Page 50 - Improving Machinery Reliability

22 Improving Machinery Reliability

modated by the "uprate spare" if insertion of the spare rotor is required before actual
plant expansion.
In a similar vein, uprateability for reciprocating compressors might require pro-
curement of stronger drive elements or a frame with blanked-off spaces for future
connection of additional cylinders. Pumps would be purchased with one or more
impeller locations "de-staged," Le., spaces left blank for future installation of addi-
tional impellers. Steam turbines and large drive motors can be executed with
through-shafts (double-ended shafts) for future addition of tandem drivers, etc.
Typical questions to ask or to consider are:

1. Power capability-will the driver (electric motor, gear-speed increaser, steam
turbine, or gas turbine) handle the uprate requirements?
2. Capacity-will the casing be rated for the anticipated uprate pressures and will
equipment nozzles be sized to pass the flow?
3. Speed-can the machine handle the uprate speed without exceeding critical
speed and tip-speed criteria invoked by API or self-imposed by qualified manu-
facturers?
Screening studies may be conducted with the assumption that machine input
power requirements increase in direct proportion to increased mass flow rates. Addi-
tionally, it is good engineering practice to add an overload contingency of roughly
10% to the uprate factor. Example: The uprate will be from a present 100 mass units
per unit time to a future 130 mass units per unit time. The probable new power
requirement will be 1.3 times the present requirement. The conservative approach
would thus require driver sizing for (1.1) (1.3) = 1.43 times the present requirement.
Capacity uprate capabilities must take into account not only the manufacturer's
casing design pressure but also the pressure ratings or relief-valve settings of down-
stream equipment. In the case of centrifugal compressor uprates, the desired uprate
pressure ratio will result in a new polytropic head, H,. Using the symbol n to denote
polytropic exponents, Z for the compressibility factor, R for gas constant, T for
absolute suction temperature, and r, for compression ratio:

H, = ZRT [n/(n - 1) [rp(n-')'" - I]

This calculation is needed to determine later the approximate uprate speed.
Uprate throughput limitations will usually be encountered if inlet nozzle velocities
exceed 140 fps (42.6 m/sec) for air and lighter gases. For heavier-than-air-gases,
maximum permissible inlet velocities may be significantly lower. Figure I- 13 gives
a rule of thumb for permissible inlet velocities as a function of gas molecular weight
and temperature.
Approximate uprate speeds can be calculated with the help of the H, formula
given earlier:

N uprate = (N original) H, rerate / H, original