Code Velocity Limits Plastic Pipe

[stanier]

Please advise what Code limits there are for the use of plastic pipe materials in building services. I am not looking for generalisations or manufacturers ideas but hard and fast clauses that may appear in a specification.

The situation I have is where energy costs a fraction of what it does in the USA and therefore the NPV of Capex and Opex is different. So generalisations of maximum velocites are meaningless.

 

[749KDV]

Stainer,

You will not find a hard and fast number for maximum velocity in a Code, especially in your case where local plumbing codes and Manufacturer's standards are applicable. With that being said, I suggest that you design your pipe for the maximum velocity that it can withstand based on the limiting factor for plastic pipe. That limiting factor is water hammer. The Plastic Pipe Institute has a document called, "Water Flow Characteristics of Thermoplastic Pipe", TR-14, 2000. This paper can be downloaded from their site. Appendix I titled, "Limiting Water Velocities in Thermoplastic Piping Systems" gives guidelines for designing plastic piping systems to their maximum velocity potential.

PPI link:

http://www.plasticpipe.org/pdf/pubs/reports/tr-14.pdf

I hope this helps.

 

[stanier]

Thanks 749 KDV.

I am trying to get the client to undertake a surge analysis. Its one of the engineering specialities of my company. I am finding most people ignoring the requirements of Codes, Standards and government regulations that require surge to be taken into account in design. What is your experience?

 

[749KDV]

Honestly,

We just design within Manufacturer's recommendations for velocity (5-10 fps) and surge analysis is normally not needed. You would have to weight the cost of the engineering analysis with the extra cost of the larger diameter pipe to keep velocity down.

 

[mfontenot29]

Stanier,

It has also been my experience that most people ignore the requirements of Codes, standards, and regs when it comes to surge analyses.

I think even when using "classical techniques" like the method of characteristics (Wylie,Streeter,"Fluid Transients in Systems"), many people ignore the fact that much aboveground piping is "loose enough" that fluid-structure interaction may play an important point with both surge and vibration.

As the velocity goes up, the following items occur:
1. If any solids are present, increased erosion will take place.
2. Flow induced vibration will occur and progressively increase with velocity, with the most structural force "felt" at elbows, valves, etc. Flow-induced vibration in combination with structural interations can produce increased piping vibration to the point of long term fatigue. (Blevins, "Flow Induced Vibrations" is a good starting point.)
3. And of course, as you pointed out, the need for surge/waterhammer analyses becomes increasingly important.

Don't forget that plastic piping has relatively low allowable stresses/strains and can fatigue crack.

I hope these points help.

 

[749KDV]

Wouldn't proper design of pipe supports (minimum spacing requirements per ASME B31.1, B31.3, etc.) in conjunction with design of the piping system within the velocity limits specified by the manufacturer preclude the necessity for an elaborate surge analysis?
I guess what I'm trying to say is that the further you push the limits of your piping system design away from the established recommendations, a surge analysis becomes increasingly important. Trying to save the client a few dollars in material will cost him/her more dollars in engineering analysis.

 

[stanier]

Hi 749KDV,

The reason for a surge analysis is that you look at the whole system and the way it behaves. Surge can be caused by :-

Pumps tripping which causes column separation and very high transient pressures when the columns recombine

Valve closure/opening speeds

In appropriate selection of check or air valves

The mere design of pipe supports does not prevent these events from occurring. They may protect the piping from failure, they may not.

I have seen a 16" LPG line jump from the rack because of surge.

Refer to

http://www.pipingdesign.com

where you will find a paper on the risks in ignoring surge.

Just because its hard it doesnt mean you dont have to do the design. Check your current piping standards for words like, surge, waterhammer, transient loads. Then ask your self are you designing for them?

 

[rconner]

You may be interested in the high-lighted guidance below the calculator at the site

http://www.irrigationtutorials.com/sprinkler09.htm

specifically,

<<<Velocities over 7 feet per second should not be allowed in PVC pipe.
Velocities over 5 feet per second should be avoided, especially with pressures higher than 80 PSI, or in pipe larger than 2".>>>

[While I suspect there are good reasons for these particular concerns, perhaps you could contact the site developer for his reasons and/or references for these stipulations.]

 

[stanier]

Rconner WHY???

In gravity flow at critical velocities water will run at 10m/s. There are plenty of floor wastes and drains that have free surface flow! They appear to survive.

Can you provide any references that you have used to make this statement?

 

[jdarco]

Stanier,
The reason for limiting velocity to 5ft/sec is to minimize the risk of pressure surges damaging plastic pipe, which are significantly weaker than steel. If you do the math, you will find significant differences in surge pressure with 5' per second vs. 6,7,8, etc...The plastic pipe people apparently have found 5fps to be the upper limit to minimize these surges, which as you mentioned in a earlier post, many people fail to design for or consider (until they get a costly failure!) Then they blame that "cheap plastic stuff!

