Ball valves consist of a spherical obturator with a cylindrical hole, usually of the same diameter as the pipe, although it can be smaller. Operation is by rotation (1/4 turn) of a shaft mounted, often horizontally, with its axis at right angles to the cylindrical hole. Seals are usually resilient and can provide drop tight shut off. Ball valves are commonly used in small diameters (up to DN 300) although at least one manufacturer can make ball valves up to DN 1200. Ball valves are manufactured in one-piece, top entry, two-piece (Fig. 18.6) and three-piece bodies. A top entry body allows access to the ball and seats for maintenance without the need to remove the valve and is preferred for larger sizes.
A ball valve consists of a valve body in which a large sphere with a central hole equal to the inside diameter of the pipe is mounted. As the ball is rotated, in the fully open position the valve provides the through conduit or full bore required for unrestricted flow of the fluid and scrapers or pigs. Compared with a gate valve, a ball valve has very little resistance to flow in the fully open position. When fully open, the L/D ratio for a ball valve is approximately 3.0. The ball valve, like the gate valve, is generally used in the fully open or fully closed positions. A typical ball valve is shown in Figure 12.10.
Figure 12.10. Typical ball valve.
Unlike a gate valve, a ball valve requires a one-quarter turn of the hand wheel to go from the fully open to the fully closed positions. Such quick opening and closing of a ball valve may be of importance in some installations where isolating pipe sections quickly is needed in the event of emergency.
The ball valve is an inexpensive alternative to other valves. Ball valves use a metal ball with a hole bored through the center, sandwiched between two seats to control flow. Used in many hydrocarbon process applications, ball valves are capable of throttling gases and vapors and are especially useful for low-flow situations. These valves are quick opening and provide a very tight closure on hard-to-hold fluids (see Figure 5.13).
Figure 5.13. Ball valve.
Courtesy of Jenkins Bros.
Ball valves do not use a handwheel but instead use a wrench to control the flow. A 90° turn of the wrench opens or closes the valve. This simple design yields a nonsticking operation that produces minimal pressure drop when the valve is in its full-open position. Drawing symbols for the ball valve are shown in Figure 5.14.
Ball valves are not recommended for FO applications. Generally, it is possible to reduce the opening time of the fail open actuated valve by installing a quick exhaust valve on the control panel to release the instrument air from the pneumatic actuator in the fail mode quickly. However, a ball valve’s seat and disk are in contact during the opening and closing, which can jeopardize FO. In addition, moving the relatively large and heavy ball requires a higher stem torque, a larger actuator, and perhaps a longer opening time. The ball valve manufacturer was asked about the possibility of using a soft seat ball valve for this application. The manufacturer believed that FO of the soft seat ball valve in 2 s could cause damage to the soft seat because of the very quick contact with the ball. On the other hand, the manufacturer stated that a 2-s opening time can be achieved with a metal seat ball valve. But a metal seat has the disadvantage of possible leakage, unlike a soft seat, and it is a more costly solution than butterfly and axial control valves due to the valve and the large mounted actuator.
Unlike FO applications, a ball valve is a good choice as a blowdown valve with less opening time than an FO valve. Fig. 12.25 shows a blowdown ball valve to release the overpressured fluid from the equipment in an emergency mode. The blowdown ball valve is an 18″ Class 2500 in a 6MO body and a metallic Inconel 625 seat, which may need 18 s for opening. Blowdown or FO valves on flare lines usually see low operating temperatures because of the released gas pressure drop. Gas pressure drop reduces the operating temperature to − 46°C or even lower, so the minimum design temperature is typically below − 100°C. The low temperature application makes it impractical to use 22Cr duplex with a minimum design temperature of − 46°C for the valve, so 6MO or Inconel 625 are the correct choices of materials. An extended bonnet is used for the valve to keep the packing away from the relatively cold service, similar to cryogenic valves.
Fig. 12.25. 18″ Blowdown ball valve 6MO material CL2500.
