Using pipe made from PE100 material to install a 29-km gas main provided Manitoba Hydro a C$200,000 advantage over the second-best alternative. In fact, the avoided cost was sufficient to render the expansion project economically feasible.
PE 100 – ISO MRS/CRS. PE 100 is a bimodal polyethylene material that has been used in Europe and Asia for over 20 years. Key characteristics of PE 100 are a higher pressure rating for a given pipe wall thickness, and excellent resistance to both slow crack growth and rapid crack propagation.
In Europe and Asia, an ISO pressure rating method is used that is based on testing pipe samples at three different temperatures with the linear log stress-log time 20 C regression line extrapolated to 50 years. The lower confidence level of the extrapolated value is called the lower predictive level (LPL) and the categorized value of the LPL is called the minimum required strength (MRS) in accordance with ISO 9080 and ISO 12162. The MRS for different resins is published in the Plastic Pipe Institute (PPI) Technical Report TR-4: PPI Listing of Hydrostatic Design Basis (HDB), Hydrostatic Design Stress (HDS), Strength Design Basis (SDB), Pressure Design Basis (PDB) and Minimum Required Strength (MRS) Ratings for Thermoplastic Piping Materials or Pipe.
Using the ISO methodology for MRS, the maximum operating pressure is calculated as:
MOP= [20 (MRS)/ (DR-1) (C)] (Eq. 1)
Where: MOP= Maximum Operating Pressure (bar)
MRS= Minimum Required Strength (MPa)
C= Design Coefficient (minimum of 2.0 for gas applications – from CSA Z662)
DR= Dimension Ratio
An example calculation for a DR11 PE 100 pipe for gas applications with an MRS of 10 MPa is:
MOP= [20 (10)/ (11-1)(2)]= 10 bar = 145 psig
The MRS is always defined at 20 C and 50 years. The ISO system recognizes that polyethylene behaves differently at different temperatures and provides the gas designer a method to design a system at the actual use temperature and desired system life. The categorized required strength (CRS) is the lower predictive level at a desired use temperature and desired time. For gas applications, lower ground design temperatures with the same design life, will typically result in a CRS value exceeding the MRS. As the calculation of the maximum operating pressure using CRS is the same as shown in Equation 1 with the substitution of CRS for MRS, a higher MOP will be realized.
An example calculation for a DR11 PE 100 pipe with a CRS of 11.2 MPa at 14 C (57 F) is:
MOP= [20 (11.2)/ (11-1) (2)] = 11.2 bar = 160 psig
PE 2708/PE 4710 – ASTM/HDB. The calculation of maximum operating pressure using the North American standard ASTM methodology is:
MOP= [2 (HDB)(F)/(DR-1)] (Eq. 2)
Where: MOP= Maximum Operating Pressure (psig)
HDB= Hydrostatic Design Basis (psi)
F (Design Factor) = 0.4 (From CSA Z662)
DR= Dimension Ratio
As a comparison, the pressure rating of medium density polyethylene, PE 2708, and high density polyethylene, PE 4710 would be calculated as:
MDPE – PE 2406/PE 2708 [HDB = 1250 psi, DR 11]
MOP= [2 (1250) (0.4)/ (11-1)] = 100 psig
For the common DR 11 pipe size, the MOP for PE 2708 in a Canadian gas application is only 100 psig. A DR 7 would be required for the desired MOP of 160 psig.
HDPE – PE 3408/PE 4710 [HDB = 1600 psi, DR 11]
MOP= [2 (1600) (0.4)/ (11-1)] = 125 psig
For the common DR 11 pipe size, the MOP for PE 4710 in a Canadian gas application is only 125 psig. A DR 9 would be required for the desired MOP of 160 psig.
These lower DR pipes have a thicker wall and smaller inside diameter that result in lower flow and lower capacity, which are not desired. Also, standard squeeze-off tool DR 11 gap stops cannot be used, different metal insert stiffeners are required, and butt fusion fittings may not always be available.
