National Iranian Gas Company Column Internals

GS 146-1 COLUMN INTERNALS September 1995

Copyright © The British Petroleum Company p.l.c.

Copyright © The British Petroleum Company p.l.c. All rights reserved. The information contained in this document is subject to the terms and conditions of the agreement or contract under which the document was supplied to the recipient's organisation. None of the information contained in this document shall be disclosed outside the recipient's own organisation without the prior written permission of Manager, Standards, BP International Limited, unless the terms of such agreement or contract expressly allow.

Copyright © The British Petroleum Company p.l.c. All rights reserved. The information contained in this document is subject to the terms and conditions of the agreement or contract under which the document was supplied to the recipient's organisation. None of the information contained in this document shall be disclosed outside the recipient's own organisation without the prior written permission of Manager, Standards, BP International Limited, unless the terms of such agreement or contract expressly allow.

BP GROUP RECOMMENDED PRACTICES AND SPECIFICATIONS FOR ENGINEERING

Issue Date Doc. No.

GS 146-1

September 1995

Latest Amendment Date

Document Title

COLUMN INTERNALS

(Replaces BP Engineering Standard 168)

APPLICABILITY

Regional Applicability:

International

SCOPE AND PURPOSE

This document covers general requirements for the design, fabrication, supply, testing and installation of trays, packings and other column internals.

AMENDMENTS Amd Date Page(s) Description _______________________ ___________________________________ _______________________ _______________________ _____________________ _________

CUSTODIAN (See Quarterly Status List for Contact)

Fluid Separation Issued by:-

Engineering Practices Group, BP International Internationa l Limited, Research & Engineering Centre Chertsey Road, Sunbury-on-Thames, Middlesex, TW16 7LN, UNITED KINGDOM Tel: +44 1932 76 4067

Fax: +44 1932 76 4077

Telex: 296041

CONTENTS

Section

Page

FOREWORD ............................................................................................................... iii 1. INTRODUCTION................................................................................................... 1

1.1 Scope 1 1.2 Application of this Specification........................................................................... 1 1.3 Quality Assurance................................................................................................ 1 2. COLUMN TRAYS.................................................................................................. 1

2.1 General Arrangement and Size Requirements....................................................... 1 2.2 Fractionator Tray Design Details ......................................................................... 2 2.3 Materials and Thickness....................................................................................... 4 2.4 Structural Details................................................................................................. 5 2.5 Clamping and Bolting Arrangements.................................................................... 7 2.6 Seal Welding ....................................................................................................... 8 2.7 Manways and Access........................................................................................... 8 2.8 Gasketting ........................................................................................................... 8 2.9 Drainage.............................................................................................................. 9 2.10 Tolerances for Fabrication and Assembly .......................................................... 9 2.11 Vortex Breakers ............................................................................................. 10 2.12 Blanking on Sieve or Valve Trays ................................................................... 11 2.13 Tray Installation.............................................................................................. 11 2.14 Reflux Inlets ................................................................................................... 11 2.15 Feed Inlets ...................................................................................................... 12 3. PACKED COLUMNS........................................................................................... 13

3.1 General Arrangement......................................................................................... 13 3.2 Materials and Thickness..................................................................................... 13 3.3 Structural Detail ................................................................................................ 14 3.4 Packing.............................................................................................................. 14 3.5 Distributors - Gravity......................................................................................... 14 3.6 Distributors - Pressure ....................................................................................... 16 3.7 Ancillary Equipment .......................................................................................... 16 3.8 Tolerances ......................................................................................................... 17 4. ACCUMULATOR TRAYS .................................................................................. 17

4.1 General Arrangement......................................................................................... 17 4.2 Manways and Access......................................................................................... 17 4.3 Structural Requirements .................................................................................... 18 4.4 Seal Welding ..................................................................................................... 19

GS 146-1 Column Internals

PAGE i

5. VESSEL FEATURES ........................................................................................... 19

5.1 Internal Pipes..................................................................................................... 19 6. DESIGN, DELIVERY AND INSTALLATION................................................... 20

6.1 Design calculations ............................................................................................ 20 6.2 Drawings ........................................................................................................... 20 6.3 Finishing, Handling and Storage......................................................................... 21 6.4 Testing and Inspection before Installation .......................................................... 22 6.5 Testing and Inspection after Installation. ............................................................ 23 7. GUARANTEES..................................................................................................... 23

7.1 Mechanical ........................................................................................................ 23 7.2 Process.............................................................................................................. 24 8. REGIONAL ANNEX ............................................................................................ 24

8.1 USA

.............................................................................................................. 24

APPENDIX A.............................................................................................................. 25

DEFINITIONS AND ABBREVIATIONS .............................................................. 25 APPENDIX B .............................................................................................................. 26

LIST OF REFERENCED DOCUMENTS............................................................... 26


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FOREWORD Introduction to BP Group Recommended Practices and Specifications for Engineering

The Introductory Volume contains a series of documents that provide an introduction to the BP Group Recommended Practices and Specifications for Engineering (RPSEs). In particular, the 'General Foreword' sets out the philosophy of the RPSEs. Other documents in the Introductory Volume provide general guidance on using the RPSEs and background information to Engineering Standards in BP. There are also recommendations for specific definitions and requirements. Value of this Guidance for Specification

This document covers the major requirements of column internals appropriate to all BP Businesses. Adherence to the procedures will ensure that the equipment is fit for purpose whilst being cost effective Application

This Guidance for Specification is intended to guide the purchaser in the use or creation of a fit-for-purpose specification for enquiry or purchasing activity. Text in italics is Commentary. Commentary provides background information which supports the requirements of the Specification, and may discuss alternative options. This document may refer to certain local, national or international regulations but the responsibility to ensure compliance with legislation and any other statutory requirements lies with the user. The user should adapt or supplement this document to ensure compliance for the specific application. Principal Changes from Previous Edition

This is a new BP Guidance for Specification which incorporates the substance of three previous standards listed below: BP (America) Refining Practice RP 7-2-2 (Fractionating Trays and Column Packing) - April 1992. BP Chemicals Engineering Standard ES/ENG/520/01 (Column Internals) - April 1987 BP Engineering Standard No 168 (Distillation, Absorption and Extraction Column Internals) April 1987.


