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Pipe supports as we stated in Part 1 (of Pipe Supports) is a much more complex subject than the term would first suggest.  We also want to make it clear that there are many ways that errors can be made when designing or selecting pipe supports this includes the various secondary pipe supports.

In Part – 1, we saw a chart that described some of the many different types of secondary pipe support devices.  In this, Part – 2 of Pipe Supports we are going to focus on specific data required to properly size, qualify and select a support.  To do this we will look at one specific device.  The specific device we will focus on is the Hanger Rod. 

 

You will remember that in Part – 1 we said there are three basic types of Hanger Rod support devices: (type 1) beam-to-pipe, (type 2) pipe-to-pipe and (type 3) beam-to-beam (or trapeze).  They all have three major components, a top connection component, middle or connector component and a bottom component.  For the type 1 Hanger the top component normally connects to a structural beam.  The connector component is normally steel rod.  The bottom component is normally a pipe clamp.  We also said that the danger with the design of these items is in the lack of knowledge of some of the people doing the design.  They do not know how to calculate all the actual dead and live loading that the Hanger will support.  Then they choose the wrong type or strength of component for the intended load.  

In order to bring attention to some of the potential problems lets take a hypothetical piping configuration and plant situation for study.  We will look at two cases. We will use the same configuration with different conditions for each case.

Case #1

Let’s take the following as an example scenario for the basis for our discussion.

  • The project is a process plant in a multi-story structure
  • The line is 12”, standard weight carbon steel pipe located in a lower level of the structure
  • The line will carry a process liquid with a specific gravity of .85
  • The line is subject to hydrotest
  • The line is not insulated
  • The piping travels horizontal north in a well supported manner, then after crossing the last normal pipe support (support ‘a’) it travels 40 feet, then drops down (3’-0”) and turns east (right) with two elbows (fitting-to-fitting) and travels another 40 feet to the next normal support (support ‘b’).
  • There are no additional horizontal support beams available at or near the turn point and at the exact piping elevation.
  • The closest steel available as a possible support point is 24” deep major equipment support beam located 6’-0” (top-of-pipe to bottom-of-beam) above the pipe and 4’-0” from the pipe drop.

 

It is logical and factual that structural support ‘a’ will carry one half of the pipe load of the north-south run.  And the structural support ‘b’ will carry half pipe load of the east-west run.  However, the L-shaped “dog-leg” in this scenario is obviously excessively overspanned and the pipe will be over stressed.  The piping designer must provide some type of additional support at or near the corner.  Because of the availability of the overhead beam a hanger rod is chosen as the best possible and most economical method of support for the pipe.

 

We must now look at the factors so we can choose the correct Hanger Rod assembly.  The factors include all the weight to be supported.

The component weights are as follows:

  • 20’-0” of pipe in the north-south run (1/2 the 40’ run)
  • 20’-0” of pipe in the east-west run (1/2 the 40’ run)
  • Two 90 degree elbows
  • 43 lineal feet of hydrotest water in the 12” Standard Weight pipe

 

With this information the next step is a simple look-up of the correct data.

 

Case #1-12” Standard Weight, Carbon Steel Pipe

Pipe Weight Fitting Weight Insulation Weight Water Weight Total Weight
1984 lbs. 246 lbs. 0 2107 lbs. 4337 lbs.



We now have what we need to select a hanger rod assembly to support our pipe. There are two ways that this can be done.  First, the designer can use the “pick-and-choose” or “do-it-your-self ‘ method.  This is the process of picking up a hanger parts catalog and then selects each individual piece and part.  The hope is that the designer knows what they are actually doing.  

The second method is that we select from a pre-packaged Hanger Rod assembly that fits our need.  One that comes complete with all the proper and matched pieces and parts.  The term “pre-packaged hanger assembly” also means that the assembly has been “tag named,” has been pre-designed, pre-engineered, pre-qualified and fully documented including the related needs for the applicable computer aided design system, material procurement and installation.  

