How plot plans and plant layout can affect operations, safety, reliability, and performance

Sept. 9, 2021
Everything from maintenance to the performance of the plant relies on a good layout.
Plot plans show positions of major equipment, machineries, and items within units and their associated infrastructure. The plant layout and plot plan can significantly affect the operation, safety, reliability, maintenance, and performance of the plant. The decisions related to plot plans and layout are critical ones, such as how to locate different equipment, the sequence of installation, distances, clearances, access, roads, maintenance, etc.

Platforms, walkways, stairways, ladders, and similar structures and provisions are required for safe access to facilities and equipment. These are required for access and safe working at places normally used by operation, maintenance, inspection, and servicing personnel. This article discusses plot plan, layout, and safe access for plants.

Plot plan and layout

The preparation of plot plan or layout involves knowledge of a wide range of engineering and operational topics such as process, operation, maintenance, mechanical, piping, safety,  and reliability. This pool of knowledge usually extends beyond those of an individual engineer or expert. Therefore, in most cases, a team of experts should be assigned for this great task. However, often one person takes the lead and responsibility for the layout of each unit or area.

In many cases, the most experienced expert on the team is assigned as the leader (say for instance, the plant layout engineer or plot plan lead). The layout is an extremely important matter and any mistake in the layout would costs lots of time and money to rectify later. The plant layout engineer/expert is responsible for collecting all comments and recommendations about the layout, compiling them, and incorporating them. Some of requirements may conflict, and therefore optimization is required. The capability to resolve any issue (as raised) and make an optimized decision is clearly needed.

Specific knowledge and experiences of each unit or each type of production/processing are required for the layout/plot-plan, as each unit or type of processing/production mandate certain needs for its layout. The feedback from the operation team, safety team, reliability/maintenance team, and piping team are critical, as the layout should encourage smooth operation while safety, reliability, and easy access/maintenance are ensured.

The clearances and distances between equipment and machineries should be optimum so that piping can easily be routed between them and access ensured. Likewise, distances should not be so long that excessively long piping required. The piping configuration, the layout of the facility, and the nozzle orientation of equipment/machineries can affect each other. Therefore, all these three key elements should be considered simultaneously for any facility. The access for the installation, for the operation and for all maintenance works (for instance, during major overhauls) should be ensured.

Key layout points

For a plant, all operational needs should be considered while adhering to all applicable local and international codes/regulations and good engineering/operational practices. The first step in the preparation of a layout (or plot plan) is to identify the real needs and requirements. This is often the most difficult task. It is essential to clearly define the objectives of the task and to re-confirm or slightly adjust the objectives as time progresses.

A plant layout should facilitates the reception of raw materials and the dispatch of finished products in the shortest possible time with minimum capital tied up in work in progress. However, other key requirements that should be considered are safety, access to the site, maintenance, easy transfer/handling of materials, and necessary intermediate storages. Key operational requirements that should be seriously considered and incorporated:

  1. Production should be sales driven. Therefore, operational/production flexibility is extremely important. This is applied to all levels of flexibility, such as production capacity variations, production quality improvements, spares for key equipment, or high reliability.
  2. Make every part right the first time. This key requirement should be applied on all levels for layout, equipment, items, processes, or production.
  3. Typically, equipment and machineries should be configured in “process flow” sequence (when possible, where applicable). There have been some exceptions and deviations to this rule. But this is a good guide to start the layout.

A wide range of requirements and topics should be considered for plot plan and layout. Many considerations related to expected and unexpected events, such as wind direction, possible storms, potential fire, or possible flooding, need attention. The different roads, waterways, pipe-racks, major buildings, and other infrastructures should also be carefully thought-through. Drainage and how all different drains and effluent flow need attention. Topology of the land and all surrounding battery limits and neighboring should also be considered.

Future expansions and renovations

A key consideration is areas needed for possible extensions and future expansions. Many unexpected things could happen at the commissioning, start-up, and operation of the plant. Some additional piping lines or in-lines may be added to facilities. Even a few new items or small equipment might be added to the layout.
Therefore, enough space and areas should be considered for these unexpected additions. The future expansion area is for future debottlenecking or renovation/expansions when new equipment and machineries will be added or other improvements to increase the production capacity of the facilities. 

Vendor supplied machineries and packages

A key challenge is many equipment, packages, and machineries will be supplied by vendors. When the first version of plot plan is developing, certified vendor data may not be available. Using primitive general arrangements from preliminary proposals or finding data of other (similar) projects/plants may be a good solution for this stage. However, the final equipment layout of the selected vendor later on might be different. Therefore, some extra margins and contingency plans are needed.

Safe access

Permanent and fixed platforms, walkways, stairways, and ladders are needed for many places in facilities and units. Judgment should be made to define where permanent and fixed access provisions are required and where temporary provisions such as mobile platforms or portable ladders might be acceptable instead. Temporary, portable, and mobile provisions may be used for some places as permanent or fixed provisions may not be required, suitable, or appropriate for those specific places.

