Gas Compressors and Systems

Gas compressors and systems are used to pressurize and circulate gas through a process, enhance conditions for chemical reactions, provide inert gas for safety or control systems, recover and recompress process gas, and maintain correct pressure levels by either adding and removing gas or vapors from a process system. Gas compressors work in two stages. In the first stage, gas flows through the inlet check valve and fills the larger diameter first-stage cylinder. Pressurized hydraulic fluid, acting on the hydraulic piston, strokes the piston assembly to the left compressing the gas in the first-stage cylinder. Gas in the first-stage cylinder flows through the check valves into the smaller diameter second-stage cylinder.

At the end of the first stage, the four-way valve changes position and directs pressurized hydraulic fluid to the left side of the hydraulic piston. The piston assembly moves to the right compressing gas in the second-stage cylinder. Gas flows out of the second-stage cylinder into the discharge gas line. The piston assembly reverses direction at the end of the second-stage stroke and the cycle repeats.

There are four broad categories of compressor types. There are many variations within each type: reciprocating compressor, fan / blower compressors, rotary compressors, and ejector compressors.

In reciprocating compressors, the thrust of a positive displacement pump, within the cylinder, moves the gas through the system. This thrust enhances both the pressure and the density of the gas being transported. Examples of reciprocating gas compressors include piston compressors, lubricated and non-lubricated, and metal diaphragm compressors. In general, reciprocating pumps offer low rational and piston speed, leading to a high reliability factor with a minimum of maintenance.

Fan / blower compressors contain high-speed impellers through which a dynamic head is imparted to the gas. This category of gas compressors includes axial flow, radial, centrifugal and fan-blower compressors. They may also be referred to as turbomachinery.

Rotary compressors and rotary screw compressors move gas through the system by the positive displacement of two rotating lobes or by oscillating vanes confined in an eccentric cylinder. Rotary screw compressors are well known for their robustness, compactness and reliability. They are designed for long periods of continuous operation, needing very little maintenance. The smooth running action of the rotors enables the screw compressor to handle the most difficult of gases, contaminants, or liquid slugs without vibration.

Ejector compressors move gas via kinetic energy induced through high-velocity nozzles. The advantage of ejector gas compressors over mechanical pumping machinery is that it has no moving parts and as such requires very low maintenance.

Storage Tanks and Process Tanks

Storage tanks and process tanks are general purpose industrial containers. Storage tanks and process tanks can have many configurations depending upon dimensions, orientation, placement, and wall configuration. Materials of construction will dictate the application that is suitable for the tank. Storage tanks and process tanks are used in a number of applications including short term storage, long term storage, mixing, blending, metering and dispensing.

The most important parameters to consider when specifying storage tanks and process tanks are their capacity and dimensions. The capacity of the storage tank or process tank is the internal volume available for the storage of materials. The diameter of the tank is typically expressed in feet units. The length of the tank is measured in feet. The orientation of the tank can be vertical or horizontal. Vertical tanks stand vertically and typically have access ports on the bottom. Horizontal tanks are often mounted on stands or saddles and can have access ports on the bottom or top. The placement of tanks is typically either above ground or underground, depending on the construction. Portable tanks can be moved from one place to another, via wheels or other moving device. The wall construction of the tank may dictate the application that the tank is suitable for. Single wall tanks are common for various applications. Double wall tanks are used in applications where higher-pressure considerations are necessary.

Materials of construction for storage tanks and process tanks include fiberglass FRP, galvanized steel, plastic, stainless steel, steel, and titanium. Fiberglass reinforced polyester (FRP) is made of a series of long glass fibers embedded in a resin. It can be formed into almost any shape before curing. Once cured it is light in weight, very strong material that has excellent corrosion resistant properties. In some cases, fiberglass is used along with the plastic in the body material of the tank. Galvanized steel is cold rolled steel that has been surface treated with a layer of zinc. Stainless steel is a type of metal that resists corrosion. Steel is a ferrous-based metal having a variety of physical properties depending on composition. Steel used in tank applications is typically rolled sheet steel. Titanium is a lightweight, very strong metal used in applications where there are temperature extremes or extraordinary stresses. Storage tanks and process tanks with special linings are constructed of special materials for corrosive or other special processes. Considerations might also include glass lined or special coatings.

Common industries and applications that use storage tanks and process tanks include chemical processing, cosmetics processing, food and beverage processing, oil and fuel processing, paper and pulp processing, pharmaceutical processing, plastic processing, power generation and energy processing, and water applications.

Inside an Extinguisher

In the last section, we saw that there are three essential elements involved in producing fire -- heat, oxygen and fuel. To put a fire out, you need to effectively remove one of these elements.

• The best way to remove heat is to dump water on the fire. This cools the fuel to below the ignition point, interrupting the combustion cycle.
• To remove oxygen, you can smother the fire so it is not exposed to air. One way to smother a small fire is to cover it with a heavy blanket. Another way is to dump nonflammable material, such as sand or baking soda on top of it.
• Removing the fuel is the most difficult approach for most fires. In a house fire, for example, the house itself is potential fuel. The fuel will only be removed once the fire has burned all of it up.

Fire extinguishers are sturdy metal cylinders filled with water or a smothering material. When you depress a lever at the top of the cylinder, the material is expelled by high pressure, similar to the way material is forced out of an aerosol can. The diagram below shows a typical design.

In this extinguisher, a plastic siphon tube leads from the bottom of the fire-suppressant reservoir to the top of the extinguisher. A spring-mounted valve blocks the passageway from the siphon to the nozzle. At the top of the cylinder, there is a smaller cylinder filled with a compressed gas -- liquid carbon dioxide, for example. A release valve keeps the compressed gas from escaping.

Most dry-chemical fire extinguishers have a built-in pressure gauge. If the gauge indicator is pointing to "recharge," the pressure in the extinguisher may be too low to expel the contents. The National Fire Protection Association recommends having dry extinguishers inspected every six years, even if the gauge indicates correct pressure.

To use the extinguisher, you pull out the safety pin and depress the operating lever. The lever pushes on an actuating rod, which presses the spring-mounted valve down to open up the passage to the nozzle. The bottom of the actuating rod has a sharp point, which pierces the gas cylinder release valve.

The metal safety pin prevents the operating lever from closing accidentally.

The operating lever pushes down on an actuating rod (the blue piece).

The compressed gas escapes, applying downward pressure on the fire-suppressant material. This drives the material up the siphon and out the nozzle with considerable force. The proper way to use the extinguisher is to aim it directly at the fuel, rather than the flames themselves, and move the stream with a sweeping motion.
In the next section, we'll look at the major types of extinguishers.

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