Manitoba Hydro is a Crown Corporation, fully owned by the Province of Manitoba. Manitoba Hydro is the prime supplier of natural gas and electricity to the Province of Manitoba serving about 275,000 natural gas customers and more than 530,000 electrical customers. The Manitoba Hydro natural gas distribution system operates in a regulated environment under the auspices of the Public Utilities Board of Manitoba.
As a Canadian gas utility and operating under Province of Manitoba Regulations, the natural gas system is to be designed, constructed, operated and maintained in accordance with CSA Z662 Oil and gas pipeline systems. CSA Z662 explicitly lists acceptable materials and makes direct reference to CSA B137.4 Polyethylene (PE) piping systems for gas services as the material standard governing the polyethylene materials used in the natural gas system.
In 2005 at the time of the initial project development, the 2003 edition of CSA Z662 was in effect with this standard referencing the 2002 edition of CSA B137.4. In this edition, the acceptable listed materials were limited to HDPE3408 and MDPE2406. Material ratings were listed in accordance with ASTM standards. Between Z662 and B137.4, the acceptable method of determining the maximum operating pressure of a pipe was through the hydrostatic design basis.
The Project. In response to customer demand, a project was initiated to expand the natural gas distribution system to provide natural gas to the Town of Shoal Lake, Manitoba. The project would require the installation of a 29-km main between the existing gas distribution system and the proposed distribution piping system for the Town of Shoal Lake. The distribution piping system would include approximately 9 km of medium density polyethylene. The route of this main generally follows a highway and is through a sparsely populated agricultural area. The required system design gas capacity was determined based on a five year period to connect identified interested customers and an allowance for growth.
As per typical practice, designs using alternative materials were evaluated to identify the most cost effective approach to provide the defined gas supply volumes. Preliminary hydraulic design using the standard steel, medium density polyethylene (MDPE) and high density polyethylene (HDPE) pipe materials was performed.
A budget estimate indicated that the use of the combination of 114.3 mm (4-inch) and 168.3 mm (6-inch) HDPE PE3408 pipe was the least expensive option but this was still beyond the economic feasibility limit for the project. A regulatory requirement that existing gas customers not subsidize the provision of gas to new customers required that other options be investigated to make this a viable project.
Information on PE 100 had previously been provided to Manitoba Hydro. A preliminary hydraulic design based on the use of PE 100 with a minimum required strength (MRS) rating was found to meet the five-year system capacity requirements. A maximum operating pressure based on a categorized required strength (CRS) would be required to meet the growth allowance. For Shoal Lake, the design winter temperature is – 35 degrees C and the peak natural gas flow is associated with winter heating loads. Ground temperature at pipe line depth during the winter is typically a nominal 0 degrees C while the annual average temperature is 5 to 8 degrees C. Selection of a PE 100 material with a CRS rating of 11.2 at 14 degrees C was considered conservative.
The budget estimate for the installation of the PE 100 indicated material and installation savings of about C$200,000 over the lowest cost HDPE option. This cost reduction was sufficient to provide favorable project economics and for the project to proceed. Details of the hydraulic system design results for the different pipe materials are shown in Table I.
Table I: Hydraulic Design for Various Pipe Materials.
Non-Listed Materials. An application to the Manitoba Public Utilities Board was required to obtain permission to use a material that was not listed as an approved material in CSA Z662/ CSA B137.4. The application requested the approval to use PE 100 pipe for the specific project with the material rated using the categorized required strength and the ISO methodology for determination of maximum operating pressure. The application documented the details of our investigation of PE 100 and the testing and work performed to gain familiarity with the material. This work generally included: 1) testing to establish acceptable butt fusion parameters, 2) test installations of electrofusion couplings, 3) installation and tapping of electrofusion saddle fittings on a main pressurized to 160 psig, and 4) squeezing of a main at 160 psig.
