Hydrocarbon Processing - July 2021 - 42

Plant Design, Engineering and Construction
If the width is short and cannot accommodate
thermowell projection, it can be
made inclined, as shown in FIG. 6.
Liquid
withdrawal nozzle connections.
In a distillation column, partial or
total liquid withdrawal from the tray is a
In a distillation column, fixing nozzles elevation and
orientation are based on fixed guidelines. This knowledge
helps design engineers shorten the engineering and
procurement cycle.
Safety valves (pressure relieving devices).
Safety valves are normally provided
on the top of the vessel or column (FIG. 7).
Normally, the column/vessel has a mist extractor
in the top section. Under pressure
surges where chances exist of dislodgement
of the mist extractor and choking of the
safety valve inlet, a good engineering practice
is to provide a pipe sleeve larger than
the safety valve inlet size and pass it through
the mist extractor (FIG. 7B). Under fouling
service conditions, it is good engineering
practice to provide a safety valve connection
below the mist extractor (FIG. 7C).
regular requirement. FIG. 8A shows details
of a partial liquid withdrawal from the tray
by providing a recessed seal pan and inlet
weir as a downcomer seal. Partial withdrawal
from the lowest trays can be accomplished
by providing a seal pan with a
draw-off pan (FIG. 8B). Here, the nozzle is
kept flush with the draw-off pan level.
For the total withdrawal of liquid, a
collector (chimney tray) is normally provided,
as shown in FIG. 8C. The nozzle elevation
is kept flush with the recessed pan.
Such trays require zero leaks and are generally
seal welded.
FIG. 9. Level measurements, chimney tray.
Level measuring instruments. Most of
the instruments in this category are level
gauge, level transmitter/controller, level
switches, etc. (FIG. 9). Normally, each of
these instruments has two connections:
top and bottom. Depending on instrument
selection, these can be arranged
side-side, side-bottom, top-bottom, etc.
Before fixing the elevation and orientation
of such nozzles, the operating level, alarm
level or shutdown levels of each situation
must be obtained from the process engineer.
The top connection of the instrument
must always be above the highest
liquid level (HLL) in vapor phase, while
the bottom connection must always be
below the lowest liquid level (LLL). The
elevation of each instrument
must be determined based
on the operating levels and
shutdown or alarm levels.
If the tray has a recessed
pan, as shown in FIG. 9, the
advantage of seal pan depth
can be taken to have a wider
operating range.
Level control instruments in a reboiler
feed baffle. For thermo-siphon
reboilers, the reboiler feed from a distillation
column is controlled by a pass partition
baffle (FIG. 10). The baffle overflow
is taken out as product. Baffle height is
fixed based on the static head requirement
of the reboiler.
In this section, all level instruments
and controls are provided in the overflow
section, which goes to the bottom product
outlet. To fix level instruments in this
section, an engineer requires operating
levels, alarm levels and shutdown levels.
Caution must be taken to provide all
these instruments in the product section
only. The author noticed a mistake in one
case where the sections were reversed.
Sampling connections. Sampling connections
are small nozzles sizes ranging
from 40 mm-50 mm. Sample collection
requirements can be for liquid or vapor;
this should be clearly specified in the datasheet.
Liquid samplers are placed in the
liquid zone, either in the downcomer area
or close to the tray level. Vapor collection
is done from the vapor zone, mostly from
below the tray active area.
Takeaway. With this information about
various types of nozzles, their technical
requirements and placements, working
engineers can apply the given logic and
decide for themselves the nozzle elevations
and orientation.
PRAMOD DIXIT works as a
consultant at Baroda India. He
spent 20 yr at Engineers India Ltd.
as a trays and tower internals
specialist, and another 20 yr in EPC
companies Larsen and Toubro and
Essar Offshore in different senior
FIG. 10. A reboiler feed baffle in a one-pass tray (A), and a reboiler feed baffle in a
two-pass tray (B).
42 JULY 2021 | HydrocarbonProcessing.com
positions. He provides training to operating company
inspection and maintenance engineers in related field
and advisory services in the troubleshooting of
engineering issues. Mr. Dixit holds a B.Tech degree in
mechanical engineering from IIT Roorkee India.
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