PAGE iii

Specification Ready for Application

A Specification (BP Spec 146-1) is available which may be suitable for enquiry or purchasing without modification. It is derived from this BP Group Guidance for Specification by retaining the technical body unaltered but omitting all commentary, omitting the data page and inserting a modified Foreword. Feedback and Further Information

Users are invited to feed back any comments and to detail experiences in the application of BP RPSE's, to assist in the process of their continuous improvement. For feedback and further information, please contact Standards Group, BP International or the Custodian. See Quarterly Status List for contacts.


PAGE iv

1.

INTRODUCTION 1.1

Scope

This Specification covers general requirements for the design, fabrication supply, testing and installation of trays, packings and other column internals. 1.2

Application of this Specification

This Specification shall be applied as the sole means of specifying column internals to a manufacturer, and forms a working document which shall be transmitted directly to manufacturers' without being rewritten as a project or contractor document. 1.3

Quality Assurance Verification of the vendor's quality system is normally part of the pre-qualification procedure, and is therefore not specified in the core text of this specification. If this is not the case, clauses should be inserted to require the vendor to operate and be prepared to demonstrate the quality system to the purchaser. The quality system should ensure that the technical and QA requirements specified in the enquiry and purchase documents are applied to all materials, equipment and services provided by sub-contractors and to any free issue materials. Further suggestions may be found in the BP Group RPSEs Introductory Vol ume.

2.

COLUMN TRAYS 2.1

General Arrangement and Size Requirements

2.1.1

For vessel of 840 mm (33") diameter and larger, tray panels, downcomer sections and other tray components shall be of sectional construction with sections removable through the vessel manways.

2.1.2

When a lifting beam is not available within the column, formed deck sections shall have a maximum component weight of 50 kg and a maximum length of 3000 mm (118"). A minimum number of plate sections shall be used in the floors of downcomers and accumulator trays.

2.1.3

When a lifting beam facility can be provided within a column, the length and weight restrictions shall be governed by the ability to install sectional components with the vessel in both the horizontal and vertical. The factors which govern the maximum weight/length are those necessary to ensure safe working in a confined space.


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2.1.4

All tray components shall be designed to permit installation and removal from the top side. To design the internals to be installed from both sides is an expensive addition that should be avoided unless absolutely necessary.

2.1.5

For vessels smaller than 840 mm diameter, cartridge tray assemblies shall be provided and the vessel shall be flanged to permit installation and withdrawal of the cartridge. Cartridge trays are invariably assembled in the fabrication shop.

2.2

Fractionator Tray Design Details

2.2.1

Sieve tray deck hole sizes should normally be selected between 4 mm and 12 mm (5/32 - 1/2") diameter. Larger hole sizes shall be used when the specification sheet specifies a fouling or corrosive service. The ratio of the hole pitch to hole diameter shall be a minimum of 2.0. Hole sizes larger than 25 mm (1") are more likely to weep, will restrict capacity and cause a lower efficiency. They are not normally used.

2.2.2

Holes in sieve trays shall be formed with the punch exit on the upperside of the tray. The manufacturer shall ensure through fabrication and installation that this requirement is met for all deck sections. Burrs on the sieve tray holes which would cause injury to personnel, should be removed.

2.2.3

On sieve and valve trays the hole area shall be uniformly distributed in the contacting area approximating equilateral triangular pitch. Hole area per tray shall be within ±1% of the area specified.

2.2.4

In the closed position the valve shall not make full peripheral contact with the tray deck. Valve caps should be provided with an indentation which prevents such peripheral contact and thus helps to prevent the valve from sticking closed.

2.2.5

Trays having more than two passes should be designed for equal bubbling area per pass rather than equal flow path length per pass subject to a minimum flowpath length of 450 mm (18"). In some multi pass designs this may cause problems with meeting minimum manway dimensions. In such cases this should be discussed with the owner..

2.2.6

Trays with three or more passes shall be provided with vapour and liquid balance ducts.


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These are tubes or ducts that connect the different vapour flow areas such that pressures over the columns are equalised. The design should be discussed with BP.

2.2.7

Trays shall be designed to handle the vapour and liquid loads given on the tray data sheets. Standard sieve tray performance data shall be specified on an FRI basis. It is recognised that proprietary trays using sieve decks will be designed according to the manufacturer's own procedures and in this case the basis used for calculating jet flood, entrainment, maximum sustainable loading and downcomer backup shall be provided to the purchaser.

2.2.8

Where the column is to be used offshore it may be subject to sway. In such cases the trays should be designed such that they operate satisfactorily within the sway tolerances approved by BP. In the absence of stated tolerances, a sway of ±0.5 degree may be assumed in addition to the conventional fabrication tolerances used although it will be necessary to consider motion in all six axes and, to some extent, the accelerations and periods of motion. It is the responsibility of the manufacturer to ensure that the hydraulic design meets these criteria.

2.2.9

No sieve tray shall be designed for more than 80% (FRI basis) of jet flood. Where sieve trays are used for vacuum service a maximum of 75% (FRI basis) of jet flood shall be used. No valve tray shall be designed for more than 85% of jet flood. Where valve trays are used for vacuum service a maximum of 77% of jet flood shall be used.

2.2.10

Downcomer clear liquid back-up shall be limited to a maximum percent of tray spacing plus weir height as listed below: Downcomer Backup Criteria System Pressure barg < 2.0 2.0 to < 5.5 5.5 to < 10 10 to 14 > 14 barg

2.2.11

psig < 30 30 to < 80 80 to < 150 150 to 200 > 200

Maximum % Liquid Backup

60 55 50 45 40

Where the design liquid loading on a straight segmental overflow weir would be less than 0.9 litre/min. cm (0.6 USGPM/inch), 'picket fence' type overflow weirs shall be provided. The minimum horizontal distance between pickets shall be 50 mm (2"). The minimum vertical height of picket above the weir shall be 150 mm (6"). The effective weir rate at normal design flows shall be no less than 0.9 litre/min. cm (0.6 USGPM/inch).


