Piping Hanger Support Rigidity vs. Fatigue Life

[GrimesFrank]

First time post so forgive if not complete.
Standard Grinnell stock we have (specifically weldless eyenuts [Fig 290] & pipe clamps [Fig 212]) when put together have no gap (Dim C = 1/2" = Dim D). A colleague suggested this is undesirable. If the piping has vibrations the rigidity of this joint will lower its fatigue life considerably. My stance is Grinnell (the godfather of supports) most likely gave an inherent bit of play between the components. I looked up the parts in the Bergen-Paterson catalogue (Fig 279 & 175 respective) and they have a gap (Dim C = 1/2" > 3/8" = Dim D). Can I get your opinions about the situation?

 

[GrimesFrank]

C'mon guys are there no opinions on this post? The key is there is no lateral flexibility for this joint. That would mean to me any vibratory energy is being absorbed by the hanger rod.
Does anyone see this as a problem? I would think not since this has been Grinnell design since at least the 70's.

 

[unclesyd]

I don't have any specifics as to which hangers but it can be a problem.

It is a problem in lines that are highly flexible and are moved around a lot when you have something like hammering.

I've ween several cases where pumps and valves have been changed without any consideration of the consequences.
We also had problems on feed lines to centrifuges due to machine induced vibration and the intermittent flow of material to the machine.

This problem occurred mostly on lines 4" and smaller. I can only recall a couple of problems on larger lines like steam and condensate that were in highly cyclic service. One in particular was with 5 large evaporators using 300# steam operating 24/7/365 and each cycling ever 20 minutes or so.

Anecdotal:
One 3" SS line that started to give problems with broken hanger rods was modified by the area mechanics. They replaced the rods with chains. When I asked about this they stated that the process people would not change the valve that was causing the problem so they let the line swing.
With the aforementioned Steam line the problem was mediated by not using hard anchors for the hanger rods. As a stop gap measure I replaced the double nuts with a swivel washer and nut. This system has been in service many years.

 

[GrimesFrank]

Thanks unclesyd my vibes are coming from a PD pump, piping is 2 NPS. Don't get me started with the problems those things provide. Thats a forum to itself.
I am not familiar with swivel washers, but inferring from your talk, I assume they must allow lateral swinging of the rod at the beam attachment. Is it conical? I googled it and got limited hits. Do you have a supplier as it sounds interesting. To add to the fun though I'm dealing with ASME III NF, field hangers.

 

[unclesyd]

If I would use the correct nomenclature it would have been no problem. A quick call to yee old tool room got this result. Look at both the washers and the flanged nut.

One thing that troubles me is with a PD pump you will have a much higher frequency that what I was addressing.

Do you have a pulsation dampener in your system?

http://www.northwesterntools.com/wash_sph.html

 

[GrimesFrank]

Higher frequency? Affirmative.
Dampeners? Certainly, bladder accumulators, I can only imagine what the thing would be like without them. Like I said, a whole forum could be started on the mis-design of this system. We actually have our operators trained to recognize "Vibe Normal" & "Vibe Panic" mode.

In fact our techs were hanging the rod with a nut above and below steel until I noticed......that the rods were being bent and fracturing at the steel. I think the added flexibility provided by this washer/nut assembly is interesting.