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Troubleshooting Surface Condenser Venting Systems

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Troubleshooting Surface Condenser Venting Systems Troubleshooting Surface Condenser Venting Systems J.R. LINES, R.E. ATHEY AND L.L. FRENS GRAHAM MANUFACTURING COMPANY ABSTRACT tem related problems designed into the system, or caused by a venting system malfunction. Steam condenser venting equipment In an ideal situation, the condensing pressure achievable in a steam is sometimes misdesigned, improperly installed, or required to surface condenser is determined by the exiting cooling water tem- operate beyond its capabilities. The recent trend to part load perature. However the failure of the venting system to properly operation has accentuated the problem. remove noncondensible gases from the steam condenser will result in elevated condenser pressures. Information is presented in this If the venting system is unable to remove the noncondensible paper relative to the most common venting systems available, as gases at the pressure which is achievable by the condenser the well as providing procedures for troubleshooting each type of sys- condenser back pressure will rise. In this case the condenser does tem. A description is given of the various operating characteristics, not control the back pressure. Rather the venting system is the along with narrative discussions of field problems and experiences. controlling factor. It is worth noting that any time the back pres- Simple visual, audible and physical guides to the analysis of vent- sure is higher than necessary due to air retained in the condenser ing system problems are discussed. Power plant operating the plant heat rate will reflect this condition. personnel will be able to utilize this information when investigat- ing steam condenser performance problems. A checklist is Manufacturers that provide both condensers and vacuum equip- provided which can be used to isolate these performance prob- ment to the power industry are involved in the design, lems. fabrication, testing, and repair of condensers, ejectors, and liquid ring vacuum pumps. In this capacity, those manufacturers under- stand the relationship between the steam surface condenser and INTRODUCTION the venting system. The authors of this paper have accumulated a great deal of experience in field service troubleshooting having The condensing pressure achieved in a steam surface condenser is various types of venting problems. The methods outlined in the determined by the exiting cooling water temperature if the condi- following analysis should prove to be a valuable tool in the assess- tions are ideal. All other factors steam condenser design operate ment of steam condenser performance and in solving venting to limit this optimum condition and to raise the condensing pres- related problems. sure, which results in increasing the plant heat rate. Some of the factors which have a negative impact on the condenser pressure include inadequate tube surface area, both vapor and cooling CHARACTERISTICS OF water maldistribution, air inleakage, resistance due to tube bundle SURFACE CONDENSER VENTING EQUIPMENT layout and/or baffle placement, and an inadequate venting system capability. These factors may act independently, or concurrently. Before examining the various methods of identifying and dealing with condenser venting systems, it is essential to have an under- Although it would seem obvious that the failure of venting sys- standing of the tems to properly remove noncondensible gases from the steam several types of condenser must result in elevated condenser pressures, customer vacuum produc- requests for field service have often revealed a lack of understand- ing devices that ing of the relationship between the steam condenser and the are most com- venting system servicing it. Unfortunately this can lead to delays monly used in in remedying many problems, and in a costly waste of effort in conjunction attempts to repair that which was not defective in the first place. with steam sur- face condensers. The operating characteristics of the venting system are often mis- These include understood, which can result in inherent limitations of steam ejector systems condenser performance. Sometimes condenser fouling or con- (usually com- denser design deficiencies are suspected and investigated, when prised of several failure to achieve the required vacuum level is due to venting sys- stages with The American Society of Mechanical Engineers 1 intercondensers and aftercondensers), liquid ring vacuum pumps motive nozzle than is necessary for compression. If the actual (either single stage or two stage pumps), and the hybrid system motive steam pressure is below design, or if the steam tempera- (consisting of a combination of ejectors and a liquid ring vacuum ture is greater than intended, then, within limits, an ejector’s pump). nozzle can be rebored to a larger diameter. The larger nozzle diameter allows more steam to flow through and expand across Ejector Venting Systems the nozzle. This increases the energy available for compression. The ejector manufacturer should be consulted when considering Figure 1 is a schematic