Mitsubishi Heavy Industries Technical Review Vol. 53 No. 4 (December 2016)18

Development of Highly-durable Steel Pipe
Lining for Seawater

Kobe Nuclear Energy BusinessO p e r a t i o n D e pa r t m e n t ,Nuclear Energy S ystems Division,
Energy & Environment

In nuclear power plants in Japan, seawater is used as the cooling water. The inner surface ofpiping for circulating the seawater to the cooling system of each facility is lined with polymericmaterials such as rubber or polyethylene (PE) to prevent corrosion caused by the seawater. In
recent years, cases of degradation caused by aging of the lining materials have occurred, such as
cavitation erosion in high flow velocity sections including the narrowed portion of valves andorifices, slurry abrasion, and cracking due to the degradation of physical properties, and demand
from power companies, which are the users of the equipment, for the development of a lining
material with higher durability and reliability, has been increasing. Therefore, we have developed ahighly-durable lining material that has higher cavitation erosion resistance and greater cracking
resistance than the existing lining materials in cooperation with Ube-Maruzen Polyethylene Co.,
Ltd., and started applying the developed lining material to nuclear power plants in Japan as a lining
material that can withstand harsh environments.

|1. Features
The developed highly-durable steel pipe lining has not only all the advantages of various
lining materials that were used separately depending on the use environment in the past, but alsosignificantly higher performance (in particular, the cavitation resistance and cracking resistance)
than existing lining materials, and can characteristically be produced using the same manufacturingmethod and at an equivalent cost as conventional polyethylene steel pipe lining. The developed
highly-durable steel pipe lining meets almost all the use requirements of seawater piping in nuclear
power plants in Japan, while attaining significant improvement in durability, and allows theunification of lining materials that were used separately depending on the use condition and areduction of the maintenance and inspection burden of seawater piping.

|2. Specifications
The basic physical properties of the developed highly-durable seawater pipe lining material
are as noted below (Table 1).
The material used as the base of the developed lining material is linear low density
polyethylene resin (LLDPE), which has conventionally been used as a general lining material, andthe performances of the developed material were improved by adjusting the molecular structure ofthe resin.
Table 1

Basic physical properties of highly-durable seawater pipe lining material

Item
Test method
Tensile stress at break (MPa)JIS K 7161
Tensile strain at break (%)
JIS K 7161
Melting point (℃)
JIS K 7121
MFR** (grams per ten minutes)
JIS K 7210
Hardness (HDD)
JIS K 7215
Adhesion strength (N/mm)
JWWA K-132* Representative value** Melt Flow Rate

Characteristic value*
24880
126
2.7
59
18

Mitsubishi Heavy Industries Technical Review Vol. 53 No. 4 (December 2016)
192.1

Cavitation erosion resistanceFor the lining around narrowed sections such as flow control valves and orifices, measuresfor cavitation erosion need to be taken. Figure 1 shows the verification results of the cavitation
erosion rate of conventionally-used chloroprene rubber/PE resin (low density PE resin (LDPE) andLLDPE) and the developed highly-durable PE implemented using a cavitating jet test method inaccordance with ASTM G134-95.

Figure 1 Comparison of cavitation erosion
rate of various lining materials

Figure 2 Erosion state of Cavijet irradiation
partFigure 2 shows the external view of the test pieces after the cavitation erosion test. In thiscavitation erosion test, multiple kinds of lining materials with different physical property values
were made on a trial basis and their cavitation erosion rates were measured. The test results show astrong correlation between the tensile product (equal to the tensile stress multiplied by the tensile
strain at break) and the cavitation erosion rate. Through improvement in the tensile product, thecavitation erosion rate was successfully reduced to about 1/100 of that of rubber and about 1/10 ofconventional LLDPE. The tensile product is used as an index of the tensile fracture energy ofrubber and plastic materials. When the tensile product value is larger, fractures are less likely tooccur.

2.2

Cracking resistance

The mechanical properties of polymeric lining materials deteriorate over time resulting in anincreased risk of cracking. Therefore, it is important to enhance the initial mechanical properties.
The developed highly-durable PE shows less degradation of the initial mechanical properties evenafter a degradation acceleration test corresponding to 60 years of actual service, has a tensile
product about 1.4 times higher than that of brand-new conventional PE (Figure 3), and reduces therisk of cracking caused by age degradation.

Figure 3 Comparison of tensile product between newand aged materials

Mitsubishi Heavy Industries Technical Review Vol. 53 No. 4 (December 2016)202.3 Seawater permeability
When piping is used for seawater for a long-time, the seawater penetrates into the lining
material and finally reaches the carbon steel base material, resulting in corrosion, degradation of
the adhesion strength and peeling. Figure 4 shows the water absorptivity measurement results forvarious lining materials. The water absorptivity of highly-durable PE is about 1/10 of that ofchloroprene rubber and about 1/2 of that of conventional PE, and thus the highly-durable PE is
effective in reducing the risk of corrosion of the base metal and peeling of the lining.

Figure 4 Comparison of water absorptivity of various
lining materials

2.4 Slurry erosion resistance
The lining abrasion rate was measured by an abrasion test using high-concentration slurry inorder to evaluate the characteristics of lining material abrasion due to sand in seawater (Figure 5).
The slurry abrasion rate of the developed lining material is equivalent to that of conventional PE
and apparently slower than that of chloroprene rubber. Thus, the developed lining material exhibits
excellent slurry erosion resistance performance even under the condition of seawater containingsand.

Figure 5 Comparison of slurry abrasion resistance ofvarious lining materials|3. Future prospects
The developed highly-durable steel pipe lining can contribute to improvement in thereliability of seawater cooling systems and a reduction in the user maintenance burden in not only
nuclear power plants, but also other industrial plants, and is a product that can be offered to wide
range of users.