Pipe Rack Design and Calculations

A Pipe rack is a structure designed and installed specifically to support multiple pipes, where an adequate building or structure is not available (mainly outside the building).

Pipe racks are necessary for arranging the process and utility pipelines throughout the plant. It connects all the equipment (installed at a different location) with lines that can not run through adjacent areas.

Pipe racks are also used in secondary ways, as it also carries the electrical wire, instrument wire, fire fighting systems, lights, etc. Air-cooled or fin-fan type heat exchangers are often supported above pipe racks to reduce the plant space requirements.

Pipe Rack Type

There are mainly three types of Pipe rack-

1. Steel Structure Type
2. RCC Structure Type
3. Sleeper Type (This is also called Pipe Track)

Steel Structure Pipe Rack

A steel structure type pipe rack is preferred for lines up to 12″ or 300 MB.

RCC Structure Pipe Rack

An RRC structure-type pipe rack is preferred for lines above 12″ or 300 MB and up-to 30″ or 750 MB. It is also used if the pipe rack height is above 10 m.

Sleeper Type Pipe Rack or Pipe Track

Sleeper type pipe racks are mainly used for pipelines above 30″ or 750 MB.

The most commonly used shapes for racks are L/T/U/H/Z. The shapes are mainly decided based on space availability and optimized use of space and material.

Note: the above conditions are not fixed, it can slightly differ from company to company.

Documents Required for Pipe Rack Development

Pipe Rack Designing involves considerable planning and coordination with other engineering groups to deliver error-free work. Following documents or engineering deliverables are required-

• PFD (Process Flow Diagram)
• P&ID (Piping and Instrumentation Diagram)
• Line List
• Line Routing Diagram or GAD
• Over All Plot Plan
• Unit Plot Plan
• Equipment Layout
• Piping Material Specifications
• Client Specification
• Fire-Proofing Information

Who Designs What?

Pipe rack designing is not a one-man army job. It involves a lot of engineering calculations and so required contribution from the different engineering disciplines. Let’s see the below table

Note: In this article, we are going to learn pipe rack width, height, and length calculations.

Before coming to the pipe rack width calculation, We must know the line placing criteria. If you don’t know how to place the pipes on the rack, you will not be able to get the accurate width of the pipe rack.

Do not worry, Line placing is not rocket science, I have described all the important points that need to be considered while placing the lines on the pipe rack-

Line Placing Criteria for Pipe Rack

Followings point are need to considered-

• Group the utility and process lines.
• Keep hot and cold lines away from each other to minimize the heat transfer.
• For ease of support to expansion loops, always try to keep the hotlines near to the stanchion or column.
• If the lines are heavy, keep those lines near to the stanchion or column to minimize the stress (bending moment) on the horizontal beam or member.
• Do not get confused that if the line size is greater the line will be heavier, no it’s not like that, as the gas-filled lines will create less stress on the horizontal beam than the liquid-filled lines.
• Once we can compromise with weight, but never ever compromise in case of temperature, always maintain enough space between the lines.
• We should avoid keeping a temperature-sensitive process line near high-temperature lines. For example, if the instrument-air line is placed near to the high-temperature line, it will absorb the temperature and can harm the instrument or instrument diaphragm.
• We should also avoid keeping the temperature-sensitive lines near to chilled lines, as the other line can absorb the moisture, and further, it can be problematic for that particular line.
• In the hydrocarbon and chemical industry, avoid keeping utility lines below the process line (means the process lines will be kept on the first tier and utility lines on the second tier. As in the case of leakage of the process fluid, water may get contaminated (as the water line is a utility line), and it can be harmful to the person.
• In the food and pharmaceutical industry, it is mandatory to keep utility lines below the process lines to maintain the purity of the product.
• If possible, keep the supply and return lines near each other, as these lines are having minimum temperature difference, and so heat transfer is less. Example: steam and condensate, cooling water supply, and chilled water supply and return.
• To balance the width of the pipe rack of different tiers, water, air, nitrogen such lines can be kept on any of the tiers, there is no restriction to such utility lines.
• Always try to keep future expansion in the middle of the beam, as it can help initially to reduce the stress in the beam.
• Future expansion shall be a minimum of 20 % of the total pipe rack width calculation, and maximum what else comes if space is not a problem.
• Make sure that the flanges are staggered to minimize the pipe rack width

