Presentation on theme: "INSTRUMENTATION AND CONTROLS FOR SAFETY"— Presentation transcript:
1 INSTRUMENTATION AND CONTROLS FOR SAFETY M. B. JenningsCHE 185
2 INHERENTLY SAFE DESIGN PROCESS RISK MANAGEMENT METHODS USED DURING THE DESIGN PHASE CAN BE PUT INTO 4 CATEGORIES:InherentPassiveActiveProceduralTARGET IS A FAIL-SAFE INSTALLATIONFROM: Dennis C. Hendershot and Kathy Pearson-Dafft, Safety Through Design in the Chemical Process Industry: Inherently Safer Process Design , AIChE Process Plant Safety Symposium, 27OCT98
3 INHERENT SAFETY DESIGN Inherent — Eliminating the hazard by using materials and process conditions which are non-hazardous.Minimize — Reduce quantities of hazardous substancesSubstitute — Use less hazardous substancesModerate — Use less hazardous process conditions, less hazardous forms of materials, or configure facilities to minimize impact from hazardous material releases or uncontrolled energy releaseSimplify — Configure facilities to simplify operation
4 PASSIVE SAFE DESIGNPassive — Minimizing the hazard by process and equipment design features which reduce either the frequency or consequence of the hazard without the active functioning of any device.Location of facilities – separation of ignition sources and fuels from other facilitiesDesign equipment for design pressure in excess of the adiabatic pressure from a reaction.
5 ACTIVE SAFE DESIGNActive — Using facilities to detect and correct process conditions:controlssafety interlocksmonitoring systems for hazards that develop over a long termand emergency shutdown systems to detect and correct process deviations.
6 PROCEDURAL SAFE DESIGN Procedural — Prevention or minimization of incident impacts using:Safe operating procedures and operator trainingAdministrative safety checksManagement of ChangePlanned emergency response
7 DESIGN IN OVERALL SAFETY MANAGEMENT Art M. Dowell, III, Layer of Protection Analysis, 1998 PROCESS PLANT SAFETY SYMPOSIUM, October 27, 1998 Houston, TX
8 DESIGN OF SAFETY INSTRUMENTED SYSTEMS ACTIVE INHERENTLY SAFE DESIGN PROCEDURE (Separate instrumentation and control component in CHE 165 Design)First Level – Alarm systems for out of range situations and operator actionSecond Level – Interlock systems to automatically activate safety devicesThird Level – Devices to minimize impact of out of control conditions
9 USE OF HAZAN AND HAZOPPHA’s (Process Hazards Analysis) Are used to define areas of concernHAZAN and HAZOP provide a summary of the type of risk associated with various process locations and operationsFrequency should be determinedIntensity should be determined
10 OVERPRESSURIZATION EXAMPLE OVERPRESSURIZATION IS THE SUBJECT OF NUMEROUS CODES & REGULATIONSAIChE Design Institute for Emergency Relief Systems (DIERS)OSHA 29 CFR – Process Safety Management of Highly Hazardous ChemicalsNFPA 30 – Flammable & Combustible LiquidsAPI RP 520 and API RP 521 – Pressure Relieving Devices and Depressurization SystemsASME Boiler & Pressure Vessel CodeASME Performance Test Code 25, Safety & Relief Valves
11 SOURCES OF OVERPRESSURIZATION API 521 LISTS THE FOLLOWING CATEGORIES OF SOURCESAPI RP 521 Item No.Overpressure Cause1Closed outlets on vessels10Abnormal heat or vapor input2Cooling water failure to condenser11Split exchanger tube3Top-tower reflux failure12Internal explosions4Side stream reflux failure13Chemical Reaction5Lean oil failure to absorber14Hydraulic expansion6Accumulation of noncondensables15Exterior fire7Entrance of highly volatile material16Power failure (steam, electric, or other)8Overfilling Storage or Surge VesselOther9Failure of automatic control
12 FIRST LEVEL DESIGN HOW ARE SOURCES ADDRESSED FOR A STORAGE TANK? Item 1 in previous list - Closed outlets on vesselsWould be a concern for a nozzle used for pressure control in the tank, during filling operations.Perhaps a temporary blind flange would have been left in place after a maintenance operation.A pressure relief valve may malfunction.A PAH pressure switch (ΔP) could be installed if there was measurable difference between the Normal Operating Pressure and the Maximum Allowable Working Pressure.
