| The underground piping market in North America has | | | | 100 25 11 |
| seen tremendous growth over the last 30 years in the | | | | 150 18 7.3 |
| use of thermoplastic materials. Benefits such as | | | | 200 14 6.3 |
| corrosion resistance, improved hydraulics, and reduced | | | | SURGES |
| installation costs have been paying large dividends for | | | | Another tremendous benefit of using thermoplastic |
| owners of watermain, sanitary and storm sewer | | | | piping is that surges created are lower than those |
| systems. | | | | associated with more rigid materials such as metallic or |
| The most widely used and accepted of this group of | | | | concrete cylinder pipe. The inherent flexible nature of |
| nonmetallic polymers is Polyvinyl Chloride, also known | | | | thermoplastics allows transient shock waves to be |
| as 'PVC' or 'vinyl'. Vinyl has a successful track-record | | | | easily dampened and absorbed. This minimizes surge |
| in the application of underground pipe dating back to | | | | effects on the entire system. |
| the rebuilding of post-WWI Germany. It has long been | | | | Positive pressure surges in pipelines can be |
| considered to be one of the most durable polymers | | | | approximated by using the following two equations. |
| for both underground and above-ground piping | | | | Equation (3) a = 4660 / [1 + (k/E)(DR-2)]^0.5where,a = |
| systems. | | | | wavespeed of surge wave (fps)k = fluid bulk modulus |
| Another thermoplastic used in the underground pipe | | | | (= 300 000 psi for water) |
| market is High-Density Polyethylene (HDPE). This | | | | E = modulus of elasticity of pipeline material (psi) |
| material has been used for well pipe, gas piping and | | | | DR = dimension ratio (= OD/t) |
| drainage tubing before recent entry into the watermain | | | | Equation (4) Ps = aV / (2.31) gwhere, |
| and sewage forcemain markets. | | | | Ps = pressure surge (psi)a = wavespeed (fps) |
| HDPE and PVC are remarkably similar in their nature | | | | V = velocity (fps)g = acceleration due to gravity |
| of responses to such stress loadings as internal | | | | = 32.2 ft/s^2 |
| pressure and soil loads. Although responses are similar, | | | | The Modulus of Elasticity of PVC 1120 at 73.4°F is |
| they are not identical. In fact the magnitudes of their | | | | 400 000 psi, while that for HDPE 3408 is 115 000 psi. |
| respective strengths are dramatically different. | | | | The table below summarizes the surge pressures |
| This report is intended to investigate some of the | | | | expected for every 1 ft/s instantaneous velocity |
| similarities and differences between the design of PVC | | | | change in both PVC and HDPE. For velocities other |
| and HDPE in terms of the application of underground | | | | than 1 ft/s, the surge will be equal to the values in the |
| pressure piping. | | | | table multiplied by the actual velocity in ft/s (i.e. if V = 3 |
| PRESSURE RATING | | | | ft/s, surge = 3 times the table value for the given |
| The long-term pressure rating of a thermoplastic pipe | | | | material and DR). |
| is defined as the maximum internal pressure at which | | | | Table 3 - One Ft/s Surges |
| the pipe can operate continuously. The ratings of both | | | | PVC HDPE |
| PVC and HDPE are found using the ISO Equation for | | | | (E=400 000 psi) (E=115 000 psi) |
| thermoplastics: | | | | DR Ps (psi) DR Ps (psi) |
| Equation (1) P = 2S / (DR-1)where P = pressure rating | | | | 51 10.8 21 8.8 |
| of the pipe | | | | 41 11.4 17 9.9 |
| S = design stress of pipe material | | | | 32.5 12.8 13.5 11.3 |
| DR = dimension ratio of the pipe, (OD/t) | | | | 25 14.7 11 12.7 |
| The main difference between PVC and HDPE | | | | 21 16.0 9 14.3 |
| pressure capacity lies in the value of the design stress. | | | | 18 17.4 7.3 16.3 |
| For PVC 1120 compounds, the design stress is 2000 | | | | 14 19.8 6.3 17.9 |
| psi while that of HDPE 3408 is only 800 psi. These | | | | Although HDPE is by nature a more flexible material |
| design stresses were both derived in exactly the | | | | than is PVC, the surges created in pipe of equivalent |
| same fashion. A factor of safety of 2.0 was applied | | | | pressure ratings are very similar. For example, for a |
