Passive Fire Protection System For Petrochemical Plants – Part II

Read the first part:
https://boilerworldupdate.com/passive-fire-protection-system-for-petrochemical-plants/

Details of various methods of Passive Fire Protection system 

1. Safe distances of chemical process plants: 

Safety of process plants is a prime consideration in Plant Siting, which means the distance between various process units, utilities of petrochemical plants and oil & gas installations. In case of fire or explosion, the distance will reduce the intensity of heat or blast over-pressures as well as damage done by missiles (i.e. flying debris) during an explosion. Single or allied processes are usually located on the individual block of land surrounded by access roads on all four sides. The main factor is that fire or spills of flammable material can be attacked from all four directions of the unit or block. Engineering for safe operations denotes, generally, a distance of 76.2M (250 Ft) between the plants and between the tanks and has been used as a desirable spacing. 

The distance between the process plants, storage and utilities should be decided based on the site levels, the gradient of the land and wind direction and should comply with the following standards.

OISD 116: Fire protection facilities for Petroleum Refineries and Oil/Gas processing plants.

&/or

IS 15394 (2003): Fire Safety in Petroleum Refineries and Oil/Gas processing & PC plants

2. Fireproof coating of structures of chemical plants 

2.1 Behaviour of steel structures in case of fire:                                                             

In petrochemical plants, various process equipment such as reactors, vessels, columns, pumps, compressors, etc., are supported by structural steel members of columns and beams. In a few plants, the supporting structure is made up of RCC; however, in general, most of the process plants are erected as unprotected steel structures made up of MS steel structural members. 

Salient features of structural steel members when it is subject to severe fire are as under:

As the steel temperature increases, the following can occur: 

• Dimensional changes: As the temperature rises, steel expands due to thermal expansion, resulting in the buckling of connected steel components
• Reduction of strength: As the temperature of the steel increases above room temperature, the strength of the steel decreases. At 550°C, carbon steel only retains approximately 50% of its yield strength
• Permanent changes in the steel microstructure: If heated to sufficiently elevated temperatures, between 704°C and 843°C for most structural steel, and rapidly cooled by suppression water during firefighting operations, some of the steel’s microstructure will be transformed into martensite. Martensite is a hard but brittle phase of steel

Petrochemical plants –  supporting structural steel columns & beams

550°C is the critical temperature for structural steel, above which it is in jeopardy of losing its strength, leading to collapse

2.2 Effect of fire on concrete: 

Concrete is one of the best fire-resistant building materials for construction. Because of its strong fire resistance, the effects of fire on concrete will be insignificant, and in most cases, fire-damaged concrete is fixable by patching cracks or tuck-pointing mortar. However, if a fire exceeds 500 degrees Fahrenheit, it can cause concrete to lose its integrity and break. 

2.3 Cement-based fireproofing compound:

A common type of passive fire protection, spray-applied fire resistive materials (SFRMs) are applied directly to steel structures to thermally insulate them against the high temperatures of a fire. SFRMs, like cementitious fireproofing, primarily consist of a gypsum or Portland cement binder. This is because the cement and gypsum will harden as they cure. It also contains other materials such as mineral wool, quartz, perlite, or vermiculite to help lighten the solution or to add air as an insulator. Cementitious fireproofing is typically supplied as a dry powder in a bag, which is then mixed with water onsite before spray application in the field.

Picture showing the structural steel members of a steel structure coated with cement-based fireproofing compound.

The thickness of sprayed cement-based fireproofing compound ranges from 3/8″ to 3″ (10 to 76 mm), depending on the hours of fire rating required as per IS – 1642.

Fire safety of the building:

Details of construction: Code of practice and thickness of coating material for specific period fire rating is recommended by the manufacturer of that particular fire proofing compound.

2.4 Details of fire proofing methods of structural members:

To avoid damage to the process equipment and its supporting steel structure, it is suggested that exposed steel columns and beams of the steel structures should be encased in RCC or the exposed surfaces should be coated with a cement-based fireproofing compound. 

2.5 Details of fireproofing material and method of application:

Important features of this material are as follows: 

• Cementitious inorganic polymer fireproofing formulation
• Single package mixed with clean potable water at the job site
• Easily applied on structural steel columns & beams by hand trowelling (manual application) or sprayed by use of suitable spray guns.
• Non-flammable during or after application
• Non–flammable high impact strength

 Exposed steel columns

Fireproofing compound sprayed by a spray gun

3. Fire sealing of cable crossings: 

Fire and smoke in cable trenches or cable tunnels and wall & floor crossings may spread from one end to another through the gap around the cables. In a petrochemical plant, electrical cable crossings and service pipes were found in the following places: 

• Underground cable trenches from the main electrical SS to the distribution centres of various process plants & utility SS
• Cables are laid in UG pipes crossing the plant & warehouse walls
• Above ground electrical cables from the pipe racks crossing the walls & floors
• Services pipes crossings from one compartment to another compartment of a plant

Cable crossings are sealed with a cement-based fireproof compound

Fire barrier walls shall be provided in the underground cable trenches at intervals of not less than 30m. Also, the gap around the cables should be effectively fire-sealed with a UL/FM-approved type with a cement-based fireproofing compound with a 2-hour fire rating. Similarly, the gap around the cables and service pipes crossing through the wall and floors should be fire stopped by providing sealing around the gap by an approved type of cement-based fire-proof compound. 

