Column Piping.docx

Column Piping: Study Layout, Nozzle Orientation & Platforms Requirements

1.0 Sequence of Column Piping Study

1.1  All available information / data from Equipment specification and P&ID shall be written on the

elevation view of the column as illustrated in Fig.1, 2 & 3.

1.2  The designer now starts thinking about the proper orientation of nozzles and provisions for access

to the points of operation and maintenance.

1.3  Considerations of the pipeline leaving the tower area and the adjacent piping shall be visualized.

1.4  The first step is to orient the manholes preferably all in same directions. Normally, manholes shall be oriented towards dropout area within a 30° segment of column as this facilitates the lowering of tower internals to the main access way. The manhole segment of platform should not be occupied by any piperack.

1.5  A break in ladder rise (normal 5m, maximum 7m) will occupy another segment of column for platform.

1.6  The levels of platforms are to be decided on the elevation view based on the manholes and

access to relief valves, instrument for viewing.

1.7  All platform levels in the proper segments of the tower with ladder location should be drawn on

plan view. The manhole shall be shown in proper segment with the angle of orientation, and the space

for the swing of manhole cover taking davit hinge as centre.

1.8  Layout should be started from the top of the column with the designer visualizing the layout as a

whole. There will be no difficulty in dropping large overhead line straight down the side of a column,

and leaves the column at a high level and crosses directly to the condenser. This clears a segment at

lower elevations for piping or for a ladder from grade level to the first platform.

1.9  Flexibility and thermal load connected with the large-dia overhead lines to the condenser at grade

level or higher level shall be considered. The relief valve protecting the tower is usually connected to

the overhead line. A relief valve discharging to atmosphere should be located on the highest tower

platform.

In a closed relief-line system, the relief-valve should be located on the lowest tower platform above the

relief -system header. This will result in the shortest relief-valve discharge leads to the flare header.

The entire relief-line system should be self-draining.

1.10  From layout point of view, it is preferable to space the platform brackets on the tower equally and

to align the brackets over each other for the entire length of the tower. This will minimize interferences

between piping and structural members.

1.11  Nozzles and piping must meet process requirements while platforms must satisfy maintenance

and operating needs. Access for tower piping, valves and instruments influence placement of ladders.

1.12  In routing pipelines, the problem is faced to interconnected tower nozzles with other remote

points. The tentative orientation of a given tower nozzle is on the line between tower centre and the

point to which the line is supposed to run. Segments for piping going to equipment at grade e.g.

condenser and reboiler lines are available between ladders and both sides of manhole.

See the Fig.4 / 5 for overall orientation of a distillation column.

Line approaching the yard/piperack can turn left or right depending on the overall arrangement of the

plant. The respective segments of these lines are between the ladders and 180°. The segment at 180°

is convenient for lines without valves and instruments, because this is the point farthest from manhole

platforms.

The sequence of lines around the tower is influenced by conditions at grade level. Piping

arrangements without lines crossing over each other give a neat appearance and usually a more

convenient installation.

1.13  The correct relationship between process nozzles and tower internals is very important. An angle

is usually chosen between the radial centreline of internals and tower-shell centrelines.

By proper choice of this angle (usually 45° or 90° to the piperack) many hours of work and future

inconvenience can be saved. Tower piping, simplicity of internal piping and manholes access into the

tower are affected by this angle. After this, the information produced by the designer results in

selecting the correct orientation of tower nozzles.

1.14  A davit usually handles heavy equipment such as large-size relief valves and large-diameter

blinds. If the davit is at the top of the tower, it can also serve for lifting and lowering tower internals to

grade.

Clearance for the lifting tackle to all points from which handling is required, and good access should be

provided.

1.15  Very often, interpretation of process requirements inside a tower is more exact than for exterior

piping design. The location of an internal part determines, within strict physical limits, the location of

tower nozzles, instruments, piping and the steelwork. The layout designer has to concentrate on a

large-scale drawing of tower-internal details and arrangement of process piping to finalize the piping

study.

