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HSE Design Codes for Refineries & Petrochemicals
- Shubha Deo, Deputy General Manager, Uhde India Private Limited
While designing a Chemical plant, Health, Safety and Environment (HSE) aspects should be considered from start at the concept stage of the project which gets realised through all the stages such as design, construction, commissioning and most importantly during all modes of operations such as start-up, shut down, turndown etc. The article describes a comprehensive HSE plan to achieve process safety in identifying hazards such as fire, explosion and accidental chemical release which could have serious effect on life, property & environment and to prevent/mitigate such hazards to reduce risk to an acceptable level.

Health, Safety and Environment (HSE) demand highest level of commitment while designing a Chemical plant. The objective of process safety is to identify hazards such as fire, explosion and accidental chemical release which could have serious effect on life, property and environment and to prevent/mitigate such hazards to reduce risk to an acceptable level.

Deliberation on this issue should start right at concept stage of the project which gets realised through all the stages such as design, construction, commissioning and most importantly during all modes of operations such as start-up, shut down, turndown etc.

The Safety Life Cycle covering various phases from initiation and specification of safety requirements, design and development of safety features in a safety-critical system and ending in decommissioning of that system is shown in Fig 1.

The first step towards achieving process safety objective is to formulate a comprehensive HSE plan covering the following key design aspects and activities:

1. HSE studies and reviews
2. Plant layout incorporating HSE requirements
3. Pressure relief and blow down system
4. Safety instrumented system
5. Fire and Gas detection system
6. Fire protection system

Other HSE design aspects such as hazardous area classification, emergency power supply for safe shutdown, noise mapping and control etc. must also be considered at design stage.

HSE Studies and Reviews
During the FEED and EPC phases of the project, the following design reviews are recommended as a minimum to ensure that HSE requirements are incorporated in design:

HSE Aspects in Plant Layout
Impact on environment and safety are the major concerns addressed while preparing Plant Layout. The site layout should aim to contain an accident at source to prevent escalation of events to other units.

OISD 118 provides recommendations for design of Plant layout for oil and gas installations. The safe distances indicated in OISD 118 should be treated as minimum. Whenever it is not possible to provide these minimum distances, other protection measures against fire and explosion need to be considered, for example installation of blast/fire walls.

The Design engineer must ensure maximum separation between flammable hydrocarbons and ignition sources is achieved. Off-site facilities should be located according to the prevailing wind direction. e.g.:
  • Cooling tower should be loc ated downwind of process units.
  • Heaters should be loc ated upwind of process unit.
  • Main administration building and fire stations should be located upwind of process, utility and storage areas.
  • Loading and unloading areas shall be located downwind or crosswind of process and utility areas.
  • Flares should be located downwind or crosswind to process and utility areas and remote from process, utility blocks and the administration area.
Provisions should be made to ensure that all routine operations involving handling of materials and equipment are conducted in a safe manner. Layout should take into account access requirements in case of an emergency. All escape routes shall be readily accessible and unobstructed. Escape routes should be designed such that escape may be achieved under emergency conditions without risk of serious injury or loss of life.

Safety showers and eyewash units should be located in the layout wherever there is a risk of exposure of personnel to irritants that are toxic by absorption, material being handled at elevated temperatures or chemicals that can cause immediate or irreversible damage on contact with skin. Safety shower shall be designed as per IS 10592 or ANSI Z 358.1.

Pressure Relief and Safety Instrumented System
Pressure Relief & Blowdown System : In the process industr y, an impor tant safety consideration is the prevention of loss of containment due to vessel or pipeline overpressure situations. Loss of containment ca n re s u l t i n impac t to human l i f e and t h e env i ronment, when flammable, explosive, hazardous or toxic chemicals are released to atmosphere. API 520/521 and ASME provide criteria for design and protection of vessels and pipelines from rupture or damage caused by overpressure.

Pressure relief device such as safety valves are used as the primar y means of overpressure protection. The design of each pressure relief device is based on assessment of overpressure scenario such as blocked outlet, heat exchanger tube rupture, ex ternal fire, thermal expansion, power failure etc.

In addition to traditional pressure relief valves, de-pressuring systems can be used to mitigate the consequences of vessel leak by reducing the leakage rate and/or inventor y prior to a potential vessel failure. Depressurising ser ves to reduce the failure potential from scenarios such as overheating (fire), runaway reaction etc.

