Runout

Various protective instruments are used to provide a shutdown signal (to a fast-acting trip valve at the expander inlet) that senses various things, such as overspeed, lubricant pressure, bearing temperature, lubricant temperature, shaft runout, icing, lubricant level, thrustbearing load, and process variables such as sensitive temperatures, levels, pressures, etc. However, too many safety shutdown devices may lead to excessive nuisance shutdowns. 

To achieve a vibration level off) micron in a rotating mass is practically impossible. It will possess some runout, however precisely and accurately it has been balanced. While rotating, therefore, the shaft on which the other masses rotate, will deflect to the side which is heavier and the opacity around the centre of the shaft wiii become uneven. The masses wiii rotate in smail cireies about their own geometrical axis rather than the axis of the shaft. 

Dimensional and runout inspections to determine the extent of deformation to bores, turns, flange faces, and fit areas. 

The main plant s manufaeturing faeility was also responsible for produeing the new diffuser and first- and seeond-stage stator assemblies, in addition to many other miseellaneous parts. The owner/user s spare rotor was used as part of the rebuild. Complete disassembly, dimensional, and penetrant inspeetions of the rotor were eondueted. New blades were installed in the dises. Final qualifieations of the rotor ineluded rotor stud streteh tests, balaneing, and meehanieal and elee-trieal runout eheeks. 

Onee the final balanee of the rotor is aehieved, a eheek of the eleetrieal runout is eondueted. If this runout exeeeds the standard speeifieations, eorreetive measures are taken. 

Expander-compressor shafts are preferably designed to operate below the first lateral critical speed and torsional resonance. A flame-plated band of aluminum alloy or similarly suitable material is generally applied to the shaft in the area sensed by the vibration probes to preclude erroneous electrical runout readings. This technique has been used on hundreds of expanders, steam turbines, and other turbomachines with complete success. Unless integral with the shaft, expander wheels (disks) are often attached to the shaft on a special tapered profile, with dowel-type keys and keyways. The latter design attempts to avoid the stress concentrations occasionally associated with splines and conventional keyways. It also reduces the cost of manufacture. When used, wheels are sometimes secured to the tapered ends of the shaft by a common center stretch rod which is pre-stressed during assembly. This results in a constant preload on each wheel to ensure proper contact between wheels and shaft at the anticipated extremes of temperature and speed. 

A slow roll eheek must be performed to deteet any meehanieal or eleetrieal runout of the shaft. 

The maximum allowable meehanieal-plus-eleetrieal runout shall be 0.5 mils. 

The maximum permissible vibration, in mils (0.001 in.), during shop test at rated speed shall be equal to the square root of 20,000 divided by the sum of rated speed plus meeh-anieal and eleetrieal runout for the overhung rotor design. Only the aetual total measured runout may be subtraeted from the unfiltered peak-to-peak amplitude measured during testing to attain the shaft vibration. The meehanieal-plus-eleetrieal runout subtraeted from the unfiltered peak-to-peak amplitude shall not exeeed 0.5 mils regardless of total runout. 

Readings must be taken on maehined surfaees with runout below. 5 mil up to 20,000 rpm, and below. 25 mil above 12,000 rpm. 

Gear problems such as unbalance and pitch line runout... 

The shaving proeess improves surfaee finish, involute profile, and lead, and ean be used to erown the teeth. Shaving with inaeeurate eutters will reduee bobbed aeeuraey, and it eannot improve spaeing or piteh-line runout. The shaving eutter has involute teeth and meshes with the part being shaved, thus improving the finish. 

The value thus obtained represents the maximum recommended T.I.R, diameter runout. 

Axial proximity probes are another means of monitoring rotor position and the integrity of the thrust bearing. A typical installation is shown in Figure 21-11. In this case two positions are being monitored one at the thrust runner, and one at the end of the shaft near the centerline. This method detects thrust-collar runout and also rotor movement. In most cases this ideal positioning of the probes is not possible. Many times the probes are indexed to the rotor or other convenient locations and thus do not truly show the movement of the rotor with respect to the thrust bearing. 

For new rotors, where the elements have not yet been put on the rotor, other techniques can be used. First, the components can be individually balanced on a precision mandrel. Precision means that the runout is a few tenths of a mil (.001 inch). The runout high spot should be scribed on the mandrel. The new component now can be reasonably well-balanced. As the component is removed from the mandrel, the mandrel mark should be transferred to the component. When all the components are completed, the shaft is checked for runout. The high spot should be marked. As the components are stacked onto the shaft, the marks on the shaft are aligned with those transferred to the component. This works well with keyless rotors (no key between shaft and component). Experience has shown ihat in most cases with keyless rotors when the stacked rotor is put in the balance machine and checked, the residual unbalance is within the acceptable tolerance. If not, the rotor must be unstacked and the problem located. It must be remembered, however, if the components were properly balanced and the rotor comes out with unbalance, there must be a proh-... 

Vectorial subtraction of slow-roll (300-600 rpm) total electrical and mechanical runout is permitted by the API rules. Vectorial subtraction of bearing-housing motion may be Justified if it can be demonstrated to be of significance. 

Biggs, D. H., Electrical Runout in Rotating Machinery, Hydrocarbon Processing p. 87, Dec. (1972). 

Make sure the shaft does not have an indicated runout. 

Dual runout gages are rigidly mounted on special fixtures attached to the two mating shafts. The runout gages are mounted so that readings can be obtained for both shafts with one 360° rotation. 

When the reverse-dial fixture is mounted on mating shafts, the dials initially should be adjusted to their zero point. Once the dials are zeroed, slowly rotate the shafts in 90° increments. Record runout readings from both gages, being sure to record the positive or negative sign, when the fixture is at the 12, 3, 6, and 9 o clock positions. 

To eliminate this problem, always acquire and record runout readings facing away from the stationary machine component. In this orientation, the 3 o clock data is taken with the fixture oriented at 90° (horizontal) to the right of the shafts. The 9 o clock position is then horizontal to the left of the shafts. 

As with the reverse dial indicator method, the measuring device used for rim-and-face alignment is also a dial indicator. The fixture has two runout indicators mounted on a common arm as opposed to reverse-dial fixtures, which have two runout indicators mounted on two separate arms. 

Optical or laser alignment systems are based on the same principles as the reverse-dial method, but replace the mechanical components such as runout gages and cantilevered mounting arms with an optical device such as a laser. As with the reverse-dial method, offset is measured and angularity is calculated. 

Optical-alignment systems offer several advantages. Because laser fixtures eliminate the mechanical linkage and runout gages, there is no fixture sag. This greatly increases the accuracy and repeatability of the data obtained using this method. 

Check total indicated runout (TIR) of the pump shaft by placing a magnetic base-mounted dial indicator on the pump housing and a dial stem on the shaft. Zero the dial and rotate the pump shaft one full turn. Record reading (Figure 56.7). 

The pre-installation equipment inspection should include the following stuffing box space, lateral or axial shaft movement (end play), radial shaft movement (whip or deflection), shaft runout (bent shaft), stuffing box face squareness, stuffing box bore concentricity, driver alignment, and pipe strain. 

Clamp the dial indicator to the pump housing and measure the shaft runout at two or more points on the OD... 

---