Pipe Rack Width Calculation

We will do a Case study for pipe width calculation so that we can better understand the above-discussed points. Let’s take a problem below-

Input required

Calculation Steps:

Step 1:

Place the line as per the line placing criteria-

We placed the heavier pipes near the stanchion or column (12″ &10 ” pipe). Refer fig. 4

Hot and cold pipes are kept away from each other (as 10″, 6″ & 4″ lines are hot-line, we kept these lines at the left side, and others are on the right side. Refer fig. 4

Step 2:

Calculate the distance A using the below formula-

Distance between structure (spandrel) to nearest pipe = (Haft of beam + Gap 100 mm + Insulation thickness + Flange radius)

Refer the below figure for better understanding of formula-

You can make a table as prepared below for ease of calculation

So,
A = 150+100+125+272 = 647 = 650 mm

Note: We consider the beam size is 300 mm (This size is decided by structure or civil engineer)

Similarly,
Distance G can be calculated
G = 150+100+0+337 = 587 = 590 mm

Step 3:

Calculate the pipe to pipe distance “B” using the below formula

Pipe to pipe distance = (Larger flange radius + larger pipe insulation thickness + 25 mm Gap + Insulation thickness of other pipe + another pipe radius)

Refer the below figure for better understanding of formula-

So,
B = 272+125+25+75+84 = 581 = 585 mm

Similarly,
C = 177+75+25+75+57 = 409 = 410 mm

D = 177+75+25+0+44 = 321 = 325 mm

E = 190+0+25+0+44 = 259 = 260 mm

F = 337+0+25+0+109 = 471 = 475 mm

Note: 1. We consider the minimum flange rating 300#, because the line may have an orifice meter, and the orifice meter required a flange of a minimum of Class 300 because Class 150 flange thickness is not enough for tapping.

Note: 2. If the pipe NPS is greater, it does not mean that the flange radius will also be greater. For example flange radius of two lines of 10″ & Class 600 and 10″ & Class 1500 will not be same, as the flange radius are 255 mm 292 mm respectively.

Step 4:

Add the distances A+B+C+D+E+F+G

Calculated pipe rack width = 650+585+410+325+260+475+590 = 3295 mm

Step 5:

Add 20 % future expansion to calculated pipe rack width

Pipe rack width after adding future expansion = (calculated width + 20 % future expansion) = 3295+659 = 3954 = 4000 mm

Note: Pipe rack width should be rounded to the next 500 mm, which means pipe rack will be multiple of 500 mm.

Step 6:

Now Find the actual future expansion distance by subtracting distance (A+B+C+E+F+G) from pipe rack width with future expansion

Actual future expansion = 4000- (650+585+410+260+475+590) = 1030 mm

All the calculated results are mentioned in the below fig. 7

Important Points for Pipe Rack Height

• Identify the largest process or the utility lines except for the flare line.
• Line sizes can be identified using a line list or P&ID.
• Rack height is calculated considering the largest line size of the process or the utility and the same size of the branch so that all the small branches can be accommodated in the gap between tier to tier.
• Rack height is calculated considering branching from the bottom and top on both sides.
• The clearance below the first tier or lower pipe should be a minimum of 2.2 m as per the headroom.
• The standard Pipe rack height for the first tier is 4.5 m.
• The standard tier to tier height is 3 m (thumb rule)
• Tier to tier distance can be calculated based on two elbows and one spool. It can also be considered as per the operating and maintenance requirement.
• If the pipe rack crosses the road, 4.5 m minimum height required for a general vehicle.
• 6 m for the truck.
• 7 m for the train.
• 8 m for a big crane.

Pipe Rack Lenght

The length of the pipe rack depends upon the number of units and the size of the plant. Rack length can be calculated using an overall plot plan. Rack bay length in most of the cases is 6 m.

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