There are many valve designs and types that satisfy one or more of the functions identified above. A multitude of valve types and designs safely accommodate a wide variety of industrial applications. Regardless of type, all valves have the following basic parts: the body, bonnet, trim (internal elements), actuator, and packing. Today, we will introduce the parts of ball valves.
The body is a key element. It contains the moving components in contact with the fluid and has to withstand the pressure existing in the piping system. It can be manufactured by casting or forging steel in a variety of shapes, designs and material grades.
In the oil and gas industry, the most common material for cast bodies is ASTM A216 and ASTM A105 for forged bodies (high-temperature service). For low-temperature service, ASTM A352 LCB/LCB and ASTM A350 LF2/LF3 are used, respectively, for cast and forged bodies.
As the temperature, the pressure or the corrosion increase, stainless steel bodies become necessary: ASTM A351 CF8 (SS304) and CF8M (SS316) for cast devices, and the various ASTM A182 Fxx (F304, F316, F321, F347) for forged types.
For specific applications, special material grades with even stronger corrosion resistance are used, such as super austenitic stainless steels (SMO 254), duplex and super duplex steels (F44, F51, F53, F55), and nickel alloys (Inconel, Incoloy, Hastelloy).
For marine applications, non-ferrous materials or alloys the elective choice, like Monel, Cupronickel, Aluminum-bronze alloys and other alloys combining Nickel, Copper, Aluminium. Cast iron bodies, which are the cheapest types, are used for water distribution (low-pressure applications).
(1) Valve Body Construction Type
①1 Piece Ball Valve
This design features a single, solid cast body. Since 1-piece ball valves are not designed to be easily repaired, these are often considered “throwaway” valves.
②2 Piece Ball Valve
This design consists of the main body and one end connection. Two-piece ball valves can be repaired if removed from service. However, repair can be difficult because galling can occur in the threads when the end plug is removed from the valve body.
③ 3 Piece Ball Valve
A 3-piece ball valve features a main body and two pipe connectors, which are threaded or welded to a pipe. The main body can be easily removed for cleaning or repairs, without removing the pipe connectors.
(2) Ball valve body stypes
①Split-Body Ball Valve
This design features a two-part body that is held together by a flange connection. One body part is smaller than the other, and the ball is inserted into the larger part.
② Top-Entry Ball Valve
Top-entry designs allow you to remove the top of the valve to access the interior for assembly, disassembly, repair and maintenance.
③ End-Entry Ball Valves
These ball valves have a single main body, and the ball is inserted through one end.
(3) Body End Connections
Valves can be connected to other mechanical devices and pipes in different ways. The main end types are flanged and buttweld (for devices above 2 inches) and socket weld or threaded/screwed (NPT or BSP) for small diameter devices. The butterfly has more articulated end types, like lug, wafer, and double flanged ends.
① Flanged Ends
In this case, the device has two flanged ends that can be connected with a pipe by using a mating (companion) flange.
A flanged connection requires a proper number of stud bolts and nuts, as indicated by the ASME B16.5 specification, and a suitable gasket.
Flanged connections are common for larger diameter valves, and they ensure long-lasting, and strong, joints.
The flange face may be raised, flat, ring joint, tongue and groove and male & female (the most common finish is the RF type, i.e. raised face), and be finished in any of the available variants (stock, serrated or smooth).
Valves with socket weld ASME B16.11 or buttweld ends ASME B16.25 are welded with the connecting pipe.
Welded connections are more expensive to execute than flanged joints, as they require more work, but are more reliable and less prone to leakages in the long run.
Socket weld ball valve and buttweld ball valve are used for high-pressure pipelines (socket weld for smaller sizes, below 2 inches, and buttweld for larger diameters).
Buttweld connection requires full welding of the beveled ends of the two parts to be joined, whereas socket weld connections are made by fillet welds.
③Threaded End Connection
In this case, the device is connected to the pipe by a thread (tapered thread), which may be BSP or NPT (more common in the petrochemical industry).
This type of connection is used for low-pressure pipelines with bore size below 2 inches, not subject to mechanical forces as vibration and elongation.
Threaded connection is quicker to execute and cheaper, as the pipe is simply screwed onto the valve, without the need for flanges, stud bolts or welding operations.
In case of leakage, however, threaded connection needs to be changed and can’t be repaired (which is instead possible for the two previous types of pipe to end connections).
The bonnet is the part of the body that contains the stem, packing, ball valve stem seal, ball valve packing gland and so on.
By dismantling the bonnet, the maintenance staff has access to the internal mechanisms and can replace components like the seat, the stem, etc (trim).
Valve bonnets are available in many designs and models (the most typical are: bolted, round bolted, welded, pressure seal) and are manufactured in cast steel or forged steel (generally using the same material grade of the body). The bonnet is connected with the body by a threaded, bolted, or welded joint connections – and different types of gaskets are used to ensure leak-free connections between the body and the bonnet.
① Ball Valve Stem:
The valve stem is a rotary shaft connecting the internal ball to the outside lever which when turned rotates the ball.
② Ball Valve Packing:
The valve packing prevents the media from escaping the valve body through the stem opening in the bonnet. The most common type of packing today is chevron v-ring type packing. If you look at the cross section profile of this packing, it resembles the letter “V”.
③Ball Valve Packing Gland
Packing is a gasket that provides a sturdy seal between the valve’s stem and the bonnet. In most valves, gland followers are tightened until the valve is leak proof. Loose packing results in leakage; tight packing negatively affects the valve movement and may cause damage to the stem.
The o-ring is used to create a seal between the stem and housing of the valve. NBR (Nitrile rubber) is the most commonly used material for the O-ring.