Читать книгу Practical Power Plant Engineering - Zark Bedalov - Страница 18

Detail design will follow the conceptual phase. This “detailed” phase may last anywhere from 9 to 18 months for an industrial plant or two to four years for a power plant, depending on the type of plant. The conceptual drawings will be reworked and expanded. Procurement phase will commence by preparing the purchase specifications to specify equipment performance requirements and also to make the interface diagrams to tie up with the related mechanical equipment. Often, the electrical design may have to wait a while for the mechanical design to near its completion and their suppliers' drawings are in hands to determine the electrical ratings and the interfacing connections needed.

The first effort will be to update the electrical plant design criteria and the key one‐line diagram from the conceptual design phase. These two items are your two big pictures, and the foundations for everything else you plan to build on.

 System studies: The detailed design will present final one‐line diagrams with the actual impedances and equipment characteristics. It will use the data based on the results from power system studies: load flows, motor start, voltage drops, phase and ground short circuits, arc flash, insulation coordination, and step and touch potential. The studies will determine more precise and factual system characteristics and prove that the selected equipment ratings conform to the requirements set out in the design criteria. For these calculations, we will use the software from various system houses, such as Easy Power, ETAP, Cyme, and others. The plant data will be laid out on a computer and let the computer do the math. Not only that, the computer teaches you the power system functioning. One can introduce changes and alternatives and then observe the impact of the changes on the power system performance. It allows you to select the optimal solutions.

 Interfaces: At this time, schematic and wiring diagrams for all the motors and valves, cable lists, and plant layouts will be prepared.

One of my bosses once told me: “Project usually fails at the interfaces.” He was right. The projects require a huge effort by many personnel working on the project, ranging from secretaries to the managers. Possibilities of errors are ever‐present. The interface changes may be due to a late design modification initiated by other engineering departments. If not well communicated and reconfirmed, the changes may not get on the drawings. This applies also for the communications between the engineering departments, the suppliers, and fabricators. If the equipment arrives to site with incorrect connections, it will lead to a lot of confusion on site, “throwing blame around of who said what, and so on.” This is where the experience comes in from working on large projects and by recognizing how the equipment is supposed to work and how it relates to the other equipment. Experienced engineers would notice problems if incorrect drawings cross their desks.

Every discipline can use approximations, add (+) or delete (−) a few inches or feet here and there on the drawings. The electrical engineers have no such a benefit. We have to produce drawings that match the equipment perfectly. Electrical drawings show several hundreds of thousands of wires, power, and controls interfacing between the various electrical and mechanical equipment. The only grace we get is that we can bend the plant cables around in the cable trays.

You may have done your job to perfection, but unfortunately, when you come to the construction site, you may face some disappointments. You will notice the supplier's actual equipment does not match the drawings you received to prepare your diagrams from. The suppliers have just got confused and sent you drawings they had engineered for a previous customer, or they had made changes but failed to inform you.

Do not panic now. This is something to get used to. It happens. Once the wires are connected, you may notice different problems stemming from errors, suppliers' incorrect designs, and of course, the wiring errors. This is where precommissioning and commissioning comes into play to make sure everything is properly tested and made to work as intended.

Everyone can make mistakes. Let us be honest about it. Even mechanical engineers can make a mistake here and there. But there is nothing like what the electrical engineers face. Thousands and thousands of wires are laid out in the field, and each one must find its proper place or it may turn out to be a major mistake and error, which will have to be troubleshooted later during the plant commissioning. Fortunately, with the advances in technology, a half of wiring in the modern plant is now replaced by communication cables, coax, a pair of wires, etc., carrying thousands of signals which can be shaped and configured as part of the plant control system. But that is another story. That certainly is a wiring relief, but our problems will now likely resurface in the software during commissioning (see Chapter 17).

You as an electrical engineer will prepare or work on the following drawings and documents:

 Equipment and installation specifications.

 System studies: Load flow for voltage drops, short circuits for the equipment ratings, large motor starts, and relay coordination.

 One‐line diagrams.

 Design criteria.

 Layouts for electrical equipment, lighting, cable trays, load Lists, cable schedules and terminations, embedded grounding, equipment grounding, lightning, and power corridors.

 Prepare schematic and wiring diagrams for each motor, valve, and feeder,

 Review of civil, mechanical, and instrumentation drawings.

 Review of suppliers' drawings, and more.

That is a lot. A project of this magnitude may require thousands of electrical drawings and hundreds of documents.