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2.2.12

If the method of weir attachment is such that gaps could be left between the weir plate and the shell wall and/or between the weir plate and the tray weir edge, slide plates shall be provided to seal these gaps.

2.2.13

For sloped or stepped downcomers, the bottom downcomer area must not be less than 50% of the top downcomer area. In many cases the difference will be 20-40%.

2.2.14

Side downcomers shall be no less than 150 mm (6") in horizontal distances from the column shell. This distance to be measured at the centre line of the downcomer perpendicular to the inside face of the downcomer. Capsulated downcomers are exempted from this specification. The spacing between interior downcomers on proprietary multi-pass trays is often small, but these are acceptable.

2.3

Materials and Thickness

2.3.1

The minimum design thickness of tray components shall be as follows:-

Material

Thickness nearest mm

Material

Thickness nearest mm

gauge

Carbon steel Stainless steel Monel Copper

3 mm 2 mm 2 mm 2 mm

10 14 14 14

gauge

Aluminium Nickel Alloys Hastelloy Zirconium

2 mm 2 mm 1.5 mm 1.5 mm

14 14 16 16

2.3.2

The thickness of pressed alloy steel bubble caps and the riser thickness shall not be less than 1.5 mm (0.059") and 2.0 mm (0.079") respectively.

2.3.3

The thickness of proprietary valve units shall be set by the manufacturer based on process requirements for the tray.

2.3.4

More than one weight of proprietary valves shall normally be achieved by using thicker gauge material. The spot welding of a single weight on a standard valve is an alternative but in cases where severe corrosion is anticipated any weights shall be fully seal welded.

2.3.5

In all cases proprietary valve units shall be manufactured from corrosion resistant material for the intended duty and stainless steel is a minimum requirement. Carbon steel valve units should never be used.


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2.3.6

Tray rings and downcomer bolting bars shall be in the same material as the vessel. Tray rings and downcomer bolting bars for clad or lined vessels shall be in the same material as the vessel cladding or lining, subject to thermal stress limitations.

2.3.7

The dimensions of the tray support rings and downcomer bolting bars shall be provided by the manufacturer. The following minimum and maximum dimensions shall be used:-

Column ID

Ring Width

mm

mm

less than 900 901-1800 1801 - 3500 3501 - 6000 above 6001

40 50 65 75 100

Minimum excluding thickness corrosion allowance Corrosion Other resistant material material mm ins. mm ins. 6 0.24 6 0.24 6 0.24 6 0.24 6 0.24 6 0.24 6 0.24 10 0.39 6 0.24 10 0.39

Maximum Thickness

mm 15 15 15 20 20

ins. 0.59 0.59 0.59 0.79 0.79

2.3.8

Vessel Corrosion allowance shall apply to all surfaces of welded-on items in contact with the process fluids.

2.3.9

Vessel Corrosion allowance shall apply to one side of all removable support beams. Vessel Corrosion allowance does not have to be applied to replaceable/removable tray parts.

2.3.10

For trays of corrosion resistant materials all bolting shall be of the same material as the removable internals with which it is connected .

2.3.11

Monel valve trays are prohibited.

2.4

Structural Details

2.4.1

Loose beams unsupported by shell brackets are not acceptable. In the context of deck support beams, a loose beam is defined as a completely separate member whose position is maintained only by tray to support ring clamps and tray to beam clamps.

2.4.2

Certain larger diameter columns will require support beams that are distinct members separate from the stiffened tray sections. These


PAGE 5

separate beams, whether rolled joists or channels, shall be securely restrained against vertical and horizontal movement. 2.4.3

At least 360 mm (14") clearance shall be maintained between the bottom of the beams and the tray deck below when the main beams are located at right angles to the liquid flow direction on the tray below. This applies to both integral stiffeners and separate beams. Beams running parallel to the liquid flow on the tray below are not subject to the clearance requirement.

2.4.4

The design load shall include the weight of the internal element (with its corrosion allowance) plus the following uniform downward loads at design temperatures. (a)

Fractionation trays. The greater of 1470 N/m2 (150 kg/m 2, 30 psi) or the weight of water 50 mm (2") over the highest weir setting.

(b)

Areas under downcomers. The greater of 2940 N/m2 (300 kg/m2, 61 psi) or a head of water one half the height of the downcomer.

(c)

Draw-off pans. The greater of 6865 N/m2 (700 kg/m2, 143 psi) or a head of water to the top of the pan.

(d)

Total draw-off trays. The greater of 6865 N/m2 (700 kg/m2, 143 psi) or a head of water to the top of the vapour riser or the overflow weir.

(e)

Baffle tray with a weir. The greater of 6865 N/m2 (700 kg/m2, 143 psi) on the projected area or the weight of water 50 mm (2") over the weir height.

(f)

Vacuum residue stripping trays where steam stripping is used. An upward force of 49 N/m 2 (5 kg/m3, 1.0 psi) shall be used.

Calculations shall exclude the corrosion allowance. 2.4.5

Stiffeners or a retaining device shall be provided on downcomers to limit the horizontal movement of the bottom edge of the downcomer when the bottom section of the downcomer exceeds both 250 mm (10") in depth and 600 mm (24") in length.

2.4.6

Structural shapes or clips shall be provided along the bottom edge of the downcomer to ensure proper clearance between the bottom of the downcomer and the tray deck. The design and installation of these shapes or clips shall be such as to not interfere with the liquid flow


PAGE 6

exiting from under the downcomer. These clips may also be designed to satisfy 2.4.5. 2.4.7

Adjustable weirs are not acceptable unless specifically required by the project. The specification of adjustable weirs is now an uncommon practice due to the ease with which these can be incorrectly installed or otherwise maladjusted during operations. The levelness of the tray therefore depends on the levelness of the tray support ring and this requires an appropriate check.

