NFPA 24 and Thrust Block Design

[CPENG78]

Hi Everyone,
In recent design of thrust blocks for a fireline installation, I came across values reported on the NFPA 24 code which raised a few questions. More specifically the NFPA 24 2002 edition.

In design of thrust blocks, NFPA 24-02 calls out the thrust equation under figure A.10.8.2(a) based on the applicable variables. There is also Table A.10.8.2(a) which reports thrust values for pipe bends based on pipe size, pipe angle bend, and 100 psi pressure. Under the table there are instructions that if higher pressures are required, then to just multiply the values by the factor of your desired pressure to the 100 psi value. That is, since the table is based on 100 psi, the value for 200 psi, would jsut be double of the value reported on the table.

With that said, during the calculations for thrust block design I noticed that the value reported on the table is higher that what I would expect to get based on using the equation described in the same code. Almost like there is some factor of safety embedded which the code table doesn't call out. Has anyone come across this?

Furthermore, the immediate section after the equation and table section, shows the calculation for block sizing based on the allowable soil bearing and the desired factor of safety under figure A.10.8.2(c). So I wouldn't expect to have a factor of safety embedded in the table as the "factor of safety" discussion comes at a later section. By the way, the facor of safety that seems to be embedded in the table is not constant, it fluctuates between 1.3 and 1.5 (at least for the values I checked).

Has anyone come across this before? I was looking to not use a factor of safety as the geotech already has a factor of safety in the allowable bearing of the soil (the FS in the geotech's report exceeds the 1.5 FS generally used in fire line design thurst restraint). Would this be a correct approach?

This has come up as an issue now, as in previous projects I had designed to higher thrust values than those reported on the table via requested larger factors of safety. Your imput is greatly appreciated.

 

[rconner]

I noticed your inquiry was as yet unanswered. When you do your calculation, are you using the actual outside diameter of the ductile iron or pvc piping to calculate the pressure/thrust cross-sectional area(which is larger than nominal and normally used in e.g. AWWA or DIPRA thrust calculations, due to the hydraulic cylinder or piston-like loading on a rubber-gasketed joint with a gasket carrried in the socket)?
I don't know the answers to your questions about the NFPA code nor also how you know you don't need a "factor of safety" e.g. concerning horizontal bearing strength (that I believe is also normally applied as per AWWA and DIPRA).

 

[CPENG78]

Rconner,
Thank you for your input. Can you elaborate as to why the outside diameter is used as opposed to the nominal diameter? While you would have a hydraulic cylinder, I would expect the total magnitue of the force to be created by the water within the pipe (the cylinder of water) or water restricted to the inside diameter.

 

[rconner]

Uhmm... Thrust blocks are of course absolutely required in pressure pipelines when one is using unrestrained push-on or mechanical joint e.g. bends, plugs or caps etc. For perhaps easiest or most simplistic understanding of what I am about to describe, lets just say we’re trying to determine how big a force must be blocked at a plug or valve etc. location (though I believe the same principle would apply for other unrestrained joint thrust foci, at least with similar design joints). I guess the first principle to understand is what has sometimes been described as the “second law” of Blaise Pascal, i.e. internal pressure acts normal or perpendicular to submerged surfaces. Then, it may be easier to see (maybe even from my cave man scribbling partial cross-sectional view attached) that not only is there an inner pressure/thrust area defined by the internal diameter of the pipe and any lining itself, but the pressure is also applied on, and applying additional force tending to separate (the joint) and to the block, in the annular (ring or donut-like) area defined by the radial pipe/plug spigot and lining end thickness, outside the internal diameter or ”I.D.” of the piping. In other words the outside of the plug spigot is like the outside of a hydraulic "ram", with a packing seal on that outside ram surface, and the pressure area (or load applied) by the "ram" is defined by the O.D., not I.D. of that cylinder.
[It is even true that the water pressure gets even further out into the joint beyond the outside diameter or “O.D.”, e.g. applied to the back of the gasket in the gasket groove; however, the thrust force on the gasket with ductile iron joint designs is typically not exerted on the block but is instead carried internally by friction and/or the outer buttress of the gasket groove (if the pressure were to get high enough), and then resisted by very slight axial metal stress in the ductile iron bell immediately outside the gasket groove.

 

[bimr]

Simple answer, the bell section has a larger ID. The OD of the pipe is used because the ID of the pipe where the bell is located is enlarged to about the pipe OD.

Review the diagram of the pipe joint that shows the construction of a bell joint.

 

[CPENG78]

Rconner / Bimr,
Thank you for your input. It certainly has cleared up my questions.