Download the Tech Manual from Nibco/Chemtrol, see page 13 for more info.

http://www.nibco.com/assets/TermoPlastic.pdf

 

[rconner]

I have noticed there are many contemporary "references" to limiting velocity in addition to the one I initially provided. The one I also noticed at

http://www.agr.gc.ca/pfra/water/pastpipe_e.htm

says to limit velocity to <5 fps and then additionally provides one sort of qualified explanation, "If flow velocities are kept below about 1.5 m/s, and if the maximum pressure determined by adding items 1 to 3 in the preceding list is less than about 70% of the pressure rating for the pipe, it is very unlikely that transient pressures will cause problems."

I guess that in the absence of a local governing "code" (at least if energy/environmental costs are not an object) one might argue there could be (material etc. and/or other) cost advantages to allowing higher velocities with plastic pipes; however, there could also appear to be some increased long-term risk to responsible parties as a result of thinking/stepping in or out of this particular box (I guess at least with contemporary quality plastic pipe and installation). I also agree that all folks might not thoroughly understand what velocities (can/do?) eventually occur in at least some systems in rather common? occurrences (e.g. line filling and air evacuation, line flushing, hydrant flow tests, water column movement as a result of line breaks due to whatever cause, etc. etc.??)

With regard to partially-full flowing gravity drainage etc. pipes, I doubt that clean water at even much higher velocities would damage any material type pipes; however, I also suspect if some amounts and nature of abrasive particles are in the flow, they could conceivably over time and particularly at very high velocities and with turbulence wear on all pipes. I also suspect that if the flow is wastewater e.g. containing high amounts of hydrogen sulfide, that sort of turbulence/cascading down from higher velocities could conceivably result in greater hydrogen sulfide emission problems at that point. More unforeseen problems are perhaps possible the more "outside the box"?

 

[stanier]

jdarco

Rather than guess that 5fps is the upper limit I do a comprehensive surge analysis of a system using sophisticated software, reference books etc. I take into account the performance curves of pumps, including four quadrant data, if available, check, automated and air valves.

I can see your point where engineers dont have that capability. Its actually the business that I am in. But there are systems where the velocity is less than 5fps that will create high surge pressures. Whereas there are others that can exceed this velocity and do not suffer from pressure transients exceeding the rating of the pipe. The wave speed in a thermoplastic pipe is proportional to the modulus of the pipe. The pressure transient is related to the change in momentum and wavespeed provided that column speapration doesnt occur. Thermoplastic pipe materials have lower modulus properties than ferrous materials and hence wave speeds are higher. ABS and PE have lower wave speeds than PVC-U, PVC-O or PVC-M. thus are preferred materials where you might encounter transients.

I have been researching the impact of soils compaction of the apparent modulus of thermoplastics. unfortunately there has been very little testing in this field by the plastics industry. plainly if a pipe is buried the soil support will increase wave speed. But by hw much is an unknown.

My original post was seeking any standards or codes that have this magic figure stated rather than esoteric argument, motherhood statements or generalities.

So if you know of any standards or codes that specific limit upper velocities I would like to hear from you again.

 

[749KDV]

Stainer,

I look forward to reading your technical publication on this issue when you are done. It is obvious that you are in the business of performing sophisticated analyses. However, admitidly, when it comes to piping design, I cookbook with the help of publications like piping codes and others like the aforementioned plastic piping institute limits on velocity. These publications established by experts in their fields (much like yourself) guide the engineer to design a system that includes a reasonable safety factor preventing catastrophic failure of piping systems whether it be from temperature, pressure, water hammer, etc. I believe that you would have a hard sell in this day and age of "higher value engineering" to convince clients to analyse a piping system to push the limits of its design instead of following established guidelines for maximum velocity which in turn inherently keeps the system from tearing itself apart from surge, waterhammer, and the like.

Please don't take this post as an insult. I understand your frustration in your search for information. This post is just my 2 cents.

 

[stanier]

Hi 749KDV

I appreciate your response. Having worked in the plastic pipe industry for thirty years I know many of the cook book solutions that have been published. Yes, for the majority of low pressure systems they do work. Then again it is this dumbing down of engineering expertise that results in infrastructure not lasting 100 years, court cases where things have gone wrong and inefficient design costing industry millions.

I am often in a position of engineering a solution to meet a contractors budget. If more money is invested in engineering there are great savings to be made. For instance the Naberlek uranium processing facility in Australia had a design life of 3 years. After that the deposit would be exhausted. What was the point in using design guides based on a 50 year life? Did the pipe deflection have to be within a certain % of span or didnt it matter? After all the plant was in the desert.

Piping Codes such as ASME B31.3 do a very poor job at covering FRP & thermoplastic pipe materials for the chemical industry. Other standards that cover these materials are geared towards the water and waste water industry and frankly dont have the engineering rigour of ASME B31.3. For FRP I recommend ISO 14692 be used. I think the ASME and other standards committees realise the enormity of the task of putting the technical requiremnents of plastic pipe into a standard of such standing as ASME B31.3. With reduced funding for such fine bodies and less spare time at the hands of engineers they suffer in trying to keep existing projects going let alone embarking on new ones.

