DISTILLATION TOWER LAYOUT & PIPING

 DISTILLATION TOWER LAYOUT & PIPING

• Distillation tower is a very sophisticated equipment in a process plant. It separates an input stream into various components based on their boiling point range. Its critical to get their layout right first time as layout of other related equipments is based on them.

Locating distillation towers.

• Towers / columns should be located along the pipe rack towards open areas for unobstructed erection as well as maintenance of internals at grade. Tall towers requiring frequent operating attention at upper levels may be located at one place so that common connecting platform can be provided.

  Towers and vertical vessels shall be arranged in a row with common centre line, decided by the largest vessels, placing O.D. of the equipment minimum 4M away from the pipe rack.

• A minimum clearance of 3M shall be allowed between tower shells, but in any case platforms of adjacent towers shall not overlap (minimum 100 mm horizontal gap shall be provided between platform of adjacent towers) and that a minimum 900mm is left between tower plinths. The gap between vertical vessels shall allow full opening of manhole covers without restriction.

Locating Related Equipments

• owers are not standalone equipment. They need other connected equipments such as Reboilers, Pumps, Reflux Condenser, Air Cooler, Reflux Drum etc.

  Thermo-siphon reboilers should preferably be placed close to their associated towers.

  Pumps shall be placed near the tower and along the pipe rack. Air cooler is placed above pipe rack as close as possible to distillation tower, considering required flexiblity in vapor line.

  Reflux Drum and Reflux condensers are located on a technological structure near the distillation tower.

  Tower elevation to be set considering NPSH, operator access, maintenance access, minimum clearance and common access

Platforms and ladders requirements

• Distillation towers are very tall equipments and they require platforms at various levels for access and maintenance purpose.

1. Manways and platforms shall be located on the access side, with manways on or about the same centerline.
2. Maximum Ladder Length shall be 9 Meters.
3. Access platforms/ladders shall to be provided along the column for valves and instruments.
4. Platform width shall be such that minimum 1M space is available beyond manhole for movement.
5. Minimum clear width of platform shall be 750mm.
6. For ease of operation and maintenance, column and vessels which are grouped together, shall have their platforms at the same elevation and should be interconnected by walkways.
7. However each column / vessel shall have an independent access also.
8. Column / vessel platforms should be designed in such a way so that all the nozzles should be approachable from platforms.

Piping Layout.

• Piping at columns shall be located, where possible, radially about the column on the pipeway side. Valves and flanges shall not be located inside vessel skirts.

  Vents, drains and utility connections shall be arranged to prevent unintentional or undetected leakage. Piping from column shall drop or rise immediately upon leaving the nozzle and run parallel and close as practicable to vessel.

• Reboiler outlet piping shall be as short as possible with minimum bends. Piping at columns/vessel nozzles shall be arranged so that blanks can be easily installed for hydrotesting.

  Unless specifically indicated in P&IDs control valves shall preferably be kept at grade. Piping intended for vacuum services shall be routed as short as possible with minimum bends and flanged joints.

COLUMN PIPING STRATEGY

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

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.

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.

PHOTO GALLAERY