Page 335 - Subyek Teknik Mesin - Forsthoffers Best Practice Handbook for Rotating Machinery by William E Forsthoffer
P. 335

Steam Turbine Best Practices  Best Practice 5 .14


               Best
               Best Practice 5.14Practice 5.14
               Install a low pressure steam seal eductor and oil condition  Lessons Learned
               monitoring bottles on single stage turbines to prevent and  Failure to design single stage turbine steam seal systems
               monitor bearing oil contamination.                  for vacuum systems and bearing bracket oil condition
                  A common reliability issue with single stage steam turbines is the  monitoring bottles has resulted in frequent turbine shut-
               undetected steam seal wear and entrance of condensate into the small
                                                                   downs for seal and oil changes and catastrophic bearing
               volume bearing housings.
                                                                   failure and fires (resulting from ‘red hot’ bearing housings).
                  Steam seal wear will rapidly displace bearing oil with condensate
               causing Babbitt-lined, sleeve bearing wear with potentially dangerous
               consequences.                                       Benchmarks
                                                                   This best practice has been used since the mid 1990s for recom-
                  Installation of a low pressure steam eductor and associated piping
                                                                   mended field modifications and project requirements. Major oil com-
               with a vacuum gauge from each seal leak off port will function in the same
                                                                   panies have adapted this best practice to produce steam seal MTBFs
               manner as special purpose steam turbine seal systems (see BP 5. 8), and
               positively prevent the entry of condensate into the bearing housing.  in excess of 100 months.
                  Installation of oil condition monitoring bottles at the bottom of each
               bearing bracket will allow operations to monitor bearing oil condition
               by observing water accumulated in the bottom of the bottle.

              B.P. 5.14. Supporting Material                       system linkage during start-up, and during normal operation
                                                                   when steam conditions change. It appears as if the governor is
                                                                   not responding because speed will not be controlled when it
              Single stage turbine guidelines                      should. Typical examples are:

              The five common problems with single stage turbines are noted  - Speed will continue to increase when throttle valve is
              in Figure 5.14.1.                                      opened; turbine will trip on over speed
                                                                   - Speed will increase or decrease when:
                                                                     - Steam conditions change
                                                                     - Driver equipment changes
                  Bearing bracket oil contamination (inadequate carbon ring
                  steam seal design)                                 These facts are presented in Figure 5.14.6.
                  Slow governor system response (inadequate governor linkage  Since most single stage steam turbines are not supplied with
                  maintenance and governor power)                  tachometers, it is difficult, if not impossible, to condition
                  Hand valve(s) closed on critical services
                  Bearing bracket oil viscosity reduction and bearing wear (high  monitor this problem. A condition monitoring action plan is
                  pressure service)                                provided in Figure 5.14.7.
                  Use of sentinal valves on turbine cases            The usual root cause of the problem is that the friction in the
                                                                   mechanical linkage and/or valve stem packing exceeds the
                                                                   maximum torque force that the governor output lever can de-
              Fig 5.14.1   Single stage steam turbines: common reliability  liver. The governor designations TG-10, TG-13 and TG-17
              problems
                                                                   simply mean ‘turbine governor with FT-LB torque’. Therefore,
                                                                   if a TG-10 governor is installed, and the torque required to
                We will now discuss each problem in detail. Please refer to  move the valve steam exceeds the value of 10 FT-LBs, the
              Figure 5.14.2 which has each problem area circled.   governor system will not control speed. Taking the governor to
                                                                   the shop will not solve the problem. Causes of excessive friction
              Bearing bracket oil contamination                    are shown in Figure 5.14.8, and a plan to eliminate this reliability
                                                                   problem is presented in Figure 5.14.9.
              Please refer to Item 1 in Figure 5.14.2.
                The most common reliability problem with single stage
              steam turbines is the contamination, with water, of the oil in  Hand valve(s) closed on critical services
              thebearing housing. Theroot cause of theproblem is   Most single stage steam turbines are supplied with one or more
              the ineffectiveness of the floating carbon ring shaft seal system.  hand valves in the steam chest. Refer to Figure 5.14.2, Item 3.
                Unless site systems are modified to eliminate the root cause,  The purpose of the hand valves is to allow more or fewer inlet
              the best plan is to minimize the effect of the contamination so  steam nozzles to be used during operation. Optimizing the
              a bearing failure will not occur. Such an action plan is presented  steam nozzles used maintains turbine efficiency during load
              in Figures 5.14.3, 5.14.4 and 5.14.5.                changes. However, the efficiency of single stage steam tur-
                                                                   bines is only 35% at best! Therefore, adjustment of hand
              Slow governor system response                        valves, other than during start-up or during slow roll, should
              Please refer to Item 2 in Figure 5.14.2. Another very common  not be required. Figure 5.14.10 is atop view of aonehand
              reliability problem is the slow or non-movement of the governor  valve.

                                                                                                               307
   330   331   332   333   334   335   336   337   338   339   340