diagram of a typical two stage steam moti- reboring a motive nozzle. vated ejector system. Air and water vapor are removed from the main steam surface condenser, enter the first stage ejector and are Another potential ejector performance problem that is related to compressed to the intercondenser operating pressure by means of motive steam occurs if the supply pressure is greater than 20% the motive steam. After exiting the first stage ejector both the above design. When this happens, too much steam will expand load (noncondensible gases and associated water vapor) and the across the nozzle. This has a tendency to choke the diffuser. Whenever this occurs, less suction load can be handled by the ejector, and the suction pressure rises. If an increase in suction pressure is not acceptable, then the ejector nozzle must be replaced with one having a smaller throat diameter, or the steam pressure must be corrected. Steam quality is another important performance variable. Wet steam is generally damaging to an ejector. Moisture in the motive steam is noticeable when inspecting ejector nozzles, because the moisture droplets in the steam lines are accelerated to near sonic velocities. This causes erosion of the nozzle internals by etching a striated pattern on the diverging section of the nozzle, which may actually wear out the nozzle mouth, or the inlet diffuser taper(s) and throat will show signs of erosion. On larger ejectors, the exhaust elbow located at the discharge of the ejector can erode completely through the metal. Severe tube impingement in the motive steam are discharged to the intercondenser where a major intercondenser can also occur; but this is dependent upon the portion of the water vapor load and the motive steam are con- ejector orientation relative to the intercondenser. Finally, wet densed. Noncondensible gases (air) and the remaining water steam can cause performance problems. When water droplets pass vapor are then directed to the second stage ejector where further through an ejector nozzle, they decrease the energy available for compression to atmospheric pressure takes place. Finally, the gases compression. The effect is a decrease in load handling capability are discharged through the aftercondenser. and/or instability of the ejector. Furthermore, water droplets vaporize within the diffuser and then act as additional load, Two stage condensing ejector systems can be designed to operate which must also undergo compression. To solve wet steam prob- at any reasonable condenser pressure, and the design is not limit- lems, all lines leading to the ejector should be well insulated. In ed by the temperature of the available cooling water to the addition, a steam separator with a trap should be installed imme- intercondenser. These systems have no moving parts, are the most diately prior to the motive steam inlet connection. reliable, require the least maintenance of all venting systems, and are the least expensive in their initial cost. The ejector systems The maximum discharge pressure (MDP) is the highest pressure require a reliable motive steam source, generally in the range of that an ejector can attain while utilizing a given amount of 100-150 PSIG. One drawback to this type of system is that the motive steam having a specified amount of energy. If the actual motive steam pressure must be maintained at a relatively constant discharge pressure exceeds the MDP the ejector will become value in order to prevent instabilities (accompanied by a resulting unstable and break operation. When this occurs, a dramatic loss of vacuum). increase in suction pressure is common. As an example, when an ejector designed to produce 1 inch HgA of suction pressure Proper motive steam conditions are always essential to the satis- breaks operation, the suction pressure increases sharply to 2-3 factory operation of an ejector. The manufacturer will have inches HgA. Therefore, it is important to make certain that the designed the system to maintain stable operation with steam pres- ejectors do not exceed their MDP. sures at, or above, a minimum value. If the motive steam pressure falls below the minimum design value, then the motive nozzle Since increasing the discharge pressure above the MDP causes a will pass less steam than required to operate the ejector. When loss of performance, it seems logical that lowering the discharge this happens, the ejector is not provided with sufficient energy to pressure below the MDP should have the opposite effect. This, compress the design load to the design discharge pressure. The however is not the case. Ejectors with a compression ratio (dis- same problem occurs when the motive steam temperature rises charge pressure divided by suction pressure) higher than 2:1 are above the design value, resulting in a larger specific volume than termed “critical ejectors.” The performance of a critical ejector is acceptable. Again, this results in less steam passing through the The American Society of Mechanical Engineers 2 will not improve even if the discharge pressure is reduced. This is the con- primarily due to the presence of the shock wave in the diffuser denser is throat of the ejector.
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