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