Chemical Technology February 2015

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Chemical Technology • February 2015
PLANT MAINTENANCE, HEALTH,SAFETY AND QUALITY
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
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.
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 informationcontact Claudio
Darpin at GEMÜ Valves Africa at
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