«The Function of Globe Valves Used in the Oil & Gas Industry Instructor: Peter Smith, HNC (Mech) 2012 PDH Online | PDH Center 5272 Meadow Estates ...»
PDHonline Course M160 (1 PDH)
The Function of Globe Valves Used in
the Oil & Gas Industry
Instructor: Peter Smith, HNC (Mech)
PDH Online | PDH Center
5272 Meadow Estates Drive
Fairfax, VA 22030-6658
Phone & Fax: 703-988-0088
An Approved Continuing Education Provider
54 Valve Selection Handbook
Globe valves are closing-down valves in which the closure member is moved squarely on and off the seat. It is customary to refer to the closure member as a disc, irrespective of its shape.
By this mode of disc travel, the seat opening varies in direct pro- portion to the travel of the disc. This proportional relationship between valve opening and disc travel is ideally suited for duties involving regu- lation of ﬂow rate. In addition, the seating load of globe valves can be positively controlled by a screwed stem, and the disc moves with little or no friction onto the seat, depending on the design of seat and disc.
The sealing capacity of these valves is therefore potentially high. On the debit side, the seatings may trap solids, which travel in the ﬂowing ﬂuid.
Globe valves may, of course, be used also for on-off duty, provided the ﬂow resistance from the tortuous ﬂow passage of these valves can be accepted. Some globe valves are also designed for low ﬂow resistance for use in on-off duty. Also, if the valve has to be opened and closed frequently, globe valves are ideally suited because of the short travel of the disc between the open and closed positions, and the inherent robustness of the seatings to the opening and closing movements.
Globe valves may therefore be used for most duties encountered in ﬂuid- handling systems. This wide range of duties has led to the development of numerous variations of globe valves designed to meet a particular duty at the lowest cost. The valves shown in Figure 3-2 through Figure 3-11 are representative of the many variations that are commonly used in pipelines for the control of ﬂow. Those shown in Figure 3-12 through Figure 3-14 are specialty valves, which are designed to meet a special duty, as described in the captions of these illustrations.
An inspection of these illustrations shows numerous variations in design detail. These are discussed in the following section.
Valve Body Patterns The basic patterns of globe-valve bodies are the standard pattern, as in the valves shown in Figure 3-2, Figure 3-3, Figure 3-5, Figure 3-6, Figure 3-9 and Figure 3-11; the angle pattern, as in the valves shown in Figure 3-4 and 55 Manual Valves Figure 3-2. Globe Valve, Standard
Figure 3-8; and the oblique pattern, as in the valves shown in Figure 3-7 and Figure 3-10, Figure 3-12 and Figure 3-13.
The standard-pattern valve body is the most common one, but offers by its tortuous ﬂow passage the highest resistance to ﬂow of the patterns available.
If the valve is to be mounted near a pipe bend, the angle-pattern valve body offers two advantages. First, the angle-pattern body has a greatly reduced ﬂow resistance compared to the standard-pattern body. Second, the angle-pattern body reduces the number of pipe joints and saves a pipe elbow.
The oblique pattern globe-valve body is designed to reduce the ﬂow resistance of the valve to a minimum. This is particularly well achieved in the valve shown in Figure 3-10. This valve combines low ﬂow resistance for on-off duty with the robustness of globe-valve seatings.
56 Valve Selection Handbook
Figure 3-10. Globe Valve, Oblique Pattern, Pressure-Seal Bonnet, External Screw, with Impact Handwheel, Plug Disc. (Courtesy of Edward Valves Inc.)
Figure 3-12. Globe Valve, Oblique Pattern, Split Body, External Screw, with Seat-Wiping Mechanism for Application in Slurry Service. (Courtesy of Langley Alloys Limited.)
Figure 3-14. Globe Valve, Three-Way, Used as Change-Over Valve in Pressure Relief Valve Installations: One Pressure Relief Valve is Isolated While the Second One Is in Service. (Courtesy of Bopp & Reuther GmbH.) Valve Seatings Globe valves may be provided with either metal seatings or soft seatings.
In the case of metal seatings, the seating stress must not only be high but also circumferentially uniform to achieve the desired degree of ﬂuid tightness.
These requirements have led to a number of seating designs. The ones shown in Figure 3-15 are common variations.
