Piping Layout: PipeRack Piping Arrangement Drawing Part-2

YARD PIPING ARRANGEMENT

The plant layout determines the main-yard piping runs. Refer sketch, which shows typical piperack layouts for various plant arrangements.

Smaller plants usually have the simplest yard piping as shown in sketch No.. In Fig., the process and utility lines enter and leave at the same end of the plot and Fig., presents a frequently adopted layout, with utility lines entering at one end and process lines at the opposite end.

In larger plants, yard piping becomes more complicated as shown in sketch. The piperack arrangement results from an overall plant arrangement, site conditions, client's requirements and plant economy.

Pipelines in the piperack are classified as process lines, relief-line headers and utility headers.

Process lines :

Process lines are those

(a) which interconnect nozzles on process equipment more than 20ft. apart (closer process equipment can be directly interconnected with pipelines)

(b) Product lines which run from vessels, exchangers, or more often from pumps to the unit limits to storage or header arrangement outside the plant.

(c) Crude or other charge lines which enter the unit and usually run in the yard before connecting to exchangers, furnaces or other process equipment e.g. holding drums or booster pumps.

Relief-line headers

Individual relief lines, blow down 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. A pocketed relief line system is more expensive, because usually an extra condensate pot is required with instruments, valves and pumps. To eleminate pockets some relief line headers must be placed at higher elevation above the main yard usually on a tee support on the extended piperack column. However, on some noncondensing gas systems self drainage is not so essential. Relief lines can be individual, some with large diameters and occasionally high temperatures.

Utility lines

Utility lines in the piperack can be put in two groups :

(a) Utility headers serving equipment in the whole plant. Such lines are : low and high pressure steam lines, steam condensate, plant air and instrument air lines. If required, cooling water supply and return and service water can also be arranged on the piperack.

(b) Utility lines serving individually one or two equipment items or a group of similar equipment (furnaces, compressors) in the plant. Such lines are : boiler feedwater, smoothering steam, compressor starting air, various fuel oil lines, lubricating oil, cooling oil, fuel gas, inert gas and chemical treating lines.

Steam header should drain to the steam separator for more effective condensate collection. Branch connections to steam headers usually connect to the top to avoid condensate drainage to equipment.

Instrument lines and Electrical cables

Instrument lines and Electrical cables are often supported in the yard and extra space should be provided for these facilities. The best instrument line arrangement eleminates almost all elevation changes between the plant and the control room. This can be easily achieved when instrument lines are supported outside the piperack column on a suitable elevation.

Line Location

Refer sketch. It shows single-level yard piping.

Regardless of service, heavy lines (very large diameter lines, large lines full of liquid) are placed over or near the piperack columns. Centrally loaded column and reduced bending moment on the beam will result in a lighter structural design. Next to these lines are placed all process lines and relief lines. Utility lines are in the centre portion of the yard. A general sequence of uitility lines is also shown on the sketch.

It is advantageous, from point of view of support, to group hot lines requiring expansion loops.

Refer sketch. Loops elevated horizontally over the yard is the most common adopted design, having the hottest and largest diameter line outside.

Line guides, line stops and anchor points are usually required along a hot line somewhere in the piperack.

Piperack Elevation

Refer sketch for typical cross-section of piperack piping.

Elevation of yard piping is determined by the highest requirement of the following :

a) Headroom over a main road.

b) Headroom for access to equipment under the piperack.

c) Headroom under lines interconnecting the piperack and equipment outside the piperack.

The size of beam supporting the piperack piping should also be taken into account when

considering headroom.

Generally those process lines should be located in the top tier which interconnect two nozzles

elevated higher than the top tier. Process lines with one end lower than the bottom tier elevation can run either in the top or the bottom rack. If both ends of a process line are lower than the bottom tier elevation, the line should be located in the bottom tier.

The elevation of a line can also be influenced by valves and instruments in the line. Often a more convenient access platform can be provided for valves arranged in the top tier of piperack. The preferred location of lines with orifice runs is near the edge of the piperack with orifice flanges near the column for more convenient protable ladder access. Refer sketch.

The sketch, upper right corner shows platform and walkway arrangements to valves, relief valves and instruments located in the piperack.

Piperack Piping Economy

The economy in piperack piping depends primarily on the length of lines arranged on the rack. Fittings, valves and instrument are relatively few in the piperack compared to pipelength.

Sketch shows those critical dimensions (A,B,C,D & E) which will influence piping cost from piperack piping layout point of view. These dimensions depend on the overall plant layout and should be carefully considered when the plot is developed.

Dimension 'A' is the total length of the piperack and is governed by the number and size of equipment, structure and buildings arranged along both sides of the piperack. On an average,

about 10 feet of rack length is required per process equipment (exchanger, drum, tower, unhoused compressor etc.)

A control room located along the rack will increase the piperack piping cost because all lines must pass by without really being associated with the relatively long control room.

To shorten piperack length (Dimension 'A'), various methods are applied viz. equipment in pairs, stacked exchangers, exchangers under elevated drums, drums or exchangers supported on towers with common platforms, drums supported on exchangers, process equipment located under the rack etc. These arrangements not only shorten the process interconnecting equipment directly but also shorten those lines which pass through this area and utility headers serving this area.