Protect control valves when production slows
With many plants running at reduced rates or on shutdown, control valve operation faces severe challenges to maintain throughput. Solutions are at hand.
ZANE BODENSTEINER and MATT GULLEEN
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Covid-19’s effects have forced many petrochemical and refinery units to run at reduced rates or shut down temporarily. Abnormally low production rates, extended shutdowns, and reduced staff all create significant challenges for continued control valve reliability. This article will focus on the problems that each of these situations cause and offer solutions to deal with the issues in a cost-effective manner.
Falling output and rising problems
Most petrochemical and refinery units are designed to run at maximum rates, and many have undergone numerous debottleneck projects to push capacity even higher. However, decreased demand now calls for many units to produce just enough to keep the plant operating. Such abnormal conditions can have unexpected effects on the control valves that are crucial for plant operation. The following problems may arise.
A control valve is normally sized to operate around 40-60% open, with a minimum opening of 10-15% and a maximum opening of up to 90%. Under these conditions the valve will provide consistent, stable flow control. However, low production rates can drop the minimum opening to 10% or less, forcing the valve to control with the plug very close to the seat. Under this condition, the flow will be erratic and difficult to control since a minor stem movement can generate a very significant and non-linear flow response. Additionally, extremely low travel operation under a clearance flow condition may cause additional damage to the seat. Clearance flow occurs below the minimum controllable flow because the plug is not fully seated.
At low production rates, line pressure losses may be significantly reduced and a larger than normal pressure drop could appear across the valve. This condition tends to create or exacerbate cavitation in the valve, which can cause significant damage to the plug, seat, and valve internals (see Figure 1).
During normal plant operation, a compressor anti-surge valve runs closed since the flow through the compressor is well above surge conditions. Low production rates often reduce the compressor load and force the anti-surge valve open to maintain flow through the compressor stages. These continuous high flow, high noise, and high pressure drop conditions can damage the anti-surge valve over time and force an unexpected outage should the valve fail.
Low flow rates rarely extend critical valve maintenance
It is easy to assume that reduced production rates will extend critical valve maintenance cycles, but this is rarely the case. Valves exposed to corrosive conditions will continue to corrode at the same or even accelerated rates. Flow phenomena, such as cavitation and flashing, can also cause accelerated erosion. SIL-rated safety valves will still require periodic testing regardless of the production rate, and high pressure drop valves may sustain damage faster than normal. Therefore, critical valves should continue to be inspected, maintained, and tested at the same frequency as they were during normal production rates to avoid unexpected downtime.
Extended shutdown Issues
When a plant shuts down temporarily, the operational condition of the control valves must be maintained and preserved so the unit can come back online quickly when product demand recovers. If specific precautions are not followed, many of the critical valves may not operate as expected when recalled to duty. Any or all of the following situations can occur during an extended outage.
Damaged valves will not fix themselves
When a unit is shut down, there is often little incentive to focus on the current state of the equipment. Leaking and/or poorly performing valves are often forgotten until the unit is restarted, and the same problems reappear. In many cases, the valves may perform even less effectively than prior to the shutdown.
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 the air pressure is restored, the collected particulates and water will often be blown through the tubing, plugging valve positioners and damaging sensitive pneumatic valve components.
If a valve is left in a closed position for an extended length of time, this can damage the seat, or corrosion may set up on the trim and keep the valve from opening.
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.
When a valve actuator is immobile for an extended time, the diaphragm can become brittle and may leak when brought back into service. Similarly, O-ring seals in the actuator, positioner, and/or the valve body can become brittle with age.
During difficult economic times, management tends to reduce staff to cut operating costs. However, the effort required to run and maintain the plant at low production rates is often nearly the same as required to run the plant at full capacity. Under such conditions, the staff can only do so much and maintenance activities are deferred or eliminated. To keep the plant operating, the remaining staff must focus on the most critical issues, leveraging the product and application expertise of the control valve vendor whenever necessary.
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