tower piping

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BECHTELPLANT DESIGN & PIPING

ENGINEERING DESIGN GUIDE FORFRACTIONATION TOWERS3DG P22 001, Rev. 00, 12/92Prepared by: E.F. BausbacherApproved by: E.F. Bausbacher

TABLE OF CONTENTS

Page No.

LIST OF FIGURES 3

1.0 PURPOSE 4

2.0 INTRODUCTION 4

3.0 TERMINOLOGY 4

3.1 Safety 43.2 Operation 43.3 Maintenance 43.4 Economics 5

4.0 INITIATE TRAYED TOWER LAYOUT 5

4.1 Establish Tower Elevation 64.2 Establish Tray Orientations 6

5.0 TOWER LAYOUT FROM TOP DOWN 8

5.1 Vapor Overhead Line 85.2 Top Head Platform 85.3 Closed Relief System 9

Page No.


5.4 Reflux Nozzle 95.5 Instrument Nozzles 95.6 Preferred Component Locations 105.7 Miscellaneous Nozzle Considerations 105.8 Tray Details 115.9 Packed Bed Sections 125.10 Platform Details 125.11 Optimizing Tower Layout 135.12 Reboiler and Pipe Support Considerations 13

6.0 REFERENCE DRAWINGS 14


LIST OF FIGURES

Figure 1 Establish Tower Elevation

Figure 2 Set Tray Orientation

Figure 3 Alternative Methods of Reboiler Support

Figure 4 Feed Nozzles

Figure 5 Manway Locations

Figure 6 Vapor Overhead Line

Figure 7 Top Head Platform

Figure 8 Closed Relief System

Figure 9 Reflux Nozzle

Figure 10 Instrument Nozzles

Figure 11 Preferred Component Locations

Figure 12 Miscellaneous Nozzle Considerations

Figure 13 Tray Details

Figure 14 Packed Bed Section

Figure 15 Platform Details

Figure 16 Optimizing Tower Layout

Figure 17 Reboiler and Pipe Supports

Figure 18 Trayed and Packed Towers


1.0 PURPOSE

To provide a layout designer with the guidelines to develop a comprehensivefractionation tower design which considers safety, operation, maintenance andeconomics.

2.0 INTRODUCTION

The philosophy addressed in this guide primarily deals with trayed fractionationtowers. It is the responsibility of the Plant Design and Piping group to developa layout based upon a combination of certain specific rules and logic. Theoptimum tower design is achieved through a trial and error approach when notgoverned by specific industry codes and regulations. This includes striking abalance between setting tray orientations, use of internal piping, and propergrouping of valves and instruments to minimize platforming requirements.

3.0 TERMINOLOGY

3.1 Safety

Design of towers must give proper attention to safety for all plant personnelwho will be required to work within its confines. As one travels from grade tothe upper most platform, the area must be free of dangerous obstructions whenattempting to gain access to values, instruments or exit the tower in anemergency.

3.2 Operation

Modern technology rarely requires constant attention on trayed towers. Whenrequired, access to valve handwheels and instrumentation should be placed insuch a way that a minimum amount of time is required to perform the function.

3.3 Maintenance

Features to be considered for towers include providing davits at the top oftowers for handling large relief valves, placing manways in a way to facilitateremoval of internals to grade, locating davits or hinges on manway covers in away it does not obstruct other required maintenance or access, provide


removable platform sections if necessary to accommodate maintenance below,etc.

3.4 Economics

Financial impact of one design over another is one feature that is oftenoverlooked when engaged in development of a tower layout. Setting thebottom tangent line at the true minimum elevation; arranging internal andexternal piping in such a way that platforming can be minimized and weighingthe cost of increasing the vessel wall thickness to directly support a verticalreboiler over a support from grade requiring foundations, steel structure, etc.