Ball valves can be full-bore (FB) or RBbore (RB) design. With an FB (sometimes called full port) valve, the internal flow passage is equal to the full area of the inlet port. With an RB valve, the flow area of the port (closure member) is less than the area of the inside diameter of the pipe and inlet of the valve. Closure member refers to the ball in a ball valve, also referred to in some international valve standards as the obturator. An FB valve allows for the use of a pipeline injected gadget (PIG) in the pipeline. A PIG is designed and run into the pipeline for inspection or cleaning purposes such as wax or scale buildup.
Both ball valves in Fig. 1.12 should be FB to facilitate quick and full flow release of fluid to the flare line. FB is also a requirement for ball valves upstream and downstream of pressure safety valves (PSV), as shown in Fig. 1.12.
Fig. 1.12. Full-bore ball-valve upstream and downstream of the PSV.
API 6D, the standard for pipeline valves, gives a minimum bore diameter for rating 150–600 equally up to 60″ and separate minimum bore columns for CL900, CL1500, and CL2500 as shown in Table 1.1. But the standard does not provide the minimum bore diameter for large size and high-pressure classes (maximum 20″ bore in CL2500 and 36″ bore in CL1500). API 6D bores are counted as full bore but they are not really full bore—which means that the bore of ball valves as per the API 6D standard is less than the pipeline (piping) bore. Therefore, the valve bore should be equal to the pipe diameter when conducting PIG running for API 6D pipeline valves. The minimum bore in API 6D is usually larger than the ASME B16.34 standard for valves. An API 6D FB ball valve in larger sizes such as 24″ and pressure classes 150–600 has a bore much closer to the pipe. For example, a 24″ ball valve in duplex material and class 300 has about 2 mm less bore than the pipe. However, a 20″ class 150 ball valve as per API 6D standard could have a bore that is approximately 8 mm smaller than the pipe.
Table 1.1. Minimum bore diameter based on API 6D.
DN (mm)
NPS (in.)
Pressure class
PN 20–100 (class 150– 600)
PN 150 (class 900)
PN 250 (class 1500)
PN 420 (class 2500)
15
½
13
13
13
13
20
¾
19
19
19
19
25
1
25
25
25
25
32
1¼
32
32
32
32
40
1½
38
38
38
38
50
2
49
49
49
42
65
2½
62
62
62
52
80
3
74
74
74
62
100
4
100
100
100
87
150
6
150
150
144
131
200
8
201
201
192
179
250
10
252
252
239
223
300
12
303
303
287
265
350
14
334
322
315
–
400
16
385
373
360
–
450
18
436
423
–
–
500
20
487
471
–
–
550
22
538
522
–
–
600
24
589
570
–
–
650
26
633
617
–
–
700
28
684
665
–
–
750
30
735
712
–
–
800
32
779
760
–
–
850
34
830
808
–
–
900
36
874
855
–
–
950
38
925
–
–
–
1000
40
976
–
–
–
1050
42
1020
–
–
–
1200
48
1166
–
–
–
1350
54
1312
–
–
–
1400
56
1360
–
–
–
1500
60
1458
–
–
–
According to the API 6D standard, an RB ball valve has one size reduction up to and including 12″ (e.g., 12″ × 10″) and two size reductions for sizes above 12″–24″ (e.g., 24″ × 20″), and customer and manufacturer agreement for sizes above 24″. This could result in three size reductions for above 24″ (e.g., 36″ × 30″). Body-piece bolts for FB valves usually have more flange bolts compared to RB valves. An RB ball valve has a full bore at the end flange (Parameter B on Fig. 1.13, right valve), which is reduced gradually (Parameter B1 on Fig. 1.14, right valve). Therefore, both bore sizes are shown on the general arrangement drawing of RB ball valves. However, the bore of a full-bore valve is constant (Parameter B on Fig. 1.14, left valve).
Some instruments such as venture flow meters may need some length of straight pipe upstream or downstream to avoid flow turbulence and accurate measurement. Fig. 1.14 shows an 18″ ball valve in class 150 upstream of a flow element (FE) that should have the same bore as the pipe to avoid flow turbulence in the flow element.
