Hello again.

More questions about pipe supports

1. Heard of a wear pad and reinforcing pad. I could have information on the difference of the two, but if the trunnion fails and we have an option to put a reinforcing pad, could wear pad provide the same strength that of the reinforcing pad? If you could have more and very good explanation on the difference of the two, please provide me such. Thanks!

2. Does CAESAR II plan to include analysis of the pipe supports? I am looking forward that even this program could check its strength and suitability in the near future (e.g. we can graphically view pipe shoe, pads, u-bolt, etc)

Hello NYX

In piping design the term "reinforcing pad" is generally used in reference to a ring of additional metal that is welded to the pipe in the immediate area of a fabricated branch connection with the purpose of providing additional reinforcement of the pipe to compensate for the material removed in creating the branch connection. This reinforcement is added to accommodate the internal pressure in the run and branch pipe at the fabricated branch connection. I would suggest that you review B31.3, paragraph 304.3.3.

A wear pad (or sometimes "wear plate" or "doubler plate") is a section of metal (it could be circular but more typically is rectangular) that is curved to conform to the outside diameter of the pipe and is welded to the pipe to provide additional wall thickness local to a point of support (i.e., away from a branch connection). The purpose of this wear plate is to provide a wear surface at a point where a pipe is supported directly upon a section of structural steel or upon a concrete "sleeper" (without the benefit of a "shoe"). If a pipe moves in response to thermal expansion and contraction (or other reasons) there will be a constant wearing away of the pipe wall at the point of contact with the support. Also, in many cases the point of contact between the pipe wall and the supporting member is a point of greater corrosion. The wear plate provides additional (additional to the thickness required for internal pressure) thickness of metal at a specific point to compensate for the loss of material due to wear and corrosion.

It is not clear to me exactly what you mean by "but if the trunnion fails". However, it is sometimes beneficial to provide a "doubler plate" at a location where a supporting trunnion (used on a straight pipe or "base elbow") will be welded to the process pipe. In this case the "doubler plate" is provided to "spread" the "point loading" of the trunnion over a larger area of the process pipe. The "doubler plate" is welded to the process pipe after a "vent hole" is drilled through the doubler plate and after the plate is curved to conform to the outside diameter of the pipe. Then the trunnion is welded to the "doubler plate" after a vent hole is drilled through the trunnion wall. The additional effective wall thickness provided by the "doubler plate" addresses local membrane bending stresses and "punching shear" at the point where the trunnion intersects the process pipe. It may be of interest to you to read all the previous postings on this discussion board regarding the design and use of trunnion supports. You should use the "search" utility that is provided at the top of the "page". Also read the "sticky" that is provides as a guide for using the search utility.

Regards, John.

Thanks John for that information!

Anyway, in "but if the trunnion fails", different ways have been done in order for the piping system to pass yet not compromising the integrity of pipe support. And one of these ways include putting reinforcing pad. If in the standard support, there is a wear pad (doubler plate), will that be a good choice?

regards,

John / NYX,

I guess that by 'failing' the reference is to failing the criteria of a 'Kellogg' trunnion calc or similar ?

I agree with a doubler pad as described where the trunnion is vertical and loaded predominantly axially, punching shear etc will be reduced. But if the trunnion is horizontal and loads the pipe either in the longitudual or circumferential plane, a ring reinforcement and weld detail similar to that of a tee as shown in B31.3 fig 328.5.4B and detail (3) is surely a better option ? The reason being that if you weld a trunnion to a doubler plate alone (and not the parent pipe) the doubler plate has some flexibility and risks tearing away from the pipe, by stressing the back of what is basically an un-controlled fillet weld, particularly if some cyclic loading is involved.

On a slightly related topic .... what opinions are abroad regarding supporting bare pipe on a re-bar ? Good practice ... avoids crevice corrosion ? bad, local stress ? ??

NYX / MoverZ

MoverZ makes a good point regarding using the “doubler plate” on a vertically oriented pipe. When a trunnion (“dummy leg”) is attached to the vertical process pipe in concert with a doubler plate the peripheral edge welds that attach the doubler plate to the process pipe wall will be subjected to shear loadings and if the doubler plate is large, it may well “flex” under the loading. In some cases (vertical or horizontal orientations) the doubler plate is drilled through in some number of places (reasonably close to the trunnion) and “plug welds” are used in conjunction with the peripheral edge welds to increase the total weld area and to reduce the doubler plate’s tendency to “flex”. Obviously however, the better way to support a vertical process pipe when heavier loadings demand it is to use a riser clamp in concert with lugs welded to the process pipe per the requirements of Pipe Fabrication Institute Standard ES-26. In referencing PFI ES-26 one can also see details for the installation of “bat wings” for supporting vertical piping but this practice has fallen out of favor in recent years.

Dummy legs have often been attached to vertical process pipes by welding them directly to the process pipe using fillet welds. However, I would not opt for “creating” a branch connection (making a hole in the process pipe) for the purpose of being able to use the weld detail shown in B31.3 fig 328.5.4B and detail (3). Removing the process pipe wall material through-the-wall in the vertical process pipe (especially when done on opposing sides of the vertical pipe) unnecessary weakens the vertical process pipe (and the resulting area inside the “dummy legs would fill with product with unpredictable results). For heavier loadings I would be more inclined to use a short “pup piece” of vertical piping with a heavier wall thickness in concert with trunnions of greater diameter and wall thickness, such that the total area of trunnions-to-process pipe fillet weld develops the required strength. Of course when you do this you must taper bore the ends of the heavier wall “pup piece” to get proper “fit-up” alignment at the circumferential welds with the normal thickness process pipe (see B31.3, Figure 328.4.3 (b).

