PVC 40 vs JIS K 6741 2

[a10jp]

I have a question on standards for PVC sch 40 vs Japanese standard JIS K 6741. Specifically, I am trying to determine the relationship between, norminal operating water pressure, and max hydrostatic pressure to see whether the 2 different pipes are equivalent. I might be going about this the wrong way, but I am looking for some guidance. In my case, we are trying substitute existing irrigation pipes (2" and 6" PVC sch 40) with local made products in Japan (uses JIS standards). The operating pressure of the irrigation system ranges between 150psi to 200psi. However, as I understood, sch 40 (or sch 80) are merely rating for the wall thickness. So with that thought I assume the same for JIS products. But the question arise in that there is a marking on the surface of the existing US pipes that says 200psi. I NEVER thought pvc pipes has pressure rating. WHat does that mean? So I wonder, is that nominal operating pressure or max hydrostatic presssure? The ASTM chart seems to correlate sizes of pipes vs operating temperature (at 73 deg F), and for example for 2" pvc sch 40, the pressure is 280psi and drops to 180 psi for 6" pipes. As for the JIS products, it says design pressure (7.5 kg/cm2 + 2.5 kg/cms waterhammer = 10.0kg/cm2 = 150psi) and the JIS standards says max hydrostatic pressure is 25.5 kg/cm2 (= 362 psi). However, the testing method at max hydrostatic pressure is only for 1 min. for JIS standard, whihc is different than ASTM testing method. So how should I compare the 2 products to know if they are equivalent. (I know in the back of my head they probably the same, but I need help to determine their engineering equivalence, whether I can use JIS products for my application (at 200psi?? - not sure) Any idea?

 

[RWF7437]

Can you get a sample of each type and have them tested by the same test method ?

 

[stanier]

The pressure rating of PVC pipes is common outside the USA. The hoop stress is determined by test and a 97.5% lower confidence limit applied for a fifty year life. See ISO 9080. The pressure rating is at 20C. At higher temperatures the material properties decay.

HYdrostatic test pressure is not held for a long time such as in metals as thermoplastics have creep properties. A factor of 1.3 over the rated pressure is often referenced on ISO DIN and JIS standardfs.

ASTM test methods differ from European and other parts of the world's methods. They are a little behind the times preserving American industry I suspect. Most countreies have moved to metric dimensions and pressure classes in their standards and away from pipe schedules.

Geoffrey D Stone FIMechE C.Eng;FIEust CP Eng

http://www.waterhammer.bigblog.com.au

 

[a10jp]

The 50 years life span, and 20C testing condition are also referenced in the JIS standards, but I am still tyring to determine at what test pressure is the 50 years based on. In Japan, there is no schedule as stated in postings above. There is only 1 kind of each category of VP, VU, VM type pipes, and in each type, they are either thin wall or thick wall. Based on this fact, I am pretty sure that the choice of pipe is based on the application, and not just pressure.

 

[stanier]

Suggest you contact a local supplier and ask them for advice.

Kazuyoshi Abe, General Manger Eura International Ltd T171-0032 Zoshigaya 2-23-9, Toshima-Ku Tokyo 03 5957 7075 tel 03 5957 7067 fax may be able to help. He specialises in ABS pipe which is superior in most applications to PVC but he can also supply PVC. (The ABS is to AS3518 NOT the material to ASTM standards that provided so many problems in the past. This is a pressure pipe available up to Class 15.

As I have said previously thermoplastic pipes are defined by pressure class (PN6, 9 12, 15 or 20) This is in bar at 20C. Alternatively they are defined to a standard dimension ratio (SDR) which is the ratio of diameter to wall thickness ie akin to sch 40 or 80. That is all that "schedule" means a reference to the pipes dimensions not the capability of a pipe to withstand pressure.

European standards require material properties for a certain dimensioned pipe to achieve a pressure at 50 years at 20C within a 97.5% lower confidence limit. You may find that the VM, VP or VU refers to PVC-U, PVC-M etc. These are different types of PVC. PVC-M is a modified pvc (by additives) that is claimed to give superior performance. There are also PVC-C, PVC-O available.