4.0 INITIATE TRAYED TOWER LAYOUT (Refer to Figure 18)

The following approach is recommended when the design of a fractionationtower is initiated.

a. Verify all documentation to be used is the latest available issue.

b. Set tower bottom tangent line elevation (see 4.1).

c. Establish tray orientation based upon review of all governing criteria (see4.2).

d. Start layout by working from the top of tower downward (see 5.0).

e. Utilize internal piping whenever possible to maximize exterior layout.

f. Work closely with stress/support engineers throughout design.

g. When locating nozzles and manways, be sure vessel internals, such astray supports or internal piping or tray configurations will not impedemaintenance or operation.

4.1 Establish Tower Elevation - Figure 1

The three prime factors listed below should be considered when attempting toset the bottom tangent line elevation include:


4.1.1 Pump NPSH

The Process/Project engineer is responsible to supply the pump NPSHrequirements for all pump circuits.

4.1.2 Thermosyphon Reboiler Requirements

The project process engineer is responsible to establish the dimension from thevessel tangent line to the centerline of a thermosyphon reboiler. When avertical reboiler is used, adequate clearances must be given to remove thelower channel section.

4.1.3 Piping Clearances

Should the pump head requirement or reboiler dimension be a non-issue in alayout, operator clearance under the liquid outlet line to adjacent equipmentmay be the factor which sets the tangent line elevation. All factors must bereviewed before a final elevation can be transmitted to a vessel vendor. Shouldnone of the above factors be relevant, the vessel tangent line may be set at theminimum practical elevation.

4.2 Establish Tray Orientations

a. The two primary piping circuits which normally impact tray orientation arethe feed nozzle and reboiler connection.

b. The feed nozzle may have one or a multiple of external connections withan array of internal piping configurations. See Figure 4. Three typicalfeed arrangements are:

(1) single feed nozzle with two possible orientations.

(2) double feed nozzle with two possible orientations.

(3) single feed nozzle with a multiple of orientation options.

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


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

d. Should the feed nozzle not be a deciding factor, refer to the reboilerpiping requirements in paragraph e.

e. Since the preferred reboiler piping arrangement is the most direct route,coming off the bottom tray downcomer now comes into play in settingtray orientations. A stress engineer should approve the proposed layoutbefore assuming the selected nozzle location will work for the piping.Scheme (A) in Figure 2 is an example of an optimum design for thereboiler inlet line. Scheme (B) shows one of a number of arrangementsfor the reboiler return line. Tray orientations are unaffected by draw-offnozzles located in the bottom head of the tower. These nozzles may belocated at any orientation to suit.

f. Method of support for vertical reboilers needs to be established as soonas reliable design data becomes available. See Figure 3. Direct supportoff the vessel are shown in scheme (A) and (B) which will require athicker wall thickness, but a normally less complex piping system.Scheme (C) shows an independent support from grade. While itnormally does not effect the vessel wall thickness, it requires a supportstructure, foundations and generally a more complex piping system tocompensate growth differential between the tower and the stationarystructure. Refer to 5.12, paragraph a.

g. One remaining item to establish at this time is to set the generalorientation of manways, as shown in Figure 5. The most commonlocation is opposite the piperack or that direction maintenance is mostlikely to be performed. When selecting the orientation, be sure entranceinto the tower is not obstructed by internal piping, downcomers, traysupports and is over the tray below avoiding the downcomer area asmuch as practical. Swing manway cover away from high activity areaand away from ladders when possible.

5.0 TOWER LAYOUT FROM TOP DOWN

5.1 Vapor Overhead Line


The first order of business is to select the routing of the vapor overhead line.This line is normally run to a condenser located in an adjacent structure or overthe piperack directly in front of the tower. Consideration for flexibility will mostlikely determine what segment of the tower facing the piperack the line will belocated. See Figure 6. The most direct route may apply if no flexibility concernexists, assuming the line can span the horizontal distance from the tower to thecondenser header. Locating the line along the 0° axis would provide amoderate "leg" for stress while routing the line away from the condenser wouldallow greater flexibility should it be necessary. This issue must be worked witha stress engineer as the design is developed. The vapor overhead line mayexit the tower in one of two general locations. The most common is shown inscheme (A) off the top head. A variation of this design would be the eliminationof the flanged nozzle for a butt weld connection for very large O.D. lines. Thiswould most likely require client approval, but should be considered foreconomic reasons. The second approach, scheme (C), utilizes an internal pipeexiting the top side of the vessel just below the tangent line. This design mayeliminate the need for a top head platform. As with all lines at similarequipment, the piping will be supported as close to the top tangent or nozzle aspossible. Additional items normally found on vapor overhead lines mayinclude, temperature connections, inhibitor injections, relief valves and vents.

