Chemical Technology December 2015

PUMPS & VALVES

what is available, and what the product size ranges are, including dimension tables; • Drawings or descriptions showing how the valve operates (for unusual or proprietary designs); • Confidence that these valves really exist, and have been built before. Describing the valve There are two types of valves used in plants, namely ‘tagged items’ and ‘bulk items’. Any valve that is sized specifically for the flow conditions that exist in one location is assigned a tag number. One ex- ample of this is a control valve, whose functioning depends on process data taken from points nearby and whose output is a process variable for the next piece of equipment in line. This valve is a unique item whose tag number is assigned based on a structured format which is easily identifiable by process control systems (DCS or PLC, for example). Other tagged items may include specially-sized valves such as boiler blowdown, which are not connected to any control systemand thusmay be numbered as specialty items or minor equipment items. There are also automated on-off valves, which are not control valves but are connected to a control system and thus have an ‘address’ for the control system to interface with. These valves are described on a data sheet, one for each valve, with its process data, actuation data, power source data, and a brief description of the valve type and material. These forms are now often automated, wherein the data can be input and maintained in an electronic format, often

rule, the activities are the responsibility of the engineering contractor. Determining the requirements In the typical design process for a new unit, after the feasibil- ity studies as to the economics of the unit and the concep- tual engineering has been done, the first set of drawings to be produced are the metallurgical flow diagrams. These drawings show most of the equipment and the major lines connecting them, and are used to work out heat and mate- rial balances and as aids in sizing the major equipment. The only valves that are shown are the principal control valves, and those are primarily shown to diagram the logic used to control the process (flow into a particular column being dependent on the temperature in the column, for instance). After these drawings are approved, engineering proceeds on the next set of drawings, where the majority of the valves are displayed for the first time. These are known as the piping and instrument diagrams (P&IDs), also known as process flow diagrams and utility flow diagrams, depending on the system they portray. The flowsheets are used to help write piping line classes. The line class groups related services together, to organise the requirements for specific services as to corrosivity, wall thickness and so on. This information, along with design conditions, determines what material is required for valve body and trim, and pressure rating and size range. What a contractor needs frommanufacturers at the design and selection stage is: • Catalogue data (published on paper or online) showing

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Chemical Technology • December 2015