per IEC 60534-4 is classified as six different classes whereas for on-off valve the leakage mostly complies as per API 598 . Hence dual-purpose valve needs to accept the leakage requirement as per inherent design . Also , the bidirectional flow requirements require two separate dedicated valves , as in general dual operation valve would add more complexity for bidirectional leakage class requirements . Air consumption of the valve is again an important factor for dual purpose valves . If the actuator is single acting , then it is more feasible since for double-acting actuator the air consumption would be designed considering the inherent design in general considering seat closure and the stroking time requirements . For new dual-purpose valve procurement , additional checking parameters are available to measure valve performance with valve signature testing , response test , diagnostic test , hysteresis , friction test , etc . These test reports can be used as reference data . Valves can then be retested following a period of operation to determine whether the valve performance is in accordance with the process requirements or is showing signs of deterioration .
Conclusion
Dual purpose valves are a well-established concept , having been used in process installations since the early days . Speaking generally , assuming the process is low critical then dual-purpose valve can fulfill the process requirement by compensating on certain criteria like leakage and precise control . Moreover , dual-purpose valves can have advantages in retrofit projects due to space constraints , cost reduction , faster turn-around , etc . Also , good performance of an installed dual-purpose valve can promote a ‘ like-for-like ’ replacement strategy . However , if a situation arises requiring these valves to be revalidated , for example , due to model obsolesce , plant revalidation and turn around , performance issues , etc , then there can be compelling reasons to replace the dual-purpose valve with separate valves categorized into control and on-off valves , respectively . For new-build plants it is recommended to have separate valves for dedicated control and on-off use considering reliability and safety .
Disclaimer : The views and opinions expressed in this article are those of the author -Mr Murugappan V and do not necessarily reflect the views and opinion or position of Dow Chemical International Pvt . Ltd . or its affiliates .
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Meet the author
V Murugappan is Instrumentation Engineer at Dow Chemical International Pvt Ltd . His career to date spans 15 years . During this time he has acquired diversified experience , including working on water treatment plants as well as power plant erection and commissioning . As a senior application engineer with Fisher-Emerson he was responsible for sizing almost 6000 valves including control valves , block valves , desuperheater and critical applications . His professional interests include valve technology , core Instrumentation , Innovation of wireless automation etc . Mr Murugappan can be reached on Email : muru17 @ gmail . com