2.4.8

Tray support rings shall be welded to the wall of the vessel to provide a flat support for the assembled tray sections. The upper surface of the tray support ring shall conform to the stated tray elevation. The ring shall be level to within the tolerances given in 2.10.4.

2.4.9

When the data sheet specifies a corrosive or vibrating service no notched corners shall be provided. All corners shall be formed using a 5 mm (0.2") radius minimum; minimum bend radii shall be three plate thickness.

2.5

Clamping and Bolting Arrangements

2.5.1

Downcomers shall be designed to ensure that no vapour by-pass will occur. Where the downcomer is assembled in sections the top section shall be bolted through at least two matching holes or horizontal slots in the bolting bar and the plate. One clamp or bolt hole shall occur as close to the end of each section as possible. Bolts or clamp pitch shall be a maximum of 100 mm (4"). All bolt holes shall be sealed by washers.

2.5.2

Where carbon steel decks are specified, the non-threaded components of clamping assemblies may be fabricated from carbon steel. Threaded components shall be compatible with the process requirements and the bolting material shall be AISI 304 stainless steel as a minimum.

2.5.3

Where the decks are specified in stainless steel, the clamp assemblies shall be fabricated from the same material.

2.5.4

The maximum pitch between clamps shall be 180 mm (7") in the tray active area and 100 mm (4") on the downcomer bolting bars and, where used, on the downcomer floor. Each separate tray and downcomer section shall have a clamp located as close to each corner as possible.

2.5.5

The bolting for attaching major beams shall be provided with double nuts or equivalent device.


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2.5.6

Fasteners must not be welded to tray components.

2.6

Seal Welding

Where seal welding is required, for various tray sections please refer to Section 4.4. 2.7

Manways and Access

2.7.1

Tray manways shall provide a minimum access opening size of 380 mm x 600 mm (15" x 24").

2.7.2

Tray manways shall be removable from both above and below the tray, unless access is only from one side. The bolting shall be designed for positive closure orientation.

2.7.3

Tray manways shall be vertically aligned to allow maximum working access. This clause is given in order to ease maintenance. The growing legalisation to stagger manways on alternate trays so that injury due to a fall is minimised may become an important issue in some projects. Advice must be sought in cases of dispute.

2.7.4

A tray manway shall be provided on each liquid pass.

2.7.5

Where practicable, access through an accumulator tray shall be via one or more risers. The cap(s) covering the riser(s) concerned shall be detachable. Where a riser is used as a manway, its width shall not be less than 450 mm (18") in the longer direction and 380 mm (15") in the shorter direction. The top of such a riser shall have a welded brim bar to prevent injury to personnel. The maximum width shall be governed by the need to provide an adequate number of risers to ensure good vapour distribution.

2.7.6

Minimum inter-tray work space shall be 300 mm (12").

2.8

Gasketting

2.8.1

Gaskets shall not be used to seal the tray active area.

2.8.2

Gaskets shall not be used to seal horizontal downcomer plates and bolting bar junctions and any baffling that may be used in the base of the column.

2.8.3

Gasketted joints for total draw-off trays are permitted.


PAGE 8

Where a stringent (Cat 1) leak test is required, gaskets may not be used as it is impossible to achieve a suitably leak tight tray.

2.8.4

Gasket material shall not contain any asbestos and shall be an asbestos substitute suitable for both the process fluid and steam, up to the vessel design temperature.

2.8.5

The use of packing, cements, caulking, etc. to achieve liquid or vapour tightness or for any other purpose is prohibited.

2.8.6

Tray spacing requirements shall be as follows:Tray location Above top tray

Feed tray Shell manhole

Spacing A minimum of 1.5 times the normal tray spacing Normal tray spacing plus 150 mm (6 in) whichever is the greater. Manhole internal diameter plus 150 mm (6 in) or the design tray spacing, whichever is greater.

The feed tray spacing should not be less than 600 mm (24").

2.9

Drainage

2.9.1

Trays shall have drain holes only where items, e.g. bubble caps, giving a positive seal are used.

2.9.2

All seal pans and sumps shall have drain holes. Holes, where appropriate, shall be located in the pan floor closest to the column centre where appropriate to avoid vapour ingress into the downcomer liquid.

2.9.3

For weir loads exceeding 1.2 m 3/h.m3 (.5 USGPM/ft2) the drain hole area should be based on 0.1% of the area to be drained. For lower weir loads the basis should be 0.01%. Hole diameter should be between 513 mm (0.2" - 0.5"), limited to a maximum of 2 holes per section.

2.9.4

Drain holes shall not be provided on trays or pans having a nozzle positioned so as to allow complete drainage.

2.10

Tolerances for Fabrication and Assembly

2.10.1

The mean outlet weir height shall be set within ±2.5 mm (0.1") of the specified value. The difference between the maximum and minimum height dimensions shall not exceed 3.0 mm (0.12").


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2.10.2

When inlet weirs are used, the height tolerance shall be ± 3.0 mm (0.12").

2.10.3

The under-downcomer clearance shall be set within + 3.0 mm (0.12") or - 0.0 mm of the specified value.

2.10.4

Tray levelness and tray support levelness between high and low points on the vessel circumference shall be:Column diameter mm ins. < 1000 41 1000 to < 1800 71 1800 to < 3500 138 +3500 138

2.10.5

Maximum out of level mm ins. 3 0.12 4 0.16 5 0.20 6 0.24

The location of tray support rings shall be set within the following specified distances:(a)

± 6 mm (0.24") from the reference plane with the maximum high to low difference not exceeding 6 mm (0.24").

(b)

± 3 mm (0.12") top of support to top of adjacent support.

2.10.6

The maximum uncovered joint opening between tray sections shall be 5 mm (0.2") in the shorter direction and 25 mm (1") in the longer direction. The sum total of the areas of uncovered joint openings on any one tray shall not exceed 1% of the active hole area on that tray.

2.10.7

The minimum overlap between the support ring and the outside diameter of trays, pans and similar items shall be 20 mm (0.8").

