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Reliance Engineering Associates Private Limited

PIPING DESIGN GUIDE

P-EP-PL-044-0A

Piping Design – Towers

0 26/07/00 Issued as standard SSB MGC

Rev Date Revision By Chkd Appr Client

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Contents

1. Purpose … … … … … … … … … … … … … … … … … … … … … … … … ..3

2. Introduction … … … … … … … … … … … … … … … … … … … … … … … 3

3. Terminology … … … … … … … … … … … … … … … … … … … … … … ..3

4. Initiate Trayed Tower Layout … … … … … … … … … … … … … … … … 3

5. Tower Layout From Top Down … … … … … … … … … … … … … … … .5

6. Reference Drawings … … … … … … … … … … … … … … … … … … … .10

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List of Figures
Figure 1 Establish Tower Elevation

Figure 2 Set Tray Orientation

Figure 3 Alternative Methods of Reboiler support

Figure 4 Feed Nozzles

Figure 5 Manway Locations

Figure 6 Vapor Overhead Line

Figure 7 Top Head Platform

Figure 8 Closed Relief System

Figure 9 Reflux Nozzle

Figure 10 Instrument Nozzles

Figure 11 Preferred Component Locations

Figure 12 Miscellaneous Nozzle Considerations

Figure 13 Tray Details

Figure 14 Packed Bed Section

Figure 15 Platform Details

Figure 16 Optimizing Tower Layout

Figure 17 Reboiler and Pipe Supports

Figure 18 Trayed and Packed Towers

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1. Purpose

To provide a layout designer with the guidelines to develop a comprehensive

fractionation tower design which considers safety, operation , maintenance and
economics.

2. Introduction

The philosophy addressed in this guide primarily deals with trayed fractionation
towers. It is the responsibility of the Plant Design and Piping group to develop a
layout based upon a combination of certain specific rules and logic.

Reference should also be made to Piping Design Guide 3PS-PL-003 (Piping Design &
Plant Layout) for distance limitations to additional process equipment.

3. Terminology

3.1. Safety

3.11. Design of towers must give proper attention to safety for all plant
personnel who will be required to work within its confines. As one
travels from grade to the upper most platform, the area must be free of
dangerous obstructions when attempting to gain access to values,
instruments or exit the tower in an emergency.

3.2. Operation

3.2.1. Modern technology rarely requires constant attention on trayed towers.

When required, access to valve hand wheels and instrumentation
should be placed in such a way that a minimum amount of time is
required to perform the function.

3.3 Maintenance

3.3.1. Features to be considered for towers include providing davits at the top
of towers for handling large relief valves, placing manways in a way to
facilitate removal of internals to grade, locating davits or hinges on
manway covers in a way it does not obstruct other required
maintenance or access, provide removable platform sections if
necessary to accommodate maintenance below, etc.

4. Initiate Trayed Tower Layout (refer to Fiqure 18)

The following approach is recommended when the design of a fractionation tower is

initiated.
a) Verify all documentation to be used is the latest available issue.
b) Set tower bottom tangent line elevation (see 4.1).

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c) Start layout by working from the top of tower downward (see 5.)
c) Utilize internal piping whenever possible to maximize exterior layout.
d) Work closely with stress/support engineers throughout design.
e) When locating nozzles and manways, be sure vessel internals, such as tray
supports or internal piping or tray configurations will not impede maintenance or
operation.

4.1. Establish Tower Elevation – Figure 1

The six prime factors listed below should be considered when attempting to set the
bottom tangent line elevation include:

4.1.1. Pump NPSH

The process/Project engineer is responsible to supply the pump NPSH requirements
for all pump circuits.

4.1.2. Thermosyphon Reboiler Requirements

The project process engineer is responsible to establish the dimension from the vessel
tangent line to the centreline of a thermosyphon reboiler. When a vertical reboiler is
used, adequate clearance must be given to remove the lower channel section.