Go to

http://www.pipa.com.au

for some information.

Geoffrey D Stone FIMechE C.Eng;FIEust CP Eng

http://www.waterhammer.bigblog.com.au

 

[a10jp]

Appreciate your input. This is a big help.

 

[jdarco]

Schedule 40 & 80 pipe was adopted from the steel pipe industry and relates to wall thickness. Generally speaking, the larger the diameter in schedule pipe, the lower the pressure rating.

Internationally, most pipe is "dimension ratio" pipe (see Stanier post above) and is rated at a single pressure rating for the complete size range. So, where 2" PVC 40 will have a higher pressure rating than 6" PVC 40, 2" SDR pipe will have the same pressure rating as 6" SDR pipe.

Lastly, the pressure rating printed on the pipe is usually maximum operating pressure at ambient (70 deg F) temperature and based on water. Higher temps derate the pressure rating.

 

[bimr]

You should be concerned with the design pressure rating, not pipe thickness as determined by pipe schedule.

The 200 psi rating on your PVC is a sustained design pressure rating. You have said that the Japanese product has a design pressure rating of only 150 psi so the Japanese product is not equivalent.

There is another plastic pipe test requirement called the "quick-burst pressure" test. The quick-burst pressure test was intended to be an assurance of PVC extrusion quality. The quick-burst test pressure is held for only 60-70 seconds. The quick-burst pressure test should be used for quality control purposes in pipe making, and not in the design and application of plastic piping systems.

The JIS standards max. hydrostatic pressure is a quick-burst pressure test. Instead, you should use the design pressure rating of the pipe and note that the pipe should be derated for the temperature that it will be used.

The plastic pipe test pressures have nothing to do with creep. Data has been accumulated that proves PVC pipes held at their design pressure ratings for over 11 years will exhibit no perceptible rate of creep at the end of the time. In addtion, the response of the PVC pipe to applied pressures stabilizes at 11 years with a design life of 50 to 100 years.

 

[a10jp]

Hi bimr, thanks for your input. I have done more research after my initial posting. In the absence of having a common approach to compare the 2 piping standards, I used the equation for calcuating the pressure ratings, since the same equation was posted in both US and Japanese manufacturer: P = 2S / (DR-1). When I applied this equation, first I applied it to PVC sch 40 pipes, then I applied it to SDR pipes. I noticed the values came out very close to the ASTM table. And now I understood what that 200psi rating means, but I do not know what to call it. You call it the sustained design pressure, some calls it the critical pressure, some manufacturer calls it the maximum pressure, ATSM D 1785 calls it the water pressure ratings, the Japanese calls it the Naday's equation for working pressure...So as you can see, this is very confusing for just one formula. I am not an expert in this field, so I am learning as it goes.

As for the JIS max hydrostatic pressure, it is 362psi (25.52 kgf/cm2). I have yet to compare with US standards on this.

Then I came across this technical paper (not sure if I can list the source of posting, they did not allow me last time...) where it talks about the difference between pressure class and pressure ratings. Because the Japanese uses a single pressure value for all pipe sizes, I am inclined to think that it is not pressure rating, but rather pressure class.

On another note, I need to discuss the definition of S. Earlier in the equation S = hydrostatic design stress = pressure class / safety factor = 4000psi / 2. Realize we use safety factor = 2. I have not yet determined what safety factor is being used in Japan, but the convetred S in Japan is about 1565 psi. Could it be the Japanese is using a more stringent safety factor than ours (> 2.0)...

I am still looking into this. But this is getting to be more academic than I thought.

 

[bimr]

Pressure class or pressure rating of the PVC product refers to anticipated steady-state continuous operating capability and are based on long term material strength. Desing for long term, steady state operating conditions based on short term strength of PVC pipe would be inappropriate. The Pressure class or pressure rating takes into account the long term response of PVC pipe to applied stress.

The quick-burst pressure or short-term strength divided by a safety factor of 2.5 will yield the corresponding short-term pressure rating of the pipe. The short-term pressure rating should be considered the design surge capacity limit for PVC in a transmission application. The short-term pressure rating represents a level approximately 25%a above the pressure rating for each rating class.

