Critical Control Valve Selection in Hydroprocessing Units

Control valves installed at critical locations in hydroprocessing units typically operate under high differential pressures. Under these conditions, the process fluid is prone to flashing and vaporization, making the accurate selection of valve flow capacity especially important. Failure to account for these effects during control valve sizing calculations can result in insufficient flow capacity, which may prevent the unit from meeting its performance requirements. Conversely, overestimating the effects of flashing and vaporization may result in oversized valves, which not only increase capital costs but also force the valves to operate at small openings for prolonged periods. This can lead to vibration and premature wear of the valve’s internal pressure-control components. Therefore, it is essential to select an appropriate medium vapor pressure when performing valve sizing calculations, as it serves as a basis for determining the correct valve size and rated Cv values. Before performing these calculations, it is important to understand the concept of vaporization pressure (Pv), which is the pressure at which a liquid begins to vaporize when its pressure drops below a certain threshold at a given temperature.

The oil medium in a hydrogenation unit is a complex mixture, and the reaction of the feedstock with hydrogen in the reactor involves multiple simultaneous chemical processes. For instance, polycyclic aromatic hydrocarbons (PAHs) undergo hydrocracking through ring-by-ring saturation or ring opening, producing small-molecule alkanes and cycloalkane-aromatic compounds. Cycloalkanes containing two or more rings undergo ring-opening cracking and isomerization, ultimately yielding single-ring cycloalkanes and smaller alkanes. Single-ring aromatics and cycloalkanes are relatively stable, resisting hydrogenation-induced saturation or ring opening, and primarily undergo reactions such as side-chain cleavage or isomerization. Alkanes undergo simultaneous isomerization and cracking. Consequently, oil hydrogenation is an extremely complex chemical process involving multiple components, making it impossible to define a single, unique vaporization pressure.

After heat exchange, the reactor effluent enters the hot high-pressure separator, also referred to as hot high-pressure oil. The main components of this medium are alkanes with varying carbon chain lengths, represented by molecular formulas of the form CxHy, where x and y are natural numbers ≥ 1. Alkanes of different chain lengths exhibit different vaporization pressures. During process design, engineering institutes typically estimate the vaporization pressure based on the theoretical composition of the hot high-pressure oil to guide valve selection and sizing calculations.

Experienced valve manufacturers, however, generally adopt the approach Pv=P (where P is the upstream pressure), assuming that flash vaporization occurs as the medium passes through the valve’s pressure-reducing element. This approach accounts for the complex and variable composition of high-pressure oil, which is affected by factors such as feedstock composition, catalyst uniformity, and fluctuations in temperature and pressure. These factors cause continuous fluctuations in vaporization pressure. Therefore, valve selection must fully account for extreme vaporization conditions to ensure that the valve’s flow capacity meets the plant’s operational requirements and prevents insufficient flow caused by vaporization. The following section describes the selection and calculation of the level control valve for the hot high-pressure separator in a hydrogenation unit, based on the operating parameters listed in Table 1. The valve’s rated flow coefficient is assumed to be linearly related, with Cv = 80, incorporating a 12-stage pressure reduction structure. Figure 2 illustrates the valve selection parameters without considering vaporization effects. The calculation results indicate that the hot high-pressure oil undergoes only initial cavitation downstream of the valve, with Cv values ranging from 16.62 to 33.24 and valve openings between 19% and 40%. Figure 3 presents the valve selection parameters under extreme vaporization conditions (Pv = P). The calculation results show that the hot high-pressure oil flashes downstream of the valve, with Cv values ranging from 30.24 to 60.82 and valve openings between 36% and 75%. Comparing the results in Figures 2 and 3 shows that the calculated Cv values under extreme vaporization conditions are approximately twice those obtained without considering vaporization, thereby placing greater demands on the valve’s flow capacity. Therefore, when selecting a level control valve for the hot high-pressure separator, the vaporization pressure of the hot high-pressure oil must be carefully taken into account. Otherwise, the selected valve may fail to meet the actual flow requirements. However, a higher CvC_vCv​ value does not necessarily indicate better performance.

Based on on-site experience, when fully accounting for Pv=P1, the valve position at the maximum calculated flow should be maintained between 70% and 80%. In practice, the actual operating valve position is generally 60%–70%, approximately 10% lower than the calculated value. By analogy, the selected valve’s operating position should range from 25% to 70%, offering a reasonable range that ensures reliable regulation and long service life. If the selected Cv (CvC_vCv​) is too large, the minimum valve position may drop below 15%, increasing the risk of vibration and cavitation damage.

