Page 460 - Subyek Teknik Mesin - Forsthoffers Best Practice Handbook for Rotating Machinery by William E Forsthoffer

Lube, Seal and Control Oil System Best Practices Best Practice 7 .29

Fig 7.29.3 A.C. motor-driven main pump, steam turbine auxiliary pump drive (Courtesy of Elliott Co.)

coastdown of the critical equipment unit. The DC power source flow to the system for approximately ten seconds as a minimum
can be obtained from a UPS (Uninterrupted Power Supply) since the start-up time of a steam turbine is much slower than
system or other sources. that of a motor-driven pump.
Coastdown times of the driven equipment vary between 30
seconds to in excess of 4 minutes. Sufficient emergency pump Centrifugal pump system
energy must be available. The emergency pump would be acti-
vated in this case on auxiliary system pressure drop. Note that Figure 7.29.4 depicts a dual centrifugal pump system. The main
the emergency pump is not a full capacity pump. It is sized to pump is steam turbine-driven and the stand-by pump is
only provide sufficient flow during equipment coastdown to motor-driven. Such a system can be used in areas of the world
prevent auxiliary system component damage. where large ambient temperature fluctuations are not present;
Middle East, etc. Since the pumps are dynamic type, note that
a bypass valve is not used in this system. This is because the flow
A.C. motor-driven main pump, steam rate of dynamic pumps are determined by system resistance.
turbine-driven auxiliary pump
Therefore, the control valve in this application senses pressure in
Refer to Figure 7.29.3 for a diagram of this system. A system of the system and adjusts system resistance at the pump discharge
this type would be used in a plant where dual pump driver to supply the required flow.
power is available, but steam is at a premium. Therefore, the Referring back to the concept of an equivalent vessel, let us
turbine driver is not desired to be used for continuous operation. examine the case of increased system demand as in the case of
This system requires continuous attention in terms of steam bearing wear. Bearing wear will gradually increase system
turbine steam conditions. The turbine must be capable of ac- demand and reduce system equivalent vessel pressure. Consider
celerating rapidly since a system accumulator is not included. in this case the system to be the equivalent vessel. Since the
Therefore, steam conditions at the turbine flange must be control valve senses equivalent vessel pressure, a reduction in
maintained as specified. That is, steam must be dry. A steam the equivalent vessel pressure will open the valve to provide
trap is recommended to be installed to continuously drain greater pressure at the sense point. This action in turn will
condensate from the steam. To ensure rapid turbine accelera- reduce pump discharge pressure. Referring to the characteristic
tion, the turbine inlet valve must be a snap open type (less than 1 curve of a centrifugal pump, reduced pump discharge pressure
second full open time) to minimize auxiliary pump start-up results in increased pump output flow, thereby compensating
time. Experience has shown that this type of system is not re- for the increased demand requirement.
liable in terms of auxiliary pump starts. If used, this type of In the event of a sudden system flow increase, a sudden
system should employ a large accumulator to provide sufficient change in the equivalent vessel pressure would be experienced.

431