4.1.3. Piping Clearances

Should the pump head requirement or reboiler dimension be a non-issue in a layout,
operator clearance under the liquid outlet line to adjacent equipment may be the factor
which sets the tangent line elevation. All factors must be reviewed before a final
elevation can be transmitted to a vessel vendor. Should none of the above factors be
relevant, the vessel tangent line may be set at the minimum practical elevation.

4.1.4. Operator access

4.1.5. Maintenance access

4.1.6. Common access

4.2. Establish Tray Orientation

a. The two primary piping circuits which normally impact tray orientation are the
feed nozzle and reboiler connection.
b. The feed nozzle may have one or a multiple of external connections with an array
of internal piping configurations. See Figure 4. Three typical feed arrangements
are :

(1) Single feed nozzle with two possible orientations.

(2) Double feed nozzle with two possible orietations.
(3) Single feed nozzle with a multiple of orientation options.

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A designer should investigate which single feed nozzle arrangement (#1 or #3)
would be most advantageous for the optimum tower layout, should this option be
possible.

c. For preferred internal piping and tray details see Drawings R-501, 502, 503
and 504.

d. Should the feed nozzle not be a deciding factor, refer to the reboiler piping
requirements in paragraph ‘e’.

e. Since the preferred reboiler piping arrangement is the most direct route,
coming off the bottom tray downcomer now comes into play in setting tray
orietations. A stress engineer should approve the proposed layout before assuming
the selected nozzle location will work for the piping. Schemes (A) in Fig 2 is an
example of a number of arrangements for the reboiler return line. Tray orietations
are unaffected by draw-off nozzles located in the bottom head of the tower. These
nozzles may be located at any orietation to suit.

f. Method of support for vertical reboilers needs to be established as soon as

reliable design data becomes available. See Fig 3. Direct support off the vessel
are shown (A) and (B) which will require a thicker wall thickness, but a normally
less complex piping system. Scheme (C) shows an independent support from
grade. While it normally does not effect the vessel wall thickness, it requires a
support structure, foundations and generally a more complex piping system to
compensate growth differential between the tower and the stationary structure.
Refer to 5.12, paragraph ‘a’.

g. One remaining item to establish at this time is to set the general orientation of
manways, as shown in figure 5. The most common location is opposite the pipe
rack or that direction maintenance is most likely to be performed. When selecting
the orientation, entrance into the tower is not obstructed by internal piping,
downcomers, tray supports and is over the tray below avoiding the downcomer
area as much as practical. Swing manway cover away from high activity area and
away from ladders when possible.

5. Tower Layout from Top Down

5.1. Vapor Overhead Line

5.1.1. This line is normally run to a condenser located in an adjacent structure or

over the piperack directly in front of the tower. Consideration for flexibility
will most likely determine what segment of the tower facing the piperack the
line will be located. See Fig 6. The most direct route may apply if no
flexibility concern exists, assuming the line can span the horizontal distance
from the tower to the condenser header. Locating the line along the 0o axis
would provide a moderate “leg” for stress while routing the line away from
the condenser would allow greater flexibility should it be necessary. This
issue must be worked with a stress Engineer as the design is developed. The

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vapour overhead line may exit the tower in one of two general locations.
The most common is shown in scheme (A) off the top head. A variation of
this design would be the elimination of the flanged nozzle for a butt weld
connection for very large O lines. This would most likely require client
approval, but should be considered for economic reasons. The second
approach, scheme (C), utilizes an internal pipe exiting the top side of the
vessel just below the tangent line. This design may eliminate the need for a
top head platform. As with all lines at similar equipment, the piping should
be supported as close to the top tangent or nozzle as possible. Additional
items normally found on vapour overhead lines may include, temperature
connections, inhibitor injections, relief valves and vents.

