Defining Severe Service Control Valves

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Ross Waters, President, CGIS
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Right now there are no national standards that define Severe Service Control Valves (SSCVs) or set them apart from general purpose valves. A recognized definition would benefit users through improved process performance and increased profitability, safety and environmental protection. This high level article offers an objective definition for SSCVs. Most experts agree that SSCVs are identified by applications, and that these applications challenge the valve’s ability to provide a minimum acceptable level of performance over a minimum acceptable duration. All valve design functions require basic information, but for those valves destined for severe service it is imperative to have a comprehensive understanding of the factors that affect their in-service performance.

Identification of severe service conditions for control valves may be determined by performing sizing calculations using IEC 60534-2-1 or ISA 75.01.01 with the following information:

  • Fluid state (liquid, gas, vapour, 2-Phase, multi-phase, slurry)
  • Flow rate at max, normal and min conditions (Q)
  • Upstream pressure at max, normal and min conditions (P1)
  • Differential pressure at max, normal and min conditions ((P1-P2) or (dP))
  • Vapour pressure of liquids (Pv)
  • Temperature (T1)
  • Valve size

A determination of whether severe service exists for a control valve can be applied through thresholds expressed in Table 1. It should be noted that the potential for severe services is only an indication and not proof and therefore further examination should be performed. However, it is apparent that the further beyond the condition threshold, the more severe the service. Excellent tools currently exist to reduce risk and time to perform the extensive calculations that are required to test for the condition thresholds; one of the best is Flowserve’s Performance!, which uses a Valve Selection Guide to significantly reduce sizing and selection inaccuracies, and provides clear and abundant data to assist in defining the conditions within and around the control valve.

When any of the above conditions violate the thresholds in Table 1, the application should be considered as severe service and a SSCV should be selected. Additional process information should also be reviewed.

Control valves that do not violate any of the conditions in Table 1 or are not identified in the above list of severe service applications can be identified as general purpose control valves.

By using suppliers who specialize in SSCVs, applying a series of steps in qualifying them and separating general purpose from the more challenging applications, greater and longer success for a facility’s valve population is achievable.

Ross Waters serves on the Manufacturers Standardization Society (MSS) as a task force member on C-114 Steel Valves and C-409 Knife Gates. He is currently increasing awareness of severe service valves at conferences and to industry organizations. Ross proposes the MSS establish a technical committee to develop the parameters, specifications and standards for severe service valves and that the MSS Board of Directors formally adopt the designation at the MSS annual meeting in May 2016.

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Diagrams and certain thresholds provided by Flowserve.

Image: ValvTechnologies Xactrol® severe service control valve and provided by ValvTechnologies