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1.6 Layers of Protection in Process Plant Dr. AA.

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Presentation on theme: "1.6 Layers of Protection in Process Plant Dr. AA."— Presentation transcript:

1 1.6 Layers of Protection in Process Plant Dr. AA

2 2 ALARMS SIS RELIEF CONTAINMENT EMERGENCY RESPONSE BPCS Strength in Reserve BPCS - Basic process control Alarms - draw attention SIS - Safety interlock system to stop/start equipment Relief - Prevent excessive pressure Containment - Prevent materials from reaching, workers, community or environment Emergency Response - evacuation, fire fighting, health care, etc. AUTOMATIONAUTOMATION Layers of Protection for High Reliability

3 3 Seriousness of event Four independent protection layers (IPL) In automation Key Concept in process Safety: REDUNDANCY

4 4 1. Safety 2. Environmental Protection 3. Equipment Protection 4. Smooth Operation & Production Rate 5. Product Quality 6. Profit 7. Monitoring & Diagnosis We are emphasizing these topics Objectives of Process Control

5 5 First line of defense Process control maintains variables at set points, which are fixed at some desired values Technology - Multiple PIDs, cascade, feedforward, etc. Guidelines Always control unstable variables (Examples in flash?) Always control “quick” safety related variables Stable variables that tend to change quickly (Examples?) Monitor variables that change very slowly Corrosion, erosion, build up of materials Provide safe response to critical instrumentation failures - But, we use instrumentation in the BPCS? Basic Process Control System (BPCS)

6 6 Where could we use BPCS in the flash process?

7 7 The level is unstable; it must be controlled. The pressure will change quickly and affect safety; it must be controlled. F1

8 8 Alarm has an anunciator and visual indication - No action is automated! - require analysis by a person - A plant operator must decide. Digital computer stores a record of recent alarms Alarms should catch sensor failures - But, sensors are used to measure variables for alarm checking? 2. Alarm System

9 9 Common error is to design too many alarms - Easy to include; simple (perhaps, incorrect) fix to prevent repeat of safety incident - One plant had 17 alarms/h - operator acted on only 8% Establish and observe clear priority ranking - HIGH = Hazard to people or equip., action required - MEDIUM = Loss of RM, close monitoring required LOW - LOW = investigate when time available 2. Alarm System

10 10 Where could we use alarm in the Flash Process ?

11 11

12 12 Automatic action usually stops part of plant operation to achieve safe conditions - Can divert flow to containment or disposal - Can stop potentially hazardous process, e.g., combustion Capacity of the alternative process must be for “worst case” SIS prevents “unusual” situations - We must be able to start up and shut down - Very fast “blips” might not be significant 3. Safety Interlock System

13 13 Also called emergency shutdown system (ESS) SIS should respond properly to instrumentation failures - But, instrumentation is required for SIS? Extreme corrective action is required and automated - More aggressive than process control (BPCS) Alarm to operator when an SIS takes action 3. Safety Interlock System

14 14 The automation strategy is usually simple, for example, If L123 < L123 min ; then, reduce fuel to zero steam water LC PC fuel How do we automate this SIS when PC is adjusting the valve? 3. Safety Interlock System

15 15 If L123 < L123 min ; then, reduce fuel to zero steam water LC PC fuel LS ss fc 15 psig LS = level switch, note that separate sensor is used s = solenoid valve (open/closed) fc = fail closed Extra valve with tight shutoff

16 16 The automation strategy may involve several variables, any one of which could activate the SIS If L123 < L123 min ; or If T105 > T105 max ……. then, reduce fuel to zero SIS 100 L123 T105 ….. s Shown as “box” in drawing with details elsewhere 3. Interlock System3. Interlock System

17 17 The SIS saves us from hazards, but can shutdown the plant for false reasons, e.g., instrument failure. 1 out of 1 must indicate failure T100 s 2 out of 3 must indicate failure T100 T101 T102 Same variable, multiple sensors! s False shutdown Failure on demand 5 x x Better performance, more expensive 3. Safety Interlock System3. Safety Interlock System

18 18 We desire independent protection layers, without common- cause failures - Separate systems sensors SIS system i/o …………. sensors Digital control system i/o …………. BPCS and Alarms SIS and Alarms associated with SIS 3. Safety Interlock System3. Safety Interlock System

19 19 These layers require electrical power, computing, communication, etc. KEY CONCEPT IN PROCESS SAFETY - REDUNDANCY! What do we do if a major incident occurs that causes loss of power or communication a computer failure (hardware or software) Could these all fail due to a common fault?

