Ball Valve, DBB vs DIB

API 6D/ISO 14313

Trunnion Ball Valve

API vs OSHA

The term “Double-Block-and-Bleed” may sound straightforward enough – two barriers and a bleed to confirm absence of leakage – but it only makes real sense when related to a specific standard or regulation.

OSHA – Occupational Safety and Health Administration – is an agency in the United States government responsible for setting and enforcing standards about workplace safety. For OSHA, double-block-and-bleed means “the closure of a line, duct, or pipe by closing and locking or tagging two inline valves and by opening and locking or tagging a drain or vent valve in the line between the two closed valves.” That is, this definition is clearly concerned with control of hazardous energies. For OSHA, double-block-and-bleed is a system: two independent valves installed in series with a third valve providing a means to bleed or vent the fluid in the piping. For works about to break a line for maintenance, this system allows them to safely check if pressure is contained and the job may proceed. API, the American Petroleum Institute, has a different definition for double-block-and-bleed. Now we are not talking about a system of valves, but isolation characteristics for a single valve. In its standard API 6D (Specification for Pipeline and Piping Valves), there are three important definitions regarding double block: DBB (double-block-and-bleed), DIB-1 (Double-Isolation-and-Bleed type 1), and DIB-2 (Double-Isolation-and-Bleed type 2). However, in order to understand these API definitions, we need first to understand piston effect for seats.

Disc Function

SPE

• Uni-directional valve seats or Single Piston Effect (SPE) seats are self-relieving seats which provide sealing if the upstream pressure is greater than the cavity pressure, but will leak if the cavity pressure is greater than the upstream pressure.
• Seats of the ball valves are pressed on the ball by means of spring load.
• As the body cavity pressure increases than the spring load, the seats are pushed back and the pressure is released in the line. This is called a single-piston effect (the pressure in the body cavity is the only acting parameter)
• Cavity relief to the downstream side, if both the ball valve seats are of single-piston effect design.
• Each seat is self-relieving the body cavity overpressure to the line.

DPE

• Bi-directional valve seats or Double Piston Effect (DPE) seats will provide sealing regardless of whether the cavity or upstream pressure is higher. A DIB valve utilizes one or two bidirectional seats. The valve provides double isolation from pressure at both ends of the valve, but requires an external relief system to relieve pressure.
• In this seat design, medium pressure, as well as the body cavity pressure, creates a resultant thrust that pushes the seat rings against the ball. This is called a double piston effect (the pressure in the pipe & that in the body cavity both are acting parameters)
• Ball Valves with this design require a cavity pressure relief device to reduce the body cavity pressure.
• DPE is synonymous with “bi-directional”, and SPE is synonymous with “unidirectional” as defined by API 6D/ISO 14313.

L&T Ball Valves - Single Piston & Double Piston Concepts Explained

Valve Function

A trunnion ball valve has two seats, one upstream and one downstream, and they may be combined in three different arrangements:

1. Two SPE’s seats
2. Two DPE’s seats
3. One SPE seat and One DPE seat

Each of these arrangements is classified by API with a different name: Double-block-and-bleed (DBB), Double-Isolation-and-Bleed type 1 (DIB-1), and Double-Isolation-and-Bleed type 2 (DIB-2), respectively.

DBB

Single valve with two sealing surfaces that, in the closed position, provides a seal against pressures from both ends of the valve with a means of venting/bleeding the cavity between the seats.

The double-block-and-bleed design is the most commonly used for trunnion ball valves, responsible for around 85% of global oil and gas applications. It is a compact, low-weight valve that provides reliable isolation in critical areas, reducing piping requirements. To summarize, a DBB is a valve that seals from either direction with a bleed between the two, if the first seal leaks, the second will not seal in same direction and it does not require an external mechanism to relieve pressure. A DBB valve implies double security, providing security in two different directions, each with a separate seal. When a DBB valve is in use and the first seal leaks, the second seal will not seal in that same direction.

DBB

Single valve with two seating surfaces, each of which, in the closed position, provides a seal against pressure from a single source, with a means of venting/bleeding the cavity between the sealing surfaces. This feature can be provided in one direction or in both directions.

In API’s DBB and DIB definitions the difference is that a double-block-and-bleed valve seals against pressures from both sides of the valve while a double-isolation-and-bleed valve provides an additional seal against pressure building in the valve’s cavity.

Two configurations are available for DIB ball valves — DIB-1 and DIB-2. The DIB-1 configuration features bi-directional seats upstream and downstream. The DIB-2 ball valve design features one bidirectional (DPE) seat and one uni-directional seat (SPE).

DIB-1

DIB-1 ball valves have DPE seats both upstream and downstream to provide a seal in both directions. For the DIB-1 configuration, over pressurization of the cavity is avoided by the use of an external relief valve. With the ball in the closed position and pressure on the upstream side, the cavity pressure will increase in case of upstream seat failure. The cavity pressure will cause a double piston effect on the downstream seat creating a second seal on the ball.

DIB-2

For the DIB-2 configuration, one seat SPE and the other DPE, cavity over-pressurization is internally controlled within the line. If the upstream seat leaks, then there is automatic cavity pressure relief on the upstream side and the downstream seat will provide isolation due to double piston effect.