Layout mistakes continue to plague chemical plants. Deficiencies in design and engineering cause overcrowding and poor access to equipment. So, in this article, we will look at some key failings as well as lessons learned.
Thorough simulations underpin many current designs of chemical plants. However, numerous projects still require adequate and reliable tests to obtain the information necessary to develop elements of the basic design including flowsheets, design criteria, etc. Often, getting essential data calls for operation of a pilot plant.
Unfortunately, many things can go wrong with tests. Samples, methods and details of those tests need great care. Non-representative samples have caused a multitude of issues and problems. The test results only are as reliable and dependable as the sample tested. Small-scale laboratory work often doesn’t provide definitive results.
While the quality, details and methodologies of tests always are important, so, too, is the interpretation of the results; this requires close examinations and sound judgment.
In too many cases, the design for a large expensive unit has been based on poor and ill-conditioned tests and inaccurate reports. This has led to serious problems during commissioning and start-up such as the inability to meet product quantity, quality or specification. Sometimes, the severity of problems has necessitated completely redesigning a unit and ordering additional equipment as well as extensive new piping or material-handling units — resulting in considerable delay and financial losses. Such mistakes also can significantly affect the layout and configuration of the unit or facility.
Overcrowding And Access
There has been an unfortunate tendency to use the smallest possible footprint. A variety of motives can prompt this, e.g., the desire to decrease the amount of piping; to cut frictional losses and, hence, overall power consumption; and to have smaller structures to reduce the cost of foundations and footings. However, adopting such an approach makes installation, operation and maintenance far more difficult.
Overcrowding of equipment and machinery has led to many problems, such as operational issues, maintenance difficulties, safety risks and others. Another major challenge often comes when the installation of additional equipment, piping or other items is desired, say, to improve product quality, increase capacity, or as part of a broader revamp or expansion. Too many facilities find they lack sufficient space to make the wanted alterations.
Adequate access, i.e., the space required between components and equipment to permit operations such as operating valves, viewing instruments, and safely actions in an emergency, maintenance activities, etc., is an extremely important consideration. It not only is essential for installation, operation, inspection and maintenance but also for safety, e.g., to enable plant personnel to exit a potentially hazardous area and for fire fighters to work effectively. Access also encompasses providing ladders, stairs, platforms, etc.; meeting safety requirements usually includes providing a sufficient number of ladders and stairways.
A common shortfall in numerous facilities is that frequently inspected platforms and areas such as busy pipe racks with many valves or units with lots of valves and instruments only have access with ladder(s). I strongly recommend providing stairs for access to frequently inspected places such as facility levels, major platforms, etc. Of course, ladders still will suffice for many less frequently inspected places (such as equipment platforms).
Multilevel Versus Horizontal Layout
To save space and cost, some designs adopt a compact multilevel layout, locating equipment at different elevations but close together and with minimum lengths of piping. Often referred to as a structure-mounted vertical arrangement, this puts the equipment in a rectangular multilevel steel or concrete structure (Figure 1). The structure can be several bays long and either open-sided or fully enclosed (depending upon, e.g., operating company preference, climate conditions, etc.). Stairs or elevators provide access. Piping, cabling, utilities, etc., often enter and exit the structure at one level and gain access to each floor by chases (or similar). It often is difficult, expensive and challenging to provide desired clearances and access. Equipment maintenance usually requires use of hitch points, trolley beams or traveling cranes. Ideally, each item should have an adequate open area around it as well as a clear drop zone at grade for equipment removal. Theoretically, designing and building such a multilevel facility with desired access, clearances and safety is possible. However, doing so is very expensive and challenging. In practice, to save some money, contractors may not strictly adhere to some requirements.
In contrast and still preferable for many applications is the traditional concept of using a horizontal wide layout with ample clearances and excellent access. This usually involves locating a horizontal inline unit within a rectangular area, with equipment and machines arranged adjacent to a central pipe-rack network. Such horizontal layouts obviously incur higher costs for land and connecting piping. However, they far better address and manage issues of access, maintenance, safety and operation.
The land available always has been a major consideration. When space is scarce, such as in a renovation, or when special requirements demand an enclosed building, then the only option is multilevel compact installation. However, I’ve observed several cases where ample land was available but the designer still opted for a congested multilevel facility that used under 60% of the available land and provided less-than-desired access and clearances. Why? Because such a multilevel compact installation was cheaper to design and procure. However, over the long term, such an installation is expensive to operate and maintain, and poses more difficulties and risks.
This points up a crucial lesson. If sufficient land is available, deciding upon an optimum layout requires considering all factors including operation, maintenance, access, etc. Many modern plants and facilities use a combination of both multilevel and horizontal concepts as appropriate.
Key Factors
The layout reflects the design team’s ability to incorporate and anticipate design, operational safety and maintenance requirements while providing the required access and clearances. Layout of an area heavily depends on piping.