13 SECOND LEVEL DESIGN HOW ARE SOURCES ADDRESSED FOR A STORAGE TANK? Item 1 in previous list - Closed outlets on vesselsAdd a pressure relief valve to allow gas to leave the tank and be directed to an appropriate flare or scrubber.Set point needs to be at or slightly above the Maximum Allowable Working PressureNeed an interlock to:Alarm to indicate valve has been activated and receiving unit (flare or scrubber) is activated.Shut down a valve in the tank fill line and/or shut off a pump used for filling.
14 THIRD LEVEL DESIGN HOW ARE SOURCES ADDRESSED FOR A STORAGE TANK? Item 1 in previous list - Closed outlets on vesselsAdd a rupture disc to relieve to either a flare or scrubber.This level is to protect the equipment from failure on a major scaleNeed to have an indication that the rupture disc has opened – typically a wire across the discNeed to determine actions necessary when the disc opens – stop filling, start flare, etc.
15 OTHER DESIGN CONSIDERATIONS A large storage tank is filled manually by an operator opening and closing a valve. Once a year, the tank overfills as the operator is distracted by other activities. A high pressure alarm is added to the tank. After the alarm is added, the tank is typically overfilled twice a year.Why?
16 EXAMPLE 1After the alarm was installed, the operator relied on it to indicate a high level and did not supervise the filling closely. The alarm loop turned out to have a failure rate of twice per year, so the system was not as reliable as the manual operation.
17 OTHER CONSIDERATIONS – EXAMPLE 2 Fail-safe valves are either Air-to-Open or Air-to-Close, which equate to Fail Closed and Fail Open, respectively. Recommend the correct valve for the following processes:Flammable solvent heated by steam in a heat exchanger. Valve is on the steam supply line.Exothermic reaction. Valve is on the reactant feed line.Endothermic reaction. Valve is on the reactant feed line.Gas-fired utility furnace. Valve is on the gas supply line.
18 EXAMPLE 2 - CONTINUEDSPECIFY EITHER FAIL-CLOSED OR FAIL- OPEN FOR THE VALVES IN THESE SYSTEMSRemote-operated valve on the drain for a storage tank.Remote-operated valve on the fill line to a storage tank.Gas-fired Combustion furnace. Valve is on the air supply line.Steam supply line. Valve controls the downstream steam pressure from the boiler.
19 EXAMPLE 2 – SOLUTIONS 1Valve to FAIL-CLOSED to prevent overheating the solventValve to FAIL-CLOSED to avoid a runaway reactionValve to FAIL-CLOSED to avoid reactor thermal stresses.Valve to FAIL-CLOSED to stop gas flow to uncontrolled combustion.
20 EXAMPLE 2 – SOLUTIONS 2Valve to FAIL-CLOSED to prevent draining material from tankValve to FAIL-CLOSED to prevent overfilling tankValve to FAIL-OPEN to maximize air flow to furnaceValve to FAIL-OPEN to avoid localized overpressure of line
21 EXAMPLE 34 kg of water is trapped in between inlet and discharge block valves in a pump. The pump continues to operate at 1 hp.What is the rate of temperature increase in C/hr if the cP for the water is constant at 1 kcal/(kg C)?What will happen if the pump continues to operate?
22 EXAMPLE 3 SOLUTION - 1Assume adiabatic conditions for the calculations:
23 EXAMPLE 3 SOLUTION - 2Allowing the pump to continue to run will eventually result in high pressure steam formation. This could result in the pump exploding.Adding a thermal switch or a high pressure switch to shut down the pump can prevent this from occurring.