20 20 Entirely self-contained, no external power required The action is automatic - does not require a person Usually, goal is to achieve reasonable pressure - Prevent high (over-) pressure - Prevent low (under-) pressure The capacity should be for the “worst case” scenario 4. Safety Relief System

21 21 Increase in pressure can lead to rupture of vessel or pipe and release of toxic or flammable material - Also, we must protect against unexpected vacuum! Naturally, best to prevent the pressure increase - large disturbances, equipment failure, human error, power failure,... Relief systems provide an exit path for fluid Benefits: safety, environmental protection, equipment protection, reduced insurance, compliance with governmental code RELIEF SYSTEMS IN PROCESS PLANTS

22 22 Identify potential for damage due to high (or low) pressure (HAZOP Study) In general, closed volume with ANY potential for pressure increase - may have exit path that should not be closed but could be - hand valve, control valve (even fail open), blockage of line Remember, this is the last resort, when all other safety systems have not been adequate and a fast response is required! Location of Relief System

23 23 BASIC PRINCIPLE: No external power required - self actuating - pressure of process provides needed force! VALVES - close when pressure returns to acceptable value - Relief Valve - liquid systems - Safety Valve - gas and vapor systems including steam - Safety Relief Valve - liquid and/or vapor systems Pressure of protected system can exceed the set pressure. Standard Relief Method: Valves

24 24 BASIC PRINCIPLE: No external power required - self acting RUPTURE DISKS OR BURST DIAPHRAGMS - must be replaced after opening. Standard Relief Method: Rupture Disk

25 25 Two types of designs determine influence of pressure immediately after the valve - Conventional Valve -pressure after the valve affects the valve lift and opening - Balanced Valve - pressure after the valve does not affect the valve lift and opening ConventionalBalanced Relief Valves

26 26 ADVANTAGES - simple, low cost and many commercial designs available - regain normal process operation rapidly because the valve closes when pressure decreases below set value DISADVANTAGES - can leak after once being open (O-ring reduces) - not for very high pressures (20,000 psi) - if oversized, can lead to damage and failure (do not be too conservative; the very large valve is not the safest!) Some Information about Relief Valves

27 27 ADVANTAGES - no leakage until the burst - rapid release of potentially large volumes - high pressure applications - corrosion leads to failure, which is safe - materials can be slurries, viscous, and sticky DISADVANTAGES - must shutdown the process to replace - greater loss of material through relief - poorer accuracy of relief pressure the valve Rupture Disk/Burst Diaphragm

28 28 Spring-loaded safety relief valve Process To effluent handling Rupture disc Process To effluent handling Symbols used in P&I D

29 29 Add Relief to the Following System

30 30 The drum can be isolated with the control valves; pressure relief is required. We would like to recover without shutdown; we select a relief valve. Add Relief to the Following System

31 31 Positive displacement pump Add Relief to the Following System

32 32 Positive displacement pump The positive displacement pump will be damaged if the flow is stopped; we need to provide relief. We would like to recover without shutdown; we select a relief valve. Add Relief to the Following System

33 33 Why are all those valves in the process? Add Relief to the Following System

34 34 The extra “hand”`valves enable us to isolate and remove the heat exchanger without stopping the process. The shell side of the heat exchanger can be isolated; we need to provide relief. We would like to recover without shutdown; we select a relief valve. Add Relief to the Following System

35 35 Why is the pressure indicator provided? Is it local or remotely displayed? Why? What is the advantage of two in series? Why not have two relief valves (diaphragms) in series? In some cases, relief and diaphragm are used in series – WHY?

36 36 Why is the pressure indicator provided? If the pressure increases, the disk has a leak and should be replaced. Is it local or remotely displayed? Why? The display is local to reduce cost, because we do not have to respond immediately to a failed disk - the situation is not hazardous. What is the advantage of two in series? The disc protects the valve from corrosive or sticky material. The valve closes when the pressure returns below the set value. In some cases, relief and diaphragm are used in series – WHY?

37 37 Structure vent closed Structure explosion Vents required to control or direct vapour/dust explosion effect

38 38 From relief To environment Vent steam, air Holding for later processing Waste water treating Recycle to process Fuel gas, fuel oil, solvent Recover part to process Immediate neutralization Flare, toxic materials Materials from relief must be process or dispose safely

39 5. Containment Use to moderate the impact of spill or an escape Example –Bund containment for storage tanks –Location of relief valves and vents –diversion to temporary storage /drain system (following breakage of rupture disk) –Safety management in containment areas. –Containment building (if applicable)

40 6. Emergency Response Management Also used to moderate impact on incidents All plants should ERP (emergency response plan) –Assembly, head-counts, evacuation etc…

41 Summary ALARMS SIS RELIEF CONTAINMENT EMERGENCY RESPONSE BPCS 1.Inherent design starts at project conceptualization 2.Three main strategy Substitution Intensification Attenuation 3.Six Layers of Protection


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