Gas Cabinets and Distribution Systems Information
Gas cabinets and distribution systems are designed to deliver bulk and specialty gases to process equipment. These systems are manufactured based upon the type and nature of the gas they will deliver, and can consist of vaporizers, gas cabinets, gas manifolds, mass flow controllers, valves, pressure sensors and associated interconnect tubing.
A gas distribution piping system must have high quality inner surfaces to ensure that there is no contamination of the gas by particles from the system. Distribution systems are designed for safety and reliability. Safety is paramount in the midst of high pressures and hazardous gases (such as NH3, HCl, and SiH4) which could harm equipment or personnel. Reliability is important to ensure the success of gas delivery in critical gas feed applications.
Gas Cabinet Composition
At its simplest level, a standard gas cabinet consists of a vessel chamber with a panel or door, one or multiple cylinders with pressure regulators, a manifold, valves, and tubing. Valves, tubes, and fittings are connected with gas tight tapered threads. Other components that may be incorporated include purge assemblies and pressure switches. Purge assemblies consist of an additional cylinder of inert gas set up to provide a convenient means for purging a regulator or the entire delivery system prior to and after use. Pressure switches are used to activate automated functions (such as valve control or emergency shut-off) based on the pressure levels in the system.
Gas cabinets may operate in parallel or series depending on the design of the manifold used to deliver the gas. In series, the manifold circuit is designed so that the pressure supply is ported from one valve to the next. In contrast, parallel inlet ports all share a common pressure supply.
Gas cabinets are gas storage units which are designed to contain and vent leaking gas. They typically also accommodate manifolds and gas handling systems for distribution applications. Gas cabinets are distinguished based on the automation of the delivery system, which often correlates to the type of gases that are handled.
Manual gas cabinets - All functions in these systems are controlled manually by the user. These are typically used for inert, non-toxic, and non-reactive gases where additional safety factors for gas isolation are not needed.
Semi-automatic gas cabinets - These systems contain sensors which trip safety features automatically if leaks or abnormal pressures are detected, but some other controls are manually operated. They are used for less reactive or toxic gases where safety factors for gas isolation are needed.
Fully automatic gas cabinets - These systems are designed to monitor a large number of process sensors and inputs; all functions including gas delivery and cleaning/purging can be automated and driven by a PLC controller. They are used for toxic, corrosive, and reactive gases where gas isolation safety factors are paramount.
Auto-changeover gas cabinets - These systems contain multiple cylinders and are designed for uninterrupted gas flow during changeover and purging cycles. They are used in applications requiring constant flow or where a drop in delivery pressure could adversely affect instrument performance.
Ports and Valves
The port and valve specifications for gas cabinets and distribution systems are an important part of the selection process.
Gas cabinets may be distinguished by the types of valves used to control flow.
- Manual valves - valves are manually adjusted or actuated via a knob, lever, or other manual device.
- Solenoid valves - valves are opened and closed via a solenoid magnet activated by an electrical signal.
- Air pilot valves - valves are actuated by a pneumatic signal.
Ports are openings in the manifold or distribution system where the inlet and outlet connections are made. Each opening is either an inlet (supply) port or an outlet port; the number of each corresponds to the requirements of the application.
- The number of supply ports defines the number of independent fluid supplies that can be interfaced with the manifold or manifold system.
- The number of outlet ports determines the number of outlets in the system. This is frequently specified as number of ports or valves that are or can be attached to the manifold. For example, a 6 point manifold has 6 ports or valves.
Ports are sized based on standardized National Pipe Thread (NPT) sizes. Dimensions for these sizes are given in inches; each is based on the nominal pipe size that corresponds to the connection.
The specifications for a gas cabinet describe everything from gas flow rates to the physical size of the system.
Maximum pressure - Pressure describes the amount of force exerted on the system by the contained and pressurized gas. Most compressed gases will not exceed 2,000 to 2,640 pounds per square inch gage (psig), but some can reach pressures of 6,000 psig. The system's weakest point determines the pressure limit, so any parts weakened by heat, corrosion, or stress may potentially lower the maximum pressure of the system or cause the vessel to rupture.
Maximum flow - Flow rate describes the maximum rate of flow of the gas through the system, typically measured in standard cubic feet per minute (scfm).
Temperature range - Temperature range is the full required range of safe ambient or fluid operating temperatures, given in degrees Fahrenheit or degrees Celsius.
Dimensions - Size dimensions specify the physical size of the gas cabinet, distribution system, or its components.
- Cabinet size - Indicates the physical size of the gas cabinet or the body of the distribution system.
- Port/tube size - Indicates the physical size of the tubing or exhaust port connections in the system, typically given in inches based on a sizing standard such as National Pipe Thread (NPT). Sizing is important, as an undersized tube line will result in high pressure drops, while an oversized line will be unnecessarily expensive to install.
The materials used to construct the gas cabinet are an important part of proper system selection. The materials used for the casing and outer parts must have adequate structural strength, while the materials for the gas handling components must be compatible with the media, temperature requirements, and pressure ratings to prevent leakage, rupture, or contamination.
Aluminum - A light and fairly corrosion-resistant metal which is typically anodized for increased corrosion and wear resistance. Aluminum may be used for tubing or fittings in lower pressure applications with higher surrounding temperatures or relatively corrosive gases.
Copper - A soft, ductile metal with low hardness and excellent corrosion resistance. Copper is used commonly in tubes and pipes for its inertness and resistance to corrosion.
Plastic - Any of numerous thermoplastic or thermosetting polymers of high molecular weight. Different grades (such as nylon, acetal, and polycarbonate) have varying properties, but most have strong chemical and corrosion resistance.
Steel - General purpose industrial metal with high physical strength and hardness. Steel is typically coated or finished to increase its corrosion resistance properties.
Stainless steel - A highly chemical and corrosion resistant steel alloy most commonly used to construct the gas handling components of high pressure distribution systems.
Gas cabinets and distribution systems may be comprised of a number of additional features which are important for certain applications.
- Adjustable chamber isolation - Supply chambers can be isolated or joined, depending on application or state of process cycle.
- Emergency shut-off - System is designed to shut-off fluid flow when sensors indicate emergencies such as leaks or high pressures.
- Feedback signal - Feedback signals are electrical or electronic signals used to signal the state of valves for indication or automated system response.
- Filter or trap - Traps remove unwanted contaminants (typically oxygen, moisture, or hydrocarbons) from specialty gases before they reach the instrument. They also can be used as indicators of gas contamination.
- Visual indicator - Indicators provide visual signals to describe the status of different parts of the system, such as LEDs used to signal valve state (open or closed).