1.16  Access, whether internal or external is very important. This includes accessibility of connections

from ladders and platforms and internal accessibility through shell manholes, handholes or removable

sections of trays. A manhole opening must not be obstructed by internal piping.

1.17  Reboiler-line elevations are determined by the draw off and return nozzles and their orientation is

influenced by thermal flexibility considerations. Reboiler lines and the overhead lines should be as

simple and direct as possible.

1.18  Fig.6 shows the segments of tower circumference allotted to piping, nozzles, manholes, platform

brackets and ladders as normally recommended to develop a well-designed layout.2.0  Nozzle Orientation and Level

Nozzles are located at various levels on the tower to meet the process and instrumentation requirements.

2.1 Manholes

Nozzles are to be oriented keeping provision for maintenance and operation needs.

Manholes are usually located at bottom, top and intermediate sections of tower. These access nozzles must not be located at the downcomer sections of the tower or the seal pot sections of the tower.

Where internal piping is arranged over a tray, manhole shall be provided but it should be ensured that the internals do not block the maintenance access through the manhole.

Possible location of manhole and handholes within the angular limits of b° are illustrated in detail-2

ofFig.4

Fig 1- Sample Piping and Instrumentation Diagram around a Column

Fig 2 -Evaluation of the Flow Diagram for a Distillation Column to Visualize an Orderly Arrangement of Piping

Fig 3- Typical Nozzle Location and Platform Elevations

Fig 4- Details of a Typical Distillation Column

Fig 5- Typical Tower Piping Arrangement

Fig 6- Typical Tower Area Division for Various Facilities

2.2 Reboiler Connections

Reboiler connections are normally located at the bottom section of the tower. Detail-1 of Fig.4 shows reboiler draw-off connections for single-flow tray. This connection can be very important for arranging tray orientation. The simplest, most economical location for reboiler connections with the alternative location within the angular limits of a° is shown. The angle a° depends on the size of reboiler draw off

nozzle and the width of the boot (dimension ‘b’) at the tray down flow. The return connection from the thermosyphon reboilers is shown in detail-1 of Fig.4.These lines should be as simple and as direct as possible, consistant with the requirements of thermal flexibility.For horizontally mounted thermosyphon reboiler, the draw off nozzle is located just below the bottom tray and for vertically mounted recirculating thermosyphon reboiler, the draw off nozzle is located at the bottom head. For both the systems, the return nozzles are located just above the liquid level as shown in Fig.7.

Fig 7 – Reboiler Connection

2.3 Reflux Connections

Reflux nozzles are provided with internal pipes that discharge the liquid into the sealpot of the tray

below. Detail 3 of Fig.4 shows the reflux connections. Care must be taken that the horizontal leg of the

internal pipe clears the tops of bubble caps or weirs. It must be ensured that the internal pipe can be

fabricated for easy removal through a manhole or can be fabricated inside the tower shell.2.4 Overhead Connections

The vapour outlet nozzle is usually a vertical nozzle on the top head of tower. In addition, the vent and

relief valve could be located on the top head with a typical platform arrangement for access to vent,

instrument connections and top manhole. In a closed relief line system, relief valve should be located

on the lowest tower platform above the relief system header. This will result in the shortest relief valve

discharge leads. The entire relief line system should be self draining.

2.5 Bottom Connections

The liquid outlet is located on the bottom head of the tower. If the tower is supported on skirt, the

nozzle is routed outside the skirt as shown in Fig.8. The elevation and orientation of this line is

generally dictated by the pump NPSH requirement and the pump suction line flexibility. (see

Fig.9) 

Fig 8- Details of Tower Skirt

Fig 9 -Net Positive Suction Head (NPSH) of Bottoms Pump

2.6 Temperature & Pressure Instrument Connections / Level Instruments

The temperature and pressure instrument connections are located throughout the tower. The

temperature probe must be located in a liquid space and the pressure connection in a vapour space

as shown in Fig.10.

