PB vs. PE-RT & PEX

A true cost comparison between piping systems should include installation time, jointing options, life span and SDR rating.

Polybutylene (PB-1) offers tangible benefits for piping systems compared to PE-RT and PEX

For building projects, the true cost variation of piping systems across competing materials is more than a per length cost comparison for the same outside pipe diameter.

Specifiers look at: ease of installation impacting onsite costs; jointing options, long-term system performance and projected life span; and Standard Dimension Ratio (SDR) comparing pipe materials for durability against pressure.

When compared to PE-RT and PEX systems, Polybutylene (Polybutene, PB-1) offers significant benefits across a broad range of performance categories all contributing to make Polybutylene piping systems the optimum choice for high performance piping installations.
 

What is SDR?

SDR rates pipe durability against pressure and correlates a pipe's outside diameter and wall thickness.

The SDR or the Standard Dimension Ratio refers to the geometry of a pipe. SDR is a method of rating a pipe's durability against pressure and it describes the correlation between the pipe dimension and the thickness of the pipe wall. SDR 11, for example means that the outside diameter of the pipe is eleven times the thickness of the wall.

• High SDR ratio
  The pipe wall is thin compared to the pipe diameter
• Low SDR ratio
  The pipe wall is thick compared to the pipe diameter

Example calculation:
SDR for a pipe with an outside diameter of 100mm and wall thickness of 5mm can be calculated as:
100mm / 5mm = SDR 20
 

Why does SDR matter for piping systems?

Due to the higher SDR ratio of Polybutylene (PB-1) compared to either PE-RT and PEX, Polybutylene piping systems deliver the following benefits because of its lower wall section requirements for the same pressure rating and outside pipe diameter:

• Less material for the same pressure capability
• Less weight per meter of pipe
• Lower outside pipe diameter for the same performance
• Larger inside area for the same outside diameter providing:
  - Higher flow rate at the same pressure
  - Lower pressure loss, requiring less energy to run a system or pumps with lower capacity

SDR Classes and Pipe Dimensions

SDR classes quoted in National Standards for District Energy piping systems

SDR Classes and Water Hammer

The higher the SDR Class the lower the water hammer at a given flow rate. Polybutylene (PB-1) has the highest SDR Class compared to PP-H, PP-R, PE-RT and PEX

A column of moving water within a pipeline contains stored kinetic energy arising from its mass and velocity. Since water is essentially incompressible, this energy cannot be absorbed when a valve is suddenly closed.

The result is a high instantaneous pressure surge normally referred to as 'Water Hammer'.

With people living together in greater density the noise levels and acoustic properties of piping systems are a significant issue. Piping systems that minimise fluid noise and water hammer where pipework passes through ceilings and walls are a key element in addressing residents’ noise concerns.

Five factors determine the severity of water hammer:

• Velocity
• Modulus of elasticity of the pipe material
• Inside diameter of the pipe
• Wall thickness of the pipe
• Valve closing time

Repetitive water hammer can be destructive to pipe systems. Beside the noise, water hammer can cause pipelines to break if the pressure is high enough.
 

The maximum theoretical value of pressure surge Ps is:

v₀ · a · ρ = p_s

v₀ = velocity of the medium [m/s]
a = propagation rate of the pressure wave [m/s]
ρ = density of the medium [kg/m³]
p_s = pressure surge – water hammer [N/m²]

The maximum surge pressures caused by water hammer can be calculated using the following equation taken from the 'Handbook of Thermoplastic Piping System Design', Thomas Sixsmith and Reinhard Hanselka, Marcel Dekker Inc., pp 65-69

Ps = V((3960 E t)/(E t + 3 x 105 DI))½
where:
Ps = surge pressure (psi)
V = water velocity (ft/sec)
DI = inside diameter of the pipe (in)
E = modulus of elasticity of the pipe material (psi)
t = pipe wall thickness (in)

The low elastic modulus of Polybutylene, combined with reduced wall thickness gives rise to a low surge pressure for a given pipe OD and pressure rating. The table below compares maximum surge pressure for 38.1 mm (1-1/2") OD pipes of different plastic materials, designed for the same pressure service.
 

	E	E	DI	t	V	Ps	Ps
	[psi]	[MPa]	[mm]	[mm]	[ft/s]	[psi]	[bar]
Polybutylene	65000	450	32.5 (1.28")	3.8 (0.15")	5.0	49.5	3.4
PE-X	87000	600	28.9 (1.14")	5.6 (0.22")	5.0	72.4	5.0
PP	116000	800	26.7 (1.05")	6.6 (0.26")	5.0	93.0	6.4
CPVC	507000	3500	30.9 (1.22")	4.6 (0.18")	5.0	140.6	9.7

When compared to PP-H, PP-B, PE-RT and PEX, Polybutylene has the highest SDR Class and delivers the best acoustic capabilities including the lowest level of water hammer.
 