2.11

Vortex Breakers

2.11.1

Vortex breakers shall be fitted in all vessels at nozzles leading to pump suctions. They are also required at all other pipe outlets where swirl may interfere with the separating of phases, e.g. in the separation of water in oil-water separators. Vortex breakers may be of the cross type or grating type. The choice is usually made according to the space available.

2.11.2

Vortex breakers shall be fitted to all internal pipes discharging liquid from accumulator trays.


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2.12

Blanking on Sieve or Valve Trays

2.12.1

Blanking of active holes shall be achieved using flat plate blanking strips bolted to the tray. Blanking strips shall be distributed uniformly in the active area and should be between 40 mm (1.6") to 100 mm (4") wide, so as to completely cover holes.

2.12.2

Blanking strips shall be installed at right-angles to liquid flow, covering complete rows if practicable.

2.12.3

For trays using circular valves, valves shall be removed prior to blanking.

2.12.4

Blanking shall not be used to adjust installed hole area to the specified design value.

2.13

Tray Installation

2.13.1

The manufacturer shall supply an installation procedure for each type of tray to be supplied to include tolerances for installation and cross referenced with drawings.

2.13.2

When tray installation takes place with the vessel in the horizontal position, the manufacturer shall supply an installation procedure which takes account of the vessel diameter and the out of roundness of the vessel in the horizontal. All sections of the trays including tray manways shall be fitted when traying takes place in the horizontal.

2.13.3

The manufacturer shall supply the following extra material to allow for field losses during installation:(a)

Bolt, nuts and washers

10%

(b)

Tray clamps

3%

(c)

Gasketting material

20%

Operational spares are not required. 2.14

Reflux Inlets

2.14.1

Refluxes shall enter an inactive tray area behind an inlet weir or false downcomer acting as a seal dam. The inlet pipe shall be either an open ended 'T' or a slotted branch distributor. The choice will depend on the need to maintain a certain minimum exit velocity.


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2.14.2

The seal dam height shall be 50 mm (2") greater than the inlet pipe nominal diameter up to a maximum dam height of 200 mm (8").

2.14.3

Branch pipe velocities shall be a maximum of 1 m/s (3.3 ft/sec) and slot velocities a maximum of 2 m/s (6.6 ft/sec).

2.15

Feed Inlets

2.15.1

Liquid feeds shall enter through an open ended branch to discharge vertically downward into the downcomer of the tray above the desired feed location or via a slotted branch distributor discharging onto the active side of the downcomer at the desired feed tray level. It is recognised that a number of variations in design may be used. These should be carefully reviewed.

A distributor shall be used when flashing of liquid in the downcomer could result in premature flooding. 2.15.2

Two phase, vapour-liquid, feeds shall enter the tray zone via a slotted branch distributor which discharges against a wear plate attached to the downcomer bolting bars.

2.15.3

Wear plate thickness shall be a minimum 5 mm (0.2"); spacers shall be provided to hold the wear plate off the downcomer.

2.15.4

Vapour inlets shall normally be a plain open inlet nozzle. The inlet nozzle shall be oriented approximately at right angles to the flow of liquid on the tray above.

2.15.5

For liquid feed pipes which cross the tray active area the minimum clearance from the bottom of the pipe to the tray below shall be 350 mm (14").

2.15.6

High velocity (i.e. approaching sonic velocity) vapour inlet lines shall be designed to dissipate the energy. The inlet pipe shall be sized to limit the fluid velocity to a maximum of 70% sonic velocity. A minimum straight run of 2 m shall be provided to the column entry and the feed shall enter the tower tangentially. A hood shall be provided extending at least to one quarter of the column circumference from the column inlet point.

2.15.7

For high velocity inlets the minimum distance from the top of the inlet pipe to the seal pan of the tray above shall be the greater of one pipe diameter or 500 mm (20"). The minimum distance from the bottom of the inlet pipe to the surface of any liquid below shall be the greater of two pipe diameters or 600 mm (24").


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3.

PACKED COLUMNS 3.1

General Arrangement

3.1.1

All non random packing elements, support grids or structures, and distributor systems and hold down grids shall be of sectional construction with sections removable through the column manways. Column manways shall be provided at the distributor system.

3.1.2

When a lifting beam is not available within the column any component section referred to in 3.1.1 shall have a maximum weight of 50 kg (110 lb.) and a maximum length of 3000 mm (118"). All other components, excluding structural beams, shall have a maximum weight of 120 kg (265 lb.) and a maximum length of 4000 mm (157"). When a lifting beam facility can be provided within a column the weight and length restrictions shall be governed by the ability to install sectional components with the vessel in the vertical. The factors which govern the maximum weight/length are those necessary to ensure safe working in a confined space.

3.1.3

Gravity distributors shall be provided as standard. Justification shall be provided for any alternative offered. The quality of the liquid distribution is the key factor to good packing performance. More even liquid distribution can be achieved with gravity distributors. Particular emphasis is necessary in distillation duties where efficiency is important.

3.2

Materials and Thickness

3.2.1

The standard material for all items referred to in 3.1.1 and random packing elements shall be stainless steel. The specific grade of stainless steel or alternative material shall be suitable for the process fluid. Carbon steel may be suitable for the process fluids and carbon steel will not rust less than stainless steel. Carbon steel is not recommended because of the maintenance requirements. It is very difficult to prevent surface rust by excluding air/water in a maintenance condition when column man entry is necessary.

3.2.2

The thickness of packing shall be specified so that the maximum distortion of packing elements at the bottom of the tallest packed height shall be no more than 2% of the corresponding standard dimension. Distortion under compression will effect both the efficiency and the hydraulic capacity of a packing. The thickness of packings shall be determined by the manufacturer and shall conform to his current practice. This will depend on the grade, and therefore


PAGE 13

rigidity, of the packing grade used. It is frequently 0.15 mm (0.006") but may with some packing be as low as 0.1 mm (0.004") thick.

