Chemical Technology February 2015

PLANT MAINTENANCE, HEALTH, SAFETY AND QUALITY

expectation and the maintenance cost can vary greatly. The following example shows cost development of low cost commodity valves versus standard branded valves (unit comparison by index figures). In the case of the low cost valves, they have to be replaced once a year. In the case of the standard quality valve, no replacement is required, merely only service. After ten years of operation, savings of approximately 30 % of the TCO can be recorded when using quality technology.

go well beyond the material grade alone. The liner is respon- sible for containing themedia within the valve body, providing shaft sealing and valve to flange sealing. When this primary seal fails, shaft and body will be in contact with media, and leaking of the valve will occur. Interference between seal and the disc determines pres- sure rating and operating torque. Consistent, repeatable and accurate manufacture of the seal results in reliable opera- tion, whereas compoundmix determines physical properties such as hardness, chemical resistance (eg, volume change or material degradation), tear strength and age hardening of the elastomeric material. The better the finishing or polishing of metallic discs at all sealing points is, the lower the torque value and internal tightness will be. Design of the liner and accuracy in selecting the most suitable rubber determines the stability and the reliability of actuation of the valve. Larger diameter butterfly valves (>DN300) have historically suffered from stalling, hesitation, and subsequent uncontrollable opening rates when coupled to an actuator. This usually happens as a consequence of alteration of the elastomeric liner material, and due to poor valve design that results in flange compression increasing the valve break torque. Butterfly valves using metal pins to transmit to the valve disc the rotation power when quarter-turning the shaft, repre- sent a potential danger of media penetrating to the interior of the shaft, where we then have an excellent point for galvanic corrosion with dissimilar metal contact (as the shaft, the body and the disc will be made of different materials). Coated discs (eg, Halar, Epoxy, Rilsan) generally have a coating thickness of 0,3 – 0,6 mm, which, in theory, appear very good. A coated disc is ultimately fully reliant on the total and complete encapsulation of the disc. Any breach of surface continuity will result in heavy corrosion and ultimate breakdown. The photograph shows a butterfly valve used in a demineralised water application (boiler water). Specific costs of valve replacement Depending on the quality of an installed valve, the lifetime

Low price butterfly valve, to be replaced once a year

Standard quality butterfly valve, to be serviced once a year

Purchase Cost Valve

100

140

Cost of installation

5

5

Cost of inspection

5

5

Cost of service

Replacement 15

Maintenance Service 50

Cost of new valve

100

0

Cost of spare parts

0

20

Total Costs at beginning

105

145

Total Costs after 10 years

1200

750

IInvesting in high-end technology has a direct impact on the total costs of a production plant, resulting in a posi-

tive return on investment. Consequently CAPEX are expenditures which create future benefits. Even though capital costs of industrial plants have decreased sig- nificantly over the past ten years, costs of operation, maintenance and repair are still significantly onerous. A valve is not just an irrelevant component in a process. Rather it is an element which can exert a big ef- fect on a small cause. It is therefore worth having a closer look at it when designing, building, operating and maintaining an industrial production plant. For more information contact Claudio Darpin at GEMÜ Valves Africa at claudio.darpin@gemue.co.za

13 Chemical Technology • February 2015