Hydrocarbon Processing - November 2021 - 40

Engineering and Construction
top view of the cut-out). However,
this may not be the safest solution,
so it was not implemented.
* The second proposed solution
was to change the grating around
the level gauge from carbon
steel to stainless-steel. This was
the best technical solution with
respect to safety and was therefore
implemented. The new material
was procured, and the grating was
installed in the module yard on time
for the module to be transported
as per the project schedule.
* Moving the steelworks: Where
steelworks or beams were present
in the 203-mm radius of the
gauge (FIG. 8), steelworks were
moved further away by the civil
and structural team. Even though
this required more effort, it was
necessary to avoid interference
of ferrous material and ensure
accurate level measurement.
Magnetic level gauge drain orientation.
A drain end can be threaded or
flanged based on project specifications. In
most cases, if the process fluid is non-hazardous,
the drain can remain open; however,
it is sometimes required to route the
drain to a dedicated collection tank. To
facilitate this, a closed drainpipe should
be installed from the level gauge chamber
drain connection to the collection tank.
If a closed drain is required, this should
be determined in the early stages of the
design engineering phase. The routing
of closed drain lines in plant design software
can be completed when preliminary
dimensions are received from the vendor.
The certified dimensions are received
only after an order has been placed-a
significant increase in the dimensions of
the level gauges can lead to the following
scenarios, especially in 3rd-generation
modularization projects:
Clearance from bottom/platform/
FIG. 9. Cutout in grating: for y ≥ 25 mm,
a toe guard is required; for y ≥ 75 mm,
a handrail is required.
grating. To safely remove the float during
maintenance and shutdown, a minimum
clearance is required from the bottom of
the platform to the drain flange (generally
¾ in. or ½ in.) to remove the float
without dismantling the gauge from the
vessel. If the gauge dimensions received
from the instrument vendor are greater
than preliminary dimensions, the drain
connection might have to be re-oriented.
This occurred in a modularization project:
the confirmed dimensions were received
from the vendor after the design
was fixed, necessitating a re-routing of
drain piping to accommodate the increased
dimensions of the level gauge,
which could eventually change the isometrics
of the lines that were released to
the fabricator. To minimize the change
and to meet the minimum clearance requirement
of float removal, it was proposed
to reorient the drain connection of
the level gauge. Care was taken to provide
the correct drain orientation (east, west,
north and south) based on the orientation
of drain piping. In collaboration with
the vendor, the face-to-face dimension of
the bottom flange to the drain flange was
made as small as possible. FIG. 10 shows
such a case where the drain orientation
changed from the bottom to the side.
Maintenance/access. To accommodate
new increased dimensions or a
change in dimensions of a magnetic level
gauge, access to piping items (e.g., isolation
valve) connected to the drain piping
can be compromised due to space constraints.
Since the goal of modularization
is to save space, cost and time, these challenges
are often encountered. Accurate
modeling is vital to ensure access to critical
and frequently used items.
Float removal. Float removal clearance
is the space required to remove the float
from the bottom of the platform during
maintenance or float failure. Float
removal clearance depends on the float
length-ideally, this is equal to the float
length plus 50 mm to allow easy removal
from the bottom flange of the chamber.
No overhead structure should be directly
in line with the level gauge chamber or
float removal in case of top-mounted
gauges, which require a removal clearance
equal to the float length plus the
float guide/rod length. Any additional
required clearance depends on the construction
of the level gauge. Due to space
constraints in modularization projects, it
sometimes becomes necessary to remove
the entire level gauge assembly to remove
the float. This should be documented in
the mechanical handling process, which
describes the handling of each instrument
during maintenance.
Heat
tracing:
Electric/steam. For
FIG. 10. Drain orientation changed from the bottom to the side.
40 NOVEMBER 2021 | HydrocarbonProcessing.com
applications requiring fluid to be maintained
at a positive temperature and to
avoid freezing, electric heat tracing, heat
tape or steam tracing can be fitted on the
magnetic level gauges. Trace heating is an
electric heating element or steam line run
in physical contact along the length of the
liquid chamber.
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