Hydrocarbon Processing - March 2022 - 89

Valves, Pumps and
Turbomachinery
M. ALMOUSA, A. AL-HAMOUD and A. AL-GOFILI,
Saudi Aramco, Dhahran, Saudi Arabia
Rotating equipment oil systems
optimization guideline
The industry is moving away from oil
shaft seals and hydraulic speed control for
turbines to advanced alternatives. Many
old installations have been upgraded to
dry gas seals and electronic governors.
However, few changes have been made
to the combined oil systems, leaving the
system overdesigned for its new purpose
and presenting an energy optimization
opportunity. Optimization solutions to
existing systems vary depending on system
requirements; however, the solutions
discussed in this article include utilizing
a lower-speed motor, magnetic coupling,
recycle line and resizing the pump.
Turbomachinery oil systems can be
categorized into three categories: lubrication
oil, seal oil and control oil. A lubrication
oil system aims to continuously
provide clean-cool oil at the required
pressure, temperature and flowrate to
the hydrodynamic bearings and gears
(if applicable). Typically, oil is supplied
with a pressure in the range of 1 barg-2
barg, but the required flow to each bearing
varies depending on bearing size and
equipment speed.
Seal oil systems are similar to lube oil
systems and, in many cases, combine into
a single system. A seal oil system provides
cool-clean oil to the seals at the required
temperature, flowrate and differential
pressure compared to a gas reference pressure.
The pressure at which the seal oil is
supplied varies depending on the machine's
discharge pressure, but it is much
higher than that of lubrication oil and
with a fraction of the lubrication oil flow.1
Control oil is used on turbines to
supply hydraulic power to the inlet valve
actuators, which regulate the turbine
speed based on the valve position as it
PCV
PS-1
Control oil supply
PS-2
FILT
TCV
Exchanger
P1
FILT
TCV
Exchanger
PCV
Manway/vent
Notes
1. PS-1 alarms
2. PS-2 alarms
3. PS-3 alarms
4. PS-4 trips driver
5. A = snubber valve
P1
PSV
PSV
Oil level
Oil reservoir
T1
Heater
Aux pump Main pump
FIG. 1. Lube oil supply system. Courtesy of M.E. Crane Consultant.
controls the flow of gas or steam supplied
to the turbine.
The first step in optimizing the system
is to determine the current oil requirement
regarding flow and pressure. Once
established, the options below should
be explored and evaluated based on economic
benefits.
Lower-speed motor. This option is
applicable to systems with rotary oil
pumps if the system demand pressure
remains the same but the required flow
is reduced. The capacity of rotary pumps
is linearly proportional to shaft speed,
and they provide whatever discharge
pressure the system requires. Therefore,
operating a motor at half speed will provide
half of the flow at the same pressure.
This is an excellent and simple modification
if a large quantity of oil is not required,
allowing for a lower horsepower
(hp) motor since the flow is lower. The
downside of this option is that motor
speed choices are limited to certain rpm,
depending on the motor's frequency and
the number of poles, which restricts the
system enhancement.
Permanent magnet shaft coupling.
This option is also applicable to systems
with rotary oil pumps and reduced flow
demand. The permanent fixed-gap magnetic
coupling allows the pump speed to
Hydrocarbon Processing | MARCH 2022 89
Level gage
A
P1
T1
T1
T1
T1
Turb.
Comp.
Typical lube oil supply system
A
L.O. Supply header
P1
PS-3
PS-4
Trans.
L.O. Drain
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