Alignment Of Pumps And Drivers

Outside of serious unbalance of pump components, there is no single contributor of poor mechanical performance more significant than poor alignment. Incorrect alignment between a pump and its driver can cause

• Extreme heat in couplings

• Extreme wear in gear couplings and fatigue in dry element couplings

FIGURE 8 First, second, and third resonate animated mode shapes of multistage high-pressure boiler-feed pump with plain journal bearings. Rotor weight = 377.7 lb (171.3 kg); rotor length = 84.6 in (215 cm); number of stations = 47; number of bearings or bushings = 4. (a) Mode 1: frequency = 2614 cpm; (b) mode 2: frequency = 5223 cpm; (c) modes: frequency = 8134 rpm. (in X 2.54 = cm)

FIGURE 8 First, second, and third resonate animated mode shapes of multistage high-pressure boiler-feed pump with plain journal bearings. Rotor weight = 377.7 lb (171.3 kg); rotor length = 84.6 in (215 cm); number of stations = 47; number of bearings or bushings = 4. (a) Mode 1: frequency = 2614 cpm; (b) mode 2: frequency = 5223 cpm; (c) modes: frequency = 8134 rpm. (in X 2.54 = cm)

• Cracked shafts and totally failed shafts, with failure due to reverse bending fatigue transverse to the shaft axis initiating at the change of section between the large end of the coupling hub taper and the shaft

• Preload on bearings (evident by an elliptical and flattened orbit resembling a deflated beach ball); pure asymmetry of vertical and horizontal vibration can be misleading because the bearing spring constants could vary greatly in the kyy (vertical) and the kxx (horizontal) axis.

• Bearing failures plus thrust transmission through the coupling, which can be totally locked (axial vibration checks across the coupling; that is, at each adjacent machine, will generally confirm this condition)

Significant changes in the cold nonrunning alignment of a pump and driver can take place if the temperature rise in each machine is different and if the piping imposes forces on the pump.

FIGURE 9 First three critical speed mode shapes of multistage high-speed boiler-feed pump superimposed (in > 2.54 = cm)

Therefore, alignment under actual operating conditions must be predicted or, if unknown, confirmed by instrumentation. In either case, an allowance must be made in the initial cold alignment to compensate for changes in alignment from cold idle to hot running.

There are several techniques for measuring cold and hot alignment. The cold alignment is generally measured by either face and rim (Figure 10) or reverse dial indicator (Figure 11) methods.

The face and rim method has a sensitivity advantage when the diameter of a coupling exceeds the indicator span of reverse indicator bracket tooling. This is rare, as the pump will generally have a spacer coupling and the reach of the reverse indicators can be increased by clamping onto the shaft behind each coupling half. The face and rim method would also have an advantage if either the driver or the gear could not be rotated, as it seems unlikely that the pump could not be rotated. In order to compensate for the measuring surface's not being circular or smooth, both shafts should be rotated together when using this method.

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