A ball valve features a spherical disc to open and close the flow of the fluid. They are the industry standard for pipeline shut-off applications, as their design guarantees very tight and leak-free sealing even after years of operation. The two key types of ball valves are the floating and the trunnion designs (side or top entry). The body can be cast or forged and monolithic or split (in 2 or 3 pieces). The API 6D and ASME B16.34 are key specifications.
WHAT IS A BALL VALVE?
Ball valves are key devices to shut-off and regulate the flow of a fluid (oil, gas, steam, etc) in the petrochemical industry.
ADVANTAGES AND DISADVANTAGES OF BALL VALVES
This type of valve has the following advantages:
- can be quickly opened and closed with a quarter turn movement
- ensure a very tight seal without the need for high torque forces
- have a compact design (this is the main difference between a ball valve and a gate valve – which are both on/off devices)
The disadvantages are:
- valve has poor regulating and throttling capabilities, as it is designed for pipeline shut-off instead of flow modulation (this is an ideal function for globe valves): throttling causes the partially exposed seat to erode because of the high-velocity flow and pressure experienced by the valve. The wear may lead to a leakage of the valve progressively.
- valve can’t be used for slurries, as the accumulation of suspended particles and debris would make the valve leak. These particles may solidify in the cavities in the proximity of the ball and seats. Ball valves are way more very efficient with gases and other liquid fluids (even challenging chemicals as dry chlorine, hydrofluoric acid or hydrochloric acid, and oxygen).
- may be difficult to clean (except for top entry design)
The open, close or partially open position of the valve can be detected by looking at the position of the lever:
- when the lever is aligned to the pipe, the valve is open; when it is perpendicular to the pipe, the valve is closed;
- when it is any other position, the valve is partly open (or partly closed) and is modulating the flow.
These valves belong to the family of “quarter turn” valves or “1/4 turn valve” (together with butterfly and plug valves), as the opening and closing operations are executed by turning a lever connected to the disc by 90 degrees.
The key ball valve specifications are the API 6D, BS 5351 (forged) and ASME B16.34 (pressure and temperature ratings), ASME B16.5/B16.47 (flanged ends) and ASME B16.25 (butt welding ends).
HOW DOES IT WORK?
The ball inside the valve has a hole through which the flow passes freely when it is fully aligned with the two ends of the valve. When the hole is completely perpendicular to the ends of the device, the valve is perfectly sealed.
Otherwise, when the hole is in any other position (perpendicular to the valve ends for instance) the flow is totally or partly interrupted.
Ball valve vs gate valve
This valve absolves the same function as a gate valve (shut-off). The difference between a ball valve and a gate valve is that the former uses a spherical ball to open/close the flow, the latter uses a disc sitting on a seat. The difference between these two valves is therefore in design and compactness.
Ball valve vs globe valve
These valves are not as precise as globe valves to throttle the fluid flow, as they generally have positive (at 0, 45 and 90 degrees) instead of incremental positions. Further, the valve would be damaged by partial open positions (or repeated open/close operations) in the long run (not an issue for globe valves).
BALL VALVE TYPES
Ball valves can be classified according to multiple criteria:
- Design: floating, trunnion mounted and double bleed and block
- Number of ports: the standard type is with 2 ports design (1 inlet, 1 outlet), but 3-ways design is also available (1 inlet and 2 outlets or vice-versa)
- Bore size: full bore or reduced bore (FB vs. RB), and V-notch
- Body assembly: unibody (the ball body is made out of one single piece of cast or forged steel), 2 pieces or 3 pieces (the body results from the assembly of two or three different parts)
- Ball access: side entry or top entry (depending on the fact that the ball can be accessed, for maintenance, from the side of the valve or from its top)
- Seat type: metal or soft (Teflon)
FLOATING BALL VALVE
In a floating valve, the ball is suspended in the flowing fluid and is kept in position by the compression of two elastomeric seats against it.
The shaft is connected at the top of the ball and allows the switch from an open to a closed position with a quarter turn movement (90 degrees).
When the shaft is moved, a load is applied to the ball which gets pressed against its seats.