3.3

Structural Detail

The design load shall include the weight of the packing elements plus the weight of the process fluid when the section is operating in the fully flooded regime. The minimum process fluid at flood shall be taken as equivalent to 10% of the packing volume. 3.4

Packing

3.4.1

The supply of random packing shall be made only on the basis of filling a given column diameter to a given depth. The manufacturer shall be responsible for the supply of sufficient packing to take account of any settlement allowance or dimensional variations. Column sections are often non-circular and the packing elements per unit volume quoted by manufacturers are averages. The supply of additional packing at the outset is more cost effective than the remedial work necessary to meet the specified height. Packing performance is almost always expressed in terms of packed height.

3.4.2

The depth of packing for each bed shall be stated by the manufacturer to provide the number of mass transfer stages required and to prevent physical distortion of the packing in service.

3.4.3

The manufacturer of structured packing shall ensure that peripheral sealing is provided at each layer to minimise vapour by-pass due to column ovality, manufacturing tolerances, etc. Peripheral sealing should be provided at each layer around the complete circumference. The sealing medium should be in contact with both the column wall and the packing. Visual inspection at installation will be required to check this.

3.5

Distributors - Gravity

3.5.1

The liquid distribution to the top of the packed bed shall be uniform over the vessel cross section for the whole design operating range. This uniformity of flow shall be maintained as the liquid impinges on to the top of the packed bed.

3.5.2

The outermost liquid distribution points shall be placed as close as practicable to the vessel wall and the following maximum dimensions shall be used.

Random packing nominal size mm ins.

Structured packing surface area 2 m /m3 ft2/ft3


Maximum distance from vessel wall mm ins.

PAGE 14

< or = 26 < or = 1.0 27 to 61 1.0 to 2.4 > 61 > 2.4

> 261 121 to 260 < or = 120 Grid type

> 80 37 to 79 < or + 37

20 30 40 Use manufacturers limit

The area irrigated by the distributor shall be a minimum of 95% of the tower cross sectional area, as defined by a circle linking the outermost distribution points. Particular attention shall be given to ensure that flow from the outermost orifices is not blocked by the presence of any other piece of equipment in the column. 3.5.3

Distributors shall be provided with means of levelling.

3.5.4

The design of the gravity distributor and any associated parting boxes shall allow them to be levelled to within the tolerances specified in 3.8.

3.5.5

Liquid supply to the parting box shall be so arranged that aeration of the liquid in the box and liquid swirl shall be minimised.

3.5.6

Distributor design shall be subject to a minimum orifice diameter which will be a function of the process fluid characteristics. This minimum diameter shall not be less than 2 mm (0.08"). Despite best efforts to filter all liquid streams entering packed columns, industrial fluids are not sufficiently clean to operate for prolonged periods with orifices below 2 mm (0.08") diameter. In many cases this chosen orifice diameter will be greater than this minimum and must always be considered at the design stage, bearing in mind the nature of the fluid being distributed and the process environment.

3.5.7

All orifices shall be formed with the punch exit on the liquid outlet side of the distributor. Small irregularities during manufacture are to be minimised near orifices so as to minimise differences in exit flow rates. Where orifices are laser cut this clause need not apply. Drilled holes should not be used other than in tube distributors.

3.5.8

The manufacturer shall provide within the distribution system a means to minimise particulate fouling of the distributor and define the method of cleaning such systems. Even in the cleanest process duties periodic cleaning will be a requirement. External filters on all liquid feed lines should always be provided and located as close as is practicable to the feed point. The filters will extend the period between cleaning.

3.5.9

Covers shall be provided on all final distributor channels.


PAGE 15

0.8 1.2 1.6

3.5.10

A minimum liquid depth of 30 mm shall be used in parting box or distributor channels. A minimum depth is necessary to minimise variations in head along the length of the channels.

3.6

Distributors - Pressure

3.6.1

Spray nozzle distributors shall be designed so that the theoretical coverage ratio is a minimum of 3 where the coverage ratio is defined as the ratio of the total geometrical spray footprint area to the column area. Nozzles shall be arranged so that there are no unirrigated areas. Circular full cone spray nozzles shall be used. The exact locations of nozzles to achieve the most uniform coverage is a function of the characteristic of the particular nozzle selected. This is an area which should be discussed with the owner.

3.6.2

Where ladder type distributors are selected, and where plain holes are used, all internal burrs shall be removed which would otherwise affect the liquid flowrate. Any pipe system shall have a vent hole to prevent accumulation of vapour.

3.6.3

Feed pipework to the distributor shall be sized to limit the maximum velocity to 1.5 m/s (4.9 ft/sec).

3.6.4

For fouling services the minimum free passage through the spray nozzle shall be equivalent to a diameter of 3 mm (0.12"). In many cases larger free passages will be required.

3.6.5

External filters shall be provided to prevent passage of material that might otherwise block the nozzle. The mesh size should be one third of the nozzle free passage diameter.

3.7

Ancillary Equipment

3.7.1

All packed beds shall be provided with a bed limiter, the design of which will be a function of the particular packing and distributor type being used. In all cases the bed limiter design shall be such as to offer the minimum possible coverage of the packing below. In this context angle sections are not acceptable. Where structured packing is used, any bars constituting the bed limiter shall be orientated so that they do not lie along joins between adjacent blocks of packing.


PAGE 16

3.7.2

Support grids shall be such as to impose the minimum resistance to vapour flow.

3.8

Tolerances

The elevation difference between low and high points in the gravity liquid distributor shall not exceed 2 mm (0.08"). 4.

ACCUMULATOR TRAYS 4.1

General Arrangement

4.1.1

Accumulator tray risers shall be uniformly spaced over the tray. The total riser area shall be consistent with the requirements of the column duty. Thus in vacuum operation and operations where minimum pressure drop normally is required, a maximum riser area is to be used consistent with the restraints imposed by mechanical considerations.

4.1.2

Accumulator trays used for all vacuum columns and/or where all liquid leakage has to be avoided, shall incorporate the following features:(a)

chimney caps vee shaped.

(b)

vapour emergence area at the pouring end, or ends, of the cap shall be fully blanked.

(c)

risers oriented to be parallel to the general flow of liquid across the tray.

