Interview Questions

Top 30 Refinery Instrumentation Interview Questions and Answers for Maintenance

  • Securing a position as an Instrumentation Maintenance position in a refinery requires a thorough understanding of both theoretical concepts and practical applications. 
  • Refinery operations depend heavily on precise instrumentation and control systems to ensure safety, efficiency, and reliability. 
  • To help you prepare for your interview, we have compiled a list of 30 unique questions along with detailed answers. 
  • This guide aims to cover a broad range of topics, from basic principles to advanced problem-solving scenarios, providing you with a comprehensive resource for your preparation.

Various control strategies used in refinery process control include:

  • PID Control: Uses proportional, integral, and derivative actions to maintain process variables at desired setpoints. Commonly used for simple control loops.
  • Feedforward Control: Anticipates process disturbances and adjusts control actions before they affect the process. Used for processes with measurable disturbances.
  • Cascade Control: Employs a secondary control loop to enhance the performance of the primary loop. Useful in temperature and level control.
  • Ratio Control: Maintains a specific ratio between two process variables. Used in blending and mixing applications.
  • Model Predictive Control (MPC): Uses dynamic models of the process to predict future behavior and optimize control actions. Applied in multivariable and complex process control scenarios.

Implementing and tuning a PID controller involves:

  • Identifying Process Dynamics: Analyze the process response to changes in input.
  • Selecting Tuning Methods: Choose methods like Ziegler-Nichols, Cohen-Coon, or software tools for initial parameter settings.

Click here for Excel based PID Loop Simulator

  • Fine-Tuning: Adjust PID parameters (proportional, integral, derivative) through trial and error or using software tools to achieve a balance between responsiveness and stability.

Click here for PID controller tuning

Common flow meters include:

  • Differential Pressure (Orifice, Venturi, Annubar): Measure flow based on pressure drop. Used for gas and liquid flow measurement.
  • Positive Displacement type: Measure flow by capturing fluid in compartments. Suitable for high-viscosity fluids.
  • Turbine type: Measure flow by the rotational speed of a turbine. Used for clean liquids and gases.
  • Ultrasonic type: Use sound waves to measure flow. Suitable for large pipes and clean fluids.
  • Coriolis type: Measure mass flow based on the Coriolis effect. Ideal for liquid hydrocarbons due to high accuracy and reliability.
  • DCS: Used for large-scale, continuous processes. Offers extensive data handling and integration capabilities, better for plant-wide control and advanced strategies.
  • PLC: Suited for discrete control and smaller applications. Less scalable than DCS but excellent for high-speed, repetitive tasks.

Click here for PLC Learning

Advantages of Foundation Fieldbus include:

  • Greater Data Transmission: Digital communication allows for more information exchange.
  • Device Interoperability: Supports multiple devices on a single pair of wires.
  • Diagnostics: Provides detailed device diagnostics.
  • Reduced Wiring Costs: Fewer cables are needed.

Gas Chromatographs (GC) analyze the composition of process streams, providing real-time data for:

  • Process Optimization: Adjusting processes for efficiency.
  • Quality Control: Ensuring product specifications.
  • Process Safety: Monitoring hazardous substances.
  • Function: Safely shut down processes during hazardous conditions.
  • Integration: Connected with DCS and SIS, receives inputs from sensors, and executes safety actions. Regular testing and maintenance ensure reliability.
  • Sensor Selection: Use appropriate sensors like thermocouples or RTDs.
  • Installation: Proper sensor placement and protection (e.g., thermowells).
  • Calibration: Regular calibration, signal conditioning, and compensation for accurate readings.
  • Principle: Measures mass flow by detecting the Coriolis force in a vibrating tube.
  • Calibration: Compare output with known standard, adjust settings, and verify accuracy across the flow range.

Click here for Mass Flow Controller Calibration Procedure

Challenges and solutions include:

  • Signal Interference: Use robust protocols (e.g., ISA100, WirelessHART).
  • Power Supply: Ensure reliable power sources.
  • Security Concerns: Secure data transmission.
  • Network Reliability: Proper network planning and redundancy.
  • SIL: Measure of safety system performance, indicated by the probability of failure on demand (PFD).
  • Determination: Through risk assessment considering failure consequences, event frequency, and required risk reduction.