So, final layout and arrangement should not proceed before review of a sketch with an overview of all major piping (i.e., above 3–4 inch or DN80) and all alloy steel and expensive piping in a given area. This review should check that all major piping can be routed, supported and designed in an orderly and cost-effective manner with the proposed layout. Considering all piping lines at the same time is recommended, as is routing and supporting them together as much as possible. This approach saves fittings and requires fewer supports. It is cost-effective and efficient as well as easier than the alternative, one-line-at-a-time approach, which always has been problematic and wasteful, and has led to many issues and reworks.
The effective use of available land is key for success. For example, a good strategy is to use a vertical piping configuration, if possible, rather than a horizontal one. Arranging valves, instruments, control devices, inline items, etc., in a vertical piping line with proper access rather than in a horizontal run can avoid wasting a considerable area of land. Obviously, such a vertical configuration might not suit some valves or instruments, leading to the use of a combination of vertical and horizontal runs.
To start the layout, it is best to locate equipment and machines in process sequence to minimize interconnecting piping. Exceptions exist to this rule, such as when there is an operational, maintenance or safety requirement. In addition, it makes sense to group equipment within common areas to suit independent operation, shutdown, etc., or when there is a common utility, maintenance facility, or other shared resource. In some cases, equipment location should facilitate in-place maintenance by mobile equipment (such as a mobile crane) or overhead crane in a shelter or building. Some examples include grouping all water-cooled heat exchangers together in an area and locating pumps together in a unit. Of course, some equipment must be located in a specific position due to process or operational requirements, for instance, pressure drop, line pocketing, gravity feed, etc.
Spacing And Clearances
Many pieces of equipment and machines require routine maintenance for reliability and safe operation. So, the layout should facilitate the removal of equipment or parts of items for maintenance. It should provide unobstructed space for service equipment and personnel to access and remove components without having to take out unrelated items, equipment and piping. One case that should be considered is the pulling of tube bundles from shell-and-tube heat exchangers. Other examples are the removal of internals from distillation columns, and catalyst loading/unloading of reactors.
It is very difficult to provide general rules for spacing and clearances. As a very rough indication, for typical drums or vessels, the minimum spacing might be half the diameter plus 1.2 m (½D + 1.2 m). Because usual diameters are 0.6 m to 3 m, this works out to minimum spacing of 1.5 m to 3 m. Identical equipment such as matching heat exchangers or horizontal vessels (for instance, those in “1+1” configuration) might be located closer, side-by-side, with, say, a minimum spacing of half the diameter plus 0.5 m (½D + 0.5 m). The spacing between each row of equipment often exceeds 3 m. All piping, auxiliaries, accessories, etc., should be arranged in this spacing while ensuring sufficient clearances still remain after everything is installed. Carefully consider all factors that affect spacing and clearances before finalizing the layout. For instance, locate furnaces, boilers and heaters with fired burners away from potential sources of gas leaks. For these major items, spacing from other equipment typically is 14–20 m depending on details.
Pipe Racks
These play a major role in the layout and overall configuration of facilities. Pipe racks are located in the middle of most units. It makes sense to erect them first before they become obstructed by other items or equipment. Multilevel pipe racks commonly are used, most often with process lines on the lower level(s), utility lines above them, and instrument and electrical trays on the highest level.
Pipe racks also often serve secondary functions — such as to provide a protected location for auxiliary equipment, pumps, utility stations and manifolds as well as firefighting and first-aid stations. Air-cooled heat exchangers often are supported above pipe racks for economy of plot space. Do not locate piping over columns as this will prevent adding another level. Place large, heavy liquid-filled pipelines near columns to reduce bending stresses on pipe-rack beams.
Designing and finalizing pipe racks demand close cooperation among civil, structure, piping and mechanical specialists. A key point is the load estimation of pipe racks. Often, ample margins are needed.
In one project, the initial estimation at the early stage of the detail design was that two-level pipe racks were needed; based on this, the civil team finalized the piling layout. Immediately afterward, piling and civil works began. Later, the piping and instrumentation diagram was updated with the addition of more piping lines requiring three levels of pipe rack. This changed the entire loading, civil design and piling layout — and led to many discussions, problems and reworks. The lesson learned here is: always consider the potential for a sharp increase in the number of piping lines on pipe rack and loadings, and even an additional pipe-rack level. Keep more than 25% of the final width of each level free for accommodating potential future piping lines.
Another concern is the spacing of the pipe-rack supports and the method of intermediate support to prevent pipe sagging. Electrical and instrument trays are best placed on outriggers or brackets to prevent interference with pipes leaving the pipe rack.
Mobile lifting equipment needing access often determines the minimum clearance under the pipe rack. Long lengths of rack piping may require expansion/contraction loops, especially in extreme hot or cold temperature services.
The best way to alter the direction of a pipe in a rack usually is by a change in elevation rather than a flat turn. This will avoid blocking space for future lines. In this way, each line can be routed to change elevation and direction with only two elbows and without obstructing other lines. Using this method, it is easy to move a line from one side of a pipe rack to the other side of the next pipe rack (when it turns) without obstructing other lines. This is important, for example, if you must route a large and expensive alloy steel line to move fluid from a critical piece of equipment on the left side of a pipe rack to a machinery package on the right side of the next pipe rack.