These requirements are made to minimise the possibility of liquid re-entrainment and to minimise liquid residence time, a factor particularly required in high temperature vacuum service.

4.1.3

Accumulator tray caps shall be oriented so that liquid pouring from one cap shall not enter the vapour emergence area of an adjacent cap.

4.2

Manways and Access

4.2.1

Where practicable, access through an accumulator tray shall be via one or more risers. The cap(s) covering the riser(s) concerned shall be detachable. Where a riser is used as a manway, its width shall not be less than 450 mm (18") in the longer direction and 380 mm (15") in the shorter direction. The top of such a riser shall have a welded brim bar to prevent injury to personnel.


PAGE 17

The maximum riser width should be governed by the need to provide an adequate number of risers to ensure good vapour distribution.

4.2.2

Where the height of a riser used as a manway exceeds 450 mm (18") an internal rung ladder shall be provided as necessary, with strengthening of the riser.

4.3

Structural Requirements

4.3.1

Major beams shall be attached by bolting to vertical brackets or foot step brackets, welded to the vessel shell. Bolt holes in brackets or beams shall be slotted to allow for thermal expansion of the beam. Methods of restraint which rely on indirect fastenings are not acceptable. Other methods of restraint may be used providing that both horizontal and vertical movement of the beam is limited to that strictly required to contain the effects of thermal expansion.

4.3.2

The deflection of assembled deck sections at operating temperature shall not exceed a value of 1/900 of the column diameter. Where a corrosion allowance is used, the calculations for deflection shall be made on the corroded thickness.

4.3.3

All elements shall be designed for a concentrated maintenance load of 150 kg (330 lb.) at any point on the installed assembly, based on the allowable stress at 40°C (104°F). Calculations shall exclude the corrosion allowance.

4.3.4

The design load shall include the weight of the internal element (with its corrosion allowance) plus the following uniform downward loads at design temperatures. The greater of 6865 N/m 2 (700 kg/m2, 143 psi) or a head of water to the top of the vapour riser or the overflow weir. Calculations shall exclude the corrosion allowance.

4.3.5

Allowable stresses for metals shall be as per the vessel code used for the column design and construction, such as for example ASME Code or BS 5500.

4.3.6

The distance between the upper edge of an accumulator tray riser cap and the underside of the tray above or packing support above, shall not be less than 300 mm (12"). When the riser is used as a manway the minimum distance shall be 450 mm (18").

4.3.7

The manufacturer shall ensure that the trusses above the riser caps of accumulator trays do not obstruct the flow of vapour from the risers.


PAGE 18

Such structural members shall be oriented so that they are parallel to, and whenever possible immediately above, the riser caps. 4.4

Seal Welding

4.4.1

All joints in seal pans and discharge pans shall be fully seal welded. The minimum design thickness of seal pans and discharge pan components shall be 3 mm (0.12"). Trays designated as total draw-off trays shall be designed to have all seams continuously welded, including seams in risers and the peripheral seal with the tray support ring. Accumulator trays with clamps and bolting have often failed in service. The presence of gasketting has often made failure more likely. Fully welded construction has proved to be the solution to minimising failures. Long experience has shown that provided the tray is designed for full seal welding, damage due to thermal stresses will not occur even though there may be some minor localised panel buckling.

5.

4.4.2

Seal welding shall normally be using a continuous fillet weld above the attachment and an intermittent weld below. Where the duty is specified as corrosive, continuous fillet welds should be used above and below.

4.4.3

Welding operators and procedures shall be qualified in accordance with the vessel code used for the column construction, such as Section IX of the ASME Boiler and Pressure Vessel Code or BS 4870/4871.

4.4.4

All welding shall conform to BS 5500 or ASME Section VIII.

4.4.5

Accumulator trays shall be designed to be fully seal welded, including the periphery. The peripheral support shall normally utilise a conventional tray support ring welded to the vessel wall and not to any cladding which may be present.

VESSEL FEATURES 5.1

Internal Pipes

5.1.1

Internal distribution pipes shall have flanged and gasketted connections to facilitate installation and removal.

5.1.2

Internal piping shall be the same nominal alloy as the inside surface of the vessel. For ferric, austenitic or high alloys, such as Inconels, the minimum wall thickness shall be schedule 10 or 3/16th inch (4.7mm), whichever is less. For carbon steel or low chrome steels, the minimum wall thickness shall be schedule 80.


PAGE 19

6.

5.1.3

Internal flanges shall be ASME/ANSI Class 150 weld neck or slip-on type unless otherwise approved. Internal flanges shall be of a compatible metallurgy with the material specified for the internal pipe. Internal flange bolting shall be sequentially tightened, and double nuts used to prevent loosening.

5.1.4

When an internal distributor or sparger is of the 'bayonet' type, the external flange shall be marked in such a manner that the position of the slots or holes will be known from the outside of the vessel.

DESIGN, DELIVERY AND INSTALLATION 6.1

Design calculations

The results of the rating calculations and related mechanical data shall be summarised on the data sheets. The information shall be furnished to the owner's engineer for their review prior to commencement of fabrication. 6.2

Drawings

6.2.1

The following dimensions shall be referenced to the tray floor and be consistent with the process and mechanical data sheets:-

6.2.2

(a)

Weir heights

(b)

Clearance under the downcomers

(c)

Splash baffles

Plan and elevation drawings shall be provided by the manufacturer for approval by the purchaser before fabrication commences, indicating materials specifications and all design and assembly dimensions. These drawings should contain the following minimum design information, where applicable, relating to every separate design section of the tower. (a)

clearance under the downcomer.

(b)

inlet and outlet downcomer area.

(c)

effective exit weir length.

(d)

exit weir height.

(e)

sieve hole diameter.

(f)

valve type (reference number).


PAGE 20

(g)

valve thickness.

(h)

actual number of sieve holes on valves.

(i)

normal tray spacing.

(j)

correct bolting torque requirements.

(k)

level tolerances (distributors).

(l)

fluid densities at operating conditions.