5.2. Top Head Platform

5.2.1. Potential variations found at top head platforms are shown in Figure 7. The
relief valve system is the next feature to be considered. It may be an open or
closed system, with or without block valves and bypass. Scheme A shows
the two variations for an open system, without block valves. The ideal
location for the relief valve would be off the top of the overhead line,
discharging at least 3 metres above the platform assuming there are no other
platforms at higher elevations in close proximity to this point. Should the
valve be too high, the inlet line can come off the vertical portion of the
overhead line which would enable the valve to be set at a lower elevation.

5.3.1. Davits for tower maintenance should be located on the top head platform.
The davit should preferably be located in a corner of the platform. It must
be able to swing over the relief valve to be maintained and be moved to the
specified drop zone. The centreline elevation must permit the lifting device
to clear the highest item to be handled. The lifted load of the heaviest single
component must be identified either from the vessel Engineer or relief valve
vendor. The vessel vent should be located in the most convenient place to
accommodate operator maintenance and access on the top head platform.

5.3. Closed Relief System

5.3.1. Closed relief systems require special consideration before the valve or valves
can be located. See Figure 8. There may be a economic advantage in
locating the valve at a lower platform elevation. Providing the platform can
accommodate the relief valve without significantly increasing steel cost
which may offset any piping material saving by being set at the lower
elevation. Both options should be investigated.

5.4. Reflux Nozzle

5.4.1. Figure 9 shows variations of internal piping arrangements for the reflux
nozzle. Scheme A and B are fixed arrangements, while scheme C enables
the reflux nozzle to be oriented at any desired orientation within 270o arc.

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This offers multiple opportunities to develop an optimum layout for the

reflux line.

5.5. Instrument Nozzles – Figure 10.

5.5.1. Pressure and temperature connections on trayed towers require special

Attention when attempting to set nozzle locations. Pressure connections are
located in the vapour space just below the designated tray. They should not
be located in the downcomer area. Temperatue connections must be located
in the liquid space or 50mm above the designated tray in the downcomer as
shown in the auxiliary plan. An alternate solution is shown by setting the
nozzle in the “hillside position.” This approach is only recommended if the
length of the thermowell can not be reduced. Level instruments such as
controllers and gauge glasses are commonly located on bridles in the liquid
section, at the bottom of the tower. The elevation of these nozzles is
determined by the amount of liquid being controlled for maximum
operation. The specific elevations required are shown on the vessel
instrument sketch. A preferred and alternate location is shown if a baffle
plate is required.

5.6. Preferred Component Locations – Figure 11.

5.6.1. Optimum tower layouts generally follow the following guidelines:

a) Plan to run piping down the tower on the side facing the pipe rack, away
from manways, instrumentation, ladders, etc. for small diameter towers
of 4 ½’or less, lines should be grouped for common support. (see Figure
17).

b) Locate the manways toward the maintenance access area, away from the
piperack.

c) Locate items such as bridles with level instruments at dead end of

Platforms if possible, thereby eliminating normal operator travel around
such items.

d) Ladders should be located between the main piping area and the segment
of platforming, manways, instruments etc.

5.7. Miscellaneous Nozzle Considerations – Figure 12

5.7.1. when initiating a tower layout the following information should be used to
develop nozzle locations and elevations.

a) Projections are established from the vessel inside diameter to face of

flange and vary depending on the nozzle size.

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b) Top head nozzles are set from the platform elevation (top of steel) to the
face of flange. The dimension varies with the nozzle rating.

c) The maximum distance a nozzle can be set in the top head from the
vessel center line is shown in detail “A”.

d) Platform penetrations for nozzles and lines are shown in Figure 15.

e) Liquid outlet nozzle in the bottom head may have minimum

dimensional requirements. If a valve is bolted to the nozzle, a tower
drain connection is likely to be needed. Clearance between nozzles,
and the drain nozzle flange and skirt access opening reinforcement or
fireproofing must be checked.

