Emerson Exchange 365

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: 

• Poor Control - A control valve is normally sized to operate around 40-60% open with a minimum opening of 10-15%. However, low production rates can drop the minimum opening to 10% or less, resulting in erratic flow control.
• Cavitation - At low production rates, a larger than normal pressure drop can appear across the valve. This can create or exacerbate cavitation in the valve and cause significant damage to the plug, seat, and valve internals (Figure 1).

Figure 1: Valve cavitation often occurs at low production rates and can cause significant damage to valve internals. 

• Anti-surge Valves - Normally a compressor anti-surge valve runs closed since the flow through the compressor is well above surge conditions. However low rates reduce the compressor load and force the anti-surge valve open. These high flow, high noise, and high pressure drop conditions can damage the anti-surge valve over time.
• Low Flow Rates Rarely Extend Critical Valve Maintenance - Some assume that reduced production rates will extend critical valve maintenance cycles, but valves exposed to corrosive conditions will continue to corrode at the same or even accelerated rates.

 Extended Shutdown Impacts

An extended plant outage can also create problems if certain precautions are not followed. Some of these problems might include: 

• Damaged Valves Will Not Fix Themselves - If a unit is shut down with leaking and/or poorly performing valves, those same valves will be just as bad or worse on restart.
• Bad Instrument Air - If the instrument air system is shut down during an outage, dust, rust, and water can collect in the air headers. When air pressure is restored, the particulates and water can be blown into and damage a valve’s pneumatic components.
• Stuck Valves - If a valve is left in a closed position for an extended length of time, it can damage the seat, or corrosion may set up on the trim.
• Packing Leaks - When a valve stem does not move for a prolonged period of time, the packing can lose elasticity and leak when process pressure is restored.
• Diaphragm/O-Ring Problems - When a valve is immobile for an extended time, the diaphragm or O-ring seals can become brittle and may leak when put back into service.

 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: 

• Improve Control - Sometimes poor control can be resolved by simply retuning the loop. This does not resolve a trim that is oversized, but this issue can be addressed by changing the trim.
• Evaluate Cavitating Valves - If a valve starts cavitating after rates are reduced, it is wise to have the valve vendor evaluate the valve to ensure it can handle the new service conditions.
• Ensure Anti-surge Valve Performance - Anti-surge valves should be inspected more often if they are continuously running partially opened.
• Provide Effective Maintenance - Do not assume that critical valve inspections can be delayed and extended just because the plant is running at reduced rates.
• Prepare for Restart Before Shutting Down - If a plant is going into a shutdown (extended or otherwise), diagnostic and leak tests should be run in advance (Figure 2).

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.

• Set Correct Valve Position - As many control valves as possible should be left slightly open and off their seat during an extended outage.
• Test for Leaks - Well in advance of any startup, plant personnel should pressure test as many valves as possible and ensure any required replacement packing is available.
• Deal with Diaphragms - Similar to packing, diaphragm failures in the actuators should be anticipated after an extended outage (Figure 3).

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.