5.2 Top Head Platform

Potential variations found at top head platforms are shown in Figure 7. Therelief valve system is the next feature to be considered. It may be an open orclosed system, with or without block valves and bypass. Scheme A shows thetwo variations for an open system, without block valves. The ideal location forthe relief valve would be off the top of the overhead line, discharging at least10'/3M above the platform assuming there are no other platforms at higherelevations in close proximity to this point. Should the valve be too high, theinlet line can come off the vertical portion of the overhead line which wouldenable the valve to be set at a lower elevation.

Davits for tower maintenance are commonly located on the top head platform.They handle vessel intervals and large relief valves, 4" x 6" and above. Thedavit should preferably be located in a corner of the platform. It must be able toswing over the relief valve to be maintained and be moved to the specified dropzone for tower maintenance. The centerline elevation must permit the lifting


device to clear the highest item to be handled. The lifted load of the heaviestsingle component must be identified either from the vessel engineer or reliefvalve vendor. The vessel vent should be located in the most convenient placeto accommodate operator maintenance and access on the top head platform.

5.3 Closed Relief System

Closed relief systems require special consideration before the valve or valvescan be located. See Figure 8. Will there be an economic advantage to locatethe valve at a lower platform elevation? Can a lower platform accommodatethe relief valve without significantly increasing steel cost which may offset anypiping material saving by being set at the lower elevation? Both options shouldthoroughly be investigated before taking the design further down the tower.

5.4 Reflux Nozzle

Figure 9 shows variations of internal piping arrangements for the reflux nozzle.Scheme A and B are fixed arrangements, while scheme C enables the refluxnozzle to be oriented at any desired orientation within a 270° arc. This offersmultiple opportunities to develop an optimum layout for the reflux line.

5.5 Instrument Nozzles - Figure 10.

Pressure and temperature connections on trayed towers require specialattention when attempting to set nozzle locations. Pressure connections arelocated in the vapor space just below the designated tray as shown on the P&Idiagrams. They should not be located in the downcomer area. Temperatureconnections must be located in the liquid space or 2"/.050 above thedesignated tray in the downcomer area. It is important to check the length ofthe thermowell. It may interfere with the downcomer as shown in the auxiliaryplan. An alternate solution is shown by setting the nozzle in the "hillsideposition." This approach is only recommended if the length of the thermowellcan not be reduced. Level instruments such as controllers and gauge glassesare commonly located on bridles in the liquid section, at the bottom of thetower. The elevation of these nozzles is determined by the amount of liquidbeing controlled for maximum operation. The specific elevations required areshown on the vessel instrument sketch. A preferred and alternate location isshown if a baffle plate is required.


5.6 Preferred Component Locations - Figure 11.

Optimum tower layouts generally follow the following guidelines:

a. Plan to run piping down the tower on the side facing the pipe rack, awayfrom manways, instrumentation, ladders, etc. for small diameter towers of4'/1.2 or less, lines should be grouped for common support. (See Figure17).

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

c. Locate items such as bridles with level instruments at dead end ofplatforms if possible, thereby eliminating normal operator travel aroundsuch items.

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

5.7 Miscellaneous Nozzle Considerations - Figure 12

When initiating a tower layout the following information can be used to developnozzle locations and elevations.

a. Projections are established from the vessel inside diameter to face offlange, and vary depending on the nozzle size.

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

c. The maximum distance a nozzle can be set in the top head from thevessel centerline is shown in detail "A".

