RAKI - Design for Constructability

Design for Constructability

Proper design calls for the total integration of "quality management," "value engineering" and "constructability."

By: W. J. Boyce

Designing for constructability will be just a buzzword without the right team. The team must have the right attitude, experience and authority (or the guts to take the authority) to make things happen. It is based on the same philosophy as the "Ten Commandments of KISS Design":

Keep It Straight and Simple
Keep Its Structures Simple
Keep Its Specification Simple
Keep It Shop Standard
Keep Its Standards Simple
Keep It the Same Size
Keep It Square and Squatty
Keep Its Support Simple
Keep Its Schedule Sacred
Keep It Site Suitable.

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Keep it straight and simple applies to many things, but is most important during the plot-plan and layout phase. The plot-plan phase is the general location of units. The layout phase is a much more detailed review of each individual piece of equipment. It includes not only the best location of each item, but also the orientation of each nozzle. Layouts are not just plan views, but also include elevations and sections. Access may be the best word to define layout. Layouts must consider access for construction, production and maintenance. The layout must be finalized early in the project but is often not given the attention it requires. It is often the biggest source of savings, but once finalized it's difficult or impossible to change due to schedule and permits.

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Layouts are best made with a simple block model made by an experienced layout specialist, and not by a model maker. No layout drawings are made to make the model. The model comes first, the drawings come second. A model is not only three dimensional, it also speaks all languages, not just foreign languages but also the languages of construction, production, safety, maintenance, etc. Groups of people will look at, and discuss, and maybe cuss a physical model. But groups do not conference over a 3D CAD screen. With a simple block model we first make a straight layout of piperacks, cable trays, drainage trenches, sewers, flair header and roads, and then place equipment. We never place a piece of equipment without knowing how we can get to it with piping, electrical, and sewers, etc. during construction, operation, and maintenance, Do not run the pipe-racks and roads side by side. This will sterilize one side of the rack for equipment placement. This will often force spur piperacks to be required, which will require more pipe, at least, two more fittings and four more welds. Run piperacks vertically as well as horizontally.

Keep its structures simple by using the quarter-point design concept as shown in the typical cross section. This design simply means getting heavy loads over the vertical supports (Fig. 1).

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Foundation design for structures can make a big difference in both time and cost. Most foundations are still designed from old standards based on cheap labor and expensive materials, Today the opposite is true. Mass pours of' concrete today are in the range of $100 per cubic yard, and small or complex pours approach ten times that cost. A single-mat-type foundation has been used to support, a complete process unit. Larger process units often have three mats, and all three mats are the full length of the process unit. The center mat supports the piperack, pumps, strainers and miscellaneous items. It has a high ridge in the center and drains to a precast storm water trench on each side. The trenches are used for screeds when pouring the mats. This center mat is also an unobstructed maintenance access way from end to end of the unit.

The other two mats are on each side and slope to the trenches. These two mats support the major equipment such as columns, heat exchangers and vessels. Vertical vessels are preferred. They require less space. With skirt or leg supports, they can sit directly on the mat without the additional activity of piers. Vertical reboilers can also be on legs. Inline pumps are much simpler to support and are less total installed cost. Horizontal pumps can be supported on studs drilled into the mat foundation and are doublenutted at the pump base. Again, no concrete is required. This also gives horizontal pump piping the same flexing and stress advantage of inline pumps. With mat foundations there are no octangular pedestals as columns and vertical vessels sit only on grout.

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Piperack structure is a major item and is often fire proofed. It is also an important cost factor. If the piperack is 40 it wide, don't span the 40 ft. With quarter-point design the span should be about 20 to 25 ft. This piperack and equipment support concept is based on precast concrete bents with a precast concrete deck on top. I call this "parking garage" construction. This design may require more ground area but gives an equal amount of prime space on the deck right in the center of the process area. This design has shortened the overall length of the piperack and pipes and cables by one third. With this design we now use three sides of the rack for equipment placement in lieu of two sides. This piperack design concept has many advantages:
Cost, schedule, construction, production, maintenance and safety are improved.
It gives a liberal maintenance access road under the center of the piperack.
Outside of the support columns, the outriggers give extra piperack space.
The deck gives weather protection and improved productivity for operations.
The deck is also a firebreak between the piping and the equipment on the deck.
With this wide area of access, exchanger bundles can be cleaned or pulled into the covered area. With the dirty end of the exchangers over the oily water sewer, the piping is shortened and the oily water sewer is in straight line.
The precast deck is used for items that require elevation. Often these items that require elevation are designed with separate structures alongside of the piperack; these structures limit access. The piping to items on the deck is generally shorter.
The deck covers the full width and length of the piperack in the process area and also serves to support pipes as they enter or leave the rack by rod hangers.
The deck also serves for mounting and access to control valves, relief valves, orifice runs, air coolers, etc.
Equipment on the deck would have support saddles designed to match the span of the vertical support columns for quarter point design.
Compressor structures that require overhead cranes and shelter for operation and maintenance can be designed with an 'A' frame type crane supported on rails mounted on the concrete tabletop (Fig. 2). This eliminates the heavy custom-designed steel structure to support a bridge crane. Cantilevered monolithic extensions from the concrete tabletop will support precast concrete decking on both sides of the compressors. A lightweight and inexpensive pre-engineered building can be added to the tabletop structure to give weather protection.
Silo structure designed for 12 silos, each mounted on 3 load cells, should have the load cells directly over the vertical support columns (Fig. 3). This requires breaking a paradigm that requires support columns be in a square pattern, and put them in a triangular pattern. The square pattern required 30-in. beams because the loads were not over the support columns. The triangular pattern only required 12-in. beams.

