Piping Support: Am I being unnecessarily cautious? 7

[KootK]

From time to time, I get asked to design the piping support for various mechanical and plumbing runs/stacks within the structures that I design. I mostly do regular buildings, not oil refinerys etc.

The very first time that this happened, I went out and procured a couple of good books on pipe support engineering. It turns out that it can get quite complicated: dynamic loads, vibration criteria, lateral loads... Apparently, there are specialized engineers that do piping and piping support design almost exclusively. Of course, the books that I have ARE tailored to serious industrial designs.

Now, whenever I get roped into designing piping supports, I say "I'll design the supports but I won't specify the loads. Those need to come from your mechanical consultant". I say this because, knowing how complex piping support can get, I don't feel comfortable making the call on the loads. Often, I don't even know which pipes are pressurized etc.

Invariably, I make this pronouncement at project meetings where I get accused of being uncooperative/nuts. When someone finally does specify some loads for me, they're always just straight up pipe wet weights (weight of pipe + weight of standing water) which I could have generated myself.

Am I being unnecessarily cautious with this stuff? How do others handle pipe support design? Do you work out the loads yourself? Is it the case that, for typical buildings, pipe support really is as simple as "wet weights"?

 

[apsix]

I don't do 'serious' piping support either and I use "wet weights" as well as wind, seismic and bend forces if applicable.

 

[KootK]

Are these bend forces supplied to you or do you work them out yourself?

 

[JoshPlumSE]

There is a good reason why the petrochem world has specialty engineers working on piping systems. The pipes experience temperature changes in the hundreds of degrees.

It becomes a significant engineering task designing the system to be able to withstand these thermal forces and deflections.

99.9% of this stuff would not really apply to building systems. Seriously, what is the normal temperature difference between a hot and cold line of water. 50 degrees Farenheight vs. 150 degrees.... And, that's in a system of relatively small diameter pipes with plenty of deflection absorbing bends in them.

I would design for seismic loading for them or at least a minimum lateral load (for non seismic areas). But, those shouldn't be particularlyl complicated to come up with.

 

[ajose]

Koot,

You are right, i have work with supports analisys on several stages.

I'm agree with you there are several steps when you talk about piping support. Let me roughly make some example based on my experience.

The normal level, the one used in residencial ones it is enough work with the wet weight and the seismic and other effects, that´s fair enough.

When pressure beging to grow like water distribution (aduction to populate areas) where in a change of direction you need to calculate anchors to void loose the pipe. In this level people begin to care about support distribution and longtudinal stresses.

And the top piping engineering where besides high pressures are big temperatures, high flow speed, vibration and some time big sizes. They really work with the stresses on pipes, flanges on pumps equipments etc. They have maximun stresses for severeal cases on several places (pipes and equipment). In this level each pipe is analized, in the analisys the direction of supports is one of the input needed (change of direction is change of stresses). It is not unusual that the minimun safe distance betwen two points have one or more loop in the middle to minimize axial stresses.

My experience for building in the normal range of temperature, size and pressure there are not big problems on design.


Ajose

 

[oneintheeye]

I have worked on pipe supports for industrial works. the pressure forces and expansion forces can be higher than wind and live. On water pipes (gravity) we design loads ourselves, on high pressure pipes with bends the mechanical/process guys do it. The forces generated are sometimes way higher than I would imagine. Expansion joints position play a part, sliding supports or fixed it gets prety complicated.

 

[BENDOG]

Another thing you might want to consider is that if there is un-used space on the rack, someone is going to use it in the future. For gravity loads I might factor the loads up depending on how much un-used space there is on the rack.

 

[paddingtongreen]

Send the client a letter stating the criteria that you will use, and that the mechanical consultant must supply any other loads, such as, but not limited to, surge, hammer and expansion.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[ToadJones]

Koot-
I have run into the same thing in working in the power industry. We'd be asked to design pipe supports and given literally nothing as far as loads. It is simply unprofessional on the part of the piping engineers.
Not only should they be supplying you ALL the loads (the must have had these to size their pipe properly anyway, right?)they should be specifying support conditions as well. High pressure pipes and those that go around corners or make a 90º bend vertically can exert some pretty high forces if supported improperly. They should be specifying them supports as hangers, sliders, fixed, etc.

 

[JStephen]

I think what you're getting into is that for a lot of piping, there simply isn't any design done other than selection of the pressure rating. For example, on must municipal water systems, nobody does a stress analysis of the underground piping, so nobody can tell you what the moments are, etc.

I don't think it's necessarily unprofessional. A part of engineering experience is knowing what to analyze and what not to, and that will vary according to the industry and application. It's the same reasoning by which a doghouse or outhouse are not normally analyzed.

In refinery or industrial work, in addition to the thermal effects noted, typically the consequence of a pipe failure can be much more severe, which warrants the additional work.

 

[ToadJones]

JStephen-
In my case, industrial, power plant work is to what I am referring. Coal dust lines, steam lines etc.

 

[KootK]

Thanks for all the great feedback guys. It sounds as though, for non industrial applications, I can be a bit more accomodating. My clients will appreciate that.

Paddinton: I like the disclaimer bit. I'll work that into my general notes.

BENDOG: That's a good point that you make. Unfortunately, it fouls up the work that I'm doing this morning...

 

[JLNJ]

Whatever loads you get from the piping people, be very suspicious. I have seen many try to run pipe stress analyses yet few have proper understanding about what they are modeling - especially support conditions. Press them on anchor locations, types of restraint, and real world flexibility vs. the infinite rigid supports modeled. A good piper can keep the stresses on the pipe low AND keep the forces on the supports to something reasonable. Don't be surprised if you know more about the action of the pipe then the piping guy does. Don't agree to crazy anchor locations with absurdly high reactions.

This of course is for hot pipe. Most (ambient) pipe reactions you can and should figure out for yourself.

 

[westheimer1234]

i dont have any idea what you are asking but in petrochem industry we never put pipe loads in structural drawings..

typical loads are pipe empty weight, pipe operating weight, pipe test weight, pipe horizontal friction (30%) and pipe anchor loads that comes from piping stress department..

 

[rmw]

I don't know what seismic zone you might be in, but you should check that out regarding your location and analyze seismic loadings too.

If your handiwork is any where near where a 'big one' (or even small one for that matter) will visit, you don't want your work to come crashing down do you?

rmw