Figure 3-15. Seating Conﬁgurations Frequently Employed in Globe Valves.
62 Valve Selection Handbook Flat seatings (see Figure 3-15a) have the advantage over other types of seatings in that they align readily to each other without having to rely on close guiding of the disc. Also, if the disc is moved onto the seat without being rotated, the seatings mate without friction. The resistance of the seating material to galling is therefore unimportant in this case.
Deformation of the roundness of the seat due to pipeline stresses does not interfere with the sealability of the seatings as long as the seat face remains ﬂat. If ﬂow is directed from above the seat, the seating faces are protected from the direct impact of solids or liquid droplets travelling in the ﬂuid.
By tapering the seatings, as shown in Figure 3-15b, c, and d, the seating stress for a given seating load can be greatly increased. However, the seating load can be translated into higher uniform seating stress only if the seatings are perfectly mated; that is, they must not be mated with the disc in a cocked position. Thus, tapered discs must be properly guided into the seat.
Also, the faces of seat and disc must be perfectly round. Such roundness is sometimes difﬁcult to maintain in larger valves where pipeline stresses may be able to distort the seat roundness. Furthermore, as the seatings are tightened, the disc moves further into the seat. Tapered seatings therefore tighten under friction even if the disc is lowered into the seat without being rotated. Thus the construction material for seat and disc must be resistant to galling in this case.
The tapered seatings shown in Figure 3-15b have a narrow contact face, so the seating stress is particularly high for a given seating load. However, the narrow seat face is not capable of guiding the disc squarely into the seat to achieve maximum sealing performance. But if the disc is properly guided, such seatings can achieve an extremely high degree of ﬂuid tightness. On the debit side, narrow-faced seatings are more readily damaged by solids or liquid droplets than wide-faced seatings, so they are used mainly for gases free of solids and liquid droplets.
To improve the robustness of tapered seatings without sacriﬁcing seating stress, the seatings shown in Figure 3-15c are tapered and provided with wide faces, which more readily guide the disc into the seat. To achieve a high seating stress, the seat face in initial contact with the disc is relatively narrow, about 3 mm ( 1 in.) wide. The remainder of the seat-bore is tapered 8 slightly steeper. As the seating load increases, the disc slips deeper into the seat, thereby increasing the seating width. Seatings designed in this way are not as readily damaged by erosion as the seatings in Figure 3-15b. In addition, the long taper of the disc improves the throttling characteristic of the valve.
63 Manual Valves Figure 3-16. Seatings of Globe Valves Adapted for Throttling Duty. (Courtesy of Pegler Hattersley Limited.) The performance of such seatings may be improved by hollowing out the disc to impart some elasticity to the disc shell, as is done in the valve shown in Figure 3-11. This elasticity permits the disc to adapt more readily to deviations of the seatings from roundness.
The seatings shown in Figure 3-15d are ball shaped at the disc and tapered at the seat. The disc can therefore roll, to some extent, on the seat until seat and disc are aligned. Because the contact between the seatings approaches that of a line, the seating stress is very high. On the debit side, the line contact is prone to damage from erosion. The ball-shaped seatings are therefore used only for dry gases, which are also free of solids. This construction is used mainly by U.S. manufacturers.
If the valve is required for ﬁne throttling duty, the disc is frequently provided with a needle-shaped extension, as in the valve shown in Figure 3-4;
or with a V-port skirt, as in the valve shown in Figure 3-8 and in the seatings shown in Figure 3-16. In the latter design, the seating faces separate before the V-ports open. The seating faces are, in this way, largely protected against erosion.
An example of soft seating design is the valve shown in Figure 3-2. The soft seating insert is carried in this case by the disc, and may be renewed readily.
64 Valve Selection Handbook Connection of Disc to Stem The stem of a globe valve may be designed to rotate while raising or lowering the disc, or be prevented from rotating while carrying out this task. These modes of stem operation have a bearing on the design of the disc-to-stem connection.
