Specifying Control Valves for Severe Service

2023-01-13 01:16:13 By : Mr. Varus Lee

The best severe service application solutions can be found with research, process knowledge and SME advice. This feature originally appeared in InTech Focus: Systems Integration 2022, the InTech Focus ebook for November 2022.

Severe service applications can be the bane of an automation engineer’s existence because they often have higher than necessary maintenance and repair costs. The process or application is particularly punishing and almost always critical to plant operation, so getting it right is paramount. This article demystifies the severe service control valve selection process and provides advice to help end users make the right choice. Pro Press Fittings

Specifying Control Valves for Severe Service

Severe service can mean different things to different people, but there are common themes to these types of applications. They usually involve high pressure drops that generate high noise and vibration in vapor service; flashing, cavitation, or outgassing in liquid service can occur as well. Temperature extremes, mixes of vapors, liquids, and solids, which tend to plug passages and erode internal components, and corrosion might also be encountered. While this article discusses the severe service valve selection process in general, it focuses on three applications that most engineers would designate as severe service (Figure 1). These include:

Figure 2: Angle-body style valves have inherent advantages for some severe service applications. The dirty service trim valve (left) handles outgassing liquids with entrained solids common in HHPS applications. The turbine bypass valve (right) has an integrated desuperheater to control steam temperatures. Figure 3: Anti-cavitation trims are also available in globe valves, as well as angle valves. Simpler designs (the trim on the left) use small holes to absorb multiple pressure drops and direct cavitation away from the seat and body. More complex designs (right) have larger openings to handle entrained solids. In some cases, the piping configuration does not easily accommodate angle-style valves, so globe-body styles are a better option. Fortunately, there are anti-cavitation trim designs (Figure 3) suited for these applications as well. Valves in high-pressure-drop vapor applications, such as compressor anti-surge valves, are designed to handle high-velocity vapor flows, while reducing the noise and vibration that would otherwise result. Low-noise trims (Figure 4) incorporate slots or holes to separate the flow into smaller parallel paths, and they may also use a series of pressure stages to manage pressure drops and reduce sound power level. Figure 4: Low-noise trims vary from single-stage trim (left) to multipath (middle) to multipath and multistage (right). Higher pressure drops and vapor velocities typically require these types of complex trim designs. Figure 5: Additive manufacturing has made a whole new generation of innovative designs possible. Additively manufactured noise trim (left) controls valve noise with increased flow capacity, and the anti-cavitation trim (right) allows rotary valves to be installed and perform well in cavitating applications. Low-noise trims reduce noise and shift noise frequency into ranges that are less detectable to the human ear. As valve pressure drops increase, low-noise trim designs tend to get more complex because multiple stages are required to achieve the required noise reduction. Anti-surge valves pair low noise trims with specialized high-capacity, intelligent digital valve positioners to achieve the extremely fast response required by this application. These valves also can incorporate engineered deadband to allow partial stroke testing without passing significant flow, as well as diagnostic capabilities in the digital positioner to ensure reliability. Additive manufacturing, in this case 3-D metal printing, has made a host of new designs possible that were either not economical or feasible to manufacture using conventional techniques (Figure 5). These types of designs can incorporate high-strength alloys in novel configurations to create new solutions for a variety of applications.

Material selection is critical for success in severe service applications. Trim and body components often are subjected to damage from cavitation, flashing, and erosion. Packing materials are subjected to high temperatures and pressures, and all wetted parts are subjected to corrosive attack. A severe service valve incorporates a variety of materials for different components to best address the conditions each is expected to encounter. Fortunately, additive manufacturing has improved this area of valve design as well. These manufacturing techniques allow very high-strength alloys to be used where they could not be employed before, resulting in longer service life and reduced maintenance.

It should be clear from this article that the number of valve component options can be overwhelming. When faced with evaluating the best solution, it is advisable that the user seek help and advice. Corporate engineering may have some suggestions, as may sister plants that encounter similar applications. Peer recommendations from end users at other companies can be helpful, and these types of discussions are common at International Society of Automation (ISA) meetings. It also may be wise to engage your control valve partner, who will typically have a good understanding of the myriad alternatives and can help users make informed decisions. Severe service applications are not insurmountable problems, but considerable effort is required to thoroughly research them before the best solution can be found. Taking the time to fully understand the details and to seek guidance from knowledgeable peers and partners can go a long way to ensuring a wise and successful decision. All figures courtesy of Emerson. This feature originally appeared in InTech Focus: Systems Integration 2022, the InTech Focus ebook for November 2022.

Justin Goodwin is the director of the steam conditioning group at Emerson. He has a BS in mechanical engineering from Iowa State University and a BA in applied math from Grand View University. Goodwin has been responsible for the design and technical support of steam conditioning and desuperheating equipment since 2005. Today, he provides direction, technical oversight, and training for Emerson’s global steam conditioning business.

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Specifying Control Valves for Severe Service

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