| to the long-term hydrostatic strength (i.e. the | | | | 100 psi pipeline, the surge created by a 1 ft/s velocity |
| Hydrostatic Design Basis - HDB) of each material. The | | | | change would be 11.4 psi for DR41 PVC and 9.9 psi for |
| HDB for PVC 1120 is 4000 psi while that of HDPE | | | | DR17 HDPE. |
| 3408 is 1600 psi. | | | | Overall, the surges for both materials are well below |
| The following examples illustrate the use of the ISO | | | | the values of metallic pipe which typically generate |
| Equation to determine pressure ratings. | | | | surges of 50+ psi for every 1 ft/s instantaneous |
| Example 1 - Find pressure ratings of DR21 pipe for | | | | velocity change. Continuous pressure surges should |
| both (a) PVC, and (b) HDPE. | | | | not be ignored in any pressure pipeline design, |
| Solution - use equation (1) | | | | regardless of material. |
| P = 2S / (DR-1) | | | | BUCKLING RESISTANCE |
| (a) for PVC, S = 2000 psi | | | | The ability of a soil surrounding a flexible pipe to |
| Substituting, P = (2) x (2000 psi) / (21 - 1) | | | | strengthen the pipe is numerically known as the Soil |
| = 200 psi | | | | Stiffness (E'). E' numbers are derived empirically to |
| (b) for HDPE, S = 800 psi | | | | represent the quality of soil and degree of compaction |
| Substituting, P = (2) x (800 psi) / (21 - 1) | | | | as a 'psi' value. E' values are described in detail in |
| = 80 psi | | | | standards ASTM D 2321 or CSA B182.11. A brief |
| Example 2 - (a) Find the pressure rating of PVC DR41 | | | | summary is presented below. |
| and then (b) find the equivalent DR of HDPE to yield | | | | Table 4 - Soil Stiffness |
| the same rating. | | | | Soil Stiffness E' (psi) Material Compaction (S.P.D.) |
| Solution - use equation (1) | | | | 3000 Manuf. Angular 90% |
| (a) P = 2S / (DR-1) | | | | 2000 Clean Sand/Gravel 90% |
| = (2 x 2000 psi) / (41-1) | | | | 1000 Sand/Gravel/Fines 90% |
| = 100 psi | | | | 500 Sand/Gravel/Fines 85% |
| (b) rearranging equation (1), | | | | Buckling may occur in any pipe if the total load in the |
| DR = (2S / P) + 1 | | | | inward direction (i.e. static soil + traffic + vacuum) |
| = [(2 x 800 psi) / 100 psi] + 1 | | | | exceeds the critical buckling resistance of the pipe. A |
| = 17 | | | | thermoplastic pipe must be designed to have sufficient |
| Therefore, to obtain a 100 psi pressure pipe, the 2 | | | | strength to resist inward structural collapse, or buckling. |
| options would be PVC - DR41 or HDPE - DR17. | | | | Tremendous strength can be added to any pipe's |
| The following points can be concluded from the above | | | | resistance by having solid lateral soil support in the |
| information: | | | | Haunch Zone of a buried pipe trench, i.e. a high soil |
| (a) The ratio of PVC to HDPE in terms of tensile | | | | stiffness. |
| strength is equal to the ratio of the design stresses, i.e. | | | | Below is a summary of the critical buckling strengths |
| 2000:800 which is 2.5:1, and | | | | of various DR's of PVC and HDPE for (a) Pcr, an |
| (b) The wall thickness of HDPE must be 2.5 times | | | | unsupported condition (i.e. subaqueous or |
| thicker than that of PVC to obtain pipe with equal | | | | above-ground) and (b) Pb, a buried trench condition |
| pressure ratings. | | | | with a specified soil stiffness, E' (for this example, = |
| Below is a summary of long-term pressure ratings for | | | | 500 psi). |
| both PVC and HDPE derived using the ISO Equation | | | | Table 5 - Buckling Strengths |
| and a S.F. of 2.0. | | | | PVC DR Pcr (psi) Pb (psi) |
| Table 1 - Pressure Ratings | | | | 14 425.8 530.6 |
| PVC HDPE | | | | 18 190.2 354.6 |
| DR Rating (psi) DR Rating (psi) | | | | 21 117.0 278.1 |
| 51 80 21 80 | | | | 25 67.4 211.1 |
| 41 100 17 100 | | | | 32.5 29.8 140.4 |
| 32.5 125 13.5 128 | | | | 41 14.6 98.3 |
| 25 165 11 160 | | | | 51 7.3 69.5 |
| 21 200 9 200 | | | | HDPE DR Pcr (psi) Pb (psi) |
| 18 235 7.3 254 | | | | 6.3 266.2 419.6 |
| 14 305 6.3 300 | | | | 7.3 171.2 336.5 |
| Although CSA B137.3, AWWA C905 and ASTM | | | | 9 91.4 245.8 |
| D2241 all use a S.F. = 2.0, there is one PVC standard | | | | 11 50.0 181.8 |
