DCS

Not until we know how many of several kinds of items need to be monitored and controlled in the power plant (or other process).

AI = analog input: temperatures, pressures, flow rates, liquid levels, gas analysis meters, etc. (Continuous ranges of values.)

AO = analog output: valves or actuators that modulate through intermediate positions, signals to motor VFDs or solid-state heater controllers, etc.

DI = discrete input: basically switches that are either open or closed.

DO = discrete output: solenoids, relays, contactor coils, indicator lights, etc. (On/off.)

<...Please give the basic information about the DCS I/O count?....>

It's on the P&ID.

<...How......relate this DCS configuration to the power plant?...>

Using the P&ID.

Both of the questions are somewhat ambiguous so I will respond to it in a more broad way than give a simple definition. I hope it does not insult your intelligence

In a process plant control system project, whether it is a digital computer controlled system or an old fashioned analog system, there is a basic set of documents required to convey information. Some of these are used in the design process and some are required to control hardware selection, purchase, and installation. In the case of a computerized system there is additional documentation to define the application software preparation and its installation. The particular formats and degree to which details are developed will depend on the instrument engineers and designers customs, but the overall approach to creating the document package and its use in the field is quite similar.

Proper document development effort begins with P&ID's and a functional specification setting out the control system philosophy. Certain information in these documents get reduced to lists, i.e. instrument lists, valve lists, panel lists, with cross references to mechanical information such as equipment lists, pipe lists, line specials, and other listings of components that make up the overall plant. The lists provide a reference tag number to inventory what is in the plant plus key engineering data regarding the component that is needed for designers to complete the contractor installation documents. The lists allow engineers to quantify the types of control circuits that will be included in the system, i.e. the analog ins and outs plus the digital ins and outs. If it is a DCS system this information is vital to proper specification of the computer and important in the design of the software used in the computer. All of these documents are needed in the design office to develop the construction documents. They are not usually called "hook-up" drawings.

The instrument lists are an integral part of the contract documents needed for construction and ultimately for the debugging and start-up phase of the project. Such drawings may be generated and used by multiple disciplines in the design effort.

For the instrument group most of the documents that make up an "instrument package" are schematic, that is they are not physical layout or mechanical details, although some are actually details showing true physical connection requirements. One of the principle schematics is a group called "loop diagrams". This is a diagram that shows the complete interconnection of a group of instrument components that function together as a control circuit. Typically it will define all signal connections between the control center, panels, jb's if any and field devices. In some projects the loop diagram may be used as a "hook-up" drawing. If the overall system is rather simple, all the necessary information to install tubing and wiring and make terminations can be shown in the loop diagram. Loop diagram information will feed directly into the digital system application programming when computers, PLC's, or other digital devices are used for control.

Typically however, most process plants are too complex for construction to be executed simply with loop diagrams, especially in digital control systems, so other detailed documents are made to control the installation, i.e. the "hook-up" of instruments. These include panel detail drawings that depict the layout of terminal strips and inventory the terminal numbers associated with each set of wires. These drawings may be supported by additional drawings called "wire run lists" that tabulate the particular terminations to be made along the loop defined in a particular loop diagram. They may also define routing of wires, allocation to certain cable trays, raceways, subpanels, etc. and other information needed to identify wires that is vital for debugging, ringing out, and trouble shooting. All of these drawing are sometimes referred to as "hook-up" drawings as a generalization.