THE PURPOSE OF EQUIPMENTS SPECIFICATIONS

The main purpose of preparing a specification for an item of equipment is to ensure that the purchaser, who may also become the owner, obtains the equipment required, rather than what the supplier or manufacturer thinks the purchaser should have. In many situations the difference in perception of the requirements may be small and insignificant. However, for complicated equipment such as high-voltage switchgear and generation systems the differences may be very significant.

In order to satisfy both the requirements of the owner and the available options from the supplier, it is necessary to describe the requirements in various degrees of detail. The degree of detail will be a function of the type of equipment. Complex equipment such as large motors, generators, high-voltage switchgear and variable speed drive systems will need a more detailed description than the more standardized equipment such as power cables, low-voltage motors and, to some extent, low-voltage motor control centers.

Manufacturers of complex equipment regularly meet the needs of different owners, whose requirements vary in content and emphasis. A particular owner may have different requirements for the same type of equipment when it is used in offshore, as opposed to onshore, installations. These environments may be radically different, e.g. Northern North Sea, desert conditions in the Middle East, hot and humid climates of tropical locations. For example, the methods of cooling the equipment and the ability to withstand corrosive conditions will be very different in these extremes of environment.

On the other hand, simple equipment is less sensitive to extremes of location and environment. The main aspect that affects simple equipment is its full-load rating for low and high ambient temperatures. The details of the construction will be almost unaffected. Simple equipment used in high ambient temperatures will tend to be physically larger and heavier. A motor of a given shaft output rating may have a larger standard frame size when used in a desert than one used on a North Sea production platform.

A standard specification of the owner should take account of what is generally available in the market, and what can reasonably be called for as options. It is uneconomical and impractical to over specify aspects which a manufacturer cannot fulfill at a reasonable cost and with sensible production duration. Where possible the aim should be to match what the manufacturer can offer from his standard range of equipment. An efficient approach by the purchaser is to call for equipment that is a standard but most suitable product of the manufacturer plus the options offered, if these are needed, and then design the power system around the equipment to be purchased. In general this will also reduce the amount of time needed to design the power system.

For some types of projects there has become an emphasis on ‘functionality’ when specifications are being prepared. Care needs to be exercised in describing functional aspects of a specification. Most people understand the function of basic equipment such as generators, motors and switch gear and yet, in order to obtain what is ultimately required, it is necessary to pay attention to design and performance details. Functionality implies a more interrelated type of existence, as is the case with systems of equipment rather than individual items of equipment. A few good examples of applying a functional approach in the specification of process control systems are SCADA systems, modern protective relaying systems, variable speed drive systems and power management control systems. These equipments comprise a system of computers, measuring devices, controller set points, switchgear and rotating machines. Here the whole system must be functionally defined, and all the individual elements must be fully compatible from the conceptual stage of the specification.

With most specifications there are some key aspects that should be clearly stated or defined, the omission of which can cause embarrassment, delay and extra costs at a later date e.g. at the factory inspection, during installation and commissioning. A well-designed data sheet to accompany the specification will do much to avoid ambiguity or the omission of requirements. The data sheet should comprise two parts, a part completed by the purchaser to define the requirements and a corresponding part for the manufacturer to state what is offered.

The content of the specification should bear a relationship to the importance of the equipment in the power system and to its capital cost. If the content is too brief or too general then it may not satisfy the intended purpose of the specification and inferior equipment may be chosen.

In summary the requirements of the owner can be arranged in the following groups:

• Essential requirements
• Desirable requirements: Those which may be easily available in the market as options.
• Incidental requirements: Those which would be useful but not critical to the performance of the equipment. These may not be easily available, could be described as ‘nice to have’ and should therefore be avoided.