 

[stanier]

It is very dangerous to consider a safety factor covers the surge in establishing the design rating.

There are plenty of scenarios where the transient pressure will exceed the maximum predicted by Joukwsky.

Also care needs to be taken as to the design temperature of the PVC. PVC is notoriously poor below 5C to resist impact or fatigue loading.

Geoffrey D Stone FIMechE C.Eng;FIEust CP Eng

http://www.waterhammer.bigblog.com.au

 

[a10jp]

I am little confused of the terms being used. What is the relationship between quick-burst pressure and Hydrostatic Design Stress (S)used in the equation above? Also just for refernece I have found out that in Japan, a safety factor of 2.3 is used, instead of 2.0 in most application in US I have encountered.

 

[bimr]

The built-in surge allowance is assumed to be adequate for piping systems operating at low velocities of 2 f/s. For systems operating at higher velocities, the potential for transient surge should be investigated.

All PVC pressure pipe for water applications must have a defined Hydrostatic Design Basis (HDB) of at least 4000 psi as defined by ASTM D 2837. The stress regression and HDB are establish using long term hydrostatic pressure testing.

In the definition of PVC pipe's pressure rating, the hydrostatic design stress rather than HDB is used in caculations. The design stress is sumply the value obtained when the HDB is divided by the safety factor of 2.

The design stress serves as the maximum hoop stress value used in calcuation of PVC pipe pressure rating.

Raw water transmission and finished water supply often use the AWWA C905 standard with a safety factor of 2 and are used for piping greater than 12" dia. AWWA C900 standard is used for distribution piping 4-12" dia. and has a 2.5 safety factor.

Here is an example:

PVC pipe for transmission operating at 140 psi with a velocity of 3.5 f/s. Using DR 25 pipe pressure rated for 165 psi with a short term pressure rated for 533 psi.

Working pressure rating = short term rating - surge pressure

If you are using DR 25 pipe,

Working pressure rating = 165 psi - (3.5) (14.7) = 164 psi
where 14.7 is the pressure surge vs dimension ratio factor for DR 25 pipe.

For this application, piping with less than a 165 pressure rating would not be recommended.

Compute safety factor for surges

Pmax = P + velocity surge

= 140 + 3.5 (14.7) = 191.5 psi.

Safety factor = Short term rating/ Pmax
= 533 psi / 191.5 = 2.8

Since safety factor is greater than 2.5, DR of pipe is adequate.

The short term pressure test allows one to take advantage of plastic pipe's inherent property of being able to handle short term pressure surges. Non-plastic pressure pipes display insignificant difference betweent short term and long term design strength.

Note that surge pressures are considered to be infrequent deviations from normal steady state operating pressures. If you have extreme cyclic conditions, that is a separate topic.

 

[a10jp]

Appreciate your input!!

 

[bimr]

In rereading my post I noticed an error in the example. Here is the corrected example:


PVC pipe for transmission operating at 140 psi with a velocity of 3.5 f/s. Using DR 25 pipe pressure rated for 165 psi with a short term pressure rated for 533 psi.

DR 25 PVC Pipe:

Pressure Rating 165 psi
Short Term Strength 533 psi
Short Term Pressure Rating 215 psi

Working pressure rating = short term rating - surge pressure

If you are using DR 25 pipe,

Working pressure rating = 215 psi - (3.5) (14.7) = 164 psi
where 14.7 is the pressure surge vs dimension ratio factor specific for DR 25 pipe.

For this application, piping with less than a 165 pressure rating would not be recommended.

Compute safety factor for surges

Pmax = P + velocity surge

= 140 + 3.5 (14.7) = 191.5 psi.

Safety factor = Short Term Strength(Quick Burst Pressure)/ Pmax
= 533 psi / 191.5 = 2.8

Since safety factor is greater than 2.5, DR of pipe is adequate.

Short Term Strength(Quick Burst Pressure)divided by a safety factor of 2.5 yields the corresponding short term pressure rating. These short term may be considered the design surge capacity limits for PVC in transmission applications ( larger pipes, fewer connections, higher velocities).

Note also that surge analysis should be done on the piping system.