1. INSTRUMENTATION AND CONTROLS FOR SAFETY
M. B. Jennings
CHE 185

2. INHERENTLY SAFE DESIGN
PROCESS RISK MANAGEMENT METHODS USED DURING THE DESIGN PHASE CAN BE PUT INTO 4 CATEGORIES:
Inherent
Passive
Active
Procedural
TARGET IS A FAIL-SAFE INSTALLATION
FROM: 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 substances
Substitute — Use less hazardous substances
Moderate — Use less hazardous process conditions, less hazardous forms of materials, or configure facilities to minimize impact from hazardous material releases or uncontrolled energy release
Simplify — Configure facilities to simplify operation

4. PASSIVE SAFE DESIGN
Passive — 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 facilities
Design equipment for design pressure in excess of the adiabatic pressure from a reaction.

5. ACTIVE SAFE DESIGN
Active — Using facilities to detect and correct process conditions:
controls
safety interlocks
monitoring systems for hazards that develop over a long term
and 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 training
Administrative safety checks
Management of Change
Planned 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 action
Second Level – Interlock systems to automatically activate safety devices
Third Level – Devices to minimize impact of out of control conditions

9. USE OF HAZAN AND HAZOP
PHA’s (Process Hazards Analysis) Are used to define areas of concern
HAZAN and HAZOP provide a summary of the type of risk associated with various process locations and operations
Frequency should be determined
Intensity should be determined

10. OVERPRESSURIZATION EXAMPLE
OVERPRESSURIZATION IS THE SUBJECT OF NUMEROUS CODES & REGULATIONS
AIChE Design Institute for Emergency Relief Systems (DIERS)
OSHA 29 CFR – Process Safety Management of Highly Hazardous Chemicals
NFPA 30 – Flammable & Combustible Liquids
API RP 520 and API RP 521 – Pressure Relieving Devices and Depressurization Systems
ASME Boiler & Pressure Vessel Code
ASME Performance Test Code 25, Safety & Relief Valves

11. SOURCES OF OVERPRESSURIZATION
API 521 LISTS THE FOLLOWING CATEGORIES OF SOURCES
API RP 521 Item No.
Overpressure Cause 1
Closed outlets on vessels 10
Abnormal heat or vapor input 2
Cooling water failure to condenser 11
Split exchanger tube 3
Top-tower reflux failure 12
Internal explosions 4
Side stream reflux failure 13
Chemical Reaction 5
Lean oil failure to absorber 14
Hydraulic expansion 6
Accumulation of noncondensables 15
Exterior fire 7
Entrance of highly volatile material 16
Power failure (steam, electric, or other) 8
Overfilling Storage or Surge Vessel
Other 9
Failure of automatic control

12. FIRST LEVEL DESIGN HOW ARE SOURCES ADDRESSED FOR A STORAGE TANK?
Item 1 in previous list - Closed outlets on vessels
Would 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 vessels
Add 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 Pressure
Need 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 vessels
Add a rupture disc to relieve to either a flare or scrubber.
This level is to protect the equipment from failure on a major scale
Need to have an indication that the rupture disc has opened – typically a wire across the disc
Need 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 1
After 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 - CONTINUED
SPECIFY EITHER FAIL-CLOSED OR FAIL- OPEN FOR THE VALVES IN THESE SYSTEMS
Remote-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 1
Valve to FAIL-CLOSED to prevent overheating the solvent
Valve to FAIL-CLOSED to avoid a runaway reaction
Valve to FAIL-CLOSED to avoid reactor thermal stresses.
Valve to FAIL-CLOSED to stop gas flow to uncontrolled combustion.

20. EXAMPLE 2 – SOLUTIONS 2
Valve to FAIL-CLOSED to prevent draining material from tank
Valve to FAIL-CLOSED to prevent overfilling tank
Valve to FAIL-OPEN to maximize air flow to furnace
Valve to FAIL-OPEN to avoid localized overpressure of line

21. EXAMPLE 3
4 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 - 1
Assume adiabatic conditions for the calculations:

23. EXAMPLE 3 SOLUTION - 2
Allowing 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.