All trade publications are there to promote the product. Publications are put together by marketing types. They do draw on engineering help however the flavour is always to put the best face on a product. SOme are technically better than others. Many are copied from the major manufacturers and the company publishing them have little engineering expertise.

The companies that manufacture plastic pipes have representatives on standards committees and will try to make these less conservative. Once a product is published in a standard and anything goes pear shaped the manufacturer can maintain that it complies with a recognised standard. Catch 22.

 

[rconner]

[Some readers of this current thread might be interested in additional information on past threads e.g. at

http://www.eng-tips.com/viewthread.cfm?qid=125930&page=10

that potentially relates to some issues brought up in this thread.]

 

[PUMPDESIGNER]

Stanier, I just love your comments.
The standards are used as a design guide, then engineering firms over-push "standard practice" or "accepted industry practice", which then results in people not having to study and learn and struggle to understand how something actually works.

Stanier, (and others interested in the topic), I have a question for you.
I just wrote this today. I am not finished yet, may revise heavily, but you could speed up the revision process. Topic is pressure transients, water hammer, pressure surge.

" In real world engineering of fluid delivery systems, momentum energy is seldom directly accounted for due to the following reasons:
1
Lack of knowledge resulting in a lack of awareness about the problems that can occur.
2
Momentum energy in pipe systems is difficult to analyze and understand, leading to the use of standard or accepted practice methods to handle the problems.
3
Standard or accepted practice methods lead to over confidence that there are no issues, no problems. No problems, no need to learn more or investigate.
4
Standard or Accepted practice methods can work well enough such that complacence settles in and dominates.
5
Because the concepts of momentum energy are never learned, the limitations of standard or accepted practice are never considered, and then the methods are over extended.

Ultimately there is no substitute for deep knowledge and experience of how systems actually function, including the physics behind the engineering. There is no safety for ignorance relying upon codes and standards as a substitute for knowledge, caring, and wisdom.

PUMPDESIGNER

 

[stanier]

Hi Pumpdesigner,

If you look at my paper on

http://www.pipingdesign.com

http://www.pipingdesign.com/articles/piping_surges.pdf

some of your points may be addressed.

I prepared this paper with a view to raising the understanding about the risks associated with the ignorance of this topic.

Yes the topic is complex and I think that is why engineers shy away from it. Understanding the reality that momentum change results in energy release should be comprehended by a college student of physics. But quantifying it in the world of fluids is beyond the capability of most civl and mechanical engineers.

We have tools now to quantify and get a much better understanding of what is going on in piping systems. However the software, books and seminars cost orders of magnitude more than MS Excel, common engineering handbooks and management workshops. Engineers dont have the time or energy to convince their MBA adorned managers that they have to invest in this skill.

This I believe is a direct result of the dumbing down of the design engineering profession. Although we have centres of excellence the average project engineer is far too busy with environmental, community, quality assurance, occupational health and safety, non discrimination, planning, concept engineering and non specific training to actual learn and practice their craft of engineering in depth. Why should they for the riches come from getting your MBA and moving into management.

It has nothing to do with intellect. Rather it is the application of that intellect to matters real rather than the "fog" of all these other neferious pastimes thought up by the MBAs in management. The non technical matters are work creation for the dumb MBAs to justify their exculted position above the technically adept. A modern form of lords over the serfdom.

Young project engineers prefer to aim for lordship over serfdom.

 

[rconner]

I happened to notice the following passage on page D3-1 of the reference (I saw this provided by another in another thread on another subject) at

http://www.agr.gc.ca/pfra/pub/pipelinealta_e.pdf,

applying to the pvc and hdpe pipes they talk about for this area, that appears to limit maximum flow velocity in these plastic pipes even further, to <1 m/sec,

" D3.2 Pipe Design
D3.2.1 General
On rural pipelines, the design should be such that flow velocities are less than 1 m/s (this minimizes surge problems). The pipeline should be designed to handle the operating pressure plus surge (if applicable), as well as the pump shut off condition (if applicable). The designer
should be aware that negative pressures can occur, and should identify the conditions that may lead to their occurrence."

 

[stanier]

rconner,

As the material in questionis Dn38mm PE I dont think that surge is going to be a problem. PE has a very low modulus and the celerity will thus be low. The chance of high transient pressures in such a rural system are miniscule.

The selection of a low velocity is more to do with energy costs versus the capital cost of putting in a slighlty larger pipe than what you could get away with. The installation of a DN38 line would be by moling in using a tractor. The cost would be the same for Dn15 to Dn100. The same machine and driver is used.

The cost of rural PE is low because so much regrind is used in this low quality product.

 

[PUMPDESIGNER]

Thank you for your comments Stanier. I did not get back sooner because of Wilma, just got power back today.



PUMPDESIGNER