An API 6D full-bore ball valve usually has a smaller bore diameter than the pipe. As an example, full-bore 18″ API 6D Class 150 ball valves in 22Cr duplex material could have a bore diameter up to 10–12 mm smaller than the pipe. The pipe in 22Cr duplex has no corrosion allowance and less thickness, which makes it have a larger bore compared to the valve and also compared to the carbon steel pipe. The minimum bore diameter (flow passage) is 90% of the inside diameter of the valve end as per ASME B16.34, which is the standard for valve design.
The inside diameter of the pipe and valve are different; so, there is a step between the valve body flange and the connected flange. However, there is no need to taper any of the valve connector flanges, unlike the flange connected to the equipment. Therefore, the ball valve should be designed as a special bore to provide a flow open area equal to the pipe bore. The internal surface of the ball, seat ring, and body and seat contact may create very low turbulence. However, a special gasket may be required with the same internal diameter as the pipe bore in the valve and flange connection to avoid fluid turbulence.
Another example describes an FB ball valve that is coupled flange-to-flange to a dual plate check valve without any distance. Dual plate check valves usually require a minimum of 2D (2 times the pipe diameter) upstream and 5D (5 times the pipe diameter) downstream straight line to avoid flow turbulence and erosion inside the dual plate check valve. Therefore, it is not a good idea to couple an RB ball valve to a dual plate check valve. Dual plate check valve disk clearance should be taken into account when the check valve is installed upstream of the ball valve, as shown in Fig. 1.15. However, installation of a check valve coupled to the FB ball from the downstream side is not a risk for dual-plate disk clash since the disk opens on the opposite side of the ball valve.
Fig. 1.15. Full-bore ball valve coupled with a dual-plate check valve.
Ball valves may need to be FB upstream of the pumps to increase the net positive suction head of the pumps. It is recommended to have isolation ball valves also upstream of the control valves. Although a reducer is designed upstream of the control valve, which makes pressure drop, an FB ball valve instead of an RB valve could be a better selection upstream of the control valve as shown in Fig. 1.16. As shown in the figure, the isolation ball valve downstream of the control valve should be FB as well. Selection of an FB ball valve avoids flashing and having two-phase flow that can increase wearing, erosion, and cavitation in the control globe valve. However, an RB ball valve may be selected instead of FB to save cost.
Fig. 1.16. Full-bore isolation ball valves before and after a control valve.
In one project, an RB ball valve was selected instead of an FB ball valve in a subflare line. The process department asked for two parameters of Θ and B = d1/d2 to determine whether the flow capacity (CV value) of the RB was sufficient. These two parameters are shown in Fig. 1.17.
Fig. 1.17. Ball-valve parameters of Θ and B.
Two FB ball valves in series that are closed can be selected for manual depressurization to the flare system. As an example, 2″ class 1500 ball valves for manual depressurizing should have at least a 49 mm bore, as per Table 1.1 from the API 6D standard. If one wonders whether a wedge-type gate valve can be selected alternatively, the answer is no. A 2″ class 1500 wedge gate valve cannot provide full bore as per the API 602 standard that covers gate, globe, and check valves for sizes 4″ and smaller in the petroleum and natural gas industries. The minimum bore of a wedge gate valve in the size and pressure class mentioned above is 38 mm, which is smaller than the ball-valve bore as per API 6D.
Except for the example of the ball valve close to the flow element (meter) mentioned earlier, pipeline valves should have a special bore equal or close to the pipe internal diameter, due to PIG running. Although pipeline valves are designed based on API 6D, minimum bore diameters given in API 6D are not necessarily piggable. The bore of a valve is usually less than the thickness of the pipe, especially when the pipe is manufactured from 22Cr duplex material. 22Cr duplex pipe has no corrosion allowance with relatively high strength, which makes the pipe thickness less compared to a carbon steel pipe and the connected valve in 22Cr duplex material. Fig. 1.18 shows a drift test after the manufacturing and assembly of a pipeline ball valve by passing a tool made of a 1 m long bar with three circular-shaped plastic plates on both ends and the middle to make sure that the internal diameter of the valve is suitable for running the PIG.
