Pipe Racks (PipeWay) Arrangement Design

Introduction• Pipe-Rack Considered as Main Artery of a process

plant. Basically Overhead piping supported on steel or concrete bents.

• Located in the central part with branches to the sides.• To be Designed, Layed out & Erected first alongwith

electrical cable trays, drainage trenches, sewers, & roads. After which the equipments are to be placed.

• Pipe racks carry process & utility pipings and may include instruments & cable trays also.

• Piperack structure is a major item and is often fire proofed. It is also an important cost factor.

• Lot of care, planning & coordination with other groups needed as mistakes can be very costly.

Intro…• Pipe racks are tailor-made to a particular

plant.• Process lines are usually on a lower level,

while utility lines are on the top level.• Instrument & electrical trays are either with

utility level or on a separate topmost level.• Many times made of concrete or steel.• Pipe ways are classified by their relative

elevation to grade.

Types• Pipetracks are Above ground piping supported on concrete

sleepers at grade level. (Off site areas where equipment is well spaced out)

• Trenched Piping are Below ground piping laid in connection trenches.

• Costly and usually undesirable; unless trenches are wide, shallow and well vented, heavy gases may settle and create a fire hazard through the length of the trench.

• For these reasons, only pump out lines, chemical sewers or chemical drain collection systems are sometimes placed in trenches and routed to a pit or underground collection tank.

• Underground Piping : Piping direct buried below ground level. Due to costly maintenance and the usually corrosive nature of soil, this method of routing is generally reserved for sewer and drain lines.

• In some plants, especially in cold climates, cooling water lines are buried below the frostline. This should be determined at the beginning of a job, is generally a Client request.

Pipe Rack : Concrete Fireproofed steel structure with piping attached for oilfield project.

Primary Inputs & OutputsInputs :• Plot / Site plan• Process Flow, Utility Flow, Engineering flow diagrams and

P & I Diagrams• Plant layout Specifications• Client specs• Construction materials• Fireproofing requirements• Battery limit, valving requirements. • Catwalk, platform and ladder access to valves and relief valves

in piperack. • Minimum headroom and clearances under overhead piping or

supporting steel within areas and for equipment positioned under piperacks .

• Operating and safety requirements affecting piperack and structure design.

Outputs :• Pipe rack width

• No. of levels & elevations

• Bent spacing

• Pipe flexibility, access & maintrnance considerations

Determining Key Parameters : Evaluation & Development

1. By interconnecting P & I Diagrams, a line-routing diagram is first created.

• It is a schematic representation of all piping systems on a plot plan.

• It helps to identify which portion of process lines will be located in piperack and which lines will interconnect directly to nozzles on adjacent items of equipment.

• Some estimate of utility piping required must be established.• Coordinate with Instrument and Electrical Section to assess

what additional rack space may be required to accommodate cable trays. This action provides a preliminary visual idea of the piperack space required.

• PFDs can help to identify temperature & insulation requirements, after which parameters like rack width, Bent spacing, No. of levels & elevations can be decided.

Special ConsiderationsCertain types of piping require special consideration: 4.1 Process Lines• Lines interconnecting nozzles on process equipment more than 6M apart • Products lines which run from vessels, exchangers or pump to battery / unit

limits • Crude or other charge lines entering the unit which run along piperack before

connecting to process equipment, furnaces, exchangers, holding drums or booster pumps.

4.2 Relief Headers• Individual relief lines, blowdown lines and flare lines should be self draining

from all relief valve outlets to knock-out drum, flare stack or to a point at the plant limit.

• To achieve this, lines will connect into the top of the header and at 45 degrees in direction of flow. To eliminate pockets, and obtain required slope to knock-out drum relief headers must be placed above the main piperack.

4.3 Instrument and Electrical Cable Trays• Often instrument and electrical cable trays are supported on the piperack track.

Space must be allocated to accommodate them from the outset. Due to the possibility of induced current interference instrument and communication cable trays must be located away from electrical and power cable trays. Consult with Instrument /Electrical Department for separation requirements.

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PipeRack Bent spacing• Pipe bent includes the vertical columns & the horizontal

structural members carrying pipes.• Normal spacing between piperack bents varies between 4.6M

to 6M. May be increased to a maximum of 8M.• Pipe sizes & weights decide the bent spacing.• Spacing may be increased (To cross roadways or provide for

construction below it.) by adding intermediate bents supported from spandrels ( horizontal cross beams for stability & support).

