Chemical Engineering April 2021 - 33

ation when the turboexpander is taken
out of service (for example, to perform
maintenance or during an emergency),
whereas a pressure-reduction valve is
used in continuous operation to deliver
excess gas, when the total flow exceeds
the expander design capacity.
Emergency shutdown (ESD) valve. An
ESD valve is used to stop gas flowing
into the expander during a contingency
to prevent mechanical damage.
Instruments and controls. Important
variables to monitor include inlet and
discharge pressures, flowrate, rotational
speed and power output.
Overspeed trip. This device shuts off
the flow to the turbine, which causes the
turbine rotor to decelerate, protecting
the equipment from an excessive speed
derived from unexpected process conditions
that could damage the equipment.
Pressure safety valve (PSV). A PSV is
usually installed downstream of the turboexpander
to protect the low-pressure
line and the equipment. The PSV must
be sized for the worst contingency, with
typical cases including bypass valve failure
open. In cases where the expander
is added to an existing pressure letdown
station, the process design team
should determine if the existing PSV already
provides adequate protection.
Preheater. A preheater compensates
for the temperature decrease caused by
passing gas through the turbine, making
it necessary to preheat the gas. Its
main role is to increase the temperature
of the upstream flow to maintain the
temperature of the gas at the expander
discharge above minimum values. The
temperature increase has the added
benefit of increasing power output while
also avoiding corrosion, condensates
or hydrates that can have a detrimental
effect on the nozzle of the equipment.
In systems containing heat exchangers
(such as the one shown in Figure 3),
gas temperature is typically controlled
by regulating the flow of heating fluid to
the preheater. In some designs, a fired
or electric heater may be used instead
of a heat exchanger. In existing J-T
valve stations, a preheater may already
exist, and the addition of an expander
may not require further preheaters to
be installed, but rather an increase in
heating-fluid flowrate.
Lube oil and seal gas systems. As described
above, expanders may use different
seal designs, which may require
lube oil and seal gas. Where applicable,
the lube oil must be maintained at high
quality and purity as it encounters process
gas, and the viscosity levels of the
oil must be maintained within the required
operation ranges of the oil bearings. A
seal gas system generally accompanies
a lube oil installation to prevent the oil
from the bearing housing from entering
the expander housing. In the particular
case of companders for use in the hydrocarbons
industry, it is common to design
lube oil and seal gas systems to API
617 [5] Part 4 specifications.
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