This article examines the principles of designing technical gas supply systems at facilities using nuclear energy. It analyzes regulatory requirements for compressed air systems used to supply instrumentation and control equipment, automation systems, and personnel breathing air. It also presents safety-class classification, air quality requirements, redundancy requirements, and recommendations for equipment selection and installation quality control.
Introduction
Technical gas supply systems at nuclear power plants supply compressed air to pneumatic valve actuators, instrumentation, and process automation control systems. In addition, compressed air is used to create overpressure in rooms, protect equipment from radioactive aerosols, and provide breathing air for personnel in protective structures.
Unlike general industrial systems, technical gas supply at facilities using nuclear energy (OIAE) is governed by increased requirements for reliability, redundancy, and compressed air quality. Air supply to control systems and emergency protection systems must be guaranteed even under beyond-design-basis accident conditions.
Regulatory Framework
The design and operation of compressed air systems at nuclear power plants are regulated by the following documents:
• NP-001-15 “General Provisions for Ensuring the Safety of Nuclear Power Plants” establishes the classification of systems by safety class. Technical gas supply systems that supply safety-related equipment are assigned to Safety Class 2 or 3.
• Rules for the Design of Gas Supply Systems (SP 62.13330.2011) apply to the installation of external gas pipelines for gaseous media.
• ASME AG-1 “Code on Nuclear Air and Gas Treatment” establishes requirements for air and gas treatment equipment used in nuclear safety systems.
According to NP-001-15, compressed air supply networks must have buffer vessels (receivers) that provide air supply to control and monitoring systems for a period sufficient for the safe shutdown of the facility, but not less than one hour.
Classification of Technical Gas Supply Systems
By functional purpose, the systems are divided into three categories.
1. Compressed air systems for instrumentation and control (I&C) and automation supply pneumatic valve actuators, regulators, and positioners. They require a high degree of purification from oil, moisture, and mechanical particles (not lower than Contamination Class 1) and guaranteed supply in the event of a power outage.
2. Compressed air systems for overpressure and sealing create excess pressure in rooms with potential radioactive contamination. They require continuous supply and automatic switchover to the standby system.
3. Compressed air systems for personnel breathing provide air for operating personnel in protective structures and supply hose respirators and pneumatic suits. Air quality must comply with hygienic standards for oxygen, carbon dioxide, oil, and moisture content.
Compressed Air Quality Requirements
The quality of compressed air for I&C systems and emergency protection systems must be not lower than Contamination Class 1 under state standards. The main Class 1 parameters are:
• solid particle content of no more than 0.1 mg/m³, with a maximum inclusion size of no more than 1 µm;
• water content by pressure dew point not higher than -40°C;
• oil content in liquid and aerosol form of no more than 0.01 mg/m³.
For personnel breathing systems, oxygen content (19-23% by volume), carbon dioxide content (not more than 0.5% by volume), toxic impurities, and the absence of odor are additionally monitored.
System Composition and Arrangement
A typical technical gas supply system includes the following main elements.
The air intake unit is equipped with coarse filters to protect the compressor from atmospheric dust. The intake point must be located in an area with minimal dust levels and no ingress of exhaust gases.
The compressor unit is the main source of compressed air. Nuclear power plants use oil-injected or oil-free screw compressors. For I&C supply, oil-free compressors or oil-injected compressors with a highly efficient oil separation system are preferred.
The air receiver is a buffer vessel that smooths pulsations and ensures consumer supply when compressors stop. Its volume is calculated based on maximum flow rate and the required autonomous operating time (not less than one hour).
Compressed air dryers may be refrigerated dryers (dew point +2 to +5°C for general industrial networks) or adsorption dryers (dew point down to -40 to -70°C for critical I&C and breathing systems).
The filtration system provides staged purification: oil separators after the compressor, microfilters (0.1-1 µm), and activated carbon adsorbers before supply to I&C and breathing systems.
Pipelines made of seamless steel pipes (for pressures up to 1.6 MPa) are laid with a slope toward drainage points and equipped with condensate drains at low points.
The monitoring and automation system continuously monitors pressure, dew point, and oil content in the compressed air.
Redundancy and Reliability
Technical gas supply systems at nuclear power plants are designed with 100% redundancy. The standard solution is two independent compressor units operating in hot standby mode. When pressure in the receiver drops below the setpoint, the standby compressor starts automatically.