The selection and calculation method for the cold high-pressure separator level control valve differs from that of the hot high-pressure separator because the cold high-pressure separator receives feed from the hot high-pressure gas, which is cooled and liquefied in the cooler to form cold high-pressure oil. The composition of this medium is considerably less complex than that of the hot high-pressure separator. Nevertheless, cold high-pressure oil still contains alkanes of varying carbon chain lengths, so the vaporization pressure must still be taken into account when selecting and calculating the valve. However, it is no longer based on the ultimate vaporization condition of P₁ = P₂. This section presents a more practical selection and calculation approach, derived from empirical experience comparing theoretical valve calculations with actual field valve positions. The valve position determined using this method generally closely matches the actual operating position observed in the field.

First, the maximum calculated Cv is determined under the ultimate vaporization pressure condition (P1=P2). Next, the Cv is calculated under the vaporization pressure condition (P1=P2​, where P2 is the downstream pressure). The average of these two calculated values is taken as the Cv for the selected valve, which also serves as the basis for determining the valve’s rated Cv. The selection calculation is illustrated using the operating parameters of the cold high-pressure separator liquid level control valve in the hydrogenation unit, as shown in Table 1. The valve’s rated flow capacity is assumed to be linear with Cv = 13, while implementing a 12-stage pressure reduction structure. Figure 4 shows the selection parameters under the ultimate vaporization pressure condition (P₁ = P₂). The results indicate that when cold high-pressure oil flashes downstream of the valve, the calculated Cv values range from 5.57 to 11.20, and the valve operating positions range from 41% to 85%. Figure 5 shows the selection parameters under the vapor pressure operating condition (Pv = P₂). The results indicate that when the cold high-pressure oil experiences initial cavitation after the valve, the calculated Cv values range from 3.15 to 6.31, and the valve operating position ranges from 22% to 47%. Averaging the two calculation results yields Cv,avg = 4.36 to 8.75, with a corresponding valve operating position of 32% to 66%. This average value serves as the basis for determining the valve’s rated Cv. The Cv is also used to estimate the medium’s initial vapor pressure, allowing the valve to be verified using this inversely calculated vapor pressure. Comparing the results in Figures 4 and 5 shows that the calculated Cv (C_v) values for the two vaporization conditions do not differ significantly. This is because, in a residue oil hydrotreating unit, although the pressure and differential pressure are high, vaporization of the medium is limited in the relatively low-temperature environment. Therefore, the calculated values differ less than in a hot high-pressure separator; nevertheless, this still has a significant impact on valve selection. In a light oil hydrotreating unit, where operating pressures are lower, the Cv values calculated under the two vaporization conditions may differ by several times, substantially influencing the selection of the valve’s rated Cv. The calculation method presented here for cold high-pressure separator level control valves is based on long-term empirical experience, comparing theoretical valve selection calculations with actual field valve openings. It is a widely accepted approach, commonly used by leading international valve manufacturers.

Other critical valves include the crude oil booster pump outlet recirculation valve, the rich amine level control valve, the lean amine booster pump outlet recirculation valve, and the sour (acidic) water inlet flash tank level control valve. When selecting and calculating these valves, the vaporization pressure of the medium should generally be taken into account, with a guideline that P<0.05. For the crude oil booster pump outlet recirculation valve, although the operating pressure differential is high, the medium mainly consists of relatively stable long-chain hydrocarbons, and the low crude oil temperature results in minimal vaporization. The rich amine level control valve, which handles circulating hydrogen scrubber fluid, contains only a very small fraction of oil products. Consequently, its vaporization pressure is essentially the same as that of the lean amine booster pump outlet recirculation valve, so only minimal consideration of vaporization effects is required. The level control valve for acidic water entering the flash tank differs slightly. After three-phase separation in the cold high-pressure separator, the acidic water contains a slightly higher oil content than the rich amine solution. Therefore, the vaporization pressure must be considered in its calculation. A typical range for this pressure is P1∼P2. To include a safety margin, it is recommended to use P1=1.1P2​. Nevertheless, this adjustment has only a minor effect on the valve’s calculated Cv​ value.

Because of their unique operating conditions, control valves in hydrogenation units have specific requirements for material selection, structural design, and sealing performance. Valves in different locations must be properly designed according to the properties of the medium, and the materials of critical pressure-control components must meet operational requirements to ensure valve safety and reliability. The selection of hot high-pressure separator level control valves is particularly specialized and warrants separate discussion, while the selection requirements for other valve types are generally similar.

When feedstock reacts with hydrogen under specific conditions, the resulting products become highly corrosive. This corrosion primarily occurs due to the chemical action of hydrogen sulfide on steel. In a hydrogen-rich environment, 90%–98% of the organic sulfur is converted into hydrogen sulfide, which, in the presence of hydrogen, further accelerates steel corrosion. Therefore, all wetted metallic components of the valve must comply with the NACE MR0103 standard to ensure adequate resistance to hydrogen sulfide–induced corrosion.

