Case study: feasibility, drawbacks, convincing

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.)

(i) Techno-commercial feasibility

It is the study of whether a proposed system is technically possible (suitable technology, components, skills, compatibility with the existing plant) and commercially worthwhile (cost, benefit, payback, risk). Example from the dairy plant: technically, a PT100 + microcontroller loop can hold 72 ± 0.5 C and all the parts are available locally; commercially, the investment of Rs 12 lakh is returned in about 10 months from savings of Rs 1.15 lakh per month (assumed), so it is feasible.

(ii) Technical drawbacks of the existing MBI system

Manual valve control with ±3 C swings; analog gauges with low accuracy and no logging; no alarms or interlocks; ageing sensors and drift; relay logic that is hard to modify; dependence on operators; no remote monitoring; frequent breakdowns and poor fault diagnosis; and no protection from power-quality problems.

(iii) Three technical solutions with fault tolerance

1. Microcontroller PID loop with redundant sensors: two PT100 sensors are compared; if they differ, raise an alarm and use a safe value (with three sensors, majority voting can be used).
2. PLC-based control with HMI: reliable industrial hardware, diagnostics and modular I/O; a hot-standby CPU is optional.
3. Distributed nodes with SCADA: each section has its own controller on RS-485, so the failure of one node does not stop the others.

Solution 1 (redundant sensors, watchdog, fail-safe valve):

 PT100-A --+                    +--> Valve (fail-closed)
 PT100-B --+--> [MCU] + WDT ----+
 Level ----+      |  ^ brown-out  +--> Alarm
                  v
              RS-485 -> SCADA

Solution 2 (PLC with optional standby CPU):

 Sensors --> [Input modules] --> [PLC CPU (PID)] --> [Output modules] --> Valve / pump
                                   |        |
                          [Standby CPU]   [HMI panel]

Solution 3 (distributed nodes):

 Reception node ----+
 Pasteuriser node --+-- RS-485 bus --> SCADA PC
 Filling node ------+

Reliability features: watchdog timer, brown-out reset, redundant sensors, fail-safe valve, UPS, optical isolation, self-test, manual bypass and spare modules.

(iv) Questions from management and how to answer

1. What is the total cost and payback? Show the figures: Rs 12 lakh, 10.4 months.
2. What if it fails? Manual bypass, watchdog, spares and vendor support.
3. How many staff are required or affected? A training plan, and redeploying workers.
4. How long will the shutdown be? Install in phases during the off-peak time.
5. What is the effect on quality and sales? Records for food-safety audits.
6. Who maintains it? Training and an AMC contract.

Convince them with a pilot, numbers (savings, risk reduction), reference installations and a clear timeline.

(v) Senior engineers

The questions are more technical: accuracy and calibration, ADC resolution and noise, control algorithm and stability, EMC and grounding, protocol and timing, firmware testing, MTBF, scalability, and standards. Answer with calculations (0.037 C per count), test results, circuit and timing diagrams, and a test plan. The management discussion, by contrast, is about money and risk.

(vi) Comparison with existing design

 Existing: sensor -> operator -> valve
 Proposed: sensor -> ADC -> MCU -> valve  (+ PC log)