Hydrocarbon Processing - November 2021 - 79

Valves, Pumps and Turbomachinery
usually require a high-pressure, multistage
or sweep-flow angle valve like the
separator letdown.
Lower gas volume ratios may be handled
with other valve designs, such as
those designed for low, moderate or high
gas volume ratios. Regardless of the valve
designed chosen, proper alloy selection is
critical for long service life.
Hydrogen quench valve. Another
critical application involves the hydrogen
quench control valves feeding the reactor.
These valves control bed temperatures,
avoid a runaway reaction and maximize
catalyst life and production yield.
They should provide tight control and
good shutoff. Normally, valves in this service
will employ a balanced plug to meet
the shutoff requirements and to handle
temperatures up to 232°C (450°F) with
high pressures. The author's company's
proprietary high-pressure globe control
valvesb
(FIG. 6) are a good option for this
service. If the quench valves are exposed
to lower temperatures and pressures
and there is a concern about particulate
buildup, alternate post-guided control
valves offer a lower cost alternative.
Pressure swing adsorption (PSA)
applications. PSA units are a critical
component for the entire operation
(FIG. 7). Two (or more) adsorption beds
operate in parallel, with one bed getting
regenerated, while the others are online.
These units typically provide very-highpurity
hydrogen to the hydrotreating unit.
The PSA process typically requires
many control valves, each constantly cycling
open and closed as the beds are regenerated
and placed online. In addition
to the very high cycle counts, the valves
are exposed to high temperature differentials,
particulate and pressure from alternate
sides. More importantly, the valves
must stroke quickly and provide nearly
zero leakage because slow-acting or leaking
valves degrade purity and reduce production
for the entire hydrotreating unit.
The best valves for this application
will vary with the pressure and temperature
requirements of the process. However,
high-cycle, reliable, tight shutoff glove
valves are often a good choice. A positioner
with advanced diagnostics should
be used to monitor valve stroke times and
torque at closure, and to sound an alarm
when a valve is falling out of specification
and requires service.
Compressor anti-surge applications.
The green diesel unit will usually employ
one or more hydrogen compressors to
pressurize hydrogen and feed it back into
various points in the process. One of the
most important valves on a centrifugal
compressor is the anti-surge valve, which
protects the machine.
During low-flow, high-discharge pressure
conditions, the gas flow can instantaneously
reverse through a compressor.
When this occurs, the discharge pressure
immediately falls and the gas flow surges
forward again, repeating the cycle. Once
compressor surge starts, the repetitive
flow reversals can shear turbine blades,
destroy bearings and take a compressor
out of commission for months.
An anti-surge valve is installed on the
discharge of the compressor and tied
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Hydrocarbon Processing - November 2021

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