Special consideration must be given to : • Smaller lines which must be supported more frequently .• Liquid filled lines requiring shorter span than gas filled lines. • Hot lines which span shorter distances than cold lines of the

same size and wall thickness • Insulated lines; small bore, cold - insulated lines due to weight

of insulation must be supported at relatively short intervals. • Space requirements of equipment at grade can sometimes

influence piperack bent spacing.

figure shows typical piperack bents with tabulated dimensions. Total available piperack width of each type of support is included. This table can be used for selection. The most commonly used piperack supports are types 2, 3,

4 and 5.

Pipe Rack width

• Influenced by : – The number of lines – Electrical/instrument cable trays. – Space for future lines.

• From the line-routing diagram, a dimensioned cross-section is drawn.• 20 % space should be provided for future growth and some space for

direct electrical conduits.• Spacing chart is used to decide on pipe line spacing.• Flanges on adjacent lines be staggered.• Allow for thermal expansion / contraction• All dimensions are added & rounded off. • If width comes too large (bigger than 9M) usually two piperack levels

will be required. • The width of the piperack may be modified by the space requirement,

and/or access to equipment arranged under the piperack.

Piperack Elevation Piperack elevation is determined by the highest requirement of the following : • Headroom over main road • Headroom for access to equipment under the piperack • Headroom under lines interconnecting the piperack and equipment located outside. • The size of steel or concrete beam supporting overhead piping must be taken into

consideration. Elevation at Piperack Intersection• Where two two-tier piperacks meet, it is essential that elevations of lateral piperacks

slot between elevations of main piperack.• Generally, lines running at right angles to main piperack are assigned elevations 500

mm to 1 meter higher or lower (depending on headroom requirements) than lines running in main piperack. 500 mm differential between pipe runs is the absolute minimum.

• a piperack intersection where the respective main and lateral piperack elevations do not slot between each other is to be avoided at all costs.

• Where a single tier piperack turns through 90°, and all lines can be kept in the same sequence in both directions, no elevation difference is necessary. When lines sequence changes, introduce an elevation change at the turn.

Elevation at Piperack Intersection

No. of levels (Tiers) & Locations

One Tier Piperacks :• Heavy lines (very large diameter lines, large bore lines full of

liquid) regardless of service are placed over or near the piperack columns. This simplifies steelwork or concrete piperack design. Centrally loaded column and reduced bending moment on the beam will result in a lighter overall design.

• Place process and relief lines next to these. Lines serving left hand areas of plant on left, lines serving right hand areas on the right.

• The central piperack portion is reserved for utility lines which may serve both right and left hand areas on the plant.

• The position of product lines is influenced by their routing after leaving the unit, right, (left) turning lines should be on the right (left) hand side of the piperack.

• If possible, a centrally placed section of the piperack is reserved for future lines. This section should run the whole length of the piperack.

Other Guidelines (related to Fig. on next slide)

Locating Lines. Valves & Instruments• Two Tier Piperacks• Where the number of lines dictate the use of a two

level piperack, utility lines are placed on the top level and process lines on the bottom level.

• Instrument & electrical trays are either with utility level or on a separate topmost level, decided in consultation with the respective groups.

• Largest lines be run near the outside to reduce the overall load on beams.

• Relief headers be located above top level of the rack to allow for drain line.

• Space should be kept for Columns’expansion in vertical direction.

• Utility Shut-off and operating valves be in horizontal position & accessible from platforms or chains.

Position of Hot Lines

It is advantageous for pipe supports to group hot lines requiring expansion loops together, preferably on side of the piperack. Horizontally elevated loops over the piperack are commonly used to minimize the effects of expansion on hot lines, the hottest and largest line being on the outside.

Piperack Layout • Piping economy depends

primarily on the length of lines routed in the piperack. Fig. shows critical dimensions which influence overall cost.

• Dimension “A”, is the total length of piperack and is governed by the number and size of equipment, structures and buildings arranged along both sides of the piperack. On average, 3 meters of piperack length are required per item of process equipment, good lay-out can reduce piperack length. (Thereby costs).

