank, heat exchanger, steam-heated holding tube, cooling, filling). The existing system uses analog panel indicators, bimetal thermostats, a manual steam valve and operator logging. (If your own visit was to another plant, replace the process names; the structure of the answer stays the same.)

Block diagram of the process control

Existing process and manual control:

 Raw milk tank --> pump --> heat exch. --> holding tube
   (level sw)      (motor    (steam valve    (72 C, 15 s)
                    starter)  manual)           |
                                        cooling --> filling
 Panel meters / thermostats  <-- operator reads, adjusts

Proposed temperature control loop:

 Set-point -> [Controller] -> steam valve -> process
                  ^                              |
                  +------ PT100 sensor ----------+

Critical factors affecting production observed

1. Temperature control accuracy of pasteurisation (the manual valve gives ±3 C swings).
2. Raw milk quality and receiving time: delays cause acidity and rejection.
3. Steam pressure and boiler reliability.
4. Power cuts and voltage fluctuation stop the pumps and fillers.
5. Manual operation and human error, and poor record keeping.
6. Equipment maintenance and sensor calibration, and cleaning (CIP) time.
7. Packaging machine downtime.

Measures suggested

• Automatic closed-loop temperature control with a PT100, a microcontroller and a proportional steam valve.
• Level and flow sensors with interlocks, alarms and a diversion valve.
• UPS/stabiliser and generator backup; proper grounding.
• Data logger/SCADA for batch records; a preventive maintenance schedule and calibration.
• Operator training and standard procedures.

Key figures (calculated): a PT100 amplifier scaled 0-150 C to 0-5 V and a 12-bit ADC give a resolution of 150/4096 = 0.037 C per count (1.22 mV per count at 5 V), enough for the ±0.5 C pasteurisation limit. An 8-byte Modbus-style frame at 9600 baud takes 8 × 10 bits / 9600 = 8.3 ms, so every point can be updated several times per second.

Advantages of the suggested system

Accurate, stable quality; savings of steam and labour; fewer rejections; records and traceability; fast fault diagnosis; and a payback of about 10 months (assumed figures below).

Cost-benefit (assumed figures): investment Rs 12,00,000 (controller panel, sensors, valves, SCADA PC, installation, training). Monthly savings: manpower Rs 60,000, steam/energy Rs 35,000, reduced rejection Rs 20,000 = Rs 1,15,000. Payback = 12,00,000 / 1,15,000 ≈ 10.4 months; yearly return = 13.8 lakh on 12 lakh, about 115 percent.

Disadvantages

Initial cost; the need for skilled maintenance; sensitivity to noise and power quality; software faults; the need for training; and dependence on the supplier for spares.