June 2017

Pipe racks and tracks are the primary means of routing multiple lines through a plant facility.

Pipe Track

The pipe track or sleeperway is the less common  of the two types. The track is a series of laterally unconnected low beams supported on piles usually no more than one meter above finished grade. They are usually a single level of piping although they can sometimes incorporate an elevated partial level to support cable trays.

The beam spacing can be based on the maximum acceptable span of the smallest line, with intermediate tray supports, if required. However this will limit the track for future additions of smaller lines. If future additions are likely then the standard spacing of 6 Meters should be used.

Care should be exercised in the routing of pipe tracks as they present a continuous obstruction to access, while stiles can be erected for personnel access, providing vehicle access will usually involve elevating the pipe track to a bridge over the road.

Unlike elevated pipe racks tracks do not change elevation when they change direction. Care is therefore required to arrange the lines in the correct order as there are few opportunities to change order. The flat turn nature of the track also makes adding future lines more difficult than a rack.

The most common use of tracks or sleeperways is in offsite tank farms where they are necessary to keep the suction lines low until they reach the tank export pumps located outside the tank berm. The lines pass through the bottom of the berm with sealed sleeves.

As there is a significant  reduction in the capital cost of tracks over racks the discharge from these pumps together with the tank feed lines and utilities are usually routed on tracks until they reach the main plant process unit pipe racks..

Pipe Rack

Pipe racks are the freeways of piping through any unit or plant. They form the spine of the unit along which all plant equipment is located. They are always elevated as this provides good secondary vehicle access beneath them. The overhead clearance to the lowest side steel should be 6.0 Meters within unit battery limits but this should be increased when crossing major plant roads or railroads. See para 8.1 of Piping and Plant Layout Standard For Oil & Gas Facilities in the Procedures and Standards blog on this site.

(Historic Note :- Until the 1970’s process pump rows were commonly located each side of and underneath  the pipe rack. Motors were on the inside with the nozzles facing out, and fork lift access was provided down the middle. This was a very efficient use of plant real estate and gave good maintenance access to the pumps.

Unfortunately ruptured pump seals are a major cause of refinery fires.  With the lowest level of process rack lines running directly above the pumps fire could quickly spread into the rack which would often collapse completely bringing down the aerial coolers mounted above it.

After several major refinery fires involving significant equipment loss this practice was discontinued and the pumps were moved to the out sides of the rack in future designs.)

Rack Levels

Main unit pipe racks usually consist of a minimum of 3 levels. The top level is cable tray, the next level down is for utility lines and the bottom level is for process lines. This arrangement prevents electrical cables being subjected to leaks from any lines and prevents utility lines being subjected to possibly corrosive leaks from process lines.

The levels are usually 2 meters apart with lateral side steel positioned 1 meter below the top, middle and bottom levels. These distances can be reduced but it must be remembered that the 1 meter side steel will allow all line sizes up to 12” to drop or rise out of the rack using 2-90 deg. elbows.

If this distance is reduced the larger lines will require 1-45 deg and 1 rolling 90deg. Elbow.

If the rack is to be transported as a module this may have an impact on the level spacing and the overall height. See Modularization below.

Width and Span

There is no theoretical maximum width of a Pipe Rack but they are usually limited  to a maximum of around 10 meters to economize on plant space and keep beam sizes to a reasonable depth and avoid the need for center beam columns. If the rack is to be modularized then local road transportation permit limits will apply and place limits on the rack width.

For these reasons major racks connecting several units may require two process and or utility levels.

New pipe racks should be sized for present needs plus 25%.  This practice assumes a contingency of 10% at the outset of the job, but anticipates this will have been used upon completion of engineering, leaving 15% for future needs.

The span between ‘bents’ (the goal post arrangement of columns and beams at each support) in process units has always been 20 ft. or 6 meters. This is based upon the span and acceptable deflection of a 2” Sch.40 liquid filled line.

For this reason it is common practice for rack lines to be a minimum of 2”. Any line below this size is increased  to  2” as it enters and reduced to normal when it exits the rack.

Change of Elevation

The major philosophy of Pipe Rack design is that a change elevation must occur at each change of direction. As the rack turns through 90 deg, each level will rise or drop by the distance between the main levels and the side steel.

This allows piping to change order at the turn and the easy transition of one rack making a tee junction with another.

Change of elevation with direction applies equally to an individual line as the whole rack. Expansion loops and the rise or drop of lines leaving the rack are good examples of this.

A flat turn across a currently unoccupied rack will effectively kill that rack space for future lines.

Line Entry or Exit

Whether a line should rise or drop out of a rack is dependent on the fluid phase. Vapour lines should always branch from the top of the header and rise out of or into the rack, this prevents liquid carry over from the condensates that form in the bottom of most vapour lines.

Liquid lines can branch from either the top or the bottom of the header and rise or drop out of the rack, although top connections are still preferred if there is a choice..