• Equipment in pairs, stacked exchangers supported from towers, two vessels combined into one, closely located towers with common platforms, process equipment located - under piperack - are examples which help shorten piperack length. In a well arranged plant, average length of piperack per item of process equipment can be reduced to 2.1 meters to 2.4 meters.

• Careful selection of dimensions B and C, below figure will minimize interconnection equipment on opposite sides of piperack. C is normally no more than 1.8 meters to 3 meters.

• Dimensions D and E, (see below figure), minimize. Overgenerous dimensioning here will increase vertical pipe lengths. Maximize use of available platforms for access to valves.

•Plant layout determines the main piperack piping runs. The shape of piperack is the result of plant arrangements, site conditions, Client’s requirements and overall plant economy. (Fig.)

•Plant layout determines the main piperack piping runs. The shape of piperack is the result of plant arrangement, site conditions, Client’s requirements and overall plant economy. (Fig.)

General Points

• Initial locations of lines are kept temporary till the layouts are optimized.

• Dead spaces be avoided on racks as space is limited.• If equipments are located below the racks, space

needed to maintain or handle them must be provided. • Low point / High point pockets to be avoided in certain

lines.• On hot lines, check shoe requirements and clearances at

changes of direction (pipe expansion).• Provide vents at high points. • Provide drains at low points. • Supports : avoid long unsupported overhangs. •

Flexibility & Supports Considerations

• Pipe rack designer needs to do elementary designs so as not to require major rework later.

• Find potential flexibility problems in lines , e.g. steam headers• Estimate the amount of expansion. And provide enough space

for it and even some additional clearance.• No. of anchor points be decided.• Drip legs, steam traps be provided on either side of expansion

loop.• Dummy legs / supports may be used.• Larger lines may be used to support smaller lines.• Uninsulated lines are U-Bolted to the supports while Insulated

lines shoes are welded to the supports.• Insulated lines should not be used for supports. If used,

expansion be allowed.•

Structural Considerations

• Supports are needed when line is entering or leaving rack. Spandrels (cross beams) are used for this.

• Inside face of pipe rack column should line up with that of equipment support column.

• Fireproofing is needed in most racks. Either upto the lower rack or if equipments are located, upto its support beams.

• Upto 4in. Thickness.

Other Considerations

• Expansion of the pipe rack can be done by adding a cantilever beam on the outside of the column.

Pipetracks (Laid on Grade level)• Generally associated with offsite areas where

equipment is well spaced out, and land space is not a premium.

• Generally single tier • Pipetrack Width, Spacing of Pipetrack

Sleepers, Pipetrack Elevation, Line Location (Line routing is all important. ), Line Spacing , Road Crossings (The standard method is to provide culverts under access roads.), Access Ways , Valves, Expansion Loops all similar to Piperacks, but simpler & less expensive.

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Trenched Piping Trenches are avoided due to problems associated with this type of pipeway: • High initial cost • Fire hazard Where trenchers are used to route lines such as: • Pump out lines • Chemical sewers • Chemical drains • Trenchers must be enough to allow sufficient clearance between trench wall

and piping. 150 mm between outside of pipe and inside of wall is the minimum acceptable clearance. This will allow for installation of piping, painting and future maintenance.

• Most trenches have either a cover of concrete slabs or a grating. • Where flammable liquids are carried in trenched lines, a fire break is

provided at suitable intervals along a trench and at each intersection. This generally consists of two concrete walls 1 M - 1.25 M apart, with the space in between filled with sand. Where highly flammable gasses are carried, the whole trench, after installation of piping, is back filled with sand.

Underground Piping • Keep buried piping to a minimum. Generally only sewer drain

lines and fire mains are located below ground. In some cases due to Client or climate requirements, cooling water lines are also buried below the frost line.

• With future maintenance in mind, buried lines should be located well clear of foundations, and if running side by side, well spaced out. A minimum of 300 mm clearance is necessary between foundations and lines and between the lines themselves.

• All buried steel pipes should have applied a corrosion resistant coating and wrapping.

• Deep valve boxes for buried lines should be designed with ample room inside the box for a maintenance man to bend over and use wrenches for tightening flanges of re-packing valves. Consideration should be given to the use of concrete pipe in lieu of square boxes.

• The criteria for a good underground piping design should be ease of maintenance. Piping should be so spaced as to allow easy digging out and replacement of faulty sections;So, never run underground piping under or through foundations.