Zane Bodensteiner(Right), Applications Engineer and refining industry Control Valve SME for Emerson Automation Solution’s flow control products, and Matthew Gulleen(Left), Refining Industry Manager for Emerson Automation Solution’s flow control products recently published an article in the Q3 2020 issue of PTQ Magazine describing the control valve challenges created by low production rates and extended shutdowns. The article is titled Protect Control Valves When Production Slows and is summarized below.
Falling Output and Rising Problems
Most petrochemical and refinery units are designed to run at maximum rates. However, recent economic conditions have reduced product demand, and many units are running at throttled rates or are taking extended shutdowns. These conditions can create a number of unanticipated issues for control valves, including:
Figure 1: Valve cavitation often occurs at low production rates and can cause significant damage to valve internals.
Extended Shutdown Impacts
An extended plant outage can also create problems if certain precautions are not followed. Some of these problems might include:
These issues can potentially create numerous operational problems and significant delays on restart. However there are a number of ways to mitigate or eliminate these potential problems and keep control valves operating reliably when running at reduced rates or when restarting. The authors describe the following solutions:
Figure 2: The Fisher FlowScanner QL valve diagnostic system can be used to analyze a control valve’s dynamic response, trend valve performance, and isolate repair needs. The system is used to evaluate current operating conditions without having to disassemble or remove control valves from the process.
Address Instrument Air Issues - If at all possible, plant personnel should leave the air system pressurized during an extended outage.
Figure 3: Actuator diaphragms and associated O-rings (above), as well as O-rings and packing in the valve body, can become brittle over an extended outage. Plant personnel should plan for thorough testing and possible replacement before bringing the equipment back online.
The authors conclude with the following thoughts:
In times of low production, the unit has to run as efficiently as possible, or restart from suspended operations with limited issues. Control valves are critical for plant operation and product quality. Taking the time to address valve problems or anticipate issues brought on by running at reduced rates or temporarily shutting down will pay huge dividends upon return to full production.
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