The source for piping dimensions comparing materials Polybutylene, PE-RT and PEX

The current Russian standard for District Heating (GOST 56730 – 2015) and the Dutch guideline (BRL 5609 - and the draft of renewed BRL 5609) both include a comparison of 3 materials for District Heating piping systems: Polybutylene, PE-RT and PEX

Both the Russian standard and the Dutch guideline have the same requirements in relation to the pipe dimensions and SDR classes of the 3 service pipe materials operating at pressures of 6 bar, 8 bar and 10 bar.

Per the Russian standard and the Dutch guideline the table (at right) is an excerpt of the relevant table showing the SDR classes for the listed materials at different pressure ratings. As indicated, for each operating pressure Polybutylene is listed in the highest SDR class when compared to either PE-RT and PEX. The section below explains what this means, why standards refer to pipe dimensions and SDR classes and what are the benefits for pipe system specifiers.

 

Polybutylene pressure capability delivers benefits versus PEX and PE-RT

The higher SDR class of PB-1 delivers the following benefits:
• Thinner wall section
• Less material (less weight)
• Larger inside diameter and area

To illustrate the performance of Polybutylene, PE-RT and PEX in relation to the given operating pressure of 8 bar at the small pipe diameter of 50mm ø, the table and diagram below provide a comparison of the internal pipe dimensions required.
 

Example 1:  Small pipe – 50mm diameter @ 8 bar

PB-1 is stronger than both PE-RT and PEX, and with an operating pressure of 8 bar and an outside pipe diameter of 50mm ø the required wall thicknesses are:

Per above, at the same water pressure, the larger inside diameter of Polybutylene 50mm outside ø pipe delivers a substantially higher flow rate than the other two materials. Taken the other way, at a given flow rate PB-1 pipes yield a lower pressure loss requiring less energy to run systems and/or pumps with a lower capacity.

As shown in the graphs above and for the purposes of comparison, PE-RT may be considered the benchmark at 100%. When comparing the inside cross-section area of a 50mm ø pipe (left graph) Polybutylene clearly outperforms PE-RT with an additional 27% of volume. Also, in comparing the amount of material per meter for a 50mm ø pipe rated for 8 bar (right graph), Polybutylene pipe uses 29% less material than PE-RT.

Once again for the purposes of comparison, PE-RT may be considered the benchmark at 100%. Per the above graph (left), using the same operating water pressure, a 50mm outside diameter pipe (8 bar) made from Polybutylene delivers a substantially higher flow rate of +35% when compared to the identically rated PE-RT pipe of the same outside diameter.

Measured using the other comparison point (right): at a given flow rate (output) Polybutylene pipes yield a 44% lower pressure loss versus PE-RT pipes. This means that Polybutylene pipes require less energy to run a system - or - can accommodate pumps with a lower capacity for the same output.
 

Example 2:  Large pipe – 160mm diameter @ 10 bar

Due to a higher SDR rating (and therefore a thinner wall section) a Polybutylene pipe of 140mm ø delivers the same performance as a PE-RT pipe of 160mm ø, but with a smaller outside diameter and larger inside pipe cross-section area.

At a 10 Bar operating pressure pipe of 160mm outside diameter:

• PE-RT @ SDR 6
  160mm ø pipe has an internal cross section area of 8,958mm²

• PEX @ SDR 7.4
  160mm ø pipe has an internal cross section area of 10,605mm²

• Polybutylene @ SDR 9
  With a smaller outside diameter of 140mm ø Polybutylene has an internal cross section area of 9,263mm²

In addition, as shown in the graph (right), the weight of 160mm outside diameter Polybutylene pipe rated for 10 bar is almost half of the weight for the same outside diameter and rating pipe made from PE-RT.
 

Polybutylene is a versatile material for all available jointing techniques

Specifying Polybutylene piping systems offers:

• Substantial material saving opportunities, while at the same time increasing the capacity of the system
  – thinner walls
  – increase of the available inside cross section area
• A higher degree of design freedom for projects including District Energy grids
  – opportunity for using smaller outside pipe and fitting diameters
• A clear opportunity for reduced integral installation cost and operating cost
  – smaller pipe support frames
  – use of less insulation material
  – smaller pumps running at reduced energy consumption
  – lighter overall weight for easier handling and lower shipping costs
• The capability to utilize all available jointing techniques
• Best in class acoustics including the lowest level of water hammer
• The ability to be fully recycled

Polybutylene vs. PE-RT & PEX

Polybutene is often referred to as Polybutene-1, Polybutylene, PB-1 or PB.

Polybutene is not sold by PBPSA members for use in pipe applications intended for use in North America, and those parties require their customers or distributors not to sell products made from Polybutene into pipe applications for North America.