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The mechanical noises which accompany the heating and cooling cycles of heating pipe systems made from metals are almost eliminated when plastic piping systems are employed. Plastic pipes reduce and muffle the transmission of both mechanical noise and 'water hammer' effects.
Mechanical Noise
For sound conduction through solids, the sound insulation and absorption of the material are important criteria and this is a function of the density and elasticity of the material. The acoustic properties of plastics, when plotted over broad ranges of frequencies and temperature, are usually determined by the glass transition temperatures of the particular plastic material. At temperatures above the glass transition temperature, Tg, the sound speed is greatly reduced and sound absorption is greatly increased. The Tg of Polybutene-1 homopolymers is -6°C. The typical service temperature of a heating or potable water installation is well above this Tg so sound transmission is significantly reduced. With metals such as copper, no glass transition exists so sound transmission remains high, even at temperatures approaching the melting point of the material.
For materials generally, it is observed that the higher the density, the higher the sound speed. The data presented in the table below clearly shows this relationship for a selection of common plumbing materials.
| Sound Velocity in Materials |
| |
Density(g/cm3) |
Elastic Modulus(MPa) |
Sound Velocity(m/s) |
| Copper |
7,2 |
110.000 |
3.900 |
| PB-1 |
0,93 |
350 |
620 |
| CPVC |
1,56 |
3.500 |
2.350 |
| PEX |
0,95 |
600 |
800 |
| Soft Rubber |
0,90 |
90 |
320 |
Obviously, design of the installation is critical in ensuring the quiet operation of a plumbing system. To achieve the best results, due consideration should be given to the choice of pipe and fitting materials and the optimum layout of the piping system relative to the building's construction.
Water Hammer
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'.
Five factors determine the severity of water hammer:
1. Velocity
2. Modulus of elasticity of the pipe material
3. Inside diameter of the pipe
4. Wall thickness of the pipe
5. Valve closing time
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))Ѕ
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 Polybutene-1, 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 1Ѕ" OD pipes of different plastic materials, designed for the same pressure service.
| |
E |
E |
DI |
T |
V |
Ps |
Ps |
| |
[psi] |
[MPa] |
[in] |
[in] |
[ft/s] |
[psi] |
[bar] |
| PB-1 |
65000 |
450 |
1.28 |
0.15 |
5.0 |
49.5 |
3.4 |
| PEX |
87000 |
600 |
1.14 |
0.22 |
5.0 |
72.4 |
5.0 |
| PP |
116000 |
800 |
1.05 |
0.26 |
5.0 |
93.0 |
6.4 |
| CPVC |
507000 |
3500 |
1.22 |
0.18 |
5.0 |
140.6 |
9.7 |
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