In processing facilities, a good layout is the foundation for cost-effective piping and smooth operation. Along with safety considerations, equipment should usually be located according to process sequence to minimize overall piping. The ideal situation for speed and quality in the piping of any processing facility is to do the job right the first time.
This article studies the piping and layout of processing units, as well as associated machinery packages and packaged mechanical equipment. Topics such as layout, piping rules and practical notes on piping and flange joints are also discussed.
Layout and piping
One basic rule is to avoid considering just one line at a time. In other words, it is necessary to consider all piping lines in an area all together, rather than individually, which can cause problems or reworks. An overview of all the piping within an area should be completed before setting final arrangement of each piping line. This can usually be achieved by closely reviewing the piping and instrumentation diagrams (P&IDs) of the area and surrounding facilities first. A successful plan should develop piping as a whole unit with each line efficiently arranged based on its start and final points. This can avoid clashes, save fittings and require less supports.
Vertical, compact facilities versus horizontal layouts
In a vertical, congested and compact facility configuration, equipment and machineries are located in close distances and in different elevations with minimal piping. However, a horizontal, wide layout offers ample clearances and excellent access. Vertical installations are usually space-saving and cost-effective alternatives to horizontal layouts. In horizontal options, the cost of connecting piping is obviously more, but access, maintenance and operation are better, and safety concerns can be addressed easier and more effectively. A major consideration is the available land for the processing unit. If a small land is available, such as revamp or renovation cases, the only option is a vertical, congested facility. However, if sufficient land is available, an optimum layout should be selected considering all factors. This often results in using the available land, or a large portion of it, and if needed, appropriate some parts as a multilevel, vertical structure. Many modern plants use a combination of vertical, multilevel and horizontal concepts.
Process, mechanical and operational
The arrangement and layout can help anticipate process, mechanical and operational problems and provide necessary access and provisions for operation and maintenance. The final layout should be safe, cost-effective, reliable and operational. Maintenance, access, clearance and equipment spacing should always be highlighted.
When piping lines run in a congested area, a basic rule to follow is to change elevation to avoid interference with other lines when lines should be routed perpendicular to most adjacent piping lines.
The required space between components, items and equipment permits operating valves, viewing instruments and safely accessing in an emergency. Piping layout should also facilitate the removal of equipment or its parts for maintenance. For instance, tube bundle removal for shell and tube heat exchangers should be considered. Piping lines in a processing unit should usually run below access platforms or overhead to meet required clearances for access.
The required space between components, items and equipment permits operating valves, viewing instruments and safely accessing in an emergency.
To minimize the cost of piping, equipment should be at an optimum distance to each other to suit safety needs, access requirements and piping flexibility. An important step in layout and piping is to identify the alloy steel, expensive or heavy wall piping systems with great care. Layout and piping routes should be adjusted accordingly for an optimum overall cost. The main machineries and large equipment should be placed first and then the remainder of items placed around them at suitable locations. Care should also be taken for pressure drops, line pocketing and gravity flow piping lines.
One of the most important piping codes is ASME B31.3, which is a well-known process piping code used worldwide. This code covers the piping of chemical and petroleum plants, refineries, processing and others. Another important code is ASME B31.1, a power piping code, which covers the power and auxiliary service piping systems for power plants and steam generators. The code also covers external piping for power boilers and high-temperature, high-pressure water boilers. Low-temperature piping is covered in ASME B31.5, known as the refrigeration piping code, which prescribes requirements for refrigerant and coolant piping for temperatures as low as -196°C. There are many other piping codes used for other specific areas of piping.
Often, the group of piping in a processing unit is arranged all together in a piping row. Sometimes, grouped piping lines are routed in a pipe-rack structure or a small version of it, known as a mini-pipe-rack. Change of direction in a piping row should be accommodated by change in elevation.
Heat exchangers and air coolers are used in many processing units. These should be positioned to allow flexibility for interconnecting piping. Usually, piping lines with high temperature differences (hot or cold) are connected to them. Pumps or other equipment containing hydrocarbons or flammable fluids should not be located directly beneath air coolers.
There are certain constructability rules that should always be respected. Piping should not be arranged fitting-to-fitting; it should usually have a piece of pipe in each length to allow adjustment during construction.
Conservative supporting is always encouraged. One additional support in a piping system may not be a big issue, and small additional costs might be easily accepted; however, a single case of missed support could lead to piping failure at startup or operation and costly shutdown or even serious safety issues. Additional supports could be provided near heavy items such as valves and strainers. Also, supports are needed at proper locations near nozzles of equipment and machineries to minimize nozzle loads.
