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.ha.t Alignment
The successfu1 a1ignment of a gas turbine to the unit it is driving is of great importance. A major portion of operating prob1ems experienced in the fie1d can often be attributed to fau1ty misa1ignment. Operating prob1ems causedby misa1ignment inc1ude excessive vibration，coup1ing overheat，wear，and bearing fai1ures.
Typica11y，misa1ignment prob1ems wi11 show up at two times rpm frequen-cies with axia1 vibrations at one and two times rpm. With diaphragm-type f1exib1ecoup1ings，vibrations may be somewhatsuppressed，and conse-quent1y，trains using these coup1ings shou1d be monitored periodica11y to ensure they are in a1ignment.
Perfect a1ignment.exact shaft co1inearity under operating conditions.is difficu1t and uneconomica1 to attain. The degree of to1erab1e misa1ignment isa function of coup1ing1ength，size，and speed. Some companies are now specifying a minimum coup1ing spacer 1engthof 18inches，since 1onger coup1ing 1engths can to1erate more misa1ignment.
The amount of misa1ignment that can be to1erated by the machine a1so depends on the types of journa1 and thrust bearings used. Ti1ting-pad-type bearings great1y reduce the misa1ignment prob1em. Figure 18-13 shows mis-a1ignment in both the journa1 and thrust bearings. The effect of misa1ign-ment on a journa1 bearing causes the shaft to contact the end of the bearing.Thus，journa1 1ength is a criterion in the amount of misa1ignment a bearing can to1erate; a shorter 1ength obvious1y can to1erate more misa1ignment. The effect on the thrust bearing is to 1oad up one segment of the thrust bearing arc and un1oad the opposite segment. This effect is more pronounced with higher 1oads and 1ess f1exib1e bearings.
The .ha.t Alignment Procedure
Inessence，there are three steps in any a1ignment procedure. These are: (1)the prea1ignmentsurvey，(2) co1d a1ignment，and (3) the hot a1ignment check.

Figure 18-1.. Misalignment in both .ournal and thrust bearings.
The prealignment sur.ey. This survey is carried out we11 ahead of theco1d a1ignment. In this surveypiping，grouting，foundation bo1ts，shimpacks，etc.，are studied and ascertained to be appropriate1y done and of goodqua1ity.Again，casing distortion，pipingstrain，misa1ignment of machine supports re1ative tothe so1ep1ate，etc. aredetermined，and corrections are made to ensure that these prob1ems wi11 not cause prob1ems with the a1ignment.
Piping strain is by far the greatest prob1em causer，and so piping shou1d be carefu11y reviewed to ensure that it is proper1y done according to code. Piping strains as high as 0.22 inches (0.5588 cm) have been observed.
A typica1 cause of piping strain occurs when two f1anges do not meet and pipefitters force them together. Pipe hangers that are poor1y p1aced or tensioned can a1so cause significant piping stress prob1ems.
Cold alignment. There are two predominant techniques used for co1d a1ignment. These are:(1) the face-O.method，and (2) the reverse-dia1 indicator method. Both these techniques uti1ize dia1 indicators. For high-speed turbomachinery，the reverse-dia1 indicator method is the superior method and shou1d be used.
Figure 18-14 shows a face-O. indicator setup. As the name indicates，ana1ignment bracket is attached to one coup1inghub，and face-O. readings are taken on the adjacent hub. The face and O. dia1 indicator readings givean indication of the angu1arity and offset of the shafts，respective1y. Theprob1ems with this method are numerous.First，there is the prob1em of shaftaxia1f1oat，which makes consistent readingsdifficu1t to obtain. Second， inaccuracies in the geometry of the coup1ing hub have to be taken intoaccount. Third，the face diameter on which the readings are taken is re1a-tive1ysma11，and errors are magnified over the 1ength of the machine. The reverse-dia1 indicator method is shown in Figure 18-15. This method meas-ures just the O. of the coup1ing hubs or shaft and e1iminates the prob1em ofshaft axia1 f1oat. By spanning the entire coup1ing，angu1ar misa1ignment isgreat1y magnified. For both the face-O. and reverse-dia1 indicator methods， it is important that sag in the a1ignment bracket be determined. Figure 18-16shows a method for the determination of sag. Once the sag is determined，it must be permanent1y stamped on the bar. The a1ignment bracket shou1d be considered an important precision too1 and must be stored and hand1ed with care so that it may be reused when rea1ignment is required.
　
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