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SEP-OCT 2018

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28 INTECH SEPTEMBER/OCTOBER 2018 WWW.ISA.ORG SYSTEM INTEGRATION forming correctly, and tagging the bad actors for maintenance, requires an ap propriate tool to evaluate tempera- ture, such as an infrared viewing de- vice. These can do the job, but a techni- cian has to get to wherever the steam trap is and make the evaluation. Unless manual rounds by a very highly quali- fied and experienced technician hap- pen regularly and frequently, one or many steam traps can malfunction for quite a while. A recent study suggests that 18 percent of steam traps in a large chemical manufacturing facility can fail in a given year, resulting in wasted energy costs up to $16,000 per trap. One traditional approach to moni- tor a steam trap involves finding a way to mount a temperature sensor on the trap itself to measure the trap's condi- tion. But this is an invasive solution, and the data it provides requires exten- sive interpretation and knowledge of what the just-right temperature should be under the operating conditions. Hearing the solution Most steam traps do not release con- densate continuously. Although such situations are possible, under normal conditions and if sized correctly, all steam trap designs open intermittently and discharge condensate in slugs. The internal turbulence when this happens creates noise that transmits through the adjacent piping. Someone listening to a properly working steam trap should hear these periodic releases interrupt- ing times of silence as appropriate amounts of condensate accumulate. An acoustic transmitter mounted on the pipe adjacent to a steam trap (figure 3) can listen to the noise it makes. It is sensitive to ultrasonic fre- quencies, so it can hear the cycling, and an algorithm can be applied to learn the characteristic activity for each trap. Data can be sent from the transmitter via WirelessHART to a central data col- lection and analysis platform, where operators can see how the steam traps equipped with acoustic transmitters in all parts of the plant are performing. Dashboards display (figure 4) which steam traps are working correctly and which are in one failure mode or the wastes energy. Consequently, there are different steam trap designs that remove condensate under different circumstances. Again, there are many more in-depth resources available, but some types of steam traps only release condensate when its temperature falls below a specific threshold; whereas others are simply concerned with liq- uid volume. The application will dic- tate which is most appropriate. Functionally, a steam trap is a valve that opens and closes automatically in response to its situation. All designs, therefore, have some moving parts and a seating surface. Thermodynamic traps are very simple with only a single moving part; whereas mechanical de- signs (e.g., float and inverted bucket) are more complex. Unfortunately, where there is a mechanism, there is an opportunity for malfunction, but these types of problems can be predicted. Steam trap failure modes Steam is not always clean. Although feedwater is heavily treated, it is still possible for scale, which can break free and be carried by the steam and con - densate, to form in the system. Such particles have an uncanny ability to come to rest in problematic spots, such as valve seats or mechanisms. Similar - ly, if feedwater treatment chemicals get out of balance, excess corrosion can result. Operating conditions, such as water hammer and vibration, also take a toll on valves, fittings, and steam traps. A steam trap can fail in one of two ways: it sticks open and releases steam, or it sticks closed and does not release anything. Inspectors on plant rounds checking traps generally classify them by diagnosis: l There is an obvious steam leak—a major mechanical failure. l The trap is too hot—it is the same temperature as the steam line, be - cause it is releasing steam directly into the condensate line or to the atmosphere. l The trap is too cold—it is stuck closed, and no condensate is being released. l The trap is just right—it is releasing warm condensate. Finding these "Goldilocks" units per- differential between the steam and product is at its greatest. The steam transfers its heat into the product quickly and condenses. If the process is aiming at the quickest heat up, this is the time when steam flow is highest. As the product temperature increases and heat transfer slows, steam flow has to be reduced or steam will be blown from the outlet, which wastes heat. Condensate runs into a steam trap, which allows the liquid to escape and return to the boiler via a collection sys- tem, but the trap stops steam, trapping it in the jacket. A steam trap is actually a condensate separation device. It has an enormous effect on the efficiency of the application. If it does not remove condensate fast enough, the conden- sate backs up into the steam passages, which reduces heat transfer. If it allows steam to blow past, heat is wasted. If the steam trap is sized properly and uses an appropriate design for the ap- plication, its action should be automat- ic, provided it is functioning correctly. Steam trap designs The discussion so far suggests that all condensate needs to be removed from steam, but the situation is more nu- anced. Small amounts of condensate in a high-pressure steam line will be at a high enough temperature that it will flash into steam if it reaches a point where the pressure drops. Condensate carries a great deal of heat itself, so re moving it when it is not needed also Figure 3. An acoustic transmitter mounts next to the steam trap on the pipe, so no shutdown is required for installation.

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