An important consideration is frequency of access and use. Another is operational cost versus capital cost. If frequent access for operation, inspection, or maintenance is needed for a location (or place), the cost of temporary measures would be very high over time. Therefore, it is justified to provide permanent (or fixed) platforms and access. In fact, the overall operating cost associated with temporary access provisions for some frequently accessed places could be several times more than the cost for the installation of permanent platforms and staircases.

Other considerations include area congestion, platform or walkway support, other item/equipment/piping interference, and many more. At the end, it usually comes to cost versus benefits, considering the frequency of use and the operational/maintenance requirements. For some places due to frequent access, permanent platforms and walkways are needed and should be provided regardless of the cost and complexities of the required supports and other difficulties. For some other places, permanent platforms and associated fixed structures create more problems, and a type of temporary access is better.

For instance, a main process filter set that may need filter element replacement every month or so should be provided with a permanent stair and platform. However, the inlet filter for a small compressor package on a relatively clean gas service that may need replacement every year (or so) does not require the permanent access and fixed platform. The installation of a permanent platform in the congested area of inlet suction of the aforementioned compressor package would be very difficult and problematic. In other words, this filter on the suction line is provided for occasional dirt in an extreme event, and access may be needed every one or two years. Therefore, the best and most appropriate option in this case is the temporary access provisions such as a mobile platform.

Configuration and arrangement for access provisions

Slips, trips, and falls are major risks for access provisions. Slip-resistant features such as using grating, checkered plates, or a plate fitted with cleats is mandatory for platforms, walkways, and other access items. Platforms should nominally be horizontal. Where drainage is required, the slope should not typically exceed 1 in 35 (say 1.5 to 2°). Continuous walkways are obviously different. Walkways may have a slope up to 20°. Appropriate slip-resistant features should be provided for the slope. In general, the collection of liquids should be avoided, and proper drainage is needed for platforms/walkways.

Head clearance is an important requirement. Normally vertical clearance above platforms and walkways should be 2.2 m (or more). Wide platforms and walkways are always preferred and narrow ones are not popular. As an indication, the clear width of platforms and walkways should be 600 to 650 mm or even more.

Falling objects, even small ones, could impose great risks. Toe-boards are required at the edge of platforms where an object could fall more than 2 m. All floors should be evenly laid, and variation in height between adjacent boards or plates that could form a tripping hazard should be avoided.

Strength and loading cases

Different loads and loading cases such as those from dead and live forces, wind, operational forces, and others should be considered for platforms, staircases, ladders, and other access provisions. As a rough indication, the floor of the platform, walkways, or stairways and the treads (in stairs and ladders) should be capable of withstanding the dead load of the structure, plus a superimposed live loading of 3 to 5 kPa uniformly distributed (weight of 300 to 500 Kg per square meter), depending on the application and services, or a concentrated loading of no less than 1.5 to 2 kN (weight of 150 to 200 Kg) at any point.

All associated components and pieces such as guard-railing and handrails need strong and suitable structures. All components and parts should be securely fixed so that guardrails, posts, and intermediate rails or infill form an integral structure.

Preventing failures

Corrosion has been a major problem for platforms, walkways, ladders, and similar structures. Load bearing structures have typically been thin-walled sections and even moderate levels of corrosion can considerably impact their load-bearing capability leading to catastrophic failures and major risk of injuries. Many pieces and parts in platforms, walkways, and access provisions might be from dissimilar metals or some might be galvanized, and others just painted. Welding of dissimilar metals should be avoided. In general, attention is drawn to the corrosion that can occur when dissimilar metals are brought into close proximity with one another under certain conditions.

There were many cases of bolt failures in platforms, walkways, stairways, and ladders. Bolts of a diameter less than 12 mm should not be used for access provisions such as attaching guard-railing posts, supports, brackets, attaching ladders to the main supporting structure and others. Typically, minimum two bolts should be used in each connection. Attention is needed where there is a tensile load on the bolts or where there is eccentricity of loading. On these challenging cases, higher safety margins, larger bolt sizes, and more bolts are needed.

Creativity and common sense

The layout and plot plan have a direct effect upon safety, risk reduction, access, reliability, maintenance, operation, and many other key topics. In simple terms, the layout needs vast experience and a good understanding of the relationship between different elements of the facility (equipment, piping, or structure) and between people (operators or maintenance team) and the facility.

Technical ability along with creative talent and a common-sense approach to problem solving are key for the layout and access provisions that play a major role in the overall safety and reliability of the plant. Platforms, walkways, stairways, ladders, and generally safe access provisions are needed for many different tasks such as operation, inspection, and maintenance.

This story originally appeared in the September 2021 issue of Plant Services. Subscribe to Plant Services here.

About the Author: Amin Almasi

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