2 2
FIG74
Pressure/Temperature Ratings For Various Trim Material Options
Gasket Materials - Most control valves include two different types of gaskets; spiral wound gaskets and flat sheet gaskets. The characteristics and
temperature ratings of several common gasket materials are listed in Figure 46.
• Spiral Wound Gasket Options - A spiral wound gasket is made of a metal alloy that is formed into a V-shape and then wound into a spiral form. During the manufacture of the gasket, a filler is inserted between each coil of the V-shaped material. Of the options that are listed in Figure 46 below, Inconel is the strongest alloy material, it has the highest temperature rating, and it will maintain its spring properties longer than the other options. As a result, the Inconel/graphite gasket is typically recommended for thermal cycling applications.
• Flat Sheet Gasket Options - A standard material for flat sheet gaskets is a composition material. Options such as PTFE coated Monel provide
corrosion resistance, but at reduced temperature ratings, as shown in the table below.
The selection of a suitable gasket material is based on the following:
• The temperature rating of the gasket material.
• Whether or not thermal cycling will occur.
• The corrosion resistance of the gasket material. Standard M a t e r i a l Optional M a t e r i a l s
Figure 46
Common Gasket Materials
Bolting Materials - As mentioned previously, each different alloy has a different thermal expansion vs. temperature curve. However, different alloys in the same family generally have similar thermal expansion and contraction characteristics. Accordingly, the general guidelines for bolting material selection are:
• If possible, select steel bolting (for example, B7 or B16) for alloys steel bodies and bonnets.
• If possible, select stainless steel bolting (for example, 316 or 304 stainless steel) for stainless steel bodies.
• Whenever non-standard bolting is considered or the above guidelines cannot be followed, investigate the need for pressure and/or temperature derating to compensate for the differential in thermal expansion coefficients, differences in bolting strength, and other influences.
Packing Materials - Packing material selection is based upon the temperature at the packing bore. The temperature at the packing bore is often considerably less than the temperature of the process fluid, especially if the valve is
insulated below the packing bore or if an extended-height (extension) bonnet is specified. For temperatures below 400 degrees F, PTFE base packing
arrangements are compatible with most fluids. Above 400 degrees, packing arrangements that are based on graphite materials are the industry standard. Graphite materials are compatible with a wide range of fluids; however, graphite base packing arrangements should not be selected for hot oxidizing acids (nitric acid and sulfuric acid) or for oxygen services that operate above 700 degrees F.
Extended Bonnets For Packing Protection
An extended bonnet locates the packing at an increased distance from the process fluid, thereby reducing the influence of the process fluid on the packing temperature. Refer to Figure 47. Section 4.1.5 of SAES-J-700 requires the selection of extended bonnets or the selection of special packing materials for applications in which the fluid temperature is greater than 450 degrees F.
Use Word 6.0c or later to
view Macintosh picture.
Figure 47
Extended Bonnets That Are Used At Temperatures Above 450 Degrees F
Achieving Tight Shutoff At Elevated Temperatures
Metal Seats - ANSI Class VI shutoff is typically achieved with the use of soft- seated valve constructions. However, Saudi Aramco standards define an upper temperature limit of 400 degrees F for PTFE and many other materials that are included in soft-seating arrangements. Therefore, at temperatures above 400 degrees F, ANSI Class V shutoff or better is generally achieved by specifying an unbalanced valve construction with metal-to-metal seats that have been precision lapped to achieve the shutoff specification.
High Temperature Seal Rings For Balanced Valves - To achieve ANSI Class V or better shutoff with a balanced valve construction in a high-temperature environment, many manufacturers offer special high-temperature PTFE seal
ring designs. Refer to Figure 48 and note the following features of a soft seal arrangement that is rated for temperatures up to 600 degrees F.
• The PTFE “omni seal” is pressure loaded to improve seal performance.
• The PTFE seal includes a spring which helps to maintain a seal between the plug and cage at elevated temperatures where the PTFE material loses its elasticity.
• The PTFE material includes a high percentage of carbon and graphite to improve its high-temperature performance.
• An anti-extrusion ring prevents any of the hot and potentially flowing PTFE material from extruding out of the seal area.
Use Word 6.0c or later to
view Macintosh picture.
Figure 48
SELECTing AND Siz ing CONTROL VALVEs FOR Cavitating FLUID Applications
Cavitation And Its Consequences The Cavitation Phenomenon
Vapor Cavity Formation and Collapse - When, in a liquid flow, the fluid pressure falls below the fluid’s vapor pressure, the fluid begins to vaporize; i.e., vapor bubbles form in the flow stream. In a control valve, the onset of vaporization often occurs near the vena contracta, as shown in Figure 49. If the
downstream pressure P2 increases to a value that is greater than the fluid’s
vapor pressure, the bubbles collapse and the fluid is cavitating.