In the design of any of these supports, it would be wise for the piping engineer to assess the effect of cyclic temperature excursions on the construct applied to support the process pipe. Any metal extension that protrudes beyond the pipe insulation will act as a “heat sink” and this will result in some degree of "temperature lag” during heat-up and cool-down. There may be significant temperature gradients that may cause local stresses at the pipe wall. All this should be given due consideration.

The issue of using the Kellogg Standard for the design of trunnions in all their guises (dummy legs, base elbows, etc.) was the topic of discussion here a “few months” ago and CraigB reported that his evaluation of the design methodology determined that it left many unanswered questions. Clearly, the better (most expedient) way to design such constructs might be to use the Kellogg methodology to “get you in the ballpark” but then use Pipe/FE to check the results. I suspect that if you do not have a copy of the Kellogg Standard, you could look into the CAESAR II discussion board archive to see who it was who offered to provide the Standard via email. Or perhaps one of the mentors here could be asked to provide the Standard. But again, use it with some healthy degree of skepticism,

The use of embedded bars at the top surface of concrete “sleepers” to provide a sliding surface with a minimum contact “patch” (thereby reducing friction) is an age-old practice in the industry. CRSI reinforcing bars however, are not the way to go as they purposely have rather rough surfaces to assure that they “lock into” the hardening concrete in their normal application. These sharp deformations will cause more friction and may actually facilitate additional wear (with expansion / contraction movement) at the OD of the pipe being supported. Embedded small-bore pipe (smooth surfaces) would be a better choice for contact with the process pipe. All of this is NOT to say that any of these practices will avoid crevice corrosion at the pressure boundary (pipe wall). There is no good substitute for providing a proper pipe shoe, especially if wear and corrosion are design issues. There has been an industry practice of running “off site” (outside battery limits) piping between units at low levels where they may occasionally be flooded by slowly draining ground water. This is a formula for creating impossible maintenance issues centered around crevice corrosion. It is bad enough to have occasionally wetted surfaces (due to normal precipitation) in contact with each other, but if these surfaces are flooded, the corrosion problems will be insurmountable. At the very least, wear plates (doubler plates) should be provided locally at the contact point (area) to “buy some time”.

Regards, John.

John,

Thanks for the considered response.

Trunnions.
I should have been more clear about weld attachment .... I would never cut a hole in the pipe at a trunnion, rather I'd apply the weld detail shown .... i.e. a weld joining the parent pipe, ring and trunnion at the trunnion / pipe interface.

Re-bar ... round bar ... I have been surprised to find one large oil co still demands these (round bars) at all support points, often without a wear plate or shoe. I reckon it's poor practice.

Cheers

Hi MoverZ,

Well, if you weld the pad and the trunnion to the pipe you will still be left with using fillet welds but that is not a bad thing.

I think a lot of old "company standards" have never benefited from getting "feedback from lesson learned" in trying to maintain these systems. The design firms (and constructors) are of course required to do it the "standard way" but it end up with the plant maintenance folks bearing the burden and trying to find it in their maintenance budgets to remedy a multitude of "bad practice" sins.

It has been an interesting discussion. Thank you for your participation.

Regards, John.

One of the solution been practiced was to put a wraper plate (360 Deg RF pad) at the stanchion location and take a creadit of its half thickness in performing the stress eveluations at run pipe to stanchion junction.

Many times trunnions or dummy legs are attached to elbows or tees. An option I often use when the loadings create excessive local stresses is to specify a heavier wall elbow or tee in lieu of a re-pad. Of course the ends will need to be taper bored to match the pipe.

First, thicker is better for trunnions on elbows.... the ratios listed in the online help for the trunnion elbow calc ala Stone & Webster are good guidance.

Taper boring an elbow can be done, but what is easier perhaps is a short pup piece of tapered bore straight pipe welded to the elbow tangents it does cost you two more welds but machining is less problematic.

I strongly suggest that at least two NPD straight run lengths to the next valve or flange are preffered (though not a B31 code requirement). Markl found elbow ovalization extends quite a distance from the elbows tangent line.

To reinforce the header I prefer simply using a taper bored heavier wall pup piece in lieu of welded pads or collars, the problem with collars and pads is there is always the question of are they attached to the base metal enough to really act as a homogenous section.

Based on what I have seen over the years I will not use reinforcing pads, trunnions or dummy legs on high temperature lines and especially not on elbows. High temperature can be defined here as over the creep limit. Items simpy fail. Last item I saw was stainless steel reinforcing pad. It came off cleanly but resulted in an emergency shut down of a process plant. Possibly not a creep issue but most probably combination of poor welding and different temperature/heating rate between the pad and pipe.

I have seen very little problems with raiser clamps locked into position with shear lugs or with moderate increase in WTH. The trick is to look into item heating up rates.

These pad and trunion problems have given me plenty work over the years so maybe I should actually recommend designers to use them :-)

HI NYX,

Yes wear pad and RF pad are different. The former is more of to save pipe external from corrosion and erosion due to regular sliding. The latter is for pressure integrity of branch connection.
I suppose you are using some excel sheet (kellogs formula) to calculate trunnion stresses when you say "Trunnion failed". My priority of solution would be as follow:

1. Increase trunnion pipe diameter (The max you can go is equal to pipe size)
2. Increase trunnion pipe schedule (If may becomes out of spec item due to increased schedule, may be difficult for procurement to buy a small spool)
3. Put RF pad (When on elbow/ bends, Its a real nightmare to construction guys with artistic profiles, also it may increase pad pressure testing, NDT and heat treatment on site as per material spec)

Regards,

You might want to check the date questions were asked before responding. The intended recipient is not going to receive them, and it makes it harder to track newer questions.

Thanks Michael_Fletcher for bringing this to attention. I never realized the date thing. But I suppose this subject still has a lot of potential to be discussed and learnt.