Operating speed

Near top speed, a fan may operate at a speed that is near or above the natural frequency of the wheel and shaft. Under such conditions, the fan can vibrate badly even when the wheel is clean and properly balanced. Whereas manufacturers often do not check the natural frequency of the wheel and shaft ia standard designs, many have suitable computer programs for such calculations. Frequency calculations should be made on large high speed fans. The first critical wheel and shaft speed of a fan that is subject to wheel deposits or out-of-balance wear should be about 25—50% above the normal operating speed. 

Performance Curves. Pan manufacturers furnish fan performance curves for each type fan available. These are typically based on 61 m/s (12,000 ft/min) tip speed and 1.20 kg/m (0.075 lb /ft ) density. To select a fan for a specific duty requires knowledge of the flow, static pressure resistance, and density of the actual operating conditions. Usually the fan diameter is known as well as some idea of operating speed a 61 m/s tip speed can often be assumed. 

Hydrostatic drives allow for selection of any travel speed up to the maximum without a concurrent variance in engine speed. The engine can be operated at the governed speed to provide proper operating speeds for auxiliary elements, eg, the threshing section of a combine. A frill range of travel speeds is available to adjust to terrain or crop conditions. Industrial applications for hydraulic systems and hydrostatic transmissions include the following (16) ... 

Note in Eq. (18-112) that Vg is the settling rate under Ig, and it is a function of the particle size and density and fluid properties. The ratio ( /2Vg is then related only to the operating speed and geometry of the centrifuge, as well as to the size recoveiy. It measures the required... 

To avoid operating difficulties, the torsional critical frequencies of the combined engine and driven equipment should be calculated or measured to assure that operating speeds are removed from these criticals or that vibration dampers are provided or that the equipment is designed for the resulting cyclic stresses. 

The selection of rolling-element bearings is based on the radial and axial loads they must support, on the operating speed, and on the expected life. All bearing catalogs provide comprehensive selection data. 

There are several operational differenees between the TPG arrangement and the full FCC string ineluding the normal operating speed, synehronization, and startup. 

The normal operating speed of the full string ean vary as mueh as 5%. The normal operating speed of the TPG string is usually eonstant at synehronous speed or, at most, a -i-l% speed for an induetion generator drive. 

All rotating parts must be dynamieally balaneed to an appropriate toleranee on preeise eleetronie balaneing equipment to ensure smooth operation over the entire operating speed range. Balanee toleranees are set by API eriteria for high-speed turbomaehinery. 

Any torsional natural frequeney of the rotor system shall not be eloser than 10% to any operating speed (i.e., rated speed). 

The limits expressed in Table 7-3 (x-axis = operating speed in rpm, y-axis = overall vibration level in mils [peak to peak]) are based on experienee in refineries. The guide refleets the typieal proximity probe installation elose to and supported by the bearing housing and assumes the main vibration eomponent to be of [lx rpm] frequeney. The... 

Appears suddenly at or above rotor critical speed when critical is below one-half operating speed. Increasing speed increases vibration amplitude, blit whirl frequency remains constant. When speed IS decreased, vibration disappears below where it first appeared. Fnction-indnced rotor whirl Encountered m bnilt-np rotors or rotors with shrink fits or rotor disassembly to inspect fits, increase shrink fits Coupling friction has been known to induce whirl... 

Occurs when two or more machines mounted on common foundations operate at nearly the same speed Occasionally, a beat can develop in one machine if its operating speed is close to a structural component resonance... 

High-frequency, destructive vibrations. Unaffected by operating speed. Steam turbine valve vibration Rare occniTence change valve ping, seat shape, and/or increase valve gear rigidity... 

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