Hydrocarbon Processing - June 2021 - GP-36

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In most cases, the
required size will make
a high-performance
butterfly valve the
best choice for a
turbine trip valve.
Specification requirements. Some of
the steps in properly specifying a turbine
trip valve are similar to those taken
in specifying any type of valve. The first
is to determine how much fluid will be
flowing through the valve. The actual
flowrate will determine the size of the
valve required for the job. In most cases,
the required size will make a high-performance
butterfly valve the best choice
for this application.
Next, the typically low process temperatures
must be taken into account. It
is not unusual for natural gas trip valves
to face design conditions below -100°C
(-148°F). These cryogenic temperatures
require thick piping insulation, and
the valves need long stem extensions so
that the critical stem sealing components
are outside of the cold zone.
The next step is to determine the required
torque, which is a function of the
valve size, seat and bearing designs, as
well as process temperatures and pressures.
An analysis of the required torque
and cycle speed will allow the calculation
of the maximum rotational force
that the long stem extension will need
to withstand. It will also determine how
much " shaft windup " will result.
Once the maximum rotational force
is known, an actuator must be selected
to provide the required torque. An adequate
safety factor should be applied
to accommodate process upsets, aging
equipment, etc. The extremely fast required
cycle times frequently make oversized
air ports necessary.
Once the major components-the
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valve and actuator-have been selected,
similar care must be used to select the
control components. Similar to the computer
modeling used to size and select a
process valve, the pneumatic system also
must be modeled to allow the selection of
solenoid valves and quick exhaust devices
to vent the air from the actuator quickly
enough to meet the trip requirements. All
sources of pressure loss must be accounted
for, such as pipe nipples, compression
fittings, elbows, tees and exhaust screens.
Other considerations. A few other important
characteristics, if not recognized
and addressed properly, can be problematic.
For example, the combination of
the mass of the moving components of
the valve and actuator and the fast cycle
speeds can cause the actuator to be damaged.
In this case, shock-absorbing bumpers
must be specified for the actuator
travel stops.
The locality where the trip valve
will be installed must be accounted for.
All components must meet local piping
codes and electrical certifications,
and the ambient conditions may require
sunshades, corrosion-resistant materials
or coatings, air system " rebreathers " or
heat tracing.
Testing. Finally, each trip valve assembly
must be tested to verify that it meets all
the system requirements. Most customers
require certified material test reports
to confirm that the critical pressureretaining
components are made from
the alloys specified by the manufacturer.
Other forms of nondestructive examination
(e.g., X-rays, liquid penetrant tests,
positive material inspection, etc.) also
may be required by the customer.
After the trip valve system has been
completely assembled, it must be tested
and timed to verify that it meets the trip
speed requirement. This step cannot be
skipped, as computer modeling alone
is not sufficient for this critical piece of
safety equipment. This testing is typically
done using data loggers and sensors. The
sensors detect valve position and air pressure,
allowing the test system to graph the
valve through the complete cycle.
Heavy engineering is required to ensure
that a trip valve will work properly
and do the job needed. Understanding all
of the conditions and requirements will
ensure that a critical element is not overlooked
that may lead to damaged equipment,
unplanned shutdowns and potential
injuries or deaths. GP
PETER JESSEE, P.E., is a Process Control
Application Engineer for Valin Corp., having joined
the organization in 1990. He has many years of
experience in both technical and sales positions for
industrial and commercial control systems, control
valves, flowmeters and industrial instrumentation.
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Hydrocarbon Processing - June 2021

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