December 2010 Vol. 237 No. 12
Features
Canadian Standard Revised To Incorporate Higher Performance Plastic Pipe
Revisions have been approved to the Canadian standard dealing with the use of plastic pipe in oil and gas pipeline systems. The standard is CSA Z662-11 “Oil and Gas Pipeline Systems.” The revisions to be published next year include:
- Added rapid crack propagation (RCP) required value of a full-scale critical pressure greater than 1.5 times the maximum operating pressure for polyethylene (PE) materials.
- Added the minimum required strength (MRS) design equation so that PE 100 pipe could be installed and pressure rated using the ISO pressure rating method. Specified stringent SCG requirement of 2,000 hours PENT and S4 RCP critical pressure of 10 bar for bimodal PE 100 materials.
- Added a higher design factor of 0.45 so that the pressure rating for hydrostatic design basis-rated (HDB) and MRS-rated pipes would be the same. Introduced “PLUS” marking to indicate that the 0.45 design factor could be used.
- Specified stringent SCG requirement of 2,000 hours PENT and S4 RCP critical pressure of 10 bar for HDB-rated PLUS materials, to be consistent with the requirement for bimodal PE 100 materials. This would include the bimodal PE 2708 PLUS and bimodal PE 4710 PLUS materials.
- Incorporated the CRS methodology to provide the gas engineer with more flexibility in design his gas system. This includes pressure rating at the desired use temperature for the desired time.
- Added PA-11 as a piping material for higher-pressure gas piping applications up to 300 psig (20 bar). Also, added a chemical design factor table, which points out that PA materials are not affected by liquid hydrocarbons.
- Added reinforced thermoplastic pipe (RTP) for even higher-pressure gas piping applications up to 1,500 psig (100 bar).
Let us discuss the reasons for the revisions and look quickly at several papers on the use of plastic piping for natural gas distribution in Canada that were presented at the PP XV Conference during September 2010 in Vancouver, BC.
RCP Requirements
Although the phenomenon of RCP has been known and researched for several years (1), the number of RCP incidents has been very low. A few have occurred in the gas industry in North America and a few more in Europe. With gas engineers desiring to use PE pipe at higher operating pressures (up to 12 bar or 175 psig) and larger diameters (up to 30 inches or 750 mm), a key component of a PE piping material – resistance to rapid crack propagation (RCP) – becomes more important.
Most of the original research work conducted on RCP was for metal pipe. As plastic pipe became more prominent, researchers applied similar methodologies used for metal pipe on the newer plastic pipe materials, and particularly polyethylene (PE) pipe (2). Most of this research was done in Europe and through the ISO community.
Rapid crack propagation, as its name implies, is a very fast fracture. Crack speeds up to 200 m/sec (600 ft/sec) have been measured. These fast cracks can also travel very long distances, even hundreds of feet. The DuPont Company had two RCP incidents with its high-density PE pipe, one that traveled about 90 meters (300 feet) and the other that traveled about 240 meters (800 feet). RCP cracks usually initiate at internal defects during an impact or impulse event. They generally occur in pressurized systems with enough stored energy to drive the crack faster than the energy is released. Based on several years of RCP research, the probability of an RCP failure in PE pipe is increased with these factors (3):
1. Increase in pipe size/wall thickness,
2. Increase in internal pressure,
3. Decrease in temperature, and
4. Decrease in resistance to RCP of the PE material.
Typical features of an RCP crack are a sinusoidal crack path along the pipe, and “hackle” marks along the pipe crack surface that indicate the direction of the crack. At times, the crack will bifurcate into two directions as it travels along the pipe.
CSA B137.4 (4) recently added a requirement that RCP testing must be conducted by the resin/pipe manufacturer, but there were no required values. It was decided to leave this up to the Oil and Gas Standard in Z662 Clause 12. The following new clause for RCP was recently added to Clause 12:
12.4.3.6 Rapid Crack Propagation (RCP) Requirements
When tested in accordance with B137.4 requirements for PE pipe and compounds, the standard PE pipe RCP Full-Scale critical pressure shall be at least 1.5 times the maximum operating pressure. If the RCP Small-Scale Steady State method is used, the RCP Full-Scale critical pressure shall be determined using the correlation formula in B137.4.
A required full-scale RCP critical pressure of 1.5 times the maximum operating pressure is consistent with the RCP requirement in ISO 4437 (5). It is also consistent with the proposed requirement in the AGA Plastic Materials Committee RCP White Paper (6), which requires that the RCP full-scale critical pressure be greater than the leak test pressure. The leak test pressure is 1.5 times the maximum operating pressure.