Ball valves are used for both on/off and throttling service. Ball valves are similar to plug valves but use a ball-shaped seating element (Figure 4.56). They are quick-opening and require only a quarter-turn to open or close. They require manual or power operators in large sizes and at high operating pressures to overcome the operating torque. They are equipped with soft seats that conform readily to the surface of the ball and have a metal-to-meal secondary seal. If the valve is left partially open for an extended period under a high pressure drop across the ball, the soft seat may become damaged and may lock the ball in position. Ball valves are best suited for stopping and starting flow but may be used for moderate throttling. Compared with other valves with similar ratings, ball valves are relatively small and light.
Figure 4.56. Rotary ball valve.
Ball valve is the most used fluid shutoff valve in upstream oil and gas production facilities, both onshore and offshore. They are also used in fuel gas systems feeding furnaces. Plug valves present the following advantages:
•
Resistance to high pressure
•
Compact assembly
•
Quick opening and closing
•
Easy maintenance
•
Can be easily actuated (electrical, pneumatic, or hydraulic actuator)
•
Acceptable for pigging operation
•
Low pressure drop
•
Can be repaired in situ
•
Adapted to the different hydrocarbon phases encountered (liquid, gas, and mixture).
Typical ball valve applications include the following:
•
Pig valve: Launch and receive small scrapers
•
Rising stem: High temperature and erosive fluids
•
Q-ball: Flow control and low noise
•
Compact DBB: Secure high pressure and/or long period shutoff. Note: DBB is a valve arrangement that provides positive isolation. The two (2) block valves in series are closed and the pipe segment between them is depressurized via the bleed valve.
4.2.8.3.2 Reduced port
Most ball valves have a reduced port with a venturi-shaped flow passage that is generally one pipe size smaller than the nominal valve size. The pressure drop through a reduced port valve is generally low enough that the additional cost of a full port valve is not justified.
4.2.8.3.3 Full port
Full port ball valves are required in applications such as hot tapping operations or in a line subject to pigging.
4.2.8.3.4 Operating considerations
Since ball valves open and close so quickly, ball valves may induce water hammer or surge pressures. The hollow ball may trap fluid in the closed position and may cause problems if the valve body is not vented. Abrasive solids suspended in the fluid flow may damage the seats and ball surface because the ball moves across the seats with a wiping motion.
Ball valves handling combustible or dangerous materials should be provided with an emergency seat seal. These emergency seat seals come into operation should the soft seals burn out in a fire (fire-safe). They consist normally of a secondary metal seat in close proximity to the ball so that the ball can float against the metal seat (or vice versa) after the soft seats have deteriorated. Packing materials should be capable of lasting through a fire. Ball valves are classified as either floating ball or trunnion-mounted types.
4.2.8.3.5 Floating ball
In the floating ball configuration, the ball is free to move in the lateral direction. Fluid pressure acting on the ball forces the ball into the seats, giving a tight seal (Figure 4.57). The floating ball is not used in high pressure and large sizes for two reasons. First, the high force of the ball against the seats can deform the seats and affect the low-pressure sealing characteristics of the valve. Second, the same force makes the valve difficult to operate, thus requiring a high torque to overcome the seating force at high-pressure differentials.
Figure 4.57. Floating ball valve.
4.2.8.3.6 Trunnion-mounted
In the trunnion-mounted configuration, the ball rotates in a fixed position (Figure 4.58). The ball cannot move in the lateral direction because it is held in place by a shaft on the top and the bottom of the ball. The valve creates a seal by either fluid pressure forcing a floating seat ring against the ball or pro-stressing the seats and the ball. Stress can occur as a result of an interference fit between the ball and seal or as a result of a spring-type mechanism. The trunnion ball valve is easier to operate than the floating ball valve and is available in larger sizes and higher pressure classes.
Figure 4.58. Trunnion-mounted ball valve.
4.2.8.3.7 Orbit ball valve
The orbit ball valve uses a rotating motion and cam action to create a seal (Figure 4.59). Operation requires several turns of the handwheel. With the valve in the open position, clockwise rotation of the handwheel causes the ball to rotate clockwise until the port through the ball is perpendicular to the flow stream. The ball is held away from the seat by the stem so as to avoid abrasion. The last few turns of the handwheel cause a cam surface on the stem to contact a matching surface in the ball to force the ball against the seat for a tight seal. This action makes the orbit valve easier to operate than other types of ball valves and suitable for moderately abrasive services. Orbit ball valves are popular in larger sizes where power operators are required, which are less expensive than an operator for a conventional quarter-turn ball valve.
