What is pipeway?
The pipe way conveys all main process lines connecting distant pieces of equipment, relief and blowdown headers, all lines leaving and entering the plant, utility lines supplying steam, air, cooling water and inert gas to the plant. Electrical and instrument cable trays are usually routed in the pipeway. Pipe ways are classified by their relative elevation to grade.
Piperack
Overhead piping supported on steel or concrete bents.
Pipetrack
Above ground piping supported on concrete sleepers at grade level. (Off site areas where equipment is well spaced out)
Trenched Piping pipeway
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. Scope of trenched piping is with civil department for trench design.
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. Scope of underground piping is with civil department for underground design.
Information Required for pipeway design
Job Specification
Job specification SP….C100 contains design criteria, agreed between Company and Client, affecting piperack design:
Battery limit, valving and spade 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
Pipeways and secondary access ways
Main access roads
Rail roads
Standard to be used for minimum spacing of lines in paperacks
Handling and headroom requirements for equipment positioned under piperacks
Operating and safety requirements affecting pipe rack and structure design
Location of cooling water lines underground or above ground
Trenched piping, if any.
Process Flow Diagrams
Process flow diagrams show main process lines and lines interconnecting process equipment.
Engineering Flow Diagrams
Engineering flow diagrams are developed from process flow diagrams and show:
Pipe sizes. Pipe classes, and line number.
Valving.
Manifolding.
All instrumentation.
Equipment and lines requiring services, i.e. water steam, air, nitrogen etc.
Utility Flow Diagrams
Utility flow diagrams show the required services:
Steam
Condensate
Water
Air
Gas
And any additional services peculiar to the plant being worked on, e.g.:
Caustic
Acid
And refrigeration lines, etc.
Evaluation of Information for pipeway
Initial Evaluation
Use plot plan and process flow diagrams to make a preliminary assessment of which portion of process lines will be located in piperack and which lines will interconnect directly to nozzles on adjacent items of equipment. Draw lines to be located on piperacks on print of plot plan. Some idea of utility piping required must be established and included 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.
Development
With the receipt of engineering flow diagrams and utility flow diagrams, a more complete and accurate assessment of rack space is possible. Utility headers generally run the whole length of the piperack, so should be taken into account when estimating additional space required. To assist Process Department in sizing utility headers in the pipeway a line routing on a repro of the plot plan, showing order of take-offs is required.
Line Identification
Certain types of piping require special consideration:
Process Lines
Lines interconnecting nozzles on process equipment more than 6M apart (closer speced equipment may be directly interconnecting inside piping areas).
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.
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 some relief headers must be placed above the main piperack.
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.
Piperack Width
The width of piperack is influenced by :
The number of lines
Electrical/instrument cable trays.
Space for future lines.
The width of a piperack may be calculated using the following method : First estimate number of lines as described. Add up the number of lines up to 18” diameter in the most dense section of the piperack.
The total width in meters (W) will be :
W = ( f x N x S ) + A meters
Where f, safety factor = 1.5, if the lines have been laid out as described in initial evaluation.
Where f, safety factor = 1.2, if the lines have been laid out as described under development.
N = number of lines below 18” diameter
S = average estimated spacing between lines in millimeters.
Usually – S = 300 mm
Usually – S = 230 mm ( if lines in piperack are smaller than 10” )
A = additional width required meters for :
Lines larger than 18”.
Future lines.
Instrument and electrical cable trays.
Any slot for pump discharge lines 500 mm – 1 meter.
The total width is thus obtained. If W is bigger than 9M usually two piperack levels will be required.
Note : At the beginning of a job, `W` should usually include 30 – 40% of clear space for future lines.
The width of the piperack may be increased or determined by the space requirement, and/or access to equipment arranged under the piperack.
Below figure shows typical piperacks 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.
Spacing between Piperack Bents
Normal spacing between piperack bents varies between 4.6M to 6M.
This may be increased to a maximum of 8M consideration must be given to :
Smaller lines which must be supported more frequently ( see Company design specification “recommended span between pipe supports” )
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.
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. Headroom requirements for each job are detailed in Company specification no. C100 and coordination procedure.
