Justin Goodwin, Director of the Steam Conditioning Group at Emerson, recently published an article in the November 2022 issue of InTech Focus. It is titled “Specifying Severe Service Control Valves” and it describes techniques for correctly choosing control valves for severe service applications. The article is summarized below.
Choosing valves for severe service applications can be particularly challenging. The process is always punishing, and these valves are invariably critical to plant operation, so selecting the right valves is paramount. “Severe service” usually involves very high-pressure drops and flashing, cavitation, or outgassing in liquid service. Corrosion and temperature extremes may also be encountered, as well as mixtures of vapors, liquids, and solids—which can plug passages and erode internal components. Selecting the right valves for these applications is not easy, but a step-by-step methodology can help.
The severe service selection process starts with a solid understanding of the application. Most users have the normal process conditions well defined, with a firm grip on the inlet and exit pressures, process temperatures, and process media characteristics. However, they often fail to appreciate how those conditions can vary during abnormal situations. Justin asks:
What happens during upset conditions or when the plant is starting up? Must the valve handle markedly different flows or pressure drops at this time? Will temperatures range higher or lower than expected?
The process fluid itself can also change markedly. During abnormal situations, the valve may need to pass impurities, or may be prone to increased flashing or outgassing.
Once the range of process conditions have been defined, the user needs to evaluate how those conditions will impact the valve. Here is a partial list of potential valve challenges:
Most severe service applications involve some combination of these challenges, and all must be addressed during valve selection.
With a solid understanding of the process conditions and valve impacts, the user is now ready to start valve selection. Unfortunately, there are a dizzying array of alternatives, but only a few will perform well in a specific application.
A starting point for valve selection is to look at the body design, which should be appropriate for the application. Angle valves (Figure 1) are good for two-phase liquid cavitating/outgassing service because the liquid/vapor combination can clear the valve, minimizing damage to the seat and valve body. Angle valves are also common in turbine bypass applications.
Figure 1: Angle body style valves have inherent advantages for some severe service applications. The Fisher DST-G dirty service trim (left) handles outgassing liquids with entrained solids common in HHPS applications. The Sempell turbine bypass valve (right) has an integrated desuperheater to quench high steam temperatures.
In some cases, the piping configuration does not easily accommodate angle style valves, so straight through body styles must be used. Fortunately, there are anti-cavitation trim designs (Figure 2) suited for these applications as well.
Figure 2: Anti-cavitation trims are also available in straight through valves. Simpler designs (Cavitrol III trim on left) utilize small holes to absorb multiple pressure drops and direct cavitation away from the seat and body. More complex designs (NotchFlo on right) have larger openings to handle entrained solids.
High pressure drop vapor application valves are designed to handle high velocity flows, while reducing noise and vibration. Low noise trims (Figure 3) incorporate slots or holes to separate the flow into parallel paths, and then utilize numerous stages to manage pressure drops and reduce sound levels.
Figure 3: Low noise trims vary from single stage trim (Whisper Trim I on left) to multi path (Whisper Trim III on middle), to multipath and multistage (WhisperFlo on right).
Anti-surge valves incorporate low noise trims with specialized high capacity, intelligent diagnostic digital positioners to achieve the required extremely fast response and high level of reliability.
Careful material selection is critical for severe service applications. Trim and body components must resist damage from cavitation, flashing, and erosion, and packing materials must endure high temperatures and pressures. A severe service valve will incorporate a variety of materials for different components to best address the conditions each is expected to encounter.
The number of valve component options can be overwhelming, so it is advisable that the user seek help and advice from whatever source is available. Justin offers some ideas:
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 ISA meetings. It may also be wise to engage your control valve partner as they typically will have a good understanding of the myriad alternatives and can therefore help users make informed decisions.
The post Specifying Severe Service Control Valves appeared first on the Emerson Automation Experts blog.
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