Dual purpose valve for control & on-off
Mr V Murugappan , Instrumentation
Engineer at
Dow Chemical International Pvt Ltd . discusses the advantages and disadvantages of using a single valve for both throttling and on / off functions .
By V Murugappan , Dow Chemical International Pvt Ltd
Valves are normally the standard selection for final control elements where they perform throttling or on-off function based on application and process demand . However , about forty years ago in process plants , some processes had a single valve which was used to perform both throttling and on-off functions using an analog signal from Logic solver . In such instances these valves carried out the regulatory function during normal process operation , while during process upsets or emergency situations either direct 4 or 20mA was used for isolating the process . A single valve used for both throttling and on-off functions has its own advantages and disadvantages as discussed in this paper .
Introduction
Automated valves are important for safe , reliable , efficient operation of plant . The throttling valve plays a major role in plant control and stability . Based on the process requirement , automated valves are classified as control and on-off final control elements . By definition , a control valve is used as a throttling valve in process control . During emergencies like plant trip , system failures require the process to be isolated and control valves are operated as on-off valves in cases where dedicated block valves are not available . Also , to isolate the process for maintenance an emergency block valve function is required . In olden days , plants were restricted in quantity of IO ’ s for optimum design due to the limited availability of digital technology and Smart control systems . Even manual control loops were em- ployed to limit the quantity of signals connecting to logic systems . Such plant design constrains were managed using the dual-purpose valves , manual loop valves . A classic example for risk management in those days was a single valve used for both controlling and shutdown functions . Throttling valve used the 4-20 mA varying analog signal to achieve the desired process control and same valve was operated using either of 4 or 20 mA to shut down or drain out the process medium to safety on emergency condition . Since majority of the valve is built of mechanical parts , with proper material selection the valve was capable of meeting the dual-purpose requirements . Nowadays with plant safety and reliability as top priority a lot of emphasis is placed on final control elements design . This coupled with the fact that the DCS and PLC system have become more powerful and capable of handling very large number of IO ’ s , there is very limited restriction on the quantity of valves . In such cases every part of the process has dedicated control valves and on-off valves respectively as required . This paper elaborates the usage of a valve in dual purpose case with detailed analysis of the pros and cons of the approach .
Detailed analysis
Valves continue to be selected as first choice of final control elements . The valve industry has undergone lot of development and sophistication , far from the first ever valve which was just a pump governor to prevent spillage of a water tank . π www . valve-world . net Valve World November 2022
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