Recent Small Scale Steady State (S4) testing has shown a dramatic difference in resistance to RCP when comparing traditional unimodal (one reactor) PE materials to bimodal (two reactors) PE materials (3). Table 1 summarizes some typical critical pressure values (S4 and corresponding converted full scale) for various generic PE materials. For most cases, the pipe size tested is 12-inch SDR 11 pipe. (See Table 1.)
These data show a ten-fold increase in S4 critical pressure for bimodal MDPE compared to unimodal MDPE, and also a significant increase in S4 critical pressure for bimodal HDPE compared to unimodal HDPE.
MRS Design Equation
Canadian gas companies were aware of the superior properties of the bimodal PE 100+ materials (7), as just shown for RCP test results comparing unimodal and bimodal. They were also aware of the ISO MRS (Minimum Required Strength) pressure rating method that utilized a 50-year design basis. As a result, the Canadian gas companies requested that the MRS design equation and the corresponding design coefficient be added to CSA Z662 Clause 12, as shown below.
12.4.2 Thermoplastic piping — Design pressure
A design coefficient of 2.0 for PE 100 materials in Clause 12 is consistent with the design coefficient of 2.0 for PE materials in ISO 4437, the international standards organization PE gas pipe standard.
To ensure that only the superior performing bimodal PE 100 materials were allowed for Canadian gas distribution systems when using the MRS methodology, Clause 12 specifies a very stringent slow crack growth (SCG) requirement of 2,000 hours PENT. Clause 12 also specifies a very stringent RCP requirement of S4 critical pressure greater than 10 bar, which is consistent with the PE 100+ RCP requirement. Again, only the bimodal PE 100 materials would be able to meet this stringent RCP requirement. These SCG and RCP requirements for PE 100 materials in Clause 12 are shown below.
The minimum PENT value for MRS-rated PE 100 compounds using the 2.0 design coefficient shall be 2,000 hours, and the minimum RCP Small-Scale Steady State value shall be 1,000 kPa at 0?C per Clause 12.4.3.6.
12.5.2.4 PE 100 compounds
HDB Design Factor
The Plastics Pipe Institute (PPI) recently addressed the dilemma that the pressure rating for ISO 9080 PE 100 pipe was higher than corresponding ASTM D 2837 PE 3408 pipe by 45% (8). In an attempt to bring these two pressure ratings methods closer together, PPI increased the design factor from 0.5 to 0.63 for water pipe and from 0.32 to 0.4 for gas pipe. The materials that qualify for the higher design factor are called “high performance” PE materials and require a PENT value (ASTM F 1473) of 500 hours. The new pipe material designation code for PE 3408 became PE 4710 and the new code for PE 2406 became PE 2708 to indicate that these high performance materials qualified for the higher design factor. However, even with this higher design factor, there was still a gap of 15% between the ASTM and ISO pressure rating methods.
In Canada, CSA Z662 Clause 12 had already increased the design factor for gas applications from 0.32 to 0.40 in 1996. As a result when PE 2708 and PE 4710 were introduced in Canada there was no difference in pressure rating compared to a PE 2406 or PE 3408, respectively. The Canadian gas companies wanted to recognize the higher performance for the superior bimodal PE materials, and so they introduced a 0.45 design factor. With this even higher design factor of 0.45, the pressure rating for PE 4710 and PE 100 became the same. Canada closed the gap between the ASTM and ISO pressure rating methods with the introduction of this 0.45 design factor for gas piping applications in CSA Z662 Clause 12.
12.4.2.2
The design factor (F) to be used in the design formula in Clause 12.4.2.1 (a) for HDB-rated materials shall be 0.40, or 0.45 for PLUS performance PE compounds described in 12.5.2.3. To indicate that a PE material could use the higher 0.45 design factor, Clause 12 introduced the term “PLUS” after the pipe material designation code – for example, PE 2708 PLUS or PE 4710 PLUS.
Superior Performing PE
The minimum PENT value for HDB-rated plus performance PE compounds using the 0.45 design factor shall be 2,000 hours, and the minimum RCP Small-Scale Steady State value shall be 1000 kPa at 0?C per Clause 12.4.3.6. These plus performance PE compounds that qualify for a 0.45 design factor shall be designated with a PLUS after the pipe material designation code; for example, PE 2708 PLUS or PE 4710 PLUS.
