3 replies to this topic

[freeman]

Dear All,

 

How are the distance of side nozzles including manways, transmitters and etc. from Tangent Line (about a pressure vessel) determined? Is it the job of a Process Engineer. If you have any document that would help,I would be grateful.

 

Thanks

Freeman

[gegio1960]

height of level nozzles are defined by hold up (by Process).

manways / handholes are defined by Mechanical according to the requests of accessibility (and without interfering with Process, Utilities and Instrumentation nozzles).

[Art Montemayor]

Freeman:

 

This query regards side nozzles (on vertical vessels, I have to presume).  It might also be related to horizontal vessels.

 

When discussing this issue, it is necessary to have a complete understanding of how a pressure vessel is designed and fabricated – and with what type of materials and criteria.  Additionally, you can’t lump EVERY conceivable nozzle into the same, generalized, description.  Fluid entry, exit, drain, and level nozzles are one thing; Manways and instrument nozzles are another different type of application.

 

You should be aware that your pressure vessel will most often have either elliptical or torispherical (ASME F&D) heads.  Rarely are hemi-heads used.  What this leads up to is knowledge of what the “tangent line” on a vessel constitutes.  It is also important to know where the SEAM between the cylindrical section (“can”) and the heads is located.  Usually, you don’t want to do any cutting or welding within approximately 2 to 3” of the welded seams in order to avoid any added stresses due to heat buildup during the cutting and welding.  The specific distance to follow depends on the size of the nozzles / manways involved.

 

Both the ellipsoidal and torispherical heads have what is called a “straight flange”, an extension of the formed head that follows the diameter of the head and forms the seam with the cylindrical section – on both ends of the vessel.  This straight flange may vary with the size and fabricator of the heads, but I generally have seen 2 to 2-1/2 inches of straight length on most straight flanges.  From this length, you should be able to come to the realization that it is probably not a good idea to select the straight flange as the location for a nozzle.  This is compounded by the fact the “knuckle” radius of each formed head (the point where the head is formed into a cylindrical flange) is a site of very high, concentrated stress because the formed shape has been forced to change direction.  I have always used the welded cylindrical seam as my reference point to locate nozzles.  That way I keep out of harm’s way in building up more stresses.

 

As a process engineer, I always decided on the quantity, quality, and location of all pressure vessel nozzles.  I would expect the same to be practiced today.  It then becomes the responsibility of the vessel designer to either concur with the selections or challenge them.  The information and instructions that are transferred from the process engineer to the vessel designer is always started with a detailed Data Sheet of the vessel in question.

[Pilesar]

Depending on your company's division of responsibilities, nozzle location does not have to be the job of a process engineer, but process engineering covers a lot of territory. The process engineer should be able to calculate how much liquid is left in a drum when the level gauge reads zero. The process engineer also needs to know the height of the lower level for pump calculations. The process engineer should get these lower level nozzles high enough so that the vessel design can be done right the first time without rework.

 

Detailed vessel design is usually the job of a vessel designer who is familiar with all the vessel codes and has experience as his teacher. Make friends with your vessel designer and seek out his advice. He can either "do exactly what you tell him to do" or he can explain to you why your design is not the best. Good vessel designers have saved me from implementing several impractical vessel designs.

 

There are practical considerations for how low you can place side nozzles. Nozzles should not be in the heat affected zone of a weld seam. Horizontal vessels have additional mechanical constraints because the nozzles are not perpendicular to the vessel wall. There are various "rules of thumb" for the process engineer. For horizontal side nozzles on a horizontal drum, I use the formula [D/10 + 5 inches, rounded up to the next whole inch] for the height of the lower level nozzle. For side nozzles on a vertical drum, I use 12 inches above the bottom tangent for the lower level nozzle. Check vessel design references (perhaps Mark's Handbook?) for alternate guidelines.