Most globe valves incorporate a rotating stem because of simplicity of design. If the disc is an integral component of the stem in this case, as it frequently is in small needle valves such as those shown in Figure 3-4, the seatings will mate while the disc rotates, possibly resulting in severe wear of the seatings. Therefore, the main ﬁeld of application of such valves is for regulating duty with infrequent shut-off duty. For all other duties involving rotating stems, the disc is designed to swivel freely on the stem. However, swivel discs should have minimum free axial play on the stem to prevent the possibility of rapid reciprocating movements of the disc on the stem in the near closed valve position. Also, if the disc is guided by the stem, there should be little lateral play between stem and disc to prevent the disc from landing on the seat in a cocked position.
In the case of nonrotating stems, as in the valves shown in Figures 3-6, Figure 3-7, Figure 3-10, and Figure 3-11, the disc may be either an integral part of the stem (see Figure 3-11) or a separate component from the stem (see Figure 3-6, Figure 3-7, and Figure 3-10). Nonrotating stems are required in valves with diaphragm or bellows valve stem seal, as in Figure 3-6 and Figure 3-7. They are also used in high pressure valves such as those shown in Figure 3-10 and Figure 3-11 to facilitate the incorporation of power operators.
Inside and Outside Stem Screw The screw for raising or lowering the stem may be located inside the valve body, as in the valves shown in Figure 3-2 through Figure 3-4, or outside the valve body, as in the valves shown in Figure 3-5 through Figure 3-14.
The inside screw permits an economical bonnet construction, but it has the disadvantage that it cannot be serviced from the outside. This construction is therefore best suited for ﬂuids that have good lubricity. For the majority of minor duties, however, the inside screw gives good service.
The outside screw can be serviced from the outside and is therefore preferred for severe duties.
65 Manual Valves Bonnet Joints Bonnets may be joined to the valve body by screwing, ﬂanging, welding, or by means of a pressure-seal mechanism; or the bonnet may be an integral part of the valve body.
The screwed-in bonnet found in the valve shown in Figure 3-4 is one of the simplest and least expensive constructions. However, the bonnet gasket must accommodate itself to rotating faces, and frequent unscrewing of the bonnet may damage the joint faces. Also, the torque required to tighten the bonnet joint becomes very large for the larger valves. For this reason, the use of screwed-in bonnets is normally restricted to valve sizes not greater than ND 80 (NPS 3).
If the bonnet is made of a weldable material, the screwed-in bonnet may be seal welded, as in the valves shown in Figure 3-6 and Figure 3-7, or the bonnet connection may be made entirely by welding. These constructions are not only economical but also most reliable irrespective of size, operating pressure, and temperature. On the debit side, access to the valve internals can be gained only by removing the weld. For this reason, welded bonnets are normally used only where the valve can be expected to remain maintenance-free for long periods, where the valve is a throw-away valve, or where the sealing reliability of the bonnet joint outweighs the difﬁculty of gaining access to the valve internals.
The bonnet may also be held to the valve body by a separate screwed union ring, as in the valves shown in Figure 3-2 and Figure 3-3. This construction has the advantage of preventing any motion between the joint faces as the joint is being tightened. Repeatedly unscrewing the bonnet, therefore, cannot readily harm the joint faces. As with the screwed-in bonnet, the use of bonnets with a screwed union ring is restricted to valve sizes normally not greater than DN 80 (NPS 3).
Flanged bonnet joints such as those found in the valves shown in Figure 3-8 and Figure 3-9 have the advantage over screwed joints in that the tightenening effort can be spread over a number of bolts. Flanged joints may therefore be designed for any valve size and operating pressure. However, as the valve size and operating pressure increase, the ﬂanged joint becomes increasingly heavy and bulky. Also, at temperatures above 350◦ C (660◦ F), creep relaxation can, in time, noticeably lower the bolt load. If the application is critical, the ﬂanged joint may be seal welded.
The pressure-seal bonnet found in the valve shown in Figure 3-10 overcomes this weight disadvantage by letting the ﬂuid pressure tighten 66 Valve Selection Handbook the joint. The bonnet seal therefore becomes tighter as the ﬂuid pressure increases. This construction principle is frequently preferred for large valves operating at high pressures and temperatures.
Small globe valves may avoid the bonnet joint altogether, as in the valve shown in Figure 3-5 and Figure 3-11. Access to the valve internals is through the gland opening, which is large enough to pass the valve components.
Stufﬁng Boxes and Back Seating Figure 3-17 and Figure 3-19 show three types of stufﬁng boxes, which are typical for valves with a rising stem.