General description
By horn drop test the loading of the casing on each corner is determined. The horn drop test is repeated at various stages in individual casing i.e. first without connection of any pipe lines the horn drop readings are recorded then it is compared after welding of inlet, outlet and extraction pipe lines etc. The horn drop readings are very important in HP & IP casing. First the proper horn drop readings are made without connection of any pipe lines and then the reading are taken after welding of all pipe lines on HP & IP casing. As such the influence of these major pipe lines are notices on each corner of HP/IP casing by comparing the horn drop test readings. The horn drop test will indicate the quality of work during assembly / welding of pipe lines with the HP & IP casing.
This is a very important check and may cause serious problem in operation of the machine like high vibration in the machine failure of barring gear in hot/cold machine, obstruction during expansion of machine etc.
In horn drop test a drop is measured on an individual corner of the casing with the help of a dial indicator by removing the support of individual corner and then it is compared with the opposite corner. As such this gives an indication of indifferent loading of the casing. In Russian design machine the value of direct dyno-meter loadings are recorded in place of this horn drop test.
Procedure
a) After placement of HP and IP casing on four temporary packers ensure that the each corner of the casing is loaded on temporary packer and if necessary the shims may be given on packer for loading of the casing. The load of shaft is also taken on transport device during this stage.
b) After transferring the load of the rotor on bearing and ensuring the casing centering with respect to shaft the horn drop check may be carried out. c) Before taking the horn drop reading ensure that the enough clearance is
available on front & rear portion of the shaft with their seals in the casing inside. After completing the centering of the casing the some may be lifted by 0.20 mm on all four corners by providing shims. This will help in avoiding TS/ST -01/003/00/95
the touching of seals with the rotor during the horn drop test readings. d) No piping may be connected to HP & IP casing till initial horn drop readings
are over. The piping work should not be postponed for want of horn drop the only thing the last joint with HP/IP casing may be left free till completion of horn drop check.
e) While recording the horn drop readings the casing must be absolutely free and even the radial and axial key of front and rear portion of the casing are either to be removed fully or these are to be made completely free by removing their shims etc. Actually during this stage of erection only temporary axial and radial keys are fitted in the casing and even they carry some shims also. Before making the casings free on axial and radial key portion the dial gauges are to be installed to monitor the movement of the casing on radial and axial direction. Immediately after the completion of the horn drop test these keys/shims may be installed back and centering of casing may be rechecked before further works.
f) In HP casing the initial horn drop test may be done even after fitting of breach nut assembly and HP exhaust elbow of both sides.
g) In IP casing the initial horn drop test may also be done after fitting of IP inlet pipe lower half assembly but this pipe of upper half casing should not be fitted till completion of the horn drop test.
h) All the four Jacking screw and hydraulic jack on each corner of the casing are to be installed for the horn drop test readings. A dial indicator is also to be installed on each corner of the casing for measuring the drop of the casing.
i) After initial centering of the casing the each corner packer may be fitted with a shim of about 1 mm. for further adjustment during the horn drop test.
j) During the horn drop test the individual packer of the casing is removed and the load of that corner is supported over the jacking screw of the casing. Now gradually the jacking screw is also relieved with the help of hydraulic jack on that corner and drop reading is recorded. This is TS/ST -01/003/00/95
repeated for each corner of the casing. In case of variation in left and right side reading the drop is adjusted by ± adjustment of shims from left to right side or vise-versa. No subtraction / addition in shim sizes from outside is done here and only the shims are adjusted from left to right or vise versa till equal loading are achieved in left/right side of individual casing. The sizes of these casing packers are recorded after completion of horn drop to avoid any confusion at a later date while fitting the final packers.
k) While recording the horn drop readings the drop on each individual corner may be controlled with the help of Jacking screw so that during this test the casing is not touching with the shaft in gland portion of the seals at all and enough clearance is left there to avoid damage to the seals inside the casing.
l) During the initial horn drop the best reading within 0.05/0.06 may be achieved by fine adjustment of the packer shims. The comparison in horn drop value should always be made in left and right side only of an individual casing.
m) In case of HP casing the drop on rear end is so high that unless the casing is locked on diagonally opposite end the horn drop check is not possible. Therefore while recording drop on HP rear end the front end of the casing diagonally opposite to it should be locked with casing clamp. The pedestal is also required to be locked on all four sides with its sole plate with the help of the clamps.
In case of IP casing no locking of any corner of the casing is necessary during the horn drop test.
n) The final packers on HP & IP casing are installed only after completing the rolling test of these casing., The horn drop reading for an individual casing may be recorded after the fixing of final packers also.
If necessary a fine adjustment may be carried out at this stage to achieve correct value of the horn drop reading.
o) After completing the welding of all pipe lines with the casing, the horn drop TS/ST -01/003/00/95
readings are repeated again for comparison with the earlier readings. These readings are taken along with its radial and axial keys of the casing. During this stage exactly this can not be defined that how much variation is permitted on these values of horn drop readings. But this variation will indicate the influence of the piping load on the HP/IP casing caused due to welding/connection of various pipe lines with the casing. The variations in horn drop readings are permitted as long as sufficient positive loading is there on each corner of the casing. But if any corner of the casing is fully unloaded/heavily loaded then there is no choice left and pipe lines are to be corrected by dismantling it and making again a free joint with the casing. p) During variation of horn drop reading no correction is to be made
on casing packers by adjustment of shims etc. The variation at this stage is caused due to the piping pull/push only so if any correction is required then the same may be carried out in piping joints and support etc. And any adjustment in piping supports can be carried out only after disconnecting the pipe line with the casing. Even some time it is necessary to cut the individual or more piping joint for correction of horn drop readings.
q) The variation in horn drop reading of left and right side of a casing may be permitted upto a difference of 35% + 10% .
r) The horn drop readings are not taken for L.P. inner outer casing due to its fabricated structure and the parting plane of the casing are leveled on four corner with the help of water level jar arrangements.
s) The IP casing horn drop is done in three steps:
i. firstly, without connecting any pipe lines with the casing.
ii. then after welding of IP inlet upper half pipe (without even any temporary supports) ,and,
iii. and, finally horn drop readings are taken after welding of all pipe lines in the casing.
The horn drop readings obtained at (iii) are compared with readings TS/ST -01/003/00/95
obtained at (i) in light of readings obtained at (ii). This is necessary wherever the IP casings are supplied with one side inlet in upper half casing.
In case of a T-joint in upper half casing where the inlet steam is supplied from both left & right sides, the additional' horn drop check after welding of upper pipe, as indicated at (i) above, is not necessary.
