Why Using to Spool at Incoming Main Gas Pipe?

A spool piece is basically a piece of pipe that is installed in-between a valve and the main distribution piping. there isn't much more to it.

Here is the answer from Answer.com.

The term "pipe spool" or "spool piece" refers to a prefabricated section of a piping system that includes the pipe, fittings and flanges that are pre-assembled in the fabrication facility and then transported to the field. The reason for pre-assembly is that hoists, gauges, material and tools are available for the assembly in a controlled environment. The size of the spool pieces is limited by factors related to transportation to the site and ability to handle the spool piece at the site during assembly.

Like the original poster I would still like to know why it is there by design. One thing the OP could do is talk to the individuals whose names are on the drawing; I can't do that, of course.

I had to look it up--I had never heard the term. Apparently it is a piece which is built off-site in a more benign environment where it can be more easily constructed.

http://www.ask.com/home-garden/pipe-spool-4d318f25ff2800fb

If the left SV requires axial movement to be serviced or replaced, there needs to be somewhere a piece that can be slid straight out to one side, such as a spool.

I never heard the term called 'gas spool" in any of my previous projects or current projects.In one of my previous petrochemical plant construction project with a French EPC giant, the short spool with weld neck flanges on both ends called as 'isolation spool'.That isolation spool used as a removable bridge in between fuel gas main pipe valve to boiler gas burner. At present I donot have drawing of that piping system and I couldnot remember the name of valve used in that exact spot.However, I guess it was a control valve.

As a very new engineer I was told by a much more knowledgeable and experienced pipefitter that a spool piece got its name from its similarity in shape to the spool that sewing thread is wound onto. He taught me many useful things so I don't question him on it.

A spool piece is a usually short piece of pipe, with flanges on it or sometimes unions, that is placed near the connection to an unit which has piping connected to it. The purposes of this is several. It permits the other piping to be welded usually in a shop and then transported to the unit and installed. The spool piece is then measured and fabricated. It is done this way to avoid having to handle a long piece of pipe at the connection to the unit. It is installed to fill the gap in the piping. This short spool also facilitates the isolation of the feed or outlet from the source/down stream discharge. This is critical when the isolation is being done for confined space entry into the unit. Removal of the spool piece and installing blind flanges to the mating flanges makes a definite disconnection of the other piping from the unit.

Flanged spool pieces also facilitate the insertion of a pipe blind to block the flow of materials supplying the unit. Among others this could include natural gas, steam, water, cooling mixtures, heat transfer liquids, chemicals, etc. all things that have to be disconnected before work is done on the unit. Would you want to be working on the cooling/heating jacket of a reactor when someone suddenly turned the steam on to it?

In your case the spool piece is placed after the main supply valve but close to the natural gas supply. The valve is closed, the spool piece removed, a blind flange installed on the supply side flange, bolts tightened and you have a definite shut off/disconnect of the gas supply. If the supply valve should fail or be erroneously opened no gas will flow into your piping system. If people are working on the piping or the lines have been opened this prevents a potential deadly explosion.

There are many other uses for spool pieces but this is the reason for its installation in your gas piping. An analogy could be a garden hose can't flow water if you disconnect the hose and cap the valve that was supplying the hose.

Good Luck, Old Salt

Looking at the picture I saw that the gas pipe is an underground pipe - the length is not mentioned in the drawing. But as steel pipes are normally used a long pipe might have several cm of length difference due to the annual changes in the ground temperature. So it is common to have a "u" shaped length in the pipe to compensate for this temperature related pipe length. If the changes are relative small - due to a limited pipe length this can also be a pipe spool - especially when the space for he installation is limited or - in case of special agressive gases the installer tried to reduce the number of welds in the pipe. Aoother reason is the in certain cases the vibrations of e.g. compressors should be kept away from the rest of the pipe network - otherwise the machine hum would we hearable everywhere in the installation. (Some times you can not use flexible piping due to pressure / pipe diameter)