Joints and flanges
The material used for the pipe joint should be mechanically and chemically compatible with the pipe transporting the fluid. If welding is required, the two materials should also be chemically compatible. Materials of slightly differing chemical compositions may be welded together as long as there is no possibility of galvanic corrosion or similar problem, the correct weld procedure is in place, and the weld is executed by a suitably qualified welder.
The material used for the pipe joint should be mechanically and chemically compatible with the pipe transporting the fluid.
Flanged joints are considered to have the lowest integrity (compared to welding), and they are usually used as the basis to set the upper design limit of a piping system. Tabulated data in ASME B16.5 for steel flanges provides the maximum allowable internal pressure for a specific material in many different piping classes at a given temperature. This allowable internal pressure reduces as the temperature increases. ASME B16.5 covers flanges from 1/2 to 24 inches. For flanges 26 inches and above, reference ASME B16.47, series A and B.
Minimal flange connections should be used in piping systems because each piping flange connection can potentially be a source of leakage. Also, each flange connection needs maintenance and operational attention. For instance, gaskets should be replaced for each disassembly and assembly of the flange connection. Any flange connection also affects the dynamic responses and modal status of the piping system, and the piping requires more supports. Flange connections should only be used when necessary, for instance, for equipment nozzles, valves, strainers and necessary spools.
A gasket is a sealing component placed between flanges to create a seal between the two stationary flanges of a mechanical assembly. It maintains the seal under all operating conditions, which may vary depending on changes in pressure and temperature during the lifetime of the flange. The type of gasket chosen is based on: details of piping and flange joint, temperature, pressure, creep resistance, compressibility and nature of fluid, such as corrosive nature, density and viscosity. Other criteria to consider include: ease of handling, availability and cost. For commonly used flanges, materials fall into three fundamental types:
- Nonmetallic: flat rubber, elastomers, graphite, polytetrafluoroethylene (PTFE) and others
- Semimetallic or composite: spiral wound, jacketed, camprofile (stainless steel/graphite, Inconel/graphite and others)
- Metallic-ring type for ring-type joints (RTJ): soft iron, stainless steel, Monel and others
A majority of the nonmetallic gasket materials used comes from the elastomer and graphite families. These are commonly called soft gaskets or cut gaskets because they are cut from sheet. They are easily compressed with low bolt loads. Generally, these gaskets are used for low-pressure, for instance, 150, 300 and 600 pound classes, and for temperatures up to 150°C or sometimes 200°C. Graphite gaskets are suitable for higher temperatures, for example, up to 450°C or sometimes 550°C.
Semimetallic gaskets, also referred to as composite gaskets, are made from different materials to satisfy temperature and pressure requirements and make the complete assembly more robust. For example, a spiral-wound gasket can have four separate elements: metal windings (to hold the filler), filler (the sealing medium), outer ring (to hold the gasket in the bolt circle) and inner ring (to prevent the windings from collapsing into the process fluid). For 600 pound class and above, the gasket is constructed of nonmetallic materials suitable for higher temperatures and metallic materials for mechanical strength. The most common nonmetallic-metallic combination is the spiral-wound gasket that combines stainless steel windings with a graphite filler material and inner and outer rings. Gaskets of this construction can be used at temperatures up to 500°C and for pressure classes from 150 to 2,500 pounds. Other semimetallic gaskets include camprofile, metal jacketed and reinforced or tanged — each with its own specific sealing characteristics and associated cost difference.
Ring gaskets are used in special ring-type joint (RTJ) flanges suitable for high pressure/temperature ratings and light gases. Metallic ring gaskets are used for higher pressures and commonly used for 900 pound class and above. These rings are available in a variety of materials to suit different services and pressures/temperatures. Materials might be stainless steel (various grades), Monel, Inconel or titanium. The ring is contained within the groove; it deforms at the base when bolt loads are applied, resulting in an effective seal. The hardness differential between the gasket (softer) and the surface of the groove (harder) ensures that the gasket deforms, rather than the face of the flange.
Cost-effective piping for processing units is the result of balanced considerations for initial cost, safety, reliability, access and long-term effects on operation and maintenance. Many factors should be considered for the layout, arrangement and piping of machineries, packages and equipment.
Amin Almasi is a senior machinery and equipment consultant. He is a chartered professional engineer of Engineers Australia and IMechE. Almasi is an active member of Engineers Australia, IMechE, ASME and SPE, and he has authored more than 150 papers and articles on rotating equipment, condition monitoring, offshore, water treatment, wastewater treatment and reliability.