The inner surface of GRP pipes and fittings is very smooth.
The geometrical roughness is seldom higher than 30 microns and, most important, it remains constant in the time as shown in the following table.

Pipe material	New	old
GRP	0,02-0,10	0,02-0,10
Steel	0,05-0,10	1,00-3,00
Cast iron	0,25- 1,00	1,00-3,00
Concrete	0,40- 1,20	1,00-4,00

 

The pipe designers can take advantage of this by decreasing the pipe diameter, or by increasing the flow rate or saving on pumping energy. The fluid velocity can also be higher than in pipelines made with other materials because of the smoothness of the inner wall.

The following graph shows the head loss for various pipe diameters and flow rates.

Water hammer is seldom a problem in GRP pipes.
In fact the values of the over pressure are much less than in the case of steel pipes because of the low modulus of elasticity of GRP and are further reduced by the increase of volume due to the flexibility of the joints.
According to AWWA C950 the pipe can take short term transient overpressure up to 140% of the nominal pressure.

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