Butt Fusion. Nine butt fusions were performed using the Manitoba Hydro Polyethylene Fusion Guide parameters for medium density polyethylene as a guide. The guide requirements indicate a fusion tool temperature of 425 degrees F, a heat cycle time of 60-65 seconds, and a holding time after fusion of 90 seconds. Fusions were performed with a range of temperatures between 405-450 degrees F and times between 60-70 seconds.
Two fusions were used to create a test assembly for squeezing and installation of saddle fittings. These two fusions were soap tested at 160 psig but no further tests were performed. The remaining seven fusions were subjected to destructive testing and analysis using the McElroy McSnapper machine. This machine combines the tensile impact test ASTM D 1822 and the high speed tensile test ASTM D 2289 to permit an evaluation of a butt fusion.
Three test coupons were prepared from each fusion joint from locations at 120-degree separation from the other coupons. All three coupons were then tested on the McSnapper. All test coupons failed on the pipe material and not at the fusion which is indicative of acceptable fusions. The key parameter in the evaluation of the results is the failure energy. The test fusions should fail at or above the level of the control coupon. While four samples were seen to fail at energy levels (between 240-247 lbf/square inch) below that of the test coupon (248 lbf/square inch), they were within the test accuracy of the equipment. The results indicated that all fusions tested were acceptable for the range of fusion conditions tested and that the use of the fusion parameters currently in use within the company would be acceptable for use with PE 100.
Electro Fusion Fittings. Two electro fusion couplings were used to connect flange adapters to a 1.5-m pipe length to create a test assembly. The complete test assembly was then pressurized to 1,140 kPa (165 psig) and two high volume tapping tees were installed on the pressurized test assembly. One of the two tapping tees was then tapped on the pressurized test assembly. Destructive testing of the fittings was performed in accordance with the requirements of CSA B137.4.1 Electrofusion-type polyethylene (PE) fittings for gas service. All fittings passed the tests
Squeezing. Squeezing of natural gas pipe is commonly used as a technique to stop the gas flow during emergency response to pipeline damages or for maintenance or new construction purposes. Squeezing was performed on a test section pressurized to 1,100 kPa (160 psig). The first squeeze was performed with a manual squeezer at a nominal 600 mm (24 inches) from the electro fusion coupling at one end. Squeezing was performed in accordance with standard Manitoba Hydro procedures. During squeezing of the pipe it was found to be a difficult one person process and, towards the end of the squeeze, two people were needed to operate the squeezer.
The outlet valve on the downstream side of the squeezer was opened after the pipe could no longer be further compressed. A noticeable flow of nitrogen past the squeeze was evident. With the pressure reduced due to the release of the downstream pressure, further movement of the squeeze tool was possible and the flow was further reduced. However, the nitrogen flow past the squeezer was high enough to be audible and this was considered to be unacceptable.
The second squeeze was performed using a hydraulic squeezer located at approximately 600 mm (24 inches) from the electro fusion coupling at the other end of the test assembly and at 300 mm (12 inches) from the first squeeze location. The downstream outlet valve was then opened to release the downstream pressure. No nitrogen flow past the squeeze was evident. The outlet valve was then closed for several minutes. No pressure build-up was seen on the pressure gauge but a faint pressure release could be heard when the outlet valve was reopened.
While the minor pressure build up was indicative of a small leak, the squeeze was considered to be acceptable. The squeezed area was sectioned at 90 degrees to the squeeze and a visual examination of the squeeze in accordance with ASTM F 1734 Standard Practice for the “Qualification of a Combination of Squeeze Tool, Pipe, and Squeeze-Off Procedure to Avoid Long Term Damage in Polyethylene Gas Pipe.” The examination with the naked eye and at ten times magnification did not identify any defects considered to be unacceptable when evaluated with the requirements of ASTM F 1734.