Use Word 6.0c or later to
view Macintosh picture.
Figure 49
Cavitation Versus Other Flowstream Phenomenon
Cavitation Vs. Flashing - Up to the point where the decrease in the local
pressure causes bubbles to form in the fluid stream, flashing and cavitation are similar phenomenon. In a flashing fluid, however, the downstream pressure P2 is below the vapor pressure of the liquid and the bubbles that form near the vena contracta remain in the fluid stream as shown in Figure 50. Flashing will be discussed later in this Module.
Use Word 6.0c or later to
view Macintosh picture.
Figure 50
Cavitation Vs. Flashing
Cavitation Vs. Outgassing - When a fluid includes dissolved gasses and the fluid is subject to pressure reduction or to agitation (both of which occur as the fluid flows through a control valve), the dissolved gas may come out of solution in a process that is known as outgassing. Refer to Figure 51. Outgassing differs from cavitation and flashing in that it is not a thermodynamic event and it occurs independently of the values of the fluid’s vapor pressure and the
pressure at the vena contracta. In addition, the bubbles that form as a result of outgassing may remain in the downstream flow regardless of the value of P2. An increase in pressure and time may both be required to force the gas bubbles back into solution.
Use Word 6.0c or later to
view Macintosh picture.
Figure 51
Cavitation Vs. Outgassing
Common Forms Of Cavitation
Hard Cavitation Vs. Soft Cavitation
The term “hard” cavitation is used to describe the worst-case scenario in terms of the potential for cavitation damage. Hard cavitation implies that there are no circumstances or conditions present in the application that will have a
mitigating effect on the intensity of the cavitation or the potential for cavitation damage. Cold water is the classic example of a fluid that will exhibit hard cavitation.
The phrase “soft cavitation” is used to describe any application in which either the fluid properties or the service conditions serve to lessen the potential for cavitation damage. For example, the cavitation that occurs in a multi-species fluid such as a hydrocarbon mixture may be less likely to cause significant cavitation damage because the mixture includes components with several different vapor pressures. As the local fluid pressure is reduced, not all of the components will vaporize, and the components that remain in the liquid form may cushion the collapse of the vapor cavities. In addition, fluids that are
viscous and outgassing fluids may provide a cushioning effect on vapor cavity implosions. Refer to Figure 52.
Use Word 6.0c or later to
view Macintosh picture.
Figure 52
Hard Vs. Soft Cavitation
Specifiers typically view the cavitation that occurs in crude oil as “soft
cavitation”. In a crude oil flow, the cavitation damage that occurs as a result of vapor cavity implosion may not present as great a concern as the noise and vibration that occurs. As hydrocarbon liquids become more refined (less viscous and closer to a single species fluid), the damage from vapor cavity implosions becomes a major concern.
Incipient Vs. Full Blown Cavitation
Specifiers will often encounter the term “incipient” cavitation. The term
“incipient” cavitation defines the point at which the first vapor cavities form in the fluid stream. On a plot of flow (Q) versus the square root of the pressure drop that is shown in Figure 53, this point is observed as the first deviation of the actual flow plot from the plot of predicted flow. Incipient cavitation occurs
when the local fluid pressure first dips below the fluid’s vapor pressure. Damage may or may not occur at this point.
At increased pressure drops, more and more bubbles form and collapse in the fluid stream. At the condition of fully choked flow, the cavitation that occurs is often described as “fully blown cavitation” or as “choked flow cavitation”. These terms indicate there is a substantial potential for cavitation damage; however, they are highly subjective and they provide little real guidance to the valve specifier.
Use Word 6.0c or later to
view Macintosh picture.
Figure 53
Incipient Vs. Choked Flow Cavitation
Consequences Of Cavitation
Valve And Piping Damage - If the vapor bubbles that are formed during the
cavitation cycle implode on or near fluid boundaries such as valve components and pipe walls, high-velocity microjets and sonic waves can result in rapid and catastrophic damage to the components as shown in Figure 54.
Use Word 6.0c or later to
view Macintosh picture.
Figure 54
Cavitation Damage That Results From Imploding Vapor Cavities
Vibration - In many liquid flows, vibration of the valve and piping is as great a concern as the potential for damage from the implosion of vapor cavities. Figure 55 shows a representative plot of valve and pipeline vibration versus the value of sigma (σ = P1-Pv/P1-P2). Following the occurrence of incipient cavitation, the intensity of the vibrations increases rapidly as the value of sigma decreases. Cavitation has been known to cause vibrations of sufficient