0405process_instrumentation_and_control.pptx

P&ID Study and Process Hazard analysis.

1. Stuart Holmes Unit: Safety Process Instrumentation and Control & Process Hazard Analysis

2. Instrumentation and Control - Objectives By the end of this lecture you should: Understand basic approaches to control Be able identify how a system is controlled from the P&ID

3. Process Instrumentation & Control It is necessary for the design engineer to understand the plant control philosophy even at the Flowsheet / Process Flow Diagram stage This lecture is an overview of control for design purposes

4. Basics of Process Control Why is Process Control important? Are we wasting time & money on it?

5. Objectives of Process Control Ensure stable process operation Particularly, keep the plant operating under safe conditions Minimize damage to equipment due to variation in plant conditions Ensure operation meets product specifications Minimize impact of external disturbances Example: change in ambient temperature Optimize process performance Maintain process throughput Minimize operating costs

6. Control Loop Components The sensing instrument detects the measured variable and sends a signal to a controller, which signals the actuator to close or open a control valve and adjust the manipulated variable (usually a flow rate) Signal flows along the instrument line – symbols show how (electrical, pneumatic, etc )

7. Control Valve

8. Control Valves The final control element is usually a control valve Exceptions: electric heaters, mixers, variable speed drives The actuator is either a motor or a bellows that opens or closes the valve in response to the signal Actuator Valve Source: Valve Manufacturer’s Association, www.vma.org

9. Valve Failure Positions It is important to specify what happens to a control valve if the signal fails The final valve position has an impact on process safety and pressure relief scenarios and may affect other instrumentation Fails open Fails locked in current position Fails closed Failure mode indeterminate

10. Instrument Line Symbols Undefined – Use for discussions and diagrams were signal type is unimportant Electrical and Electronic Pneumatic Hydraulic Fibre Optic / Acoustic

11. Types of Control Loop Feedback Control system measures changes in a process output and then adjusts manipulated variable to return output to set point Can be slow if process response time is long Feed Forward Control system measures disturbance and adjusts manipulated variable to compensate for it so that controlled output is not affected Requires greater knowledge of system response Process Manipulated variable Controlled output Disturbance Controller

12. Feedback Control Control system measures changes in a process output and then adjusts manipulated variable to return output to set point Can be slow if process response time is long

13. Feed Forward Control Control system measures disturbance and adjusts manipulated variable to compensate for it so that controlled output is not affected Requires greater knowledge of system response Process Manipulated variable Controlled output Disturbance Controller

14. Feedforward Control

15. Feedforward and Feedback Control A combination – for reliability/safety

16. Ratio Control One stream is controlled in ratio to another Often used for controlling feed rates to try to maintain stoichiometry Also used in some types of distillation column control to set reflux ratios

17. Cascade Control One (primary, or outer) controller is used to adjust the set point of a second (secondary, inner) controller Only use where secondary loop responds faster than primary Better control, but harder to set up, more to maintain, etc. Example: reactor temperature. Valve may stick, flow may be non-linear, or valve too big, but temperature changes too slowly for good valve control. TIC sets the flow setpoint . FIC controls the flow directly with the valve

18. Process Hazard Analysis

19. University of Manchester Slide 20 Why do a Process Hazard Analysis ? It is vital that the plant you build is safe and efficient to operate. It is vital that it is safe – moral, legal and financial consequences for both companies and individuals. A process throughout the design Be aware of the various methods at each stage This course concentrates on HAZOP and HAZAN BUT: These are only part of the hazard analysis process

20. Inherent Safety – 4 Principles for Design /1 Can we: Substitute? Allow fewer causes of hazards to exist Solvent: higher flash point, lower toxicity Use water for heat transfer, not oil Minimise? Reduce the scale of hazards present Change large batch reactor to small continuous reactor? Reduce inventory University of Manchester Slide 21

21. Inherent Safety – 4 Principles for Design /2 Moderate? Reduce the severity of the hazards present Move control room away Reduce temperatures Simplify? Reduce opportunities for incidents Labelling Simple pipe layout Low maintenance equipment / Reliability Start early in the design process