5.8. Tray Details – Figure 13

5.8.1. There are various types of tray designs, packing, internal piping details to
affect liquid vapour contact. Common tray designs include single or double
pass bubble cap, sieve and perforated design. As the liquid flows across the
tray surface and down to the tray below through the downcomer area, the hot
vapour rises through the bubble caps and eventually out the overhead line to
a condenser. Trays are numbered, with the top tray normally being number
one, the second, number two and so on. The P& IDs will identify which tray
a nozzle is to be set at. The piping designer using the basic data outlined in
this guide, knowledge of good industry practice, safety, maintenance
operation and economics must orient all trays, locate all nozzles, piping,
instruments, ladders, platforms, davits etc.

5.8.2. Another type of tray is the chimney type. When called for, the orientation of
trays above and below the chimney tray may vary as desired. Other towers
have multiple diameters. One arrangement uses a single downcomer in the
transition section, with a feed nozzle to the tray below. Another variation
uses internal piping from the downcomer to the tray below. Both enable the
trays below to be oriented differently from the upper section of the tower.
This enables a designer to optimize the external layout for piping, platforms,
ladder etc.

5.9. Packed Bed Sections – Figure 14 and 18

5.9.1. Packed bed sections use metal rings for liquid vapor contact instead of trays.
The rings are packed into specific sections of the tower, called beds, supported
by cross grid bars. The supports are designed to allow vapor to rise and liquid
flow down. Liquid is fed in at top of each bed through a distributor pipe.
Unlike trayed towers, there are no special orientation considerations of these
Beds, distributor or packing supports.

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5.10. Platform Details – Figure 15

5.10.1. Tower platforms should enable plant personnel to work safely during normal
operations and maintenance, without being costly and excessive in size. The
minimum size for top head, crossover or normal operations can be seen in
this figure. Bracket spacing should be standardized as shown in the elevation
view of the tower. No single ladder may not exceed 9 metres. Should a
ladder service more than one platform, the platforms must be set at an
elevation that is consistent with the rung spacing. Dimension “A” shall be in
even increments or rung spacing. Avoid setting two platforms serviced by
one ladder at the same elevation for operator safety. Elevation difference
must be 600mm minimum.

5.10.2. Ladder cages are not needed for platforms whose elevation is under 6 metres.
Plant personnel should not go up a higher elevation, when attempting
emergency egress off a tower.

5.10.3. Platform penetration sizes should be consistent with the specifics of each
case. Clearance should be given to bare pipe, insulated pipe, and flanged
connections as necessary. It is not necessary to allow for clearance of
insulation on flanges, since it is likely the insulation will be removed prior to
removal of the flanged pipe.

5.11. Reboiler And Pipe Support Considerations – Figure 17

5.11.1 Vertical Reboiler - if the reboiler is supported from the vessel, the elevation of
the lug on the reboiler must be set 25 mm above the maintenance support bracket
on the tower. During turnaround periods, when the channel end must be
removed for tube maintenance. The 25mm gap is shimmed, thereby becoming
the reboiler support.

5.11.2 Pipe supports on large diameter towers normally consist of a structural bracket
off the tower in close proximity to the line being supported or guided. A dummy
leg is welded to the pipe at an orientation which will allow the load to be
transferred from the pipe to the vessel support bracket.

5.11.3 For small diameter towers, individually supporting and guiding pipe becomes
more of a problem due to its limited size. Immediately after supporting each line
close to its nozzle, the lines should be routed and grouped as if a “ vertical
piperack” down the tower. By lining up the back of pipe, common structural
members may be effectively used for supports and guides.

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6. Reference Drawings

6.1.
The following Bechtel drawings should be used as reference material when
engaged in the layout of a fractionation tower. The relevant vessel engineer
should also be consulted for specific tray and vessel drawings.

Subject Dwg no.

- Feed and Vapor Overhead Nozzles R - 501

- Transition Section R – 502

- Bottom Section R – 503

- Drawoff Details R – 504

- Circular Checker Plate Platforms R – 505

- Circular Grating Platforms R - 506

- Manway R – 507

- Davit Details R – 508

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