Elevation at Piperack Intersection
Where two two-tier piperacks meet, it is essential that elevations of lateral piperacks slot between elevations of main piperack.
Figure 2A illustrates this requirement. Choice of top elevation of lateral piperack midway between the top an bottom main piperack elevation allows turning up or down at the intersection.
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.
Figure 2B shows a piperack intersection where the respective main and lateral piperack elevations do not slot between each other. This design complicates routing of lines from piperack to the other, especially where lines run on the bottom levels of both piperacks. Avoid this design at all cost.
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. See below figures.
Line Location in Piperacks
One Tier Piperacks
Below figure shows a cross section of a single level piperack. 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. However, utility lines serving one or two pieces of equipment should be on the same side of the piperack as the equipment to which they connect.
Process lines which interconnect equipment on both sides of the piperack should be placed close to utility lines, and can be on either side of piperack, depending upon location of equipment they serve. 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. Should this be impracticable, a series of smaller sections, running the whole length of the piperack, are to be provided. (see below figure).
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. This is not a rigid rule and where piping economy dictates certain process lines may be routed on the top level. Line sequence arrangement should follow the philosophy outlined previously.
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. See below figures.
Line Spacing
For line spacing use “recommended piperack spacing” per table in Job Specification C100. It should be noted that in certain cases, it is necessary to deviate from the standards quoted above, e.g., see below figures at point “A” due to possible differential expansion line spacing may have to be increased to allow for movement of lines at start up.
To determine expansion of hot lines, use Company engineering guide, Attachment “thermal expansion of pipe materials”.
Piping Economy in Piperack and its Influence on Plant Layout
Piperack Layout
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. See below figures.
Piping economy depends primarily on the length of lines routed in the piperack. Below figures shows critical dimensions which influence overall cost. These dimensions depend on overall plant layout and should be carefully considered when the plot is arranged.
Dimension “A”, (see below figures) 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. Where air fins are located above the piperack, use associated air fin maintenance platforms, modifying their extent if necessary. This method will be cheaper than adding special platforms in the piperack.
Piperack General Arrangement Checklist
Critically review piperack layout against latest information. Some lines should require rerouting for maximum piping economy. Check loops required for nesting. As many loops as possible should be combined in a loop bay having due regard to stress requirements.
Vibrating Lines
Avoid changes in direction
Avoid long overhanging bends without support
Use bends where possible (check with job specification)
Tees to be avoided as far as possible, flow should enter along run of tee and never in the branch.
Low point pockets to be avoided in the following lines :
Steam (trap any pockets and dead ends)
Slurry
Blowdown (these lines must be self-draining)
Caustic, acid and Phenol (all these services to be self draining)
R.V.’s both inlet and outlet
Vapour to knock-out pots
Heavy products, bitumen, wax
Pump suctions
Lethal and toxic substances
High point pockets to be avoided in the following lines :
Pump suctions
Light ends
Vapour/liquid mixes (hot tower bottoms – reflux lines)
Crude 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.
Provide steam traps at : low points, up-stream of loops and dead ends, via condensate drip legs.
On steam, air and condensate headers, take-offs to be from top of headers. RV headers high or low elevation. Before finalizing elevation of RV header consider elevation of all RV discharges and elevation of knock-out drum at flare.
Bends, if used : (check with job specification).
Where lines change elevation bends may be used providing difference in elevation is adequate and specification permits
Header take-offs in piperacks, use elbows
Special piping:
Catalyst lines 5D (minimum).
Vibrating piping 5D.
Small bore usually below 2” (Client preferred).
Pipe setting: to be avoided in large bore lines. Small bore lines to be set only where absolutely necessary.
Pipe Stress Department will advise which piping systems require formal stress analysis.
Supports : avoid long unsupported overhangs.
Steelwork clearances : check steelwork clearances for addition of fireproofing (lower elevation of piperack), brackets, gussets and thermal expansion of lines.
Concrete support clearances: check for local thickening of concrete columns due to method of fabrication adopted (i.e. corbels).
Valves: check clearances and accessibility. Make full of extending platforms for operation (i.e. air fin maintenance platforms).
Chain wheels: preferably do not use. However, if necessary, check chain clearances. Spading and valving at battery limit : check for accessibility if necessary, provide access platform.