Generally speaking pocketing of any line should be avoided, all rack expansion loops should therefore rise up and over the side steel above the specific rack level. Drains or drip legs should be provided on all lines between loops.

Line Position and Expansion

The largest and heaviest (liquid  filled)  lines should always be located on the outside of the rack close to the columns.

The reasons being :-

1. To reduce mid span beam deflection and size.
2. To maximise the rack width available for the largest expansion loops.

Anchors for all lines should be placed at the same ‘bent’ or support beam. This allows the structural group to design the additional bracing required for the anchor loading in one bay rather than several. The ‘nested’ expansion loops for all lines will occur midway between the anchor points.

Most steel fabricated pipe racks are fireproofed with a fire retardant coating during construction, especially those which have aerial coolers mounted above cable tray level.

This coating can be up to 50mm thick so allowance must be made when positioning the outer lines nearest the rack columns.

For general line rack spacing see the Rack Spacing Chart in the Standards/Selection and Application Guides section of the Procedures and Standards Blog of this site.

These charts are based on maintaining approximately 25mm between any pipe, insulation or flanges.

When checking the rack layout against the stress sketch it is important to check that the expansion movement at the change of direction of each line does not interfere with the adjacent line.

If so then the line spacing must be increased or the expansion movement decreased.

Rack Access

Pipe racks are generally considered to be an accessible area for access to valves or orifice meter runs. Generally the only area where permanent platforms and ladders are always required is at the battery limit of the unit rack were it joins the main plant rack.

This is where double block and bled valves are located to isolate all lines in the unit from the headers in the main plant rack to allow a unit shut down. The process and utility lines use an elevation change ‘waterfall’  to arrange the valves vertically at the front and rear sides of the platform.

Safe isolation is achieved by inserting blinds at the valves, this is a heavy duty manual task which requires good permanent access.

Sloping Lines and Flare

With the exception of  flare lines it is unusual for any line to be required to slope in the rack  

If a P&ID shows that a non flare line is required to slope it is worth asking  the process engineer whether sloping the line is absolutely necessary. There are occasions when there is confusion over the subtle difference between ‘slope’ and  ‘do not pocket.’

If a non flare line must slope then it will have to be placed on the elevated tee posts or cantilevers described below. As a slope also implies  free draining with no pockets any expansion loops on this line must ‘flat turn’ across the rack in the same way that the flare line loops do.

Flare lines are always required to slope or drop continuously from the far end of a unit to the Flare Knock Out Drum to ensure that no condensate liquids can be trapped in the line and prevent a PSV from discharging freely and without resistance.

The flare line is usually run on a tee post extension of the main rack columns above the cable tray level on one side of the rack. Where there is an aerial cooler structure on top of the rack the line is run on cantilevered beams outside the columns.

To achieve the slope (usually 1:500 min.) a combination of variable height shoes and changing top of steel elevations is used.

The top of steel elevation stays consistent  for around 4 tee posts/cantilevers while the shoes are varied from max to min height at each subsequent support, then the tee post/cantilever is lowered and the process repeats.

Modularization

Modularization has become the preferred method of most major plant construction over the past 30 years. While it can be argued that there is a limit to how many pieces of equipment can be economically modularized and whether the amount of additional transport steel can be justified,  there is no argument when it comes to pipe racks. Their size , shape and natural framework make them ideal candidates for modularization.

One disadvantage is that the structural engineer will often design bracing on both sides and vertically across all rack levels and all bays to stiffen the structure for transport and lifting. This becomes a restriction to line exit and entry and must be carefully clash checked.

Some engineering companies feel that all module connections should be flanged for ease of construction. I personally disagree. A pair of flanges on connecting modules are far more susceptible to misalignment than  butt welds and for the sake of saving some construction time hundreds of potential flange leak points have been created.

Transport restrictions control all of the design dimensions of a module but particularly the height. The clearance for the module to pass under power lines and bridges is critical. The layout of the rack and it’s absolute highest point must be carefully checked during design and compared with the lowest permitted height clearance along the whole intended transport route. This information should be provided to the design team by the transport company early in the design phase.  To reduce the height measured from the truck bed the lower support columns, lower side steel and bracing are not included as part of the module but are ‘stick built’ (constructed on site).

The active module transport height is measured from the underside of process level beam, which is supported on the truck bed,  to the top cable tray level or the highest vent valve, the columns, which overhang and drop below the truck bed, are provided with splice plates to connect with the stick built columns.

As the truck bed is directly below the process level support beam any lines which drop down out of the bottom process level and leave the rack via the lower side steel are not included in the module and are also stick built.

The module can usually be up to 30 meters (4 bays+ 2 half bays) long (check your local transport restrictions),  the split between modules should be located  mid bay at each end.

The only additional steel required  to transport a rack are temporary line hold downs and temporary braced beams at each level to support the 3 meter line overhangs at each end during transportation.