4. Fire proof coating for electrical cables : 

Intumescent coating is a coating that is specially formulated to protect vulnerable & flammable subtract (i.e.) outer layer of cables etc., It is a paint like materials which are inert at low temperature in case of fire when temperature increases the coating materials reacts (i.e.) swells to form an expended layer of low conductivity insulating materials thus protecting the core of electrical cables. These materials are to be tested as per FM-approved class 3971. 

In petrochemical plants, cables in the following areas need to be coated with fire retardant paints:

• Cables feeding to fire pumps, fire alarm panels, lifts, emergency lighting panel and other critical equipment, if it is laid in pipe racks and cable racks along with other cables, hydrocarbon pipe lines, etc., and electrical cables which are laid within 15m of hydrocarbon plants.
• For a length of 1m from the panel bottom, the panels of the electrical panel room are located at the first floor level, and the cable cellars are located at the ground floor level of the substation building.

Fireproofing coating of electrical cables with insulating paints

5. Electrical Installation for PC plants and fire separation of electrical sub-station, DG set rooms and Electrical LT Panels & DB areas in process units and fire pump room.

As per IE Act, CEA regulations and relevant Indian standards, the following requirements should be complied with in case of electrical installation of petrochemical plants.

5.1 Electrical installation for PC plants: 

• All light fittings & other electrical equipments in the high hazards process plants & tank farm areas should be of approved type – FP (explosion proof) electrical motors, light fittings, junction boxes, switch button stations etc., FP type manual call points (MCP), FP type CCTV cameras and FP type telephones. 
• The electrical power distributing system should be completely underground and cable trenches fully covered.

5.2 Fire separation of electrical sub-station building, DG room or outdoor DG sets and switch yard/outdoor transformer areas.

In case of severe fire and explosion in the transformer, the fire may spread to the entire switch yard, causing damage to other equipment, adjacent transformers, etc. Transformer oil may spread in cable trenches, and fire & smoke may pass to the substation building, switch gear rooms and other blocks. The power transformers in the indoor substation building or the outdoor switchyard should be provided with fire barrier walls, also called baffle walls, thus preventing the fire from spreading from one transformer to another.

Fire barrier walls between the transformers

35cm thick brick wall on RCC frame or 23cm thick RCC wall and 60cm roof.

Fire separation of electrical sub-station, DG room, outdoor DG sets, HT switchyard/outdoor transformers and fire pump room/reservoir consists of the following: 

• The HT switchyard/electrical sub-station should be located a minimum of 15 mtr away from all hydrocarbon process plants. If it is communicating, then the SS block should be segregated by fire barrier walls
• In the electrical substation block, the following equipment is separated by fire barrier walls

1. Between HT cubicles/LT panel rooms/Indoor transformers/DG rooms
2. In case of outdoor transformers, if the aggregate oil capacity is more than 2000 litres.

• The outdoor-type DGs should be located on a cement platform and at least 6m away from the SS block and 15 mt away from the hydrocarbon process plants
• The fire pump room should also be located 15m away from the hydrocarbon process plants. If it is communicating, then the fire pump room should be segregated by fire barrier walls. Also, the level of the fire pump room should be less than the level of the fire water reservoir, so all fire pumps should be installed as positive suction pumps.
• In case of lengthy underground cable trenches and in cable cellars, the fire barrier walls should be provided at every 30m intervals, and also the gap around the cable crossings (aperture) should be fire sealed with 2 hour fire rated, UL/FM approved cement-based fire proof compound. 
• If the door openings of the SS building face the transformer, which is located within 6 mtr, the same should be a 2-hour fire-rated door of approved make, and window openings, if any, should be of 2-hour fire-rated glass windows on steel frame.

5.3. Process control room – Blast proof walls & blast proof type construction

 Process control room – Blast proof type construction

Process control rooms are where the monitoring of various industrial and technical processes takes place. It consists of large pieces of equipment – oversized servers, computers, and monitors- that can graphically represent the various processes taking place in the plant facility. As per IS 15394, the control room building located in a hazardous area should be a blast-resistant type. Such buildings should not be located within 15m for single process units and 30m for 2 or more process units. Beyond 120m, the control room does not need to be a blast-resistant type. 

Blast-resistant control rooms should be designed for static overpressure of 3 MPa. Software is available to decide the thickness of walls and roof, with reference to the maximum pressure attained during the process of a particular process unit. A maximum of two exits should be provided in a process control room, such that an escape route from inside can reach the exit doors. 

The control room should be a single storey, and on one side, there should be an adjoining internal road. In general, all walls and the roof of the control room are made of RCC resting on RCC raft slab foundation. The exit doors of the control room should be of approved type blast resistance door to withstand blast pressure up to 1.52 Bar, with 4 hours fire rating. 

6. Conclusion:

It is to be noted that a “Passive Fire Protection System” consists of spacing in layout, fire-rated walls, blast-proof type construction, fire sealing, etc. Most passive system installation involves planning and implementation at the project stage itself. Implementation required civil type constructions, which is possible during the project stage only. So, in the case of passive fire protection systems, meticulous planning is required at the concept and design stages only. 

Author:

T.K.Chandrasekaran
Risk Engineering Consultant & Fire Safety Engineer