(1) Pressure Shell
The valve operates under high temperature and high pressure while being exposed to hydrogen and hydrogen sulfide corrosion. Therefore, the valve pressure-containing parts (including the body and bonnet) are typically made of A351-CF8C cast stainless steel or A182-F347/A182-F321 forged stainless steel. These materials offer excellent resistance to high-temperature corrosion caused by hydrogen and hydrogen sulfide.

(2) Key Valve Trims
For valves with multi-stage pressure reduction structures, such as the Masoneilan 77000 and 78400 series, the valve plug and cage can be made of ASTM B637 N07718. When heat-treated to HRC 35–40 and in compliance with NACE MR0103, this material provides excellent erosion and corrosion resistance, making it an ideal choice for valve trim. The valve seat may be made of 347 Stellite alloy. Single-seat valves are typically used in slurry-bed residue hydrotreating units, which operate under the most severe conditions. Because the medium contains a high concentration of solids, single-seat designs—such as the Masoneilan 74000 series—are preferred for their superior anti-clogging performance. When the solids content is exceptionally high, erosion may exceed the tolerance of ASTM B637 N07718. In such cases, solid tungsten carbide with a nickel-based binder is recommended for valve plugs, seats, and other pressure-controlling components. The porosity, density, and forming process must be carefully specified to ensure long-term reliability. Valve stems may be made from ASTM A276-347, A479-XM-19, or B637 N07718, all of which offer excellent mechanical strength and toughness to withstand high-frequency vibration. When ASTM B637 N07718 is used for the valve stem, a heat-treated hardness of approximately HRC 26 is recommended, as excessive hardness may increase the risk of fracture.

(3) Sealing Components

Sealing components include gaskets and packing to prevent leakage of the process medium. For valve gaskets, graphite spiral-wound gaskets with OCr18Ni10Ti metal strips are widely used due to their simplicity, low cost, and reliable sealing performance. In extremely harsh conditions, metal ring gaskets, such as HTMS CSI series, can better compensate for high-temperature deformation. For valve stem packing, the high permeability of hot oil renders conventional graphite packing inadequate. A combination of graphite, carbon fiber, and specialized end-ring packing is therefore required. Domestic suppliers now provide well-established products capable of meeting the requirements of high-temperature oil service.

For different types of oil hydrotreating units, valve pressure-relief structures may consist of three-dimensional labyrinths, serial notches, Christmas tree configurations, or Venturi designs. However, the effluent from the hot high-pressure separator contains substantial amounts of solids—including catalyst particles, process rust, and thermally condensable hydrocarbons—making valve blockage a serious safety hazard. Valve designers have determined that maintaining a minimum throttling aperture of at least 6 mm in the multi-stage pressure relief path effectively prevents blockage. This requirement has proven reliable even under the complex operating conditions of hot high-pressure separators. Although the exact reason for this limit is not fully understood, it is a well-established guideline, particularly for units operating under process conditions less severe than those in heavy oil hydrotreating. Designers should take this into careful consideration when selecting valves.

The valve leakage level must comply with ANSI/FCI 70-2V standards, as it is subjected to high differential pressure when closed. Excessive leakage can severely damage the valve’s sealing surfaces, compromising overall sealing performance.

(1) Compliance with NACE MR0103

With the widespread use of imported high-sulfur crude oil in domestic refineries, stress corrosion in equipment operating in hydrogen sulfide environments has become a significant safety concern. Therefore, valves at critical positions in hydrogenation units must comply with NACE MR0103, providing excellent resistance to corrosion and stress corrosion. Their materials must be suitable for highly corrosive environments containing acidic gases, sulfides, and chlorides.

(2) Material Selection
The regulating medium for the crude oil booster pump outlet recirculation valve is mixed crude oil, which is low in temperature and has limited corrosiveness. Therefore, the valve body is typically constructed from carbon steel. All regulating valves downstream of the cold high-pressure separator handle hydrocarbon condensates. Except for the rich amine liquid level control valve—which employs stainless steel due to the medium’s high sulfur content—the valve bodies are generally constructed from carbon steel or low-alloy steel. To ensure long service life and compliance with NACE MR0103, it is recommended that valve trims be made of ASTM B637 N07718 and heat-treated to HRC 35–40, providing excellent resistance to cavitation.

(3) Backup Valves
Key locations in hydrogenation units are typically configured with dual lines, each fitted with a control valve. This arrangement is necessary because valves are highly susceptible to damage under high differential pressure. To ensure long-term stable operation, a backup valve is engaged if the primary valve fails. Once repaired, the previously damaged valve assumes the backup role. Typically, one valve operates while the other remains on standby, with their roles rotated periodically. In units with particularly harsh operating conditions, such as suspended-bed residue oil hydrotreating or direct coal liquefaction hydrotreating units, three or even four parallel lines may be used for hot high-pressure separator level control. These valves operate under identical conditions, so their selection and configuration are the same.