Condensate Return in a Steam System - Basic and Essential

Efficient production of steam and return of condensate
are essential to the operation of this and many other
industrial operations.

Many industrial processes and plants, as well as commercial buildings, utilize steam in their operations. The generation and use of steam is one of the oldest industrial processes and is so well understood that it may be considered more of a utility than part an industrial process. Whatever the case, if your process or installation uses steam, then steam is a necessary input for successful operation. Keeping your steam system performing at capacity frees up time and resources for the more complex aspects of your work.

If steam is not consumed directly by the process as a component input, it is steam's heat of vaporization that is utilized by the operation. Efficient use of steam as a heating medium results in the conversion of vapor to liquid (water). Returning the liquid condensate back to the boiler for conversion to vapor again is a necessary step in the efficient operation of a closed loop system.

Condensate return systems are certainly not high technology, but keep in mind that a steam system may be the lifeblood of not just one, but many operations throughout a plant. Avoiding downtime in the steam system, of which the condensate return system is an integral part, ranks highly on the list of "Important Things for Plant Operations". Condensate return is critical.

Three general methods are employed to move the condensate from a collection vessel, a trap, to the feedwater side of the boiler. Gravity can be used when conditions permit. A pressure motive return arrangement uses steam pressure and a coordinated valve sequence to drive the condensate through the piping system and back to the boiler. Condensate pumps can also be employed as a positive means of moving condensate through the return piping system.

What are some strong attributes of a good and reliable condensate return pump?

• Minimize or eliminate cavitation at high temperatures. Cavitation will impede pump performance and cause premature deterioration of pump and drive components.
• Ability to handle a high load during cold starts through motor and pump design.
• Design and configuration to handle high temperatures without deterioration of pump and motor.
• Develop higher pressure at lower motor speeds for extended service life.
• Avoidance of mechanical seals below water line.
• Consider a single unit with dual pumps for handling high loads and extending service life.

Specifying and installing a solidly designed and built condensate return pump may require an investment of your time and consideration. The return on that investment will be reduced maintenance, repair, and downtime. hare your steam system challenges, from end to end, with knowledgeable application specialists. Combining your intimate operational knowledge and experience with their deep product knowledge and experience with many installations will yield a good solution.

Control Valve Cavitation - A Demonstration and Some Considerations

Cavitation can be damaging to process components.

Consider a generic industrial fluid process control operation. There are pumps, valves, and other components installed in the process lines that, due to their interior shape or their function, cause changes in the fluid motion. Let's look specifically at control valves and how their throttling operation can create conditions able to greatly impact the valve itself, as well as the overall process.

Fluid traversing a control valve can undergo an increase in velocity when passing the constriction presented by the valve trim. Exiting the trim, fluid then enters the widening area of the valve body immediately downstream with a decrease in velocity. This change in velocity corresponds to a change in the kinetic energy of the fluid molecules. In order that energy be conserved in a moving fluid stream, any increase in kinetic energy due to increased velocity will be accompanied by a complementary decrease in potential energy, usually in the form of fluid pressure. This means the fluid pressure will fall at the point of maximum constriction in the valve (the vena contracta, at the point where the trim throttles the flow) and rise again (or recover) downstream of the trim.

This is where cavitation begins.

If the fluid being throttled is a liquid, and the pressure at the vena contracta is less than the vapor pressure of the liquid at the flowing temperature, portions of the liquid will spontaneously vaporize. This is the phenomenon of flashing. If, subsequently, the pressure of the fluid recovers to a level greater than the vapor pressure of the liquid, any flashed vapor will rapidly condense, returning to liquid. This collapse of entrained vapor is called cavitation.

Flashing, the generation of vapor bubbles within the liquid, will precede and set the stage for cavitation. When the flashed vapor bubbles condense to liquid they often do so asymmetrically, with one side of the bubble collapsing before the rest of the bubble. This has the effect of translating the kinetic energy of the bubble’s collapse into a high-speed “jet” of liquid in the direction of the asymmetrical collapse. These liquid “microjets” have been experimentally measured at speeds up to 100 meters per second (over 320 feet per second). What is more, the pressure applied to the surface of control valve components in the path of these microjets can be intense. An individual microjet can impact the valve interior surfaces in a very focused manner, delivering a theoretical pressure pulse of up to 1500 newtons per square millimeter (1.5 giga-pascals, or about 220000 PSI) in water. In an operating fluid system, this process can be continuous, and is known to be a significant cause of erosive wear on metallic surfaces in process piping, valves, pumps and instruments. As the rapid change in pressure takes place, the bubbles (voids in the liquid) collapse (implode), and the surrounding metal surfaces are repeatedly stressed by these implosions and their subsequent shock waves.

Consequences for control valves, as well as for the entire control process, vary and are often destructive. They may include:

• Loud noise
• Strong vibrations in the affected sections of the fluid system
• Choked flow caused by vapor formation
• Change of fluid properties
• Erosion of valve components
• Premature destruction or failure of the control valve 
• Plant shutdown

The video provides a visual demonstration, through clear piping, of what happens inside the piping system when a valve is operated in a manner that causes substantial cavitation.

The solution lies in minimizing the potential for cavitation to occur through proper valve selection and sizing, along with coordinating operating characteristics of pressure drop inducing components with the total system performance. One valve manufacturer's recommendations are summed up in four basic approaches.

• Avoidance of cavitation through proper valve selection. Use a valve with a rated liquid pressure recovery factor greater than that required for the application. Some applications may be suitable for the use of an orifice plate downstream of the valve.
• Cavitation Tolerant Components capable of withstanding limited amounts of cavitation without excessive wear. Increased flow noise is likely to accompany this route.
• Prevention of cavitation through the use of valve trim design that reduces pressure in several steps, avoiding excessive flashing. These valves can be expensive, but their effectiveness makes them an alternative worth considering.
• Containment of the harmful effects of limited to moderate cavitation through trim designs that eliminate contact of the fluid with metal surfaces which are more susceptible to damage.

Share your requirements and application challenges with a valve specialist and gain insight through their recommendations. Combining your process knowledge with their product application expertise will yield a great solution.