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 classPN 20–100 (class 150– 600)PN 150 (class 900)PN 250 (class 1500)PN 420 (class 2500)15½1313131320¾1919191925125252525321¼32323232401½3838383850249494942652½626262528037474746210041001001008715061501501441312008201201192179250102522522392233001230330328726535014334322315–40016385373360–45018436423––50020487471––55022538522––60024589570––65026633617––70028684665––75030735712––80032779760––85034830808––90036874855––95038925–––100040976–––1050421020–––1200481166–––1350541312–––1400561360–––1500601458–––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).
Fig. 1.13. Full-bore/reduced bore ball-valve drawings.
Fig. 1.14. Full-bore ball valves.
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.
Fig. 1.18. Drift test on a riser ball valve.