Each compressor must have its own power supply input from different sections of the distribution switchgear. For pneumatic actuators whose failure is critical to safety, local accumulators sized for one full stroke of the valve stem are additionally provided.
According to regulatory requirements, buffer vessels must provide air supply to control and monitoring systems when compressors are stopped for a period sufficient for the safe shutdown of the facility, but not less than one hour.
Installation and Quality Control
Installation of technical gas supply systems at OIAE facilities is carried out according to a work execution plan developed in accordance with industry standards (STO 95 104-2015). Quality control includes:
• incoming inspection of pipes and fittings (certificates, visual inspection);
• radiographic or ultrasonic inspection of welded joints (scope of at least 10% for Safety Class 3 systems and 50-100% for Safety Class 2 systems);
• pneumatic strength and leak-tightness testing (pressure of 1.25 x operating pressure, 24-hour hold, pressure drop of no more than 1%);
• inspection of internal cavity cleanliness (purging with filters at consumer inlets);
• in strict regime zones, inspection of weld smoothness to prevent the accumulation of radioactive contamination.
Operational Maintenance
During operation, the following activities are performed regularly: monitoring the dew point at dryer outlets; replacing filter elements according to schedule or when the pressure drop limit is exceeded; draining condensate from receivers and low points of air ducts; and periodic analysis of compressed air quality at control points (at least once per quarter for Safety Class 2 systems).
Conclusions
Technical gas supply at facilities using nuclear energy is a critical engineering system on whose reliability the safety and operability of the reactor installation depend. Compressed air for I&C, automation, and personnel breathing must meet specified cleanliness parameters (Contamination Class 1), be dried to a dew point below -40°C, and remain guaranteedly available in the event of compressor failure through air receivers designed to operate for at least one hour. Compliance with the requirements of NP-001-15, design rules, and ASME AG-1 standards makes it possible to create a compressed air system capable of operating without failures throughout the entire service life of the power unit.
To receive a commercial proposal for the design, supply, and installation of technical gas supply systems for your facility, please send the technical assignment to the commercial department of TechAtomStroy LLC via the feedback form on the website.
*Prepared based on NP-001-15, SP 62.13330.2011, and ASME AG-1.*
Introduction
Technical gas supply systems at nuclear power plants supply compressed air to pneumatic valve actuators, instrumentation, and process automation control systems. In addition, compressed air is used to create overpressure in rooms, protect equipment from radioactive aerosols, and provide breathing air for personnel in protective structures.
Unlike general industrial systems, technical gas supply at facilities using nuclear energy (OIAE) is governed by increased requirements for reliability, redundancy, and compressed air quality. Air supply to control systems and emergency protection systems must be guaranteed even under beyond-design-basis accident conditions.
Regulatory Framework
The design and operation of compressed air systems at nuclear power plants are regulated by the following documents:
• NP-001-15 “General Provisions for Ensuring the Safety of Nuclear Power Plants” establishes the classification of systems by safety class. Technical gas supply systems that supply safety-related equipment are assigned to Safety Class 2 or 3.
• Rules for the Design of Gas Supply Systems (SP 62.13330.2011) apply to the installation of external gas pipelines for gaseous media.
• ASME AG-1 “Code on Nuclear Air and Gas Treatment” establishes requirements for air and gas treatment equipment used in nuclear safety systems.
According to NP-001-15, compressed air supply networks must have buffer vessels (receivers) that provide air supply to control and monitoring systems for a period sufficient for the safe shutdown of the facility, but not less than one hour.
Classification of Technical Gas Supply Systems
By functional purpose, the systems are divided into three categories.
1. Compressed air systems for instrumentation and control (I&C) and automation supply pneumatic valve actuators, regulators, and positioners. They require a high degree of purification from oil, moisture, and mechanical particles (not lower than Contamination Class 1) and guaranteed supply in the event of a power outage.
2. Compressed air systems for overpressure and sealing create excess pressure in rooms with potential radioactive contamination. They require continuous supply and automatic switchover to the standby system.
3. Compressed air systems for personnel breathing provide air for operating personnel in protective structures and supply hose respirators and pneumatic suits. Air quality must comply with hygienic standards for oxygen, carbon dioxide, oil, and moisture content.