Redundancy can be implemented by:

  • Multiple Sensors: Dual or triple redundancy.
  • Redundant Control Systems: DCS with hot standby.
  • Redundant Power Supplies: Ensuring component failure does not compromise the system.
  • Principle: Uses microwave radar signals to measure the distance to the surface of a liquid or solid.
  • Application: Non-contact level measurement in storage tanks and process vessels, offering high accuracy and reliability.

Considerations include:

  • Type of Control: On/off or modulating.
  • Power Source: Electric, pneumatic, hydraulic.
  • Required Torque and Speed: Based on the application.
  • Operating Environment: Conditions like temperature and pressure.
  • Compatibility: With the control system.
  • Principle: Controls motor speed by varying electrical supply frequency.
  • Advantages: Energy savings, precise speed control, reduced mechanical stress, and improved process control.
  • Sensor Selection: Use appropriate sensors (e.g., glass electrodes, solid-state sensors).
  • Calibration and Maintenance: Regular calibration and maintenance.
  • Material Compatibility: Sensors should be compatible with chemicals and temperature.
  • HAZOP (Hazard and Operability Study): Identifies and evaluates potential hazards.
  • Conducting: Multidisciplinary team reviews process diagrams, identifies deviations, and analyzes consequences and safeguards.
  • Signal Conditioning: Manipulates sensor output for processing by control systems (amplification, filtering, isolation, conversion).
  • Importance: Ensures accurate, reliable, and noise-free signal transmission.
  • Benefits: High sensitivity, immunity to electromagnetic interference, operability in harsh environments.
  • Challenges: Requires specialized equipment and expertise, higher initial costs.
  • Principle: Converts electrical control signal into a pneumatic signal to control valve actuator position.
  • Role: Ensures precise valve positioning by comparing control signal with actual position and adjusting accordingly.
  • FSA: Evaluates design, installation, and operation of safety systems.
  • Process: Reviewing documentation, inspecting equipment, testing safety functions, and verifying compliance with standards (e.g., IEC 61511).
  • Thermocouples: Measure temperature based on the thermoelectric effect. Suitable for high temperatures.
  • RTDs (Resistance Temperature Detectors): Measure temperature through resistance change of metal. Offer higher accuracy and stability at lower temperatures.
  • APC: Uses sophisticated algorithms and models to optimize process performance.
  • Benefits: Improved product quality, increased throughput, reduced energy consumption, enhanced process stability and safety.

Click here to know more about Advanced Process Control (APC)

  • Managing Drift: Regular calibration, using high-quality sensors, automatic recalibration systems, and software corrections based on historical data.
  • Principle: Uses resonance frequency of a vibrating fork to detect liquid or solid presence. Frequency change triggers a switch.
  • Application: Point level detection in tanks and vessels.

Considerations include:

  • Reliable Communication Methods: Wireless, fiber optic.
  • Data Security: Ensuring secure data transmission.
  • Sensor Coverage: Adequate coverage for accurate monitoring.
  • Integration: With existing control systems.
  • Real-Time Processing: Ensuring timely data processing and alarm management.
  • Hysteresis: Difference in valve position when approaching a setpoint from different directions.
  • Effect: Can cause control inaccuracy and instability. Minimize through proper valve selection, maintenance, and using high-quality positioners.

Click here for What are the main causes of control valve hunting?

  • Diagnostics: Use diagnostic software to check performance, identify issues like stiction or wear.
  • Maintenance: Regular calibration, cleaning, and replacing worn components.

Advantages include:

  • Enhanced Diagnostics: More detailed and accurate diagnostics.
  • Remote Configuration and Monitoring: Easier maintenance and troubleshooting.
  • Improved Accuracy and Reliability: Better measurement performance.
  • Reduced Maintenance Costs: Due to advanced features.
  • Integration: Better integration with control systems for advanced process control.
  • Principle: Measures flow rate based on Faraday’s law of electromagnetic induction. Conductive fluid generates voltage proportional to flow velocity in a magnetic field.
  • Application: Measuring flow of conductive liquids like water, slurries, and chemicals

Click here for Electromagnetic Flowmeter Troubleshooting: Identifying and Resolving Issues

Click here for More Instrumentation interview Questions and Answers

Sundareswaran Iyalunaidu

With over 24 years of dedicated experience, I am a seasoned professional specializing in the commissioning, maintenance, and installation of Electrical, Instrumentation and Control systems. My expertise extends across a spectrum of industries, including Power stations, Oil and Gas, Aluminium, Utilities, Steel and Continuous process industries. Tweet me @sundareshinfohe

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