General emergency depressurisation of a plant area is activated using switches located in the control room and locally in the plant. The operator must ascertain that it is safe for depressurisation to be initiated. Depressurisation rates shall be based on the requirements of API 521. The design of the depressurisation system shall comply with the requirements of OISD 106.

Safety Instrumented System (SIS): SIS ensures safe shutdown of the plant due to unexpected emergencies and hence reduce the potential for uncontrolled release of toxic and flammable material to atmosphere. It ensures that the process conditions i.e. pressure, level, temperature etc. are maintained within the design envelope. For example, SIS can isolate hydrocarbon entering or leaving plant equipment and facilities, remove heat input from process heaters, de-energize rotating equipment and permit manual depressurization of isolated lines and equipment.

SIS is a set of hardware and software which is engineered to bring the operating plant to fail safe position or maintain safe operation of the plant when a hazardous condition occurs. SIS is independent from all other normal control systems to ensure its functionality is not compromised during emergency.

The functional requirements and reliability of SIS is determined from Hazard and operability studies (HAZOP), layers of protection analysis (LOPA), risk graphs etc. These techniques can be referred in IEC 61511 and IEC 61508.

OISD 152 gives recommendations on safety instrumentation for process system in Hydrocarbon industry.

Remotely Operated Valves (ROV) : The purp o s e o f Remote ly Operated Valves (ROV) is to provide quick shutoff in cases where loss of containment e.g. at the pump as a result of seal failure, could result in a major hazard. Typically ROV should be installed in suction of pump if suction vessel:
  • contain more than 5 t ons of C4 or ligh ter hydrocarbon
  • contain more than 5 t ons of liquid abo ve auto ignition
  • contain toxic liquids (refer API 750 A ppendix C)
Remotely Operated Valves (ROVs) shall be placed in a fire resistant box with fireproofed cables (for MOVs) for flammable fluids. The valves shall at least be TSO (tight shut off ) Class IV. In case the ROV is a motor operated valve, the valve shall be provided with emergency power supply as per requirement of OISD-149.

Fire and Gas Detection System
The purpose of the fire and gas detection system shall be to detect a fire or gas release and initiate alarm and subsequent action.

Detection devices can be classified based on their application as follows:
  • Fire detectors : Smoke, Heat, Flame
  • Gas detectors : Flammable, Toxic
  • Dust detectors : Combustible
Following points need to be considered while designing a Gas detection System:
  • Leak Size
  • Consequence analysis
  • Type of detector and working principle
  • Location of detectors based on v oting logic, leak source, coverage area
In general, gas detectors are located at following potential points of release depending on the hazardous and toxic nature of process fluid:
  • Light Hydrocarbon Pumps
  • Gas compressor seals and inside anal yser houses
  • Process cooling tower top platform in the units having pressurised cooling water return
  • Fuel gas knock-out drum
  • Suction side of forced draft air blowers if located where hydrocarbon vapours may be present
  • LPG Horton spheres, pump house
  • Hydrocarbon bulk truck loading area
  • Class A product storage tanks in tank farm
For buildings which are located in the possible path of a gas cloud, gas detectors shall be provided at the inlet of the HVAC (heating, ventilation, and air conditioning) system to initiate HVAC shut down or switch the system to recirculation mode.

Fire Protection System
The  re protection system is designed based on  re safety study with following objectives:
  • Identify the possible fire scenarios within the plant highlighting any equipment of elevated fire risk
  • Determine which fire scenarios require simultaneous operation of different fire protection systems
  • Recommend facilities required to mitigate fire risk
  • Confirm the proposed plant layout
  • Determine the design flows for the r equired fire protection system
  • Develop fire and gas det ection philosophy
Fire protection is typically achieved by means of:
  • Active fire protection such as hydrant & water sprinkler system, foam system, clean agent system etc.
  • Passive fire protection such as fire proofing, fire detection and alarm system etc.
OISD-116 provides guidelines for design of fire protection facilities for petroleum refineries and oil/gas processing plants. OISD-164 provides recommendations for fireproofing in oil & gas industry.

Conclusion
To summarise, the HSE engineer must get involved in all phases of project execution from concept to commissioning and ensure that all aspects of HSE are incorporated in design resulting in a safe, reliable, operable and maintainable plant. It is the responsibility of HSE engineer to propagate a proactive HSE culture across the multi-disciplinary project team.
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