| that uses a S.F. = 2.5, namely AWWA C900-97 (note | | | | 13.5 27.0 133.6 |
| - this standard will soon be changing to be similar to | | | | 17 17.6 107.9 |
| AWWA C905). As well in this C900 standard, the pipe | | | | 21 7.2 69.0 |
| is further de-rated by a 2 ft/s surge. (Designers should | | | | 26 3.8 50.1 |
| not confuse the 'Pressure Class' terminology of C900 | | | | 32.5 2.0 36.4 |
| with the long-term ratings of HDPE.) If one wishes to | | | | To investigate a typical situation, a pressure pipeline is |
| select a HDPE pipe that is equivalent to a particular | | | | buried 10 feet in soil with a density of 120 lb/ft^3 and |
| PVC Pressure Class, the identical design criteria should | | | | subjected to a momentary negative 10 psi vacuum |
| be used to determine a Pressure Class of HDPE. In | | | | due to a transient shockwave. A total negative load of |
| other words, the design stress must be derived using | | | | (-)18.3 psi would be created. As can be seen from the |
| S.F. = 2.5, and the pipe must be de-rated with the | | | | above table, this negative pressure would exceed |
| surge of a 2 ft/s velocity. To determine equivalent | | | | some of the Pcr values of PVC (DR41 and 51) as well |
| pressure classes of HDPE and PVC, refer to Table 3 | | | | as HDPE (DR17, 21, 26 and 32.5). By having a minimum |
| presented later in the text and use Equation (2) shown | | | | soil stiffness of 500 psi, the values of Pb for all DR's of |
| below. | | | | both materials will easily exceed the total negative load |
| Equation (2) P.C.= P'- 2 Ps | | | | and buckling will not occur. |
| Where P.C. = pressure class of pipe | | | | If any of these pipes happened to have significant |
| P' = pressure rating of pipe using S.F. = 2.5 | | | | voids in their backfill, it is conceivable that buckling |
| Ps = surge pressure for 1 fps velocity change | | | | failure could occur. It is imperative that lower pressure |
| Note: Ps for PVC and HDPE are given in Table 3. | | | | rated thermoplastic pipe be installed so as to have a |
| Example 3 - (a) Find the pressure class of DR25 PVC | | | | minimum soil stiffness, E', of 500 psi. Voids in the |
| and (b) find the DR of HDPE to give the same | | | | haunch support zone can be prevented by using |
| pressure class. | | | | proper bedding material and light compaction. This point |
| Solution - First solve for new design stresses. | | | | is especially relevant if ever considering assembling |
| PVC: S = HDB / S.F. | | | | thermoplastic pipe above the trench and rolling it in |
| = 4000 psi / (2.5) | | | | before backfilling. Buckling is a situation far less likely to |
| = 1600 psi | | | | occur if the pipe is installed using a conventional open |
| HDPE: S = HDB / S.F. | | | | trench with moderate compaction beside the pipe as |
| = 1600 psi / (2.5) | | | | the line installation progresses. |
| = 640 psi | | | | SUMMARY |
| Now use equation (2) and the values of Table 3 to | | | | To do a thorough comparison of PVC and HDPE, |
| solve. | | | | many other factors would have to be considered such |
| (a) PVC DR25 | | | | as: material cost, installation cost, connection methods, |
| P.C. = [2S / (DR-1)] - 2 Ps | | | | and manufacturing test requirements. The designer |
| = [(2)(1600 psi) / (25-1)] - (2)(14.7 psi) | | | | should also ensure that each material has a successful |
| = 100 psi | | | | track record for the application being considered. |
| (b) HDPE - trial and error using equation (2)try DR11, | | | | This report has offered a snapshot comparison of the |
| P.C. = (2) (640 psi) / (11-1) - (2)(13.4 psi) | | | | 2 thermoplastic materials used most often for |
| = 100 psi | | | | pressure pipe in North America - PVC and HDPE. The |
| Below is a table of minimum DR's of HDPE to be | | | | capacity of each material was illustrated in terms of |
| equivalent to the pressure classes of PVC as defined | | | | their pressure ratings, surge performance and buckling |
| in AWWA C900. | | | | resistance to allow designer an equal comparison |
| Table 2 - Pressure Class DR's | | | | between PVC and HDPE. |
| Pressure Class (psi) PVC-DR HDPE-DR | | | | |