Approval. Following the review of the application and responses to a number of questions, the PUB provided approval to the application with a number of restrictions/requirements including:
* The application of PE 100 using the ISO pressure rating methodology based on the minimum required strength (MRS) was accepted. The use of categorized required strength (CRS) was not accepted, until CRS was added to CSA B137.4.
* Manitoba Hydro was to monitor the work of the CSA B137.4 Technical Committee and report on the work being performed to consider the inclusion of PE 100 and the MRS rating method in the standard.
* Requirements to review and audit the manufacturing of the purchased pipe and for quality control during installation.
Construction. Prior to the start of construction, project-specific fusion training was provided to contractor and inspection staff. During construction additional fusion testing was performed and all fusion joint locations were surveyed with GPS and recorded. The pipe was installed using open cut, plowing and directional drill techniques. The route permitted extensive use of plowing which permitted installation rates in excess of 1,500 meters per day from a single crew. Several directional drills in excess of 300 meters were required in wet areas. The pipe was supplied in 400-meter-long reels.
Plastic-to-steel transitions required for connection of the PE 100 pipe to above-ground steel facilities were manufactured from the same pipe material used for the project. At the contractor’s discretion, electrofusion couplings were primarily used for joining. The contractor commented on the increased stiffness of the PE 100 over the medium-density polyethylene generally used by Manitoba Hydro but did not experience any difficulties with the installation of the material. Modifications to an existing pressure regulation station were performed at the point of gas supply for the new system and a new pressure regulating station was constructed just outside the Town of Shoal Lake.
The route permitted extensive use of plowing which permitted installation rates in excess of 1,500 meters per day from a single crew.
CSA Z662 requires a pressure test at 1.4 times the desired maximum operating pressure (MOP). Approval had been provided to establish the MOP at 145 psig based on the minimum required strength (MRS). The decision was made to test at a pressure of 1.4 times the MOP of 160 psig determined with a categorized required strength (CRS). While not approved to operate at this higher pressure, the higher test pressure permits the opportunity for a potential future increase in the line pressure rating when the CRS is accepted in the future.
For testing purposes, the line was separated into three 8-km sections and one 5-km section. To obtain the required 225 psig test pressure a two-stage air compressor was used. During one test a ductile failure of the pipeline occurred adjacent to the connection to the compressor. Investigation determined that the after cooler on the compressor had failed resulting in the supply of air at over 80 degrees C. A 10-meter section of the pipe was removed with a sample sent to the manufacturer for further testing. The manufacture’s testing confirmed that there were no issues with the pipe. Following repair of the after cooler, all pressure testing was completed.
The system has been operating since September 2006. To date the maximum pressure that the PE 100 piping system has operated at is 135 psig.
In June 2005, a project was initiated within CSA B137.4 to consider the addition of minimum required strength (MRS) and categorized required strength (CRS) to the CSA B137.4. During the course of the project, a decision was made to focus initially on the addition of the MRS. Following extensive work, the use of MRS was accepted and is included in the 2009 edition of CSA B137.4 Polyethylene (PE) piping systems for gas service which was issued in November 2009. For Canadian gas utilities, the inclusion of MRS and CRS in CSA Z662 also is required. The next edition of CSA Z662 Oil and gas pipeline standard has been issued for public review and includes the provision to use an MRS and CRS rating. It is anticipated that the new CSA Z662 will be issued in 2011. Work on the future inclusion of the categorized required strength rating (CRS) in CSA B137.4 continues.
Gene Palermo is president of Palermo Plastics Pipe Consulting. He 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 more than 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 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. Palermo is the only person to receive both of these awards.
Tim Starodub, P.E. is the natural gas standards engineer for Manitoba Hydro. He is a mechanical engineer with more than 25 years of experience in natural gas distribution and energy utilization and is responsible for the review and acceptance of new materials and systems used in the company’s natural gas system. He provides support to construction, operations and maintenance activities and provides analysis and review of failed materials.