The Canadian gas companies wanted to assure that only the superior performing bimodal PE materials would qualify for the higher, 0.45 design factor (PLUS). To assure a very high level of slow crack growth resistance, Clause 12 requires a PENT value of 2,000 hours. This is consistent with the 2,000-hour PENT requirement for PE 100 materials in Clause 12. To ensure a very high level of rapid crack propagation resistance, Clause 12 requires an S4 critical pressure of 10 bar (1000 kPa) at 0?C (32?F). This is consistent with the S4 RCP critical pressure requirement for PE 100 materials in Clause 12. Thus both the HDB rated PE 2708 PLUS and PE 4710 PLUS and the MRS rated PE 100 have the same stringent SCG and RCP requirements in Clause 12, as shown below.
12.5.2.3 PLUS Performance PE compounds
Tables 2 and 3 compare the maximum operating pressure for MDPE (medium density PE) and HDPE (high density PE) materials used in SDR 11 pipe:
Table 2: Maximum Operating Pressure (MOP) for MDPE SDR 11 Pipe – Gas Applications.
Table 3: Maximum Operating Pressure (MOP) for HDPE SDR 11 Pipe – Gas Applications.
From Table 3, you can see that 1) the pressure rating for PE 3408 and PE 4710 are the same, 2) the pressure rating for PE 4710 PLUS is 15% higher than PE 4710, and 3) the pressure rating for PE 4710 PLUS and PE 100 are the same.
CRS Methodology
The CRS (Categorized Required Strength) concept was developed within the ISO pressure rating system to provide more flexibility to the design engineer. One of the major uses of CRS is to provide a design basis of 100 years for plastic piping applications. With the CRS concept, plastic pipe can now have a design basis or design life of 100 years and be considered equivalent to other piping materials, such as steel or iron pipe, which claim a 100-year design life.
Another major application for the CRS concept is to pressure rate plastic pipe at the actual use temperature. This can be useful for high temperature applications or in areas that have a low ground temperature. The design engineer can determine the appropriate conditions for the plastic pipe application, and design accordingly.
A PP XIII paper reviewed the CRS concept, how it is validated within ISO 9080, how it is determined within ISO 12162, what its limitations are, and finally, how it is used in typical plastic piping applications to make plastic pipe more cost competitive with metal pipe (9). ISO 12162 recently completed the balloting process to incorporate CRS methodology in ISO 12162, and CSA Z662 Clause 12 has also recently incorporated CRS.
CRS = categorized required strength (MPa) – the categorized value of the long-term hydrostatic strength determined in accordance with ISO 9080 and ISO 12162 at a selected design time and at a selected design temperature. The design engineer may select the appropriate design time and/or appropriate design temperature for the application. The design pressure using CRS is calculated using the same equation as used for the MRS in 12.4.2.1.B. Typical times and/or temperatures for CRS include:
12.4.2.1
Manitoba Hydro’s gas distribution system is designed, constructed and operated under the requirements of CSA Z662 “Oil and Gas Pipeline Systems.”
Material testing including butt, electro and saddle fusion and pipe squeezing was performed to gain hands on experience with a PE 100 material while a technical investigation of the material was performed. Based on the economic advantages provided and supported by the testing and investigation of the material, application was made to, and approval was received from Manitoba Hydro’s regulatory authority to install PE 100 pipe using the MRS pressure rating method.
In 2006, Manitoba Hydro installed 29 km of 114 mm SDR 11 PE 100 pipe by plowing, horizontal directional drill, and open cut methods. This SDR 11 PE 100 pipe has been successfully operating at 145 psig (MRS) with plans to increase to 160 psig based on the CRS (Categorized Required Strength) at 15?C (10).
Polyamide Materials
Polyamide-11 pipe, tubing and fittings may be installed in distribution systems in accordance with all the requirements for polyethylene pipe, tubing, and fittings in Clause 12, except that for Polyamide-11 piping systems the maximum operating pressure at elevated temperatures up to 80 OC shall be based on the published hydrostatic design basis (HDB) value.
The chemical design factor (Fc) to be used in the design formula in Clause 12.4.2.1, when liquid hydrocarbons are present, shall be as given in Table 12.1.
Polyamide (PA) materials have been used for natural gas distribution in Australia for 40 years, and in the U.S. for 10 years. With the completion of the ISO 22621 series of standards for PA materials for pressures of 15 to 20 bar (200 to 300 psig), the use of PA for gas applications should increase. CSA Z662 also recently added PA-11 to Clause 12, since it already has an approved product standard – CSA B137.12.