Pipetracks
This type of pipeway is generally associated with offsite areas where equipment is well spaced out, and land space is not a premium.
Pipetrack Width
Pipetrack width may be estimated using the method detailed previously for piperack.
Spacing of Pipetrack Sleepers
Pipetrack sleepers are relatively cheap thus piping economy is dictated by the recommended span of the smallest line in the track.
Where small bore lines are few, sleeper spacing may be determined by the pipe span of large bore lines provided small bore lines are supported off the larger lines at adequate intervals. An angle with “U” bolts is sufficient (check with Pipe Support Section).
For recommended pipe support spans, use Company standard.
On an average minimum span = 3 meters
maximum span = 6 meters
depending on line size and substance carried in pipes, (i.e. gas or liquid).
All lines must be supported. At changes of direction due to long overhangs, for narrow pipetracks a diagonal corner sleeper is recommended. On wide pipetracks, use alternative method of two short sleepers located near corner. See below figures.
Pipetrack Elevation
Pipetrack elevation is set by maintenance access to piping items located underneath the pipetrack, i.e. drains and steam traps. A minimum of 12”/300 mm clearance between underneath of lines and grade is recommended; where necessary, this may be increased to 18”/450 mm.
As pipetracks are generally single tier, no change in elevation is necessary at changes of direction. This is effected by use of a flat turn. See below figures.
For individual line elevation above sleeper, use Company job specification no. C4 “elevation of hot insulated lines”.
Line Location
Line location with reference to bore and weight is unnecessary, as all pipes are supported on sleepers which rest directly on the ground. Line routing is all important. All lines interconnecting process equipment and/or storage tanks located on left-hand side of pipetrack are placed to the left-hand side. Similarly, all lines interconnecting equipment located on right-hand side of pipetrack are placed to the right of pipetrack. Lines interconnecting equipment located on either side of pipetrack are placed near the center of pipetrack.
Line Spacing
Use Company job specification no. C100 “recommended pipetrack spacing”
Road Crossings
The standard method is to provide culverts under access roads. Elevating piping on a cross-over rack is expensive and introduces unnecessary pockets in the lines thus routed. When determining width and height of culvert, care must be taken to allow sufficient room round the pipework for maintenance, insulation and painting. Where only one or two lines cross a road, crossing may be by means of sleeves set under roadway.
Access Ways
In areas needing frequent access, platforms may be provided across pipetrack.
Valves
Where possible, these should be grouped at the edge of an access platform for ease of operation. Drain valves where possible should be brought to outside of pipetrack for ease of operation. The same applies to steam trap assemblies.
Expansion Loops
Horizontally elevated expansion loops above pipetrack should be provided where necessary. Group all hot lines requiring expansion loops, hottest and largest line on the outside, on one side of pipetrack. (Generally, the side chosen is that side which has the highest number of take-offs serving equipment on that side).
Trenched Piping
In most plants, 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. See below figures. Total width of trench required may be determined by using method detailed above. Piping Department will advise Civil Department of requirements.
Line Spacing
Use Company job specification C100. “recommended piperack spacing”.
Line location in a trench carrying a number of lines should be carefully chosen for maximum piping economy consistent with stress requirements, if any.
Open trenches require drains to stop accumulation of surface water. Trench bottom should be sloped towards drain points. In this case, pipe supporting is by means of angle steel or I beams set into the walls, allowing bottom for free drainage to nearest drain point.
This method allows drainage of a trench by a minimum of drain points between each pipe support as would be the case of solid concrete pipe supports built up from the trench. Before proceeding on trench drainage check with coordination procedure and Civil Department for water table level.
Safety Precautions
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. Piping will advise Civil Department of requirements.
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.
Above ground safe drain-tails will enter below ground drain line via a tundish (concentric reducer normally) or if a sealed system and cooling water lines by a flanged stub raised above ground.
Flanged connections should be a minimum of 300 mm above prevalent grade level. It is advantageous to set a common level for all these take offs at the outset of the job. When locating tie-in connections to underground systems, especially from elevated drain points, and adjacent to equipment plinths, ensure adequate clearance.
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; for this reason, never run underground piping under or through foundations.
