OPUS

System Steam

Steam is one of the oldest forms of energy used in industry, yet remains one of the least understood.  Several unique components are necessary to provide safe, efficient and corrosion-free operation. Further, pipe installation practices can greatly affect the operation, efficiency and longevity of the systems.

Steam systems are actually split into two systems.  The steam system transports steam (water vapor) from the boiler to an energy-using device of some sort.  As the steam’s energy is used up, the steam condenses and is returned to the boiler as water in what is called the condensate system. The steam side of the system is really a piping system for a gas.  However, because the gas is very close to its condensation point, there will be some liquid in the system as well.  In addition, the water (feedwater) used to make steam contains several other gasses (e.g. oxygen, carbon dioxide). Finally, air enters steam systems from time to time.  Because of these factors, a steam system must be designed to circulate steam and deal with condensate, other gasses, and the acids created when gasses combine with water.

Unlike hydronic heating systems (which use liquid to transport energy), steam does not require pumps for circulation.  Because water expands when it boils, pressure is created. Nature likes a balance, so steam flows through the pipes until the entire system is pressurized.  Once the system is charged with steam, no flow occurs until some energy-using device draws heat from the steam, causing it to cool and condense, thus creating a lower pressure. In this manner, steam flows as long as some of it is condensing at some point in the system.

Control valves are needed to throttle the flow of steam.  If left to physics, steam would continue to flow in a system until the surrounding temperature reached equilibrium.  Since we are much more comfortable at about 72°F than we are at steam temperature of, say 250°F, the control valve is used to stop steam from flowing once set point has been achieved.

Steam travels at very high velocities, normally limited to between 6,000 and 10,000 ft/min, but sometimes as fast as 20,000 ft/min (about 230 mph).  At these velocities, small droplets of water can cause considerable damage to the inside of the pipe.  Since some steam will condense inside the pipe, the condensate is drained and collected in drip legs, which are vertical pipes attached to the bottom at regular intervals along the steam line.  In addition, steam pipes must be pitched in the direction of steam travel, at a minimum of 1" per 20'.  This pitch allows condensate to drain toward drip legs, and prevents condensate pooling by compensating for sags in the pipe between hangers. 

Steam traps allow condensate to pass from the steam system to the condensate drain while keeping the steam on the steam side of the system. Steam traps are installed on all drip legs and near all energy-using devices where steam is condensed and condensate must be separated and returned to the boiler.

Vacuum breakers are needed to admit air into the system. Because steam takes up 1600 times the volume of water at atmospheric pressure, a sudden condensation of steam causes a rapid drop in pressure.  When a control valve shuts off steam to a heating unit, the steam between the control valve and the steam trap will cool and begin to collapse as it condenses, causing a vacuum to form. When the vacuum exceeds the vacuum breaker set point, the vacuum breaker will open, allowing air to enter and replace the volume of steam.

Air vents are installed to remove the air where pockets accumulate (e.g., high spots in the system, coils, shell & tube heat exchangers).  Air holds gasses that are corrosive to the piping systems, and it acts as an insulator that reduces heat transfer and thus efficiency. As such, it must be removed as quickly as possible.

Deaerating systems for feedwater are installed to remove most corrosive gasses contained in the water before it enters the boiler. With a deaerator, oxygen in the water will cause rust in the pipe and carbon dioxide will form carbonic acid, which is also corrosive.

Because of the complexity, it is critical that design, installation, operation and maintenance of steam systems and components be conducted with care.

Compatible Materials

Carbon Steel Pipe

Manufactured as welded or seamless, and in a variety of wall thicknesses, (Light, Standard,...