(m)

maximum and minimum design liquid flowrates (distributors).

(n)

elevation and orientation (in diagram) of the tray.

Note:- the above measurements shall apply to actual sizes as used for process design, and not to measurements that might be used for fabrication purposes. 6.2.3

One original transparency and two copies of each Final Fabrication drawing shall be supplied to the Owner.

6.2.4

The orientation in degrees, and the elevation, shall be marked on all drawings where appropriate.

6.3

Finishing, Handling and Storage

6.3.1

With the exception of packings, all items of equipment shall be marked with a part number for identification and easy assembly. Like parts may bear the same number. Supporting documentation will cross reference between assembly procedures and drawings. Structured packing elements shall not be individually marked, but only the housing container. Items shall normally be marked by metal-stamping. For materials subject to stress corrosion cracking these shall be marked using an electric discharge pen (pyrograph) or equivalent.

6.3.2

A list with the part numbers of the items within the container shall be provided both inside and outside all packing containers.

6.3.3

As far as possible the manufacturer shall pack containers so that unpacking for installation shall be consistent with the order of installation in the column.


PAGE 21

6.3.4

Structured packings shall be packed in weather proof boxes. Consistent with standard box sizes, elements which make up complete vessel cross sections shall be packed together for large diameter vessels.

6.3.5

Random dumped packings shall be packaged in bags, and shall be clearly marked with the part number of the item.

6.3.6

Carbon steel parts shall be protected in a dry storage area. Austenitic stainless steel components shall never be exposed to wetting by salt water or salt spray. Protective packaging or a suitable removable coating shall be used to prevent such exposure.

6.4

Testing and Inspection before Installation

6.4.1

One tray per type, including accumulator trays, shall be assembled in the manufacturer's shop to the extent necessary to check dimensions. Examination shall be by the purchaser or his representative.

6.4.2

All accumulator trays shall be subject to a water leak test at site. The permitted leakage shall be as per the schedule below:Leakage category

1

2 3

Typical service

vacuum towers:lowest distillate draw vacuum towers:except category 1 all other columns

Maximum acceptable rate of level drop mm/h inches/hour

10

0.4

50

2.0

200

8.0

The water leak tests will be made (with drain holes plugged) at the overflow weir height or chimney height, as applicable.

6.4.3

One gravity distributor per type as used for fractionation service shall be tested with water in the fabrication shop before dispatch. Liquid inlet means to the distributor parting box shall be the same as that to be used in the column. The tests shall be performed at design maximum and minimum flowrates as follows:(a)

Measurement of the uniformity of flow from the parting box or predistributor.

Samples should be taken from each part of the parting box that discharges into a separate channel.


PAGE 22

Acceptance criteria: Liquid distribution at 50% of the maximum design flowrate, or a specified minimum design flowrate, to be within + or 4% of the mean. (b)

Samples taken at no less than 10 random locations over the cross-sectional area of the column according to the following schedule:Column diameter mm < 900 900 - 1800 > 1800

ins < 36 36 - 70 >70

Approximate number of holes from which liquid is collected from each location 1 5 10

Acceptance criteria. Liquid distribution at 50% of the maximum design flowrate, or a specified minimum design flowrate, to be within ± 6% of the mean. (c)

Measurement of liquid depth at each end and at the centre of each channel.

Acceptance criteria: The distance between the liquid level and parting box/channel/pan top must be a minimum of 10% of the parting box/channel/pan depth. Under no conditions will an overflowing system be accepted. 6.4.4

When a spray distributor is used for a VDU wash zone, or similar zone, it shall be tested with water in the fabrication shop before dispatch. Tests shall be performed at design maximum and minimum flowrates. A nozzle inlet pressure versus flowrate relationship shall be derived. Nozzle dimensions shall be checked against manufacturers data. Acceptance criteria: At any pressure the flow shall be within 20% of the stated catalogue value.

6.5

Testing and Inspection after Installation.

Weld spatter on downcomer bolting bars shall be removed to ensure a flush seal. In the event that the downcomer does not form a continuous seal with the bolting bars, the downcomer shall be reassembled using a gasket.


PAGE 23

7.

8.

GUARANTEES 7.1

Mechanical

7.1.1

The manufacturer shall guarantee all items supplied by him against defective material, poor workmanship, improper design and failure within design loadings.

7.1.2

The manufacturer shall repair or replace free of charge any defective material or workmanship found within the guarantee period.

7.2

Process

7.2.1

The manufacturer shall guarantee the ability to handle the vapour and liquid loads over the specified operating range whilst retaining efficiency, when hydraulic loadings are supplied by the purchaser.

7.2.2

The manufacturer shall guarantee the separation and the hydraulic throughput when the process design and the hydraulic design is his responsibility.

7.2.3

The manufacturer shall replace free of charge any new items required to meet the process guarantees.

7.2.4

The manufacturer shall guarantee that the design and fabrication of liquid distributors for packed columns will meet the criteria for acceptance given in section 6.4.3 and 6.4.4.

7.2.5

Any modification necessary to meet the leak tightness qualification given in section 6.4.2. shall be made free of charge by the manufacturer.

REGIONAL ANNEX 8.1

USA

Where tray spacing is 915 mm (36") or greater an access ladder shall be provided at each tray manway. This is a requirement of BP's refineries in the USA which at the moment (Sept '95) is not strictly required elsewhere.


PAGE 24

APPENDIX A

DEFINITIONS AND ABBREVIATIONS

Definitions

Standardised definitions may be found in the BP Group RPSEs Introductory Volume. gravity distributor:

liquid distributor using only hydrostatic head

grid packings:

ordered packing manufactured from cut out metal strips

manufacturer:

manufacturer of column internals

owner:

the operator of the column

pressure distributor:

liquid distributor with pressurised head

random packing:

packing manufactured as discrete elements distributed randomly

structured packing:

ordered packing manufactured from pressed sheet metal sheets

Abbreviations

CS FRI HF

Carbon steel Fractionation Research Incorporated Hydrofluoric acid


PAGE 25

National Iranian Gas Company Column Internals

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