Ball valves are shutoff valves that use a ball to stop or start the flow of fluid downstream of the valve. The ball, shown in Figure 7-1, performs the same function as the disc in other valves. As the valve handle is turned to open the valve, the ball rotates to a point where part or the entire hole that is machined through the ball is in line with the valve body inlet and outlet. This allows fluid flow to pass through the valve. When the ball is rotated so that the hole is perpendicular to the flow path, the flow stops.
Figure 7-1. Typical ball valve.
Most ball valves are the quick-acting type. They require a 90-degree turn of the actuator lever to either fully open or completely close the valve. This feature, coupled with the turbulent flow generated when the ball opening is partially open, limits the use of ball valves as a flow control device. This type of valve is normally limited to strictly an “on–of” control function.
Globe valves—Used for throttling and control of flow, and for control valves.
Ball valves—There are two types of ball valves: reduced port and full port, typical applications for these valves are air lines, process lines, gas lines, pigging lines (full port only) design and cost decides the type to be used.
Plug valves—used in industrial processes, oil and gas, petro-chemical, refining, and pipelines. The advantage of the plug valve is that during the opening and closing of the valve, the valve disk and valve seat is separated and there is no friction and thus the sealing face has no abrasion, a soft seal is applied for sealing, so there will be no leakage in the process of closing.
Butterfly valves—used throughout the process, oil and gas, pharmaceutical, food, brewing, and industrial industries for process, water, and fluids.
Types of butterfly valves include flanged, lug, wafer, and triple offset.
It should be noted that you never place a butterfly valve at a vessel nozzle, as the disk can penetrate inside the vessel nozzle.
A lug or wafer butterfly valve therefore could not be used as an isolation valve at a piece of equipment as for maintenance the valve could not be removed, as the line would not be isolated.
Triple offset butterfly valves—these valves are now being used more and more because of their ability to provide a good tight seal.
Check valves—used anywhere a return flow has to be prevented, used in all process facilities.
There are different types of check valves (also known as nonreturn valves).
Diaphragm valve—used throughout the process industries as well as mining, food, pharmaceutical, they are primarily used in slurry services, water, and low-pressure service.
These valves are also known as “Saunders Valves” after the inventor who came across the idea after walking over a hose pipe in a South African mine, and noticed how the flow was reduced after stepping on the hose.
Control valves—used for the control of flow or pressure, generally (but not always) globe valves are used for this purpose.
Pressure relief valves—known as PRVs (pressure relief valves) or PSVs (pressure safety valves)—are a form of control valve as they are set to open at preset pressures, and relieve into a pressure relieving system such as a Flare Header.
Ball valves are generally used on fire-truck water delivery sections of the pump, foam system, fire-extinguishing system, and where quick action for opening of flow is intended. Ball valves are also used where simultaneous opening and closing of two or three valves by quick action is needed.
●
Bodies should be of one piece or split construction: In case of split body valves, the minimum design strength of the split body. Joint(s) should be equivalent to that of the body end flange of a flange body, or the appropriate equivalent flange for a butt-weld-end, socket weld-end, or threaded end body. Bolted covers should be provided with no less than four bolts, stud bolts, stud or sockethead cap, or hexagon-headed screw (Figure 10.3).
Figure 10.3. Angle pattern valve seat details. A Seat width. B Nut or clamping ring for Resilient Seat Seal. C Clearance between the edge of the Resilient Seat Seal holder and the inside of the body. D Clearance between inside of the seat and the nut or clamping ring for the Resilient Seat Seal. E Valve stem. F Locking pin for Retainer Nut or Clamping Ring. G Resilient Seat Seal. H Holder for Resilient Seat Seal.
●
Flanged ends: End flanges should be cast or forged integral with the body or end piece of a split body design, or attached by butt-welding.
●
Stems, ball shanks, stem extensions: Stems, ball shanks, stem extensions, stem mounted handwheels, or other attachments should be provided with permanent means of indicating port position and should be designed to prevent misorientation.