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

e. Liquid outlet nozzle in the bottom head may have minimum dimensionalrequirements. If a valve is bolted to the nozzle, a tower drain connectionis likely to be needed. Clearance between nozzles, and the drain nozzle


flange and skirt access opening reinforcement or fireproofing must bechecked.

5.8 Tray Details - Figure 13

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

Another type of tray is the chimney type. When called for, the orientation oftrays above and below the chimney tray may vary as desired. Other towershave multiple diameters. One arrangement uses a single downcomer in thetransition section, with a feed nozzle to the tray below. Another variation usesinternal piping from the downcomer to the tray below. Both enable the traysbelow to be oriented differently from the upper section of the tower. Thisenables a designer to optimize the external layout for piping, platforms, ladders,etc.

5.9 Packed Bed Sections - Figures14 and 18

Packed bed sections use metal rings for liquid vapor contact instead of trays.The rings are packed into specific sections of the tower, called beds, supportedby cross grid bars. They are spaced close enough together to prevent the ringsfrom falling through. The supports are designed to allow vapor to rise andliquid flow down. Liquid is fed in at the top of each bed through a distributorpipe. Unlike trayed towers, there are no special orientation considerations ofthese beds, distributor or packing supports.

5.10 Platform Details - Figure 15


Tower platforms should enable plant personnel to work safely during normaloperations and maintenance, without being costly and excessive in size. Theminimum size for top head, crossover or normal operations can be seen in thisfigure. Bracket spacing should be standardized whenever possible. Ladderruns must adhere to certain industry and code regulations as shown in theelevation view of the tower. No single ladder may not exceed 30'/10.0. Shoulda ladder service more than one platform, the platforms must be set at anelevation that is consistent with the rung spacing of 12"/.300. Dimension "A"shall be in even increments or rung spacing. Avoid setting two platformsserviced by one ladder at the same elevation for operator safety. Elevationdifference must be 2'/600 minimum.

Ladder cages are not needed for platforms whose elevation is under 20'/6.10.Step through ladders are permitted. One rule to remember when designingany platforming: never force plant personnel to go up a higher elevation, whenattempting emergency egress off a tower.

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

5.11 Optimizing Tower Layout - Figure 16

Typical issues a designer must confront include such things as:

a. A process suggested arrangement for a feed nozzle would put the blockvalve away from any planned platforming. By employing one of thealternate feed nozzle arrangements, the internal pipe can be set at anydesired orientation. This would put the block valve over the desiredplatform layout.

b. The reboiler draw-off nozzle has a major impact on tray orientations. Besure to deal with this circuit early in the tower layout.

c. Try to plan on locating major level instruments at the dead end ofplatforms. Since they project out into the operator access space, the size


of platform can normally be reduced if not required to traverse aroundsuch items

5.12 Reboiler and Pipe Support Considerations - Figure 17.

A piping designer should have a good working knowledge of supports whenengaged in the layout of fractionation towers.

a. Vertical Reboiler - if the reboiler is supported from the vessel, theelevation of the lug on the reboiler must be set 1"/.025 above themaintenance support bracket on the tower. During turnaround periods,when the channel end must be removed for tube maintenance, the1"/.025 gap is shimmed, thereby becoming the reboiler support.

b. Pipe supports on large diameter towers normally consist of a structuralbracket off the tower in close proximity to the line being supported orguided. A dummy leg is welded to the pipe at an orientation which willallow the load to be transferred from the pipe to the vessel supportbracket.

c. For small diameter towers, individually supporting and guiding pipebecomes more of a problem due to its limited size. Immediately aftersupporting each line close to its nozzle, the lines should be routed andgrouped as if a "vertical piperack" down the tower. By lining up the backof pipe, common structural members may be effectively used forsupports and guides.

6.0 REFERENCE DRAWINGS

The following Bechtel drawings should as reference material when engaged inthe layout of a fractionation tower.

Subject Dwg. No.

- Feed and Vapor Overhead Nozzles B-501- Transition Section B-502- Bottom Section B-503- Drawoff Details B-504


- Manway Davits C-509

- Davit Details M-507- Circular Checker Plate Platforms M-513- Circular Grating Platforms M-517

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