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.
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 of Fig.4
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.
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)
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.
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.
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.
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.
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.
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 considerations.