●
Stem retention: The valve design should be such that the stem seal retaining fasteners, e.g., packing gland fasteners, alone do not retain the stem. The design should ensure that the stem should not be capable of ejection from the valve while the valve is under pressure by the removal of the stem seal retainer, e.g., gland, alone.
●
Body seat rings: Body seat rings or seat ring assemblies should be designed so as to be renewable except for those valves having a one=piece sealed (welded) body construction.
●
Ball: On full-bore valves the ball port should be cylindrical. Sealed-cavity balls should be designed to withstand the full-hydrostatic body test pressure. The typical types of ball construction are given in (Figure 10.4(b)).
Notes:
1.
The buyer should state on his inquiry or order if reduced-bore valves are required with ball valves having cylindrical ports.
2.
Solid, sealed-cavity, and two-piece ball valves are shown with cylindrical ports in Figure 10.4A.
●
Wrenches and handwheels: When used, wrenches and handwheels should be designed to withstand a force no less than that given in BS 5351.
●
Antistatic design: Valves should incorporate an antistatic feature that ensures electrical continuity between stem and body of valves DN 50 or smaller, or between ball, stem, and body of larger valves if specified. The use of conductive packing is permitted provided that the packing:
Forms part of the primary stem seal.
Is essential for the proper functioning of the valve.
Cannot be removed by removing the gland and gland packings alone.
●
Operation: Valves should be operated by a handwheel or wrench.
Note: For manually operated valves, clockwise closing will always be supplied. Anticlockwise closing to be supplied by special request.
The length of the wrench or diameter of the handwheel for direct-operated valves should (after opening and closing a new valve at least three times) be such that a force not exceeding 350 N should be required to operate the ball from either the open or closed position under the maximum differential pressure recommended by the manufacturer.
Handwheels should be marked to indicate the direction of closing.
Handwheels and wrenches should be fitted in such a way that while held securely, and they may be capable of being removed and replaced where necessary.
All valves should be provided with an indicator to show the position of the ball port. When the wrench is the sole means of indicating port position, the design should not permit incorrect assembly and should then be arranged so that the wrench lies parallel to the line of flow in the open position.
Stops should be provided for both the fully open and fully closed positions of the valve.
All valves should be provided with same form of indicator for the position of the ball port.
●
Materials: The body, body connector, insert, and cover materials should be selected to withstand at least twice the working pressure which is 12 bar (175 psi) for fire service. Valve used on foam-liquid system should be selected from materials to withstand corrosion effect. Body seat rings, stem seal, body seals, and gaskets should be suitable for use of foam-liquid concentrate. Valve used for salt water or extinguishing agents such as dry-chemical powder, should be specified in the purchasing order. Wrench and handwheel should be of steel, malleable cast iron, or spheroidal graphite cast-iron. Screwed body ends should have female threads complying with requirement of applicable ISO standard, either taper or parallel at the manufacturer’s option unless the particular form is specified in the purchasing order. Flanged-end, butt-weld end, socket weld end, extended-weld end, and threaded-end valves should comply with BS 5351.
●
Tests: All valves should be tested hydrostatically by the manufacturer before dispatch. Test should be carried-out with water. Testing requirement for the body and seat should be in accordance with BS 5146 Part 1.
●
Pressure retention: The pressure retention of ball valve used should withstand without leakage of hydrostatic test pressure of 22.5 bar for 2 min.
●
Test certificate: The manufacturer should issue a test certificate confirming that the valves have been tested in accordance with BS 5146 Part 1 and stating the actual pressures and medium used in the test.
●
Preparation for dispatch: After testing, each valve should be drained, cleaned, prepared, and suitably protected for dispatch (painting of finish valve should be specified in purchase order) in such a way as to minimize the possibility of damage and deterioration during transit and storage. All ball valves should be in open position when dispatched. Body-end should be suitably sealed to exclude foreign matter during transit. Valves should have their jointing surfaces protected.
●
Marking: Each valve should be marked clearly on the body or on a plate securely fixed to the body. Identification marking should be in accordance with Section 7 items 28 to 31 of BS 5159. Information should be specified by the buyer.
Figure 10.4. Typical variations of construction ball valve.