This design, which is lightweight and economic, suits bore sizes up to 10 inches: above this bore size, the seats of the valve would not be able to withstand heavier and heavier balls and the valve would not operate safely and efficiently.
Floating valves allow a bi-directional shut-off of the flow (as the trunnion type).
(Source: Walworth Valves Youtube Channel)
TRUNNION BALL VALVE
Trunnion valves have been introduced to cope with the size limitations of standard floating valves outlined above. Indeed, in this type of valves the shaft, the ball, and the supporting trunnion act as a single solid assembly able to withstand the heavy loads generated by balls of large dimensions.
Besides a size flexibility advantage, a trunnion valve features a lower operating torque compared to a floating valve design (which can become a benefit when the valve needs to be actuated, as smaller and economical actuators can be fitted to operate the valve).
A stainless steel ball valve, trunnion mounted type
(Source: Robert Cort – Wartsila Valves Youtube Channel)
SIDE ENTRY VS. TOP ENTRY
What is the difference between a side entry and a top entry ball valve?
These terms relate to the way by which the ball of the valve and its internal parts can be accessed, from the side (side entry) or from the top (top entry).
Floating and trunnion-mounted ball valves are available with a “side entry” and “top entry” design.
The top entry design is specified when frequent inline maintenance activities are expected on the valve.
This is due to the fact that top entry ball valves allow easier and faster access to the ball and the valve trim compared to side entry valves (which require more maintenance time and space for the same operations).
The main differences between these two ball valve design are:
- Top entry ball valves are manufactured, generally, with cast steel bodies; side entry valves are manufactured with (robust) forged steel bodies
- Top entry ball valves have a monolithic structure (are made by one single piece); side entry valves have a two or three piece assembly
- Top entry ball valves require more non-destructive tests than side entry due to their cast bodies;
- Side entry ball valves are easier to assemble and manufacture than top entry, which requires long experience and craftsmanship to work properly
- Top entry ball valves are generally more expensive than side entry ones and have longer lead times due to the casting operations required to manufacture the body
Generally, a ball valve has two entries (or “two ways”).
However, for some applications, a 3-way ball valve may be specified. A 3-way ball valve has 3 ports (entries), instead of only two.
This multiport design is needed to divert the fluid coming from the main valve inlet to two different directions (instead of just one), each served by two separate valve’s outlet. Another typical application of a 3-way ball valve is to have the main flow line served by two separate and alternative inlets.
The ball of a 3 ways ball valve may have an L or a T design as shown in the picture below. Further, this special type of ball valve is available in floating or trunnion design, with any type of valve end (flanged, socket or threaded connection) and in various materials from plastics, to carbon, alloy, and stainless steel.
FULL PORT VS REDUCED PORT
These concepts refer to the relation between the bore diameter of the valve and the connected piping system. More in detail:
- The “Full port” (FB) type features a bore size (the hole of the ball) matching the bore size of the connected piping. Full port ball valves minimize the pressure drop that is generated by reduced port valves but they are of course more bulky, heavy and expensive. FB ball valves are better suited, than RB ball valves, for throttling applications.
- The “Reduced port” ball valve (RB) type has a bore size that is smaller than the bore size of the connected piping (for example a tube). The media flowing through the valve is, in this case, limited to a percentage (generally between 70 and 80%) of the full flow in the pipeline. Reduced port ball valve creates a pressure drop in the pipeline.
This type of valve is available with cast or forged body materials (monoblock, or split).
Generally, valves below 2 inches of diameter have 2 pieces of three pieces forged bodies (the most common is ASTM A105 for high-temperature service, ASTM A350 LF2 and LF3 for low-temperature service and ASTM A182 F304, F316 for stainless steel valve or higher grades, such as duplex valves ASTM A182 F51 and super duplex ASTM A182 F53/F55).
Forged bodies are also used for high-pressure valves of higher bore sizes (with 2 or 3 pieces split body construction).
Examples of forged ball valves (small size and stainless steel valve with a forged body, trunnion mounted).
Valves of sizes above 2 inches have a cast body (the most common grades are ASTM A216 WCB for high temp. service, ASTM A352 for low-temperature service and ASTM A351 CF8, CF8M for cast stainless steel valves).
The symbol for this type of valve in P&ID diagrams is represented below