J. Goodwin and M. WAGNER, Emerson, Marshalltown, Iowa
The urea process poses extreme challenges for control valves. High pressures, large pressure drops, elevated temperatures and the particularly corrosive nature of carbamate solutions combine to create a punishing application. Historically, the only control valves available for this service were sourced from specialty manufacturers, and they required constant maintenance to address packing and trim issues.
This article describes recently introduced control valve designs for these applications that extend service life while making any required maintenance much easier. These valves are manufactured and supported through a worldwide service network, so parts and service are more readily available.
Urea process challenges. The urea process starts with heated ammonia and carbon dioxide (CO2) injected into a high-pressure reactor (FIG. 1). Under thousands of pounds of pressure, the ammonia and CO2 combine to create a very corrosive material called ammonium carbamate (carbamate), which then dehydrates to form urea.
The main reactions occur in the pool reactor, which provides residence time for the urea reaction to complete. The bottom of this reactor feeds a solution of urea, water, carbamate and unreacted CO2 and ammonia into the high-pressure stripper, which operates at a much-reduced pressure. The steam and CO2 introduced into the stripper combine to decompose any remaining carbamate back into ammonia and CO2, and those unreacted materials are returned to the reactor to be reprocessed. Urea and condensed water leave the base of the stripper and are concentrated downstream.
The urea process can best be described as a nightmarish perfect storm for control valves. The major process reaction occurs at 2,500 psig (and higher) pressures and elevated temperatures. At those conditions, the ammonia and CO2 reactants form carbamate, which acts much like a very aggressive acid, mercilessly attacking susceptible materials with vigor. Accelerated by the high temperatures, carbamate can produce corrosion rates of > 1,000 mm/yr in carbon steel. To combat this attack, many urea licensors employ a variety of exotic alloys and passivation techniques to resist corrosion and extend service life. Common materials of construction include special austenitic or super duplex stainless steels, along with specialized proprietary materials specifically formulated for urea applications.
In addition to the challenges of material selection, the reactor pressure letdown control valve is exposed to extreme pressure drops as the carbamate/urea solution is fed to the stripper. As the solution moves through the valve, the pressure reduction drives solution offgassing and carbamate decomposition, creating large quantities of ammonia and CO2 vapor. Very high velocities, two-phase flow and extreme vibration are common. A typical control valve could not last minutes in this service, so highly specialized valves have been developed for this application.
Existing urea letdown valve limitations. A handful of firms have historically offered valves for urea letdown service. Urea plant process designs vary and change frequently, so each of these valves were typically custom designed for a particular plant, utilizing specific exotic alloys for the valves, with flow characteristics and piping dimensions determined by the process licensor. The valves usually employed a bonnetless design that could only be serviced by completely removing the valve to access the bottom loaded trim. Poor packing adjustment design and a lack of live-loaded packing tended to create leaks and valve stem damage.
This makes maintenance difficult and time consuming. The valve packing typically employs many packing rings, which can lead to uneven stress, along with high thermal expansion stress on the packing at elevated operating temperatures. Furthermore, the packing is often not live loaded, so small losses of packing material due to wear or extrusion quickly lead to packing leaks, requiring continuous adjustment. Unfortunately, in some common designs, the large nut used to adjust the packing tends to gall with the valve body and/or the valve stem, requiring expensive repair or replacement of the valve components.
Obtaining quotes and parts for these specialty valves can be extremely difficult, and the equipment is usually very expensive with long lead times.
Alternative options. Improved urea letdown valve designs are now available, and these valves are fully customizable to match a wide variety of urea process licensor applications. The forged body and trim components can be manufactured from a broad selection of standard and propriety alloys, such as Ferralium 255®, Safurex®, HVD-1™, Ultimet® and many others. The valve body design, dimensions, process connections and flow characteristics can all be adjusted to meet the requirements of each application. Sizes range from 2 in.–12 in., with a broad range of end connection styles and pressure ratings. The valves are offered in both flow-up and flow-down configurations.
These new valves also offer several design improvements to significantly extend service life and simplify maintenance. Both a bonnetless and top-bonnet design are offered (FIG. 2). When process conditions permit, the bonnet design allows top entry access to the valve internals, so the valve can be inspected and serviced without removing it from the line.
Significant design improvements have been incorporated in the packing (FIG. 3), which now employs a redundant, live-loaded packing arrangement that is easily adjusted. This reduces leakage, extends service life and allows the packing to automatically and continuously adjust as the rings wear and the valve is temperature cycled, both normal operating occurrences. This design also extends valve stem life since the consistent, even pressure loading reduces galling. These packing designs also offer optional purge ports, which may be required for some specific applications.
Other valve design features include cageless trim and gasket-less seat ring designs to eliminate potential crevice corrosion, a contoured one piece forged plug and stem to extend service life, and optional body heat trace connections to eliminate cold spots and reduce localized process crystallization. These valves are offered with ANSI/FCI 70-2 Class IV or Class V shutoff.
These valves are produced by a leading global manufacturer, providing quick worldwide access to parts, services and support. This can make a dramatic difference when a valve unexpectedly fails, and specialized trim components are immediately required.
Takeaway. The urea letdown valve application poses a very vexing design problem, but recently introduced valve designs and materials offer significant improvements in service life, maintainability and availability of parts and service. If a urea plant is experiencing ongoing challenges with severe service urea and carbamate valve applications, it would be worthwhile to explore these new designs. A broad selection of customized body styles and options can match nearly any urea process configuration or licensor specification, and the latest design improvements can generate dramatic increases in plant uptime, along with reduced required maintenance. HP
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Emerson Fisher EHU control valves
Justin Goodwin is a Business Development Leader for Emerson, focused on custom control valve solutions. He has 19 yr of experience with severe service control valve design, engineering management and product management, including 5 yr leading Emerson’s global steam conditioning valve business. Goodwin earned a BS degree in mechanical engineering from Iowa State University, and a BA degree in applied math from Grand View University.
Matthew Wagner is the Chemical Industry Manager for Emerson’s Flow Controls products. In this role, he is responsible for long-term strategy planning for the chemical team, and for managing the daily chemical business of Emerson Flow Controls. Wagner earned BS degrees in chemical engineering and microbiology.