Fig 10- Typical Temperature and Pressure Instrument Nozzle Location

The level instruments are located in the liquid section of the tower usually at the bottom. The elevation

of the nozzles is decided by the amount of liquid being controlled or measured and by standard

controller and gauge glass lengths. Level controllers must be operable from grade or platform and

level gauges / switches may be from a ladder if no platform is available. Fig.11, 12, 10, 13 &

14 illustrates a few instrument connections on tower.

Fig 11- Typical Instrument Vessel

Fig 12- Typical Arrangement for Level Instrument

Fig 13- Typical Temperature and Pressure Instrument Arrangements

Fig 13- Typical Temperature and Pressure Instrument ArrangementsFig 13- Typical Temperature and Pressure Instrument Arrangements

Fig 14- Typical Common Bridle-Level Instrument Arrangement

3.0 Access and Maintenance Facility

3.1  Access whether internal or external is very important. This includes accessibility of connections

from ladders and platforms and internal accessibility through shell manholes, handholes or removable

sections of trays.

3.2  Tower maintenance is usually limited to removal of exterior items (e.g. relief or control valves) and

interior components (e.g. trays or packing rings) Handling of these items is achieved by fixed devices

(e.g. davits or trolley beams) or by mobile equipment (e.g. cranes). When davits or beams are used,

they are located at the top of the tower, accessible from a platform and designed to lower the heaviest

removable item to a specific drop out area at grade level.

When mobile equipment is used, a clear space must be provided at the back (side opposite to

piperack) of the tower that is accessible from plant auxiliary road.Fig. 15, 16, 17 & 18 illustrates the

Access and maintenance facilities to be considered in the piping arrangement around a tower.

Fig 15- Platform Width Requirements

Fig 16- Typical Arrangement of External Piping around Column

Fig 17 Typical Operator Access

Fig 18- Typical Plan and Elevation View of Tower Area

On free-standing columns, access for major maintenance to insulation or painting will usually require

the erection of temporary scaffolding. Space for scaffolding at grade level and provision of cleats on

the shell to facilitate scaffold erection should be considered.

3.3  Utility stations of two services viz. steam and air are usually provided on maintenance platforms.

Steam and air risers should be located during piping study to keep adequate cleats for support.

(see Fig.19)

Fig 19- Typical Platform Arrangement

 4.0 Platforms and Ladders

4.1  Platforms on towers are required for access to valves, instruments, blinds and maintenance

accesses. Platforms are normally circular and supported by brackets attached to the side of the tower.

Generally, access to platforms is by ladder. Fig.20 illustrates the platform requirements.

Fig 20 – Tower Platform and Ladder Elevation Requirements

4.2  Platform elevations for towers are set by the items that require operation and maintenance.

The maximum ladder run should not exceed 7m.

4.3  Platform widths are dictated by operator access. The clear space on platform width shall be

min.900mm.

For platforms with control stations, the width of platform shall be 900mm plus the width of control

station.

The platform for manholes and maintenance access, adequate space for swing the cover flange flange

must be provided.

4.4  Top-head platforms for access to vents, instruments and relief valves are supported on head by

trunions.

4.5  Access between towers may be connected by common platforming.

4.6  It is preferable to space platform brackets on tower equally and to align brackets over each other

over the entire length of shell. This minimizes the structural design and interferences from piping.

4.7  On very wide platforms or those that support heavy piping loads, knee bracing is required in

addition to the usual platform steel. The potential obstruction immediately under the knee brace must

be kept in mind during platform design

.4.8  Fig. 3, 15, 21, 22, 20 & 19 illustrates a few platform consideration

.

Fig 21- Typical Platform Orientation

Fig 22- Details of Circular Platform Bracket Spacings