22. University of Manchester Slide 23 ICI 6-Stage Hazard Study Exploratory Phase Project aims, inc. safety and environment. Inherent safety – best process? As early as possible Flowsheet Phase Key hazards, control scheme. Inherent safety – inventory? As soon as flowsheet is available After Detailed Design Will design intent be met? Safety and Env objectives? As soon as P&ID available Pre-Commissioning Design intent met? Construction as designed? As soon as construction complete Post Commissioning Operating correctly? Design intent met? Immediately after commissioning Periodic Review Design intent met? Check on modifications 6-12 months after commissioning

23. Design Process Exploratory Phase; +/- 30% Costing Study Consider the material hazards in general. Consider suitable sites University of Manchester Slide 24

24. University of Manchester Slide 25 Six Different Hazard Studies Exploratory Phase

25. University of Manchester Slide 26 1. Exploratory Phase Former Courtaulds Chemicals Site in Trafford Park Planning Development Control Committee - 20th April 2006 HEALTH AND SAFETY CONSIDERATIONS 14. The Health and Safety Executive (HSE) has stated that it advises against the granting of planning permission on this site due to the risk of harm to the occupiers of the proposed development. This concern is related to the Hazardous Substances Consents that were granted in 1992 and 1994 to the previous occupiers of the site, Courtaulds Chemicals, for the storage of chemicals including carbon disulphide and hydrogen sulphide

26. University of Manchester Slide 27 1. Exploratory Phase 15. Courtaulds Chemicals no longer occupy any part of the site originally granted Hazardous Substances Consent. However, Hazardous Substances Consent runs with the land rather than being solely for the benefit of the original occupier. The HSE is therefore concerned that the Hazardous Substances Consent is still valid and that hazardous substances could, once again, be stored on the application site or adjacent land without the need for a further consent. The current application would significantly increase the number of people employed in the immediate vicinity and the HSE therefore considers that this would pose an unacceptable risk of harm to the occupiers of the proposed development.

27. University of Manchester Slide 28 1. Exploratory Phase In this case the HSE took the opportunity of a ‘change of use’ to prevent anyone else from using this site to store large quantities of hazardous chemicals. They carried out the ‘Exploratory Phase’ in advance for anyone thinking about this in the future.

28. Design Process Exploratory Phase; +/- 30% Costing Study Consider the material hazards in general. Consider suitable sites Flowsheet Phase; +/- 20% Costing Study Consider main equipment items/unit operations but not the detailed design. Both comparatively low cost studies. University of Manchester Slide 29

29. University of Manchester Slide 30 Six Different Hazard Studies Flowsheet Phase

30. Process Hazard Analysis Lots of types of systematic review: FMEA – Failure Modes and Effects What can go wrong with this item? What would be the result? HAZID Identify Hazards Particularly for major, but very rare (< 1 in 1000 years) hazards What could happen to cause <this major hazard>? Finds interactions between: parts of the plant, surroundings and plant which could cause a major accident . University of Manchester Slide 31

31. Process Hazard Analysis HAZOP I dentify hazards and operability problems <this part of the process> should do y, what other things could happen? How? action appropriate to deal with them. E.g. Pressure hazard – solution probably a relief system More detailed than HAZID, tends to find smaller and more common (1 in 100 year) hazards Normally HAZOP is primarily for safety, but most HAZOP studies identify more operating problems than hazards . Product quality Downtime Profit BS EN 61882:2016 Hazard and operability studies (HAZOP studies) - Application guide University of Manchester Slide 32

32. Process Hazard Analysis HAZAN / Fault Tree Analysis Calculating the probability of a particular hazard occurring? How likely is <hazard>? Consider e xtent of the consequences and then comparison with a target or criterion . One HAZOP had 326 recommendations of which only 7 justified a detailed hazard analysis . LOPA – Layers of Protection Defence in Depth Are the alarms, trips, reliefs, etc. in the onion sufficient /reliable enough ? What SIL is needed? University of Manchester Slide 33

33. University of Manchester Slide 34 Acknowledgement HAZOP And HAZAN by Trevor Kletz . Pub: IChemE Held in the UoM Library.