supporting vertical pipes 2

[alpipe]

If asked to support vertical pipes of the order 3" to 6" with a horizontal run before and after were would you initially expect to place the pipe supports and guides. What are the major do's and dont's. Lets assume the pipes are inside the building, are non critical but expansion needs to be addressed say if one support will force the pipe to lift of another. drop of 40 feet.
cheers
Al

 

[JohnBreen]

Hi Al,

Yours is a very good question.

Designing vertical piping or “risers” (long sections of piping that run in the vertical direction) can be challenging. The weight of the pipe and its contents can represent a significant restraint against expansion/contraction.

In power plant piping, if we have risers on very hot piping, we try to place a (vertically) rigid rod-hanger at a point on the pipe that is at the same elevation as some terminal point (e.g., the turbine nozzle on a Main Steam pipe). This would reduce the moment on the nozzle due to expansion of the pipe. That may not work out if the riser is long because it is also important to make the riser “stable” by placing the (only) rigid hanger above the CG of the riser (make the riser a pendulum). The location of the rigid riser hanger must be chosen so that the rigid hanger will always have a positive (down) gravity load on it throughout the expansion/contraction range of the system (and with adjacent spring type hangers acting). So, sometimes the elevation of the rigid riser hanger is a compromise – and usually other spring hangers will be located elsewhere on the riser (various elevations). But wherever the rigid riser support is located. The pipe located below it will expand down and the pipe located above it will expand up – and the loading on the rigid hanger must, by design, always be down.

In buildings where the service piping is NOT a very high temperature, some of the same considerations apply. By the way I recommend that you make reference to the ASME Code for Pressure Piping, B31.9, Building Services Piping.

In building services piping, the pipes usually go through fairly evenly spaces structural floors and theses can be used as guides (effectively reducing the “slenderness ratio” of the pipe as a structural column) and supports. In these systems the riser may begin in the building basement where a horizontal run of pipe turns to run in a vertical orientation. Intermittently located pipe clamps that bear directly upon the upper surface of a floor as the pipe penetrates that floor usually support these systems. These clamps are specially designed for this application and have rather longish “ears” that bear on the structural floor. Go to the Anvill – Grinnell web site at:

http://www.anvilint.com/

Click on pipe hanger catalog and download the pdf file. Then open the catalog and go to page 32 to see riser clamps, figure 261. If you locate your rigid riser support 2/3 of the way up the riser the expansion/contraction will be in the directions away from the clamp (up and down) and you will have to have enough horizontal pipe before the first rigid rod hanger (top and bottom) to accommodate the vertical pipe movement. Note that if the pipe and contents are heavy it may be necessary to weld lugs to the pipe above the riser clamp so that it will no slide through the clamp (no not depend upon friction).

For some semi-high-rise buildings this can be complicated. If your first riser support is located, say on the 3rd floor, the piping weight of the pipe (above it and below it) will be supported by the structural floor that the support is bearing upon. The pipe will expand (with an increase of temperature) away from that point – up above the 3rd floor and down into the basement.

If the design temperature of the piping system is significant (and the vertical length is also significant) you will have to locate horizontal “offsets” (expansion loops) intermittently, as the pipe continues vertically up through the building floors. This may also be done by offsetting the piping every so many floors so that it is not all on the same vertical centerline. If this is done, the pipe may go through, say the first five floors, then at the fifth floor turn to the horizontal (via the installation of welding elbows) for a foot or so and turn to the vertical and go up through the next five floors. At the tenth floor the pipe would turn back to the horizontal for a foot or so to locate it directly above the first vertical run. The elbows in these intermittent horizontal “jogs” would provide the flexibility to accommodate the expansion/contraction. Riser clamps would be located just below the offsets, bearing on the structural floor (i.e., there would be a support associated with every offset). These offsets and their associated riser clamps would have to be located (spaced between the right number of floors) such that the weight of the pipe above the clamp was always enough to assure that the expansion would be forced into the horizontal offsets and there would be no “lifting-off” tendency at the riser clamp above (i.e. the weight acts as a restraint). Note that the weight of the CONTENTS will only be transferred to the pipe where there is a turn to the horizontal. At the extreme top of the riser, you would have to have enough horizontal pipe length before the next rigid rod hanger to accommodate the last “up” bit of expansion/contraction. It would be prudent to design such a piping system with the help of a good piping structural analysis (flexibility) program.

I feel sure that I have forgotten something here and so I am counting upon our other piping engineers in the community to add to and correct all this.

Best regards, John.

 

[alpipe]

Thanks John
Invaluable help.
Cheers
Al

 

[sprintcar]

Great post John...
IF only you could teach all these 'consulting engineer' firms how to do it the right way! Keep the wheels on the ground
Bob
showshine@aol.com