As far as I can see from your very small image, this diagram means that the flow can be changed from one vertical branch to another one by disconnecting the flanges on the elbow and "swinging" the elbow probably 180 degrees and then bolting back up to the other branch.
In theory it can save a couple of valves and a tee, but is not common.
Your diagram doesn't show the second branch at the moment so it is difficult to see how this varies from a spool piece where you drop a section out and then blank off to provide positive isolation. but maybe allows for a second branch in the future?
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
Swing elbows are used in applications when there is a need to divert the flow occasionally to another system. This is very common for furnaces (Crude, Visbreaker, Coker etc.), where swing elbows are installed in the outlet header. Swing elbows are unbolted and then connected to the de-coking header during steam/air de-coking operations. See attached.
The page is from "Process Piping Design Handbook, Volume 2: Advanced Piping Design", by Rutger Botermans and Peter Smith, 2008, Gulf Publishing Company.
I have seen this arrangement used in pharmaceutical applications where it is imperative that it is not possible to get streams mixing due to a wrong valve position, or a leaking valve. It takes a lot more effort to change the position of a swing elbow than to move a valve handle and it is not something you are likely to do accidently.
Swivel joints are different from swing elbows. They are most commonly used in sanitary piping systems and are usually pigged clean before changing connections. An example would be a food plant where a bulk storage tank is plumbed to many mixers but the recipes change often.
Another advantage of using swing elbows is the ease of replacement. Elbows in general represent areas of high turbulence and high erosion potential (particularly true for outlet header of process furnaces = two-phase flow with high velocities) where damage is likely to occur over time. Replacing an elbow is much easier than having line cut and onsite welding. You can think of them as of a very convenient removable spool pieces.
Another definition. Flow diverter boards and "swing elbows" are traditionally used in dairy plants to isolate cleaning circuits, preventing contamination of food products with cleaning solution; this provides a physical break (disconnection) between pipelines. The installation of any number of segregating valves (set of valves with no break to atmosphere) does not constitute a physical break and is not acceptable, except in the following cases: 1. Special Case - Double Block and Bleed Valve Arrangements for CIP Cleaning; 2. Special Case - Double Seat (Mix Proof) Valves for CIP Cleaning; 3. Cleaning Aseptic Processing and Packaging Systems.
Swing elbows are used where in normal process particular services needs to be positively isolated for example air and hydrocarbon or H2 and air. In special cases like in bed regeneration etc these services have to be taken in line. That time service is disconnected by removing existing elbow and fixing required elbow in air or H2 line. Swing elbows are extra precaution to avoid inadvertent mixing of hazardous services.