Compressed Air Quality Requirements
The quality of compressed air for I&C systems and emergency protection systems must be not lower than Contamination Class 1 under state standards. The main Class 1 parameters are:
• solid particle content of no more than 0.1 mg/m³, with a maximum inclusion size of no more than 1 µm;
• water content by pressure dew point not higher than -40°C;
• oil content in liquid and aerosol form of no more than 0.01 mg/m³.
For personnel breathing systems, oxygen content (19-23% by volume), carbon dioxide content (not more than 0.5% by volume), toxic impurities, and the absence of odor are additionally monitored.
System Composition and Arrangement
A typical technical gas supply system includes the following main elements.
The air intake unit is equipped with coarse filters to protect the compressor from atmospheric dust. The intake point must be located in an area with minimal dust levels and no ingress of exhaust gases.
The compressor unit is the main source of compressed air. Nuclear power plants use oil-injected or oil-free screw compressors. For I&C supply, oil-free compressors or oil-injected compressors with a highly efficient oil separation system are preferred.
The air receiver is a buffer vessel that smooths pulsations and ensures consumer supply when compressors stop. Its volume is calculated based on maximum flow rate and the required autonomous operating time (not less than one hour).
Compressed air dryers may be refrigerated dryers (dew point +2 to +5°C for general industrial networks) or adsorption dryers (dew point down to -40 to -70°C for critical I&C and breathing systems).
The filtration system provides staged purification: oil separators after the compressor, microfilters (0.1-1 µm), and activated carbon adsorbers before supply to I&C and breathing systems.
Pipelines made of seamless steel pipes (for pressures up to 1.6 MPa) are laid with a slope toward drainage points and equipped with condensate drains at low points.
The monitoring and automation system continuously monitors pressure, dew point, and oil content in the compressed air.
Redundancy and Reliability
Technical gas supply systems at nuclear power plants are designed with 100% redundancy. The standard solution is two independent compressor units operating in hot standby mode. When pressure in the receiver drops below the setpoint, the standby compressor starts automatically.
Each compressor must have its own power supply input from different sections of the distribution switchgear. For pneumatic actuators whose failure is critical to safety, local accumulators sized for one full stroke of the valve stem are additionally provided.
According to regulatory requirements, buffer vessels must provide air supply to control and monitoring systems when compressors are stopped for a period sufficient for the safe shutdown of the facility, but not less than one hour.
Installation and Quality Control
Installation of technical gas supply systems at OIAE facilities is carried out according to a work execution plan developed in accordance with industry standards (STO 95 104-2015). Quality control includes:
• incoming inspection of pipes and fittings (certificates, visual inspection);
• radiographic or ultrasonic inspection of welded joints (scope of at least 10% for Safety Class 3 systems and 50-100% for Safety Class 2 systems);
• pneumatic strength and leak-tightness testing (pressure of 1.25 x operating pressure, 24-hour hold, pressure drop of no more than 1%);
• inspection of internal cavity cleanliness (purging with filters at consumer inlets);
• in strict regime zones, inspection of weld smoothness to prevent the accumulation of radioactive contamination.
Operational Maintenance
During operation, the following activities are performed regularly: monitoring the dew point at dryer outlets; replacing filter elements according to schedule or when the pressure drop limit is exceeded; draining condensate from receivers and low points of air ducts; and periodic analysis of compressed air quality at control points (at least once per quarter for Safety Class 2 systems).
Conclusions
Technical gas supply at facilities using nuclear energy is a critical engineering system on whose reliability the safety and operability of the reactor installation depend. Compressed air for I&C, automation, and personnel breathing must meet specified cleanliness parameters (Contamination Class 1), be dried to a dew point below -40°C, and remain guaranteedly available in the event of compressor failure through air receivers designed to operate for at least one hour. Compliance with the requirements of NP-001-15, design rules, and ASME AG-1 standards makes it possible to create a compressed air system capable of operating without failures throughout the entire service life of the power unit.
To receive a commercial proposal for the design, supply, and installation of technical gas supply systems for your facility, please send the technical assignment to the commercial department of TechAtomStroy LLC via the feedback form on the website.
*Prepared based on NP-001-15, SP 62.13330.2011, and ASME AG-1.*