A maxim that is evident to most valve subject matter experts (SMEs) is that the application dictates the valve. Remarkably the selection of valves in many applications is less than best practice and process suffers when it shouldn’t. The consequences of poor control valve sizing and selection can not be understated, affecting both commissioning and startup as well as established operations, creating safety and output issues.
Through the work initiated by the Manufacturers Standardization Society (MSS) to define severe service valves (SSVs), a clear indication of severe service thresholds for control valves has been identified. Different sources estimate the number of Severe Service control valves between 3% to 10% of plant’s valve population, it was also estimated that these valves account for 40% of purchasing cost for all control valves on site, yet once supplied and commissioned in many cases these valves need redesign or replacement as a result of incorrect sizing and selection.
Although control valves sizing is performed in accordance with ISA S75.01 sizing equations and IEC 60534-8-3 Noise Prediction Methods, it may be considered compositely; its parts consist of the following:
• Personal experience
• Personal preference
• Customer history
• Application history
• Cost of ownership
• Application severity
From practical standpoint control valve selection is often based on the least expensive alternative which will meet process control requirements. Over the years control valve manufacturers developed practical guidelines to help interpret process data correctly, and establish severity of 3 major Severe Service conditions – cavitation, flashing and high noise levels; and methods to handle these conditions according to the level of severity in the most cost effective way.
This holistic approach has proven effective across various applications, some of the most challenging processes for control valves in Hydrocarbon Processing and Pulp and Paper industries include Black Liquor flow control, HP Boiler Feed Water, Wash Water, HP Steam, Superheated Steam and Turbine Steam supply, Inlet Separators feed, High Pressure Gas Separator to Gas Flare, which mostly show not one but a combination of Severe Service conditions that were addressed cumulatively in devising a solution that would meet acceptable level of performance