Discrepancies between actual operating conditions and initial design specifications often lead to equipment problems, requiring coordinated and prompt action from plant personnel and partners. K+S Potash Canada faced such challenges with their control valves in a new plant near Moose Jaw, Saskatchewan.
Our article on pages 45-48 in the April 2024 issue of Global Mining Review, titled “Under Control,” describes how K+S worked with Emerson and its local Impact Partner, Spartan Controls, to resolve these issues.
K+S has 125 years of experience mining and processing salt and potash, and its Moose Jaw solution mine began production in 2017.
Potash solution mining involves injecting water into boreholes, and dissolving potash, salts, and minerals in mining caverns. The resulting brine is returned to the surface and processed to recover potash and other minerals.
The plant uses hundreds of control valves in challenging conditions, including erosive slurries, significant chloride concentrations, and elevated temperatures.
The new plant soon encountered problems with larger valves and valves of all sizes in slurry service, mainly due to process conditions that differed from design specifications. K+S contacted Spartan Controls, who contacted Emerson experts for additional assistance. Working together, the team identified multiple issues and determined appropriate solutions for each.
The original plant design included large Bettis actuators and Fisher Vee-Ball valves paired with Fisher FIELDVUE digital valve controllers. Unfortunately, some of these larger valve assemblies were cycling too frequently, causing damage to actuator linkages and O-rings.
The troubleshooting team discovered that the digital positioners had been tuned incorrectly during commissioning, so plant personnel adjusted gain and deadband to factory-recommended settings, and they also de-tuned the control loops for these valves to reduce cycling further.
The damaged actuator linkages were repaired, and the O-rings were upgraded to a polymer better suited for high-frequency modulating service. Altogether, these interventions significantly reduced cycling and repairs.
While most control valves can operate in any orientation without issue, vertical installations increase wear and tear on very large rotary valves. Several large valves installed vertically required frequent repairs due to seal and bearing failures, and simply rotating these valves to a horizontal position eliminated these problems.
A few valves exhibited sluggish operation, flow inconsistencies, and cavitation damage, in many cases due to differences between current operating conditions and those used in initial valve selection and specification. After revising sizing calculations based on actual operating conditions, the team selected more appropriate valves and actuators for each application.
Multiple valves across the plant experienced severe erosion and corrosion, with metallurgical analysis revealing a challenging combination of pitting corrosion, erosion, stress corrosion cracking, crevice corrosion, galvanic corrosion, and chloride attack. The team addressed this by changing valve component materials to suit the service conditions, and after a year, the modified valves showed little signs of damage.
With these combined improvements, the plant saves approximately US $130,000-$140,000 on yearly repairs and maintenance. Such savings are not uncommon, and any new process facility experiencing unexpected problems should work with local partners and automation vendors to understand and resolve their issues.
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