Chapter 4 Gas System Processes and Maintenance

• Gas systems purpose and fundamental concepts are described
• Equipment and components as well as the operating principles of gas systems can be described
• Instrumentation and control of gas systems are explained
• Factors that affect normal gas systems operations are described
• A process technician's responsibilities in operating gas systems are explained
• Specific safety, health, and environmental concerns linked to gas systems are listed

Introduction to Gas Systems

• Fuel gas is used as fuel in boilers and other types of furnaces
• Nitrogen inert gas is used to purge or remove explosive gases and air from process systems and equipment, and as a backup to instrument air
• Blanketing is the process of putting nitrogen into the vapor space above the liquid in a tank to prevent air leakage
• Air has multiple uses for:
• Operative valves, drive equipment, pneumatic systems
• Drying and cleaning equipment
• Transport via pneumatic conveyor
• System purging
• Blanketing
• Breathing air supply
• Combustion with fuel gas

System Components and Operating Principles

• Gases are highly compressible
• Gases behave according to gas laws:
• Boyle's Law (pressure-volume)
• Charles's Law (temperature-volume)
• Dalton's Law (partial pressure)
• Compressors move gases around the plant; the type used depends on flow rate and pressure requirements

Air Systems

• Air systems have supply, dryers, and distribution subsystems
• Quality needs determined by the use
• Plant air provides compressed atmospheric air for equipment and tools
• Instrument air is derived from utility or plant air but must also meet requirements like moisture removal
• Breathing air is purified ambient air tested to meet requirements for human use
• Knockout pots are small vessels to remove liquid droplets or impurities from a gas system
• Dewpoint is the temperature at which a vapor or gas begins to condense to a liquid

Nitrogen Systems

• Inert gas systems introduce an inert gas, often nitrogen, at the top of a vessel to prevent combustion and maintain positive pressure
• Letdown stations use a valve or series of valves that reduce pressure in a gas stream, and is a pressure reducing regulator
• Regulators are devices used to control the pressure of a fluid or gas at a device location

Fuel Gas Systems

• Fuel gas systems use natural gas or gas from commercial natural gas and light-end gases produced by other operations

Composition of Natural Gas

• Heating value, physical characteristics, water content, and percentage of specific hydrocarbons

Composition of Fuel Gas

• Consists of lighter hydrocarbons from a variety of sources
• Fuel can be lean (BTU value less than 900) or rich (BTU value greater than 1100)

Instrumentation and Controls

• Described in figure 4.7

Abnormal Conditions

• Primarly related to proper pressure maintenance
• Reason for troubleshooting pressure drops

Process Technician Responsibilities

• For compressors, monitor:
• Lubrication systems
• Seals, flanges
• Vibration monitors
• Verify liquid levels are within control limits
• Monitor gas flow rates, pressure levels, and temperature
• For filters and dryers, monitor:
• Instrumentation
• Time intervals between filter changes, desiccant changes, or regeneration
• Parameters that indicate when the filter or desiccant needs changing or regenerating
• Proper filters and motorized equipment for vibrations and temperature
• Check strainer baskets during cleaning needs

Compressed Gas Systems

• Compressed gas systems must be kept separate with special connections to avoid cross-connection

Safety, Health, and Environmental Concerns

• Compressors and compressor systems have high noise levels, extreme temperatures, electrical hazards, and high pressures
• Natural gas and fuel systems pose fire and explosive hazards and need mercaptan added to detect leaks via smell

Process Technicians Should

• Know the content of the gas system
• Understand the surroundings of the system
• Understand system connections and watch for potential cross-connection to prevent consequences
• Be aware of the specific hazards associated with the gas in the system
• Understand systems are under pressure, representing a potential hazard

Quiz Answers

• Putting nitrogen into the vapor space above liquid in a tank to prevent air leakage is called blanketing
• A desiccant is a substance that can absorb water vapor as it passes through a fixed bed
• Water boils at 212°F at atmospheric pressure, and the boiling point will increase as pressure increases
• A process technician is responsible for looking for leaks around filter enclosures when working with compressor systems
• High noise levels are a hazard associated with compressors and compressor systems
• Fuel gas in a fuel system feeding a furnace comes from a mix of natural gas and refinery off-gas
• A process technician is responsible for switching dryers and filters when working with compressed air systems
• Utility air systems usually have the least strict requirements
• Methane has the lowest number of carbon atoms per molecule
• Plant air is used for catalyst regeneration
• Natural gas maintains constant pressure by cascading the pressure controller to the flow controller
• Electrical hazards are common hazards associated with compressors and compressor systems
• A process technician looks for leaks around enclosures when working with filters and dryers
• Monitor pressure levels when working with nitrogen systems
• Natural gas comes from off-site distribution systems
• Double block and bleed trip valves ensure safe and effective fuel gas line isolation during shutdown
• Maintaining proper pressures and ensuring material composition meets standards is important in gas systems
• Failure of screens and/or filters is a potential cause of dirt, rust, and scale in-air lines
• A knockout drum/pot removes liquids from gases
• Flare lines are used on fuel gas lines during start up and shutdown
• A change in pressure can affect the operation of gas systems
• Most important consideration in choosing the location of a portable compressor is ensuring that it does not introduce hazardous fumes into the system
• Asphyxiation is a hazard associated with nitrogen systems
• Phase transition of a fluid (water) from liquid to vapor is boiling
• Breathing air assists turnarounds/maintenance in confined spaces