The responses for this Q&A were contributed to Flow Control by the Steam System Services Group of Spirax Sarco (www.spiraxsarco.com).
Q: What makes steam such an important resource in modern-day plant environments?
A: For many reasons, steam is one of the most widely used commodities for conveying heat energy. Its use is popular throughout industry for a broad range of tasks from mechanical power production to space heating and process applications. Several key factors make steam such a popular heat transfer source. First, water is plentiful and inexpensive. It is nonhazardous to health and environmentally sound. In its gaseous form, it is a safe and efficient energy carrier.
Another factor is the ability of steam to hold and transfer heat. Steam can hold five or six times as much potential energy as an equivalent mass of water. Due to the high heat content of steam, only relatively small-bore pipe work is required to distribute the steam at high pressure. Plus, steam flows in response to the pressure drop along the line, eliminating expensive circulating pumps. And, because the heat-transfer properties of steam are so high, the required heat-transfer area is relatively small. This enables the use of more compact equipment, which is easier to install and takes up less space in the plant. All this reduces the overall cost of the heating or process system.
In addition, the heating process is easy to control because of the direct relationship between the pressure and temperature of saturated steam. Simply by controlling the saturated steam pressure, the amount of energy input to the process can be precisely controlled. Modern steam controls are designed to respond very rapidly to process changes.
Q: What are some key considerations users need to be aware of when designing steam systems for their plants?
A: There are several interrelated factors that must be considered when designing steam systems. These include piping design, equipment selection, and safety. The reason that these factors are interrelated is that poor design in one area ultimately affects the others.
When designing steam systems, piping design is critical to ensure that the correct amount of heat is distributed to the plant and delivered to operating equipment. Key steam distribution design considerations include length and pitch of piping, changes in elevation, and the location of steam traps to remove condensate formed in transit.
In the condensate-return system, the piping layout must be as efficient as possible to ensure that the energy required for pumping is kept to a minimum. In addition, the collected condensate still contains some sensible heat that should be recovered to help reduce the overall energy usage.
One cannot overestimate the importance of correct equipment selection, involving not only the type of equipment but also the size and application of the equipment. Too many errors are made in the name of convenience and cost cutting. The consequences are excess energy consumption and excessive maintenance costs.
Finally, improper piping design and equipment selection can lead to dangerous situations. One of the key causes of equipment failure and safety incidents is water hammer that develops in an improperly designed system. In addition, lack of attention to equipment selection details can be equally hazardous.
Q: What are some common causes of water hammer in steam systems?
A: There are two types of water hammer: steam-flow-driven water hammer and condensate-induced water hammer. A steam-flow-driven water hammer is an impact event, where a slug of rapidly moving water strikes a stationary object. A condensate-induced water hammer is a rapid condensation event that occurs when a steam pocket, being totally surrounded by cooler condensate, collapses into a liquid state, resulting in a tremendous collision. Severe over-pressurization can easily exceed 1,000 PSI. Common places to look for both types of water hammer are steam mains, steam-tracing lines, and air heating coils.
Condensate buildup is common to both types of water hammer. Buildup can be caused by boiler carryover, or by backflow from the condensate main driven by deaerator pressure or malfunctioning steam traps or check valves. Even reduced trap capacity caused by low steam main pressure conditions can result in backflow from condensate mains. For condensate-induced water hammer, necessary elements are steam condensation or a pressure drop, leading to a vacuum being created.
Q: What installation best practices can be employed to prevent water hammer from occurring in steam lines?
A: Steam system operators can reduce risk of water hammer by providing proper condensate drainage; keeping steam as dry as possible at all times; not allowing steam velocities to become excessive; installing an automatic valve in the steam supply line so that the valve stays closed until a reasonable pressure is attained in the boiler; ensuring that steam traps are of the proper type, capacity, and condition; correcting pipe sagging and damaged insulation; and making certain that air-heating coils accomplish both condensate removal and air venting.
Q: Why is it important to regularly inspect steam traps? How often should users inspect their steam traps to ensure efficient operation?
A: Steam traps are mechanical devices that in most cases operate 24 hours a day, 365 days a year. When the trap is operating properly it is effective in draining condensate from the system. When a trap fails it can just leak slightly, fail completely open, or fail closed. Most traps tend to fail in an open or leakage mode. A leaking trap wastes energy but still removes condensate from the system. With a closed piping system, a failed trap may not be detected without an actual test being performed.
When the trap fails closed, the condensate will back up into the system. Process applications will experience upsets quickly, since no heat will be transferred into the process. When the steam trap is used as a drip point along a steam main, water will back up, but it will be carried down stream to the next drip point. This could cause a water hammer incident to occur, resulting in possible pipe fracture.
Traps should be checked at least once per year. On very critical processes more frequent testing may be warranted. In seasonal systems, traps should be checked after the system has been operating for a day or so. Traps should also be tested if the process is not operating properly, excessive steam is being consumed, or the return line becomes noisy.
Traps can be checked by several methods:
1. Using an ultrasonic gun to hear trap flow variations.
2. Conducting visual tests at trap stations with test ports, a test tee in the line after the trap, or where a trap discharges into an open drain.
3. Installing a permanent inline trap monitoring and recording unit, based either on conductivity or ultrasonic monitoring.
Q: What are the most common causes for steam-trap failure? How does steam-trap failure affect other equipment in the steam system?
A: The majority of traps fail because of old age. A typical trap in 300-PSIG service will last eight to 10 years. The second most likely cause is contamination preventing the trap from closing. Proper boiler treatment and strainers prevent contamination. Misapplied traps, particularly grossly oversized units, can also fail prematurely.
Failed traps can affect other equipment by reducing the output of the equipment, inducing water hammer that damages the equipment, and exposing electric condensate pumps to destructively high temperatures. The most significant effect is wasted energy.
Q: How can users best assess the performance of their steam systems? Why is it important to employ a steam systems assessment program?
A: It is vital to employ a steam system assessment program to control and reduce steam energy fuel costs. The best way to assess the performance of a steam system is to install monitoring instrumentation for utilities, steam, and make-up water. By collecting daily baseline data, customers can benchmark their present energy efficiency and compare it to industry standards and also track the actual value of energy improvement initiatives. Industry averages are available on the Department of Energy (DOE) Web site (www.doe.gov).
Another assessment technique is to perform annual, quarterly, or monthly steam system assessments with the assistance of the Department of Energy Steam System Assessment modeling tool. This can be outsourced to a service company such as the Spirax Sarco Energy Services Group.
Q: What level of energy savings could an average-sized plant expect to realize by ensuring the efficient operation of steam systems?
A: By having an efficient steam system in operation, an average-sized plant can expect at least 10% to 30% savings. There are multiple ways to accomplish this:
1. Maximizing boiler efficiency.
2. Staying within the fuel range. Fuel companies are imposing tariffs if fuel consumption goes above allowed levels. Staying away from “Peak Energy Billings” and operating within energy-consumption limits results in instant savings.
3. Fixing steam leaks in steam traps, valves, etc.
4. Maximizing condensate recovery back to the boiler.
5. Installing heat recovery systems.
6. Installing boiler stack economizers.
7. Optimizing steam process usage by using controls. For example, when the system process is off, turn the steam off.
8. Using metering and measuring to get maximum output per pound of steam.
9. Installing a boiler TDS blowdown heat recovery system.
10. Using state of the art boiler controls, high efficiency burners, boiler combustion controls and oxygen gas trim controls.