Keep its specifications simple so that you can purchase shop standard equipment. A simple specification would be a blue Cadillac with four doors and loaded with standard Cadillac extras. A complex specification would be a Cadillac with three doors, Honda red paint, a Chrysler motor, a Ford radio and a Rolls Royce transmission. This sounds ridiculous, but a lot of companies are buying this type of equipment. A performance-type specification is a good approach. Select good vendors, tell them what you want to do, but not how to do it. One vendor told me that if clients would buy on the performance specification, he could deliver a shop standard unit faster, with a better guarantee. It would be a better unit and cost half as much as the complex specification item. This same vendor felt that many API specifications are an overkill put together by specialists and consultants with self protection as their goal.

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Keep it shop standard can only be accomplished when the specification is simple. The rewards for buying shop standard are substantial. When a supplier is asked to deviate from his standard, it costs more, takes longer, is a custom-designed item, may be the first of a kind and may carry a lesser guarantee because of it. It is no longer the comfortable tried-and-true unit. The owner may think he is buying a better unit but he may be buying a problem instead.

Keep its standards simple, because like the specifications in use today, so many standards are outdated and labor intensive. The major equipment for most projects is only about 25% of total installed cost. That leaves 75% subject to "designing for constructability" savings. Some standards to look at are piperacks, platforms, stairs, sewers and block valves.

Piperacks have been discussed under structures. The standard for piperack bents, designed for constructability, is more like modern highway technology (Fig. 4). Bents are poured horizontally on the ground using standard forms. They are then stood up in an empty excavation. The foundation is poured around them. There are no anchor bolts or base plates. There are metal imbedments in the top of horizontal members for pipe supports and guides and unistruct in the bottom for lighting, sprinklers, etc. Unistructs may also be put in the columns for junction boxes, utility stations and miscellaneous items. There are many details to this design, but the main obstacle is paradigms.

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Platformrequirements are based on the orientation of' manways and nozzles (Fig. 5). Often manways and nozzles seem to be located where there is room to write on the orientation plan. When the orientation of each nozzle was reviewed by all interested people, they could be all be located on one side of the column or vertical vessel. This made it possible to have rectangular platforms and not circular platforms, which one contractor estimates at 2.5 times the cost of rectangular ones. These columns have a vertical piperack running up the column on the platform and piperack side if there are no air coolers. When there are air coolers, it is too hot on the piperack side for platforms and ladders so they must be on the far side.
Stairs can be simplified by standardizing distances between all operating and maintenance floors, platforms. catwalks, stair landings and all walking surfaces (Fig. 6) These increments are proposed as 24 in. so that three 8-in. stair risers or two 12-in. ladder rungs are equal to 24 in. The treads are proposed as 10 in., which gives a good safe angle. It is more important that they are all the same so that the angle of stringers and handrails are all the same This standard is important for shop fabrication, but even more so when local field labor is used.
Sewers also follow the straight and simple concept. A precast concrete trench for storm water runs under the piperack's cantilevered extensions. The oily water sewer system is a straight pipeline inside of the precast concrete storm water trench. This design satisfies double containment requirements by installing sensing elements in the storm water trench. It also leaves the oily water sewer system exposed, when trench cover is removed, for inspection because it is not buried. Neither the storm nor the oily water sewer is sloped. They depend on head and also flow in both directions to catch basins at both ends of the process unit. Pumps are lined up along the oily water sewer. This concept is to bring pump to sewer Don't take the sewer to the pump.

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Block valves sizing is a big dollar item (Fig. 7). This applies to control valves, pumps, traps and relief valves. Control valves stations can be treated as a sub-assembly and shipped complete with or without the control valve positioned above its working position and held in place by through-bolts. These bolts also hold a straight piece of pipe between the matching flanges of the block valves. The straight piece of pipe (no flanges) is used for flushing and testing. It is then discarded and the control valve is lowered into place. To make this design work, there are two sizes required--line size and control-valve body size. The control-valve trim size can come later. The line size is usually the first to be known, and often the block valves are ordered on line size basis. A valve twice the size costs about four times the price. Once sizes are known, you can order control valves, block valves, bypass valves, reducing ells and flanges. All valves and flanges will be the same size. Block valves need to be full port. Reducing ells are used to go from line size to valve size. Buying an ell and a reducer and making three welds in lieu of a reducing ell and two welds is not designing for constructability. This block valve sizing concept was estimated as saving $10 million on one large project.