The assembly we need for our “Case #1 includes the following:

(All components and load data are taken from “PTP” Piping Technology and Products online catalog, see www . pipingtech . com)

Load Capacity*

  • Figure 110, Eye Rod (Welded), Size 1” 4960 lbs.
  • Figure 20, Welded Beam Attachment, Size #8 (for 1” Rod) 4900 lbs.
  • Figure 40, Weldless Eye Nut, Size #2 for 1” threaded Rod 4960 lbs.
  • Figure 80, Heavy Three-Bolt Pipe Clamp, for 12” pipe 7000 lbs.
  • Beam attachment welds ¼” fillet, 2 sides 12000lbs.

* It is normal practice for components of this type to be designed with a plus 50% safety factor.  The safety factor is not to be considered as available when making a selection.

**The Beam Attachment is 3” on each side, ¼” attachment fillet weld 1” long is rated @ 2000 lbs. Per inch.

We now compare our pipe weights against the Hanger Rod load capacity data and see that (not using any of the safety factor) the Hanger’ weakest link is the Welded Beam Attachment (4900 lbs.) but it is more than enough for our piping needs (4337 lbs.).  

If we were using the “pick-and-choose” method then the designer must indicate the hanger in the design then identify each and every piece and part.  The detailed part identification is required for proper procurement and installation.  

If we use the “pre-package” method the designer is only required to indicate the hanger and the item name or tag number (example: HR-1-12”.)  All the procurement and installation details are included in the hanger documentation.  

Now Case #2

Later someone else has a similar problem.  They had seen what was done by another designer with the Case #1 problem and decided they would just copy it and callout for the same Hanger Rod Assembly.  Why not?  They too had a 12” line.  They had the same configuration.  And, they also had the same span distances.  No problem, right?  However, all things were in fact not the same.

So what was different?

Case #2

  • The project is also a process plant in a multi-story structure
  • The line is 12”, Schedule 160 carbon steel pipe located in a lower level of the structure
  • The line will carry a process liquid with a specific gravity of .85
  • The line is subject to hydrotest
  • The line is insulated with 3” of Calcium Silicate
  • The piping travels horizontal north in a well supported manner, then after crossing the last normal pipe support (support ‘a’) it travels 40 feet, then drops down (3’-0”) and turns east (right) with two elbows (fitting-to-fitting) and travels another 40 feet to the next normal support (support ‘b’).
  • There are no additional horizontal support beams available at or near the turn point and at the exact piping elevation.
  • The closest steel available as a possible support point is 24” deep major equipment support beam located 6’-0” (top-of-pipe to bottom-of-beam) above the pipe and 4’-0” from the pipe drop.

 

With this information we look-up of the correct data.

Case #2, 12” Schedule 160, Carbon Steel Pipe

Pipe Weight Fitting Weight Insulation Weight Water Weight Total Weight
6412 lbs. 794 lbs. 528 1462 lbs. 9196 lbs.

 

We see here that the total load to be actually carried by the Case #2 hanger is more than twice the safe capacity any of the components included in the original Hanger Rod.  This will not work!  This is an example of the type of errors that result when there is a lack of thinking or laziness on the part of the piping designer.

 

All of the items identified, as Secondary Pipe Support Systems are subject to this same kind of miss-design and miss-use.  It is incumbent on the piping designer to become trained and knowledgeable about these issues.  


Having identified the need for the hanger in the case study above and selected the correct hanger is not the end of the piping designers responsibility.  That hanger is carrying a load and the top of that hanger is attached to a steel beam.  The load is being transferred to that beam.  That hanger and the pipe it is carrying is an abnormal load added to that beam.  It is a load that the structural engineer would not normally be aware of.  It is the piping designer’s responsibility to document that loading and advise the proper member of the structural engineering group.  That beam may be a very large beam and is at or very near it’s safe design limit.  You might think “Oh it is okay, it can carry my pipe” However, you are not a structural engineer and this is not your decision to make.  Whenever an abnormal piping load is added to a structural beam (steel or concrete) the structural group must be advised. 


About the Author

Jop

James O. Pennock has more than forty-five years in the process plant design profession. He has been involved in both home office and job site assignments on refinery, chemical, petrochemical, power and other projects. His experience ranges from entry level designer to engineering manager. Much of this was with Fluor. He is also the author of the book "Piping Engineering Leadership for Process Plant Projects." He is now retired, living in Florida, USA and does only occasional consulting work.

Mr. Pennock can be contacted via E-Mail at jopennock@netscape.net.

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