Along the same subject, while these calculations address the pressure load on pvc piping or any piping having a larger bell and installation scenario, it would appear that in the case of flanged DIP fittings, the thrust would be calculated strictly by the inner diameter. Eventually transferring the load from the fluid, to the flanges-bolt system to the outer surface of the pipe via skin friction against the soil. Would these be a fair approach and maybe even simplified of looking at flanged DIP loads?

 

[rconner]

It is perhaps easier to build a case for using I.D. area in the case involving load (or for that matter stress and deformation) calculations and self-restrained piping such as flanged; however, it should be remembered that in many systems flexible couplings are at locations still employed (including some that seal on O.D.) Also, it is common for flanged piping to at some point transition to joints that seal on the outside of the pipe, and of course at that point there is force exerted on the annular pipe wall area outside the “I.D.” In the latter regard, let’s just say one passed a pipe from the ground and from a buried push-on joint (system) through a concrete wall, using a wall sleeve device that does not restrain thrust(such as a puddle flange or “Link-Seal”?). Inside the structure or vault, let’s say there is a flanged valve or 90 bend etc. that someone for whatever reason wants to externally anchor or buttress. In that case, the required anchor force at the flanged 90 or valve location would appear to be basically as large as that defined by the O.D., even though the piping etc. inside the vault is flanged, with part of that axial thrust exerted on the inner valve gate area and part exerted in the same direction on the end of the pipe in a buried joint as previosuly depicted perhaps just a few feet away. When folks making assumption suggestions really don’t have much control over what actually goes where in various designs and in the field (and alas that most often includes me), I guess it thus has been at least a little safer or more conservative in general to use O.D. area.

 

[jgailla]

If you're worried about the difference between I.D. and O.D. on water distribution pipes for restraint design, you are splitting hairs a little too much. How do you know what the actual bearing capacity of the soil is to within 10% of what the soil will allow across the whole project?
Thrust blocks are generally unnecessary for standard water distribution projects. Many municipalities will not allow thrust blocks in favor of pipe restraints for good reason. Restraints are cheaper, easier to install, and less subject to future changes in the built environment.

 

[CPENG78]

Rconner,
Thank you once again for your insight and I agree, an entire system is composed of different subsystems that may or may not have the flexible couplings. Some areas may be based to the ID calculation and others to the OD, and thereby the OD method provides the more conservative calculation.

Jgailla,
I agree completely with your comment between ID/OD and soil parameters, but my initial post as to the difference in the numbers had to do with how I was carrying out the calculations in comparison to values reported in the NFPA 24-02 code. Not that I was worried about a certain number of lbs but rather how the code arrived at the reported value. I also mentioned that this had not been an issue before because I had to design for higher factors of safety. With all due respect, I would also not venture into making a generalization as to what water distribution projects and their need for thrust blocks. Depending on which part of the US or the world for that matter, everyone has different criteria. As in my case, almost all water projects that I have been involved with required thrust blocks. In fact various municipalities and water districts have standard thrust block sizing schedules as part of their standard details. On a related topic and part of my original question, since it appears that your background is in geotech, have you been involved in projects that the factor of safety for the bearing capacity of the soil exceeds the 1.5 threshold reported by NFPA 24 and therefore only the geotech's factor of safety is utilized in determining thrust restraint calculations?

Thank you everyone for your help

 

[jgailla]

CPENG,
Short answer, yes, the factor of safety reported by the geotech is the only F.S. required to comply with NFPA and other regulations when determining thrust restraint sizing. This F.S. in all cases I am aware of exceeds 2 at the least.

I apologize for misreading your post. I am a fairly young engineer who has never designed thrust blocks. Everything I have read and experienced on the subject indicates that pipe restraints are a much better way than thrust blocks to resolve the forces associated with small diameter ( 4" to 20") pressurized pipelines for water distribution and forced main sewage services. I do not doubt that many municipalities still require thrust block restraint. I only argue that it is outdated.

I have worked in geotech and civil positions. It is my understanding, and my practice, that there is no additional factor of safety that is applied to a civil engineer's calculations for restraint based on a geotech's given bearing capacity, as that factor of safety is already included in the geotech report. I do not have a copy of NFPA 24 available presently, but it is my recollection that the safety factor therein discussed is a broad safety factor, that is, applying to the whole system so that the geotech's safety factor is sufficient without further reduction by the civil engineer designing the restraint.

More to your question, no geotech recommendations I have seen have a F.S. less than 2 due to the inherent variability of soil conditions. However, most reports report bearing capacity as a function of the allowable settlement of a structure and not the capacity of the near surface soil to withstand thrust forces due to pipelines. Thus we are usually comparing apples with oranges in the calculations. This post is already too long, but if you wish to pursue it I can run my mouth (or keyboard as it is) further if you are interested.