Recurring mistakes continue to afflict equipment installation and pre-commissioning at plants. Here, we’ll describe some common errors regarding equipment selection, inspection, prefabrication, site assembly and on-site modifications, and cover specific pointers to avoid problems.
As a general guideline, always buy new equipment and machines rather than used ones. Rehabilitating and revamping second-hand units often wind up costing more than buying new. Used equipment can pose many pitfalls and problems (such as cracks, corrosion, erosion, etc.). Moreover, buying new allows you to get the latest technology as well as a manufacturer’s warranty. So, many operating companies forbid the application of used equipment in new plants.
Tests And Inspection
Equipment tests such as pressure tests of fixed equipment and shop performance tests of machines underpin the success of any project. All details of such tests require careful planning and implementation. For instance, pressure testing with seawater has caused damage or even destruction of many large vessels, fixed equipment and tanks. Although the pressure test was successful, seawater remaining inside due to inadequate purging with clean water prompted damaging corrosion and led to non-conformity or even scrapping of the equipment.
Shop inspection is crucial for much equipment but often is overlooked. Using third-party inspectors is a good idea and nearly always needed. Indeed, third-party inspection is mandatory for many types of equipment and packages. It alone simply isn’t sufficient, though. Inspectors from the purchaser (whether operating company, engineering consultant, main engineering/procurement/construction contractor, etc.) should conduct inspections during fabrication and assembly, and witness major tests in the vendor’s shop. Unfortunately, many projects forego this due to its costs. However, such a thorough inspection plan using different agents (third-party inspectors, in-house experts and others) results in getting high-quality equipment and machines that should work for many years without any problem.
Two key points at which to thoroughly check equipment and machinery packages are just prior to shipment from the manufacturer’s shop and before installation at the site. As a general rule, any faulty equipment should not be released for delivery or installed on site. Examples of faults and problems include cracks, poor welding, untightened bolts, lack of nuts, seized bearings, etc.
Critical Equipment And Spares
Attaining desired overall availability and reliability of a plant often necessitates having duplicate equipment on hand. Indeed, many processing steps and units require standby (spare) equipment and machinery. It’s common to use pumps, compressors, key packages and processing equipment in “n + 1” or “n + 2” configurations, where n is the number of operating units and 1 or 2 is the number of standbys. Many pump sets in particular have a “1 + 1” arrangement. If having a standby isn’t feasible, opting for special designs and concepts to make the operating unit very reliable often makes sense. For example, the highly sophisticated compressor package required in some processes frequently features a specially designed lubrication oil skid, seal systems, etc., that enable uninterrupted operation for two, three or even four years.
Such concepts may cost more during design and construction. However, the extra expenditure provides a good return during operations by maximizing plant production and avoiding unscheduled shutdowns. It increases overall safety as well.
Challenges Of Prefabrication
The idea of prefabricating complicated and large pieces (sub-assemblies, spools, etc.) in the controlled environment of a shop and then assembling them on site is very tempting. However, it also poses challenges. Difficulties often have arisen in assembling pre-fabricated complex parts and items with many connections, connecting holes, flanges, etc., because of differences in their configurations. For instance, sometimes holes on connecting plates and other mating pieces don’t line up and require correction during installation or pre-commissioning. Personnel also may discover other items don’t conform exactly to specifications or drawings. Sometimes, making suitable modifications is tough or tricky.
Numerous sources of imperfections, mismatching and errors can create such problems. For instance, in many frames, structures, piping networks, etc., the beams, members or pipes are not parallel and not in the exact design positions. Unparallel beams or shifts of members can cause a mismatch of a few millimeters, moving connecting holes, connecting plates, flanges, etc., away from their exact design positions. Accumulated errors in measurements and manufacturing of many different parts also can led to mismatching. Another reason is that lots of items come from sub-vendors and sub-contractors, and dimensions of those items can change for various reasons such as tolerances, different source of supply, new models/types, etc.
For complex packages involving many vendor-supplied items, substantial prefabrication often is easily achievable. However, complete prefabrication may not be possible, necessitating field adjustments (such as welds in piping/steel structures). Achieving the goal of 100% prefabrication — so that parts perfectly match during the first trial assembly at the site — requires making suitable provisions, such as using jigs, fixtures, templates, etc. These commonly serve in series (mass) production to ensure interchangeability and easy assembly but aren’t always applied for equipment, machines and packages for plants.
Jigs secure a workpiece on the manufacturing tool (for cutting, etc.) and direct the tool to the correct positions on the workpiece. They are not clamped to the workpiece or working table. Fixtures, on the other hand, hold the workpiece securely in position with respect to manufacturing devices.
Templates ease production of a uniform pattern. One example is the use of two identical metal templates to implement foundation bolt hole patterns on equipment. One stays in the workshop fabricating the unit while the other goes to the site to properly position the foundation bolts being installed. In this way, the equipment, when ultimately delivered, will line up with the foundation bolts.
Another important tool is a location system to prevent linear and rotary motion of the workpiece during fabrication. This system should be fool-proof, i.e., it should positively prevent loading the workpiece wrong on a fixture, jig, etc.
An alternative is modular pre-fabrication where pieces of a large package are prefabricated with the minimum feasible number of battery limits connections, then delivered to the site and installed there. For example, a large compressor package came with the compressor with its driver and seal system on one large module (skid) and the lubrication oil system on another. Here, only supply and return lubrication oil piping were needed at the battery limits; they were delivered as prefabricated piping spools needing few field welds.
Dealing With Deviations
Correcting physical (dimensional) deviations such as those on equipment, assemblies, piping, structure, etc., can pose a great challenge. Two options exist to deal with these problems.
The first is to measure all physical deviations (linear, angular, etc.) associated with the sub-assembly and consider them in the modification process. This entails measuring the imperfections and deviations at the boundaries of the modification and incorporating such deviations in new fit-up and modification plans. Doing so often is difficult because it may require special provisions such as accurate measurements, 3D visualization, etc. It’s frequently impossible or impractical to model/draw inclined base plates, unparallel frame members, etc. Therefore, this approach rarely is used.
The second and more common option is to perform one or a few cycle(s) of fit-up/checking/correction (i.e., trial and error). Here, the initial fit-up is done according to design specifications (obtained using perfect existing models/drawings) and the sub-assembly is installed on the existing system. The deviations and imperfections then can be measured directly and the fit-up corrected appropriately.
Three key observations pertain to this process:
1. Usually many sources of deviations and imperfections exist.
2. Properly assessing angular deviations in elements such as structures, piping, material handling systems, etc., is critical.
3. Initial fit-up should be reversible (i.e., modifiable if found unsuitable).
Each piece of equipment or item incurs different linear and angular deviations. These can accumulate in an assembly. Therefore, at a boundary of a modification, the total deviation can be large.
Although both linear and angular deviations are important and can pose challenges, the angular imperfections associated with long arms/beams are the most difficult to deal with. For example, 1° angular deviation in 1-m arm can lead to around 17-mm deviation at the end of the arm.
Because deviations or imperfections might be relatively large, some provisions are needed for modifications in the fit-up. For instance, you should avoid excessive cutting (modification by progressive cutting). This applies to machines, equipment, piping, material handling units, structures, package skids and other items.
Deviations and imperfections related to processing and control also can occur and require addressing. The first step is to identify and then measure all deviations and imperfections. Then, you should develop modifications to account for these as is or after they are rectified to the extent possible. When getting actual measurements is impossible or impractical, you should plan for a few cycles of trial-and-error modifications.
AMIN ALMASI is a mechanical consultant based in Sydney, Australia. Email him at [email protected].