Keep it same size is following the "cookie cutter" design concept. This does not necessarily mean an industry., standard size, but more of a project same size. This was vividly brought to mind on a project with a large and long piperack. In a sincere effort to save money, the steel was designed very exactly so that there were very many sizes of' both vertical and horizontal steel members. The standard called for 2-in. fireproofing to be formed and poured after steel erection. The steel fabricator complained about the many sizes. He fabricated and delivered to site, all members of one size before going to the next size. The fireproofing contractor complained because of the many nonstandard forms required to do his work. The electrical contractor complained because he was held up waiting for the fireproofing contractor to finish. Because o[* scheduling, bonuses and penalties, this very complex-designed project ended with several multimillion-dollar law suits.

Keep it square and squatty. Square (including rectangular) is to emphasize the extra cost of odd shaped buildings, foundations, platforms and many other items (Fig. 8 i. Squatty is to emphasize the cost of building things higher than they need to be. particularly in earthquake zones. A structure twice as high isn't twice the cost. It is more like Four times the cost.

NPSH often determines how high a structure needs to be. However, contingencies on top of contingencies have been found to add considerable cost to structures. The real situation may have the normal liquid level several feet above the outlet. The custom of calculating NPSH requirements with many contingencies costs a lot of money. An item-by-item review of a project resulted in the lowering of a complete process unit by 15 ft. The NPSH required for the most, critical pump was found to be less than half of the NPSH available.

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Control buildings can be designed based on the square and squatty design concept. This example is based on a two story design (which doesn't sound squatty), but it was the same overall height as the conventional one-story design it replaced. The one story design started with a slab on grade. There was a computer floor space. Then the rackroom and control room with a false ceiling. Then there was a 6-ft space above the ceiling for air conditioning ducts and lighting. The two-story started with the finished floor at grade. There was no computer floor as with the control room on the second floor and the rack room directly below there is no need for a computer floor. The power and instrument cables entered the building near the ceiling of the ground floor. The instrument cables feed into the top of the racks and also exit from the top. They then penetrate the floor at any point to feed the consoles. The control building structure was built using precast double "TT" design. They were used for the second floor, the roof and exterior walls. This makes a fast low-cost shell that is also an acceptable "blast resistant" building. This design also eliminates the false ceilings and simplifies the lighting and air conditioning. I call this "parking garage construction." Fluorescent lighting is mounted directly underside of the double "TT" floor and roof. Conduit and junction boxes are on top of the double "TT"s and are then buried in the concrete second floor and roof. Air conditioning ducts are large rectangular soffit type design and mounted on the walls. Distribution is controlled by thermostatically operated outlet dampers rather than duct size. Two side-by-side offices can have several degrees differential to suit the comfort of individuals.

Keep its support simple is a good example of how little things that are repeated many times become big costs. "T" supports for pipes and conduits that run to the far end of heat exchangers can be mostly eliminated when the exchangers are turned 180 so that "T" supports are not required. Supports for lights, start-stop switches, conduit, junction boxes and sprinklers are already in place when unistructs are cast into the precast concrete piperack bents. With mat type foundations, no adjustable pipe supports are required as everything is on the same base. Flair headers can be used as structural members to carry the smaller pipes required for the flair. A project used a 72-in.-dia. flair header as a support for the small lines and a cable tray/walkway. The spacing of supports was 100 ft. Pump supports can be simple double nutted studs, with no concrete as discussed above. Centrifugal compressors have been mounted on spring supports with no anchor bolts and proved to be quieter. That was also the way they were tested in the fabrication shop.

Keep its schedule sacred. Do not change the schedule every time there is a problem. Many projects change the schedule whenever they think there is a justifiable reason to change. But that doesn't change committment for sale of the product. Real project management does all it can to find a way to recover the lost time from unexpected causes of delays. They bank their reputation on their solutions.

Keep it site-suitable means considering the unique conditions of the precise location of the project. I have seen winterizing for plants on the equator; plans for importing items to countries that do not allow those items to be imported; toilets designed for the west, but installed in the middle east; projects designed with maximum modules for places that require maximum local labor; and drawings using English units for places that only use metric. Be aware of local customs, weather, labor availability, material availability, etc.

Ten commandments. Following the ten commandments of KISS design leads to a better managed and less expensive project. They are also a stimulus for creativity. Use them to free yourself from design stereotypes and ruts.