12.4.6.2 Polyamide 11 (PA-11) piping systems
Kiwa Gas Technology presented a paper at PP XV comparing the properties of PA 11, PA 12 and PA 612 (11). All three of the PA piping materials show promise as all-plastic piping systems that are an alternative to metal pipe for gas distribution pressures up to 20 bar (300 psig). These PA materials also have greater resistance to liquid hydrocarbons as shown in the Clause 12 chemical design factor table below.
12.4.2.3
Reinforced Thermoplastic Pipe
Composite pipe technology is an interesting and upcoming field, which will allow plastic pipe to be used at higher pressures, such as 1,500 psig (100 bar).
One such product is reinforced thermoplastic pipe or RTP. A Canadian RTP manufacturer and a Canadian gas company presented a paper at PP XV on using RTP for gas distribution (12). This is now possible since RTP was recently added to CSA Z662 Clause 12 for gas distribution applications. RTP is also included in Clause 13 for gas gathering applications.
Continuous length reinforced thermoplastic pipe (RTP) Type 1 may be installed in distribution systems in accordance with the requirements for RTP pipelines in Clause 13.1, except that the Maximum Pressure Rating (MPR) in the design equation in Clause 13.1.2.8 shall be established on the basis of a minimum life expectancy of 50 years. The requirements of Clause 13.1.1.3 do not apply.
12.4.6.3 Continuous length reinforced thermoplastic pipe and fittings
The future for plastic pipe is very bright in Canada!
The Authors: Rob J. Fox graduated from Queen’s University in 1978 with a BSc degree in engineering and is a licensed Professional Engineer in Ontario and Quebec, Canada. He has worked with Enbridge Gas Distribution for more than 32 years in various operations and engineering positions and currently is the company’s manager of technical training. He serves as chair of the CSA Z662 Oil & Gas Pipeline Systems, Distribution Technical Subcommittee.
Dr. Gene Palermo received a B.S. degree in chemistry from St. Thomas College in St. Paul, MN in 1969 and a Ph.D. in analytical chemistry from Michigan State University in 1973. He has been in the plastic piping industry for over 30 years. He worked for the Dupont Company from 1976 to 1995, Elf AtoChem during 1995 and 1996, and was the technical director for the Plastics Pipe Institute (PPI) from 1996 until 2003. He is president of his consulting firm, Palermo Plastics Pipe Consulting. He serves as a member of PPI, AGA, GPTC, AWWA, ASTM F 17 and D 20, CSA B137, CSA Z662 Clause 12 and ISO/TC 138. He was recently honored with the ASTM Award of Merit, which is that society’s highest recognition for individual contributions to standards activities, and the AGA Platinum Award of Merit, which is the highest award that can be achieved within AGA. Dr. Palermo is the only person to receive both of these awards. He can be reached at 865-995-1156, www.plasticspipe.com.
References
1. C.G. Bragaw, “Rapid Crack Propagation in Medium Density Polyethylene Pipe”, 7th Plastic Fuel Gas Pipe Symposium, 1980.
2. M. Wolters, “Some Experiences with the Modified Robertson Test Used for Study of Rapid Crack Propagation in PE Pipelines,” 8th Plastic Fuel Gas Pipe Symposium, 1983.
3. E.F. Palermo, “Increasing Importance of Rapid Crack Propagation (RCP) for Gas Piping Applications – Industry Status,” PP XIV, 2008.
4. CSA B137.4, “Polyethylene (PE) piping systems for gas services.”
5. ISO 4437, “Buried polyethylene (PE) pipes for the supply of gaseous fuels — Metric series — Specifications.”
6. AGA PMC White Paper, “Using RCP Data to Design Polyethylene Gas Distribution Systems.”
7. E.F. Palermo, “The Mystery Between PE 4710 and PE 100+ Revealed,” PP XV, 2010.
8. E.F. Palermo, “High Performance Bimodal PE 100 Materials For Gas Piping Applications,” AGA Operations Conference, 2005.
9. E.F. Palermo, “Using the CRS Concept for Plastic Pipe Design Applications,” PP XIII, 2006.
10. T. Starodub and E. F. Palermo, “Use of PE100 with a Minimum Required Strength (MRS) Rating in a Natural Gas Distribution System,” PP XV, 2010.
11. F. Scholten and M. Walters, “PA Pipes for 16 Bars Gas Pipelines,” PP XV, 2010.
12. B. Weller, A. Sakr and N. Lesage, “Case Study: The Use of Reinforced Thermoplastic Pipe in Gas Distribution Applications,” PP XV, 2010.
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