Page 180 - CW E-Magazine (8-10-2024)

Special Report

Material selection
The wetted parts of pumps may
be constructed from a number of
materials:
Metals (e.g., cast iron, SS, nickel al-
loys) for casings and other compo-
nents where mechanical strength is
important.
Thermoplastics (e.g., PTFE, PVDF,
PP) for casings, linings, bearings,
gaskets and seals when dealing with
corrosive chemicals.
Ceramics (e.g., alumina, silicon car-
bide) for seal faces and bearings.
Fig. 1: Corrosion damage to a stainless steel pump shaft in saltwater service due to Elastomers (e.g., Buna N, Viton) for
electrochemical reaction (left) & failure of an O-ring due to chemical damage flexible components such as shaft
Corrosion generally is accelerated when time, leading to pump failure. You must seals and O-rings.
the fluid the pump is driving/hand- know these symptoms to act on time to
ling is corrosive or, at times, a lack of save the pump. The resistance of exposed metals
regular clean-up and inappropriate ope- 1. The pump doesn’t seem to be to corrosion and of thermoplastics and
ration. In general, pump wear or mate- working as efficiently as it used to. elastomers to chemical damage (such as
rial loss due to corrosion can easily 2. You notice an increase in the en- softening or swelling) can vary widely
cause the pump to deviate from its BEP ergy required to operate the pump. with temperature, the presence of abra-
(best efficiency point) curves. This can 3. The pump makes strange noises sives, impurities or entrained gases.
reduce the MTBF (mean time between you haven’t heard before. In selecting a pump for a particular
failures) and make finding the true 4. You see water or other fluids leak- application, it is important to understand
root cause of failure difficult. Figure-1 ing from the pump. how all exposed materials (including
shows typical corrosion damage to a 5. The pump isn’t generating enough seals and shaft components) interact
stainless steel pump shaft in saltwater pressure to correctly move fluids with the pumped fluid. This is referred
service due to electrochemical reac- through your system. to as chemical compatibility.
tion (left) & failure of an O-ring due to 6. The pump is running hot, even
chemical damage. when there’s no load on it. Chemical compatibility charts are
7. The motor seems to be struggling or normally available from pump manu-
It has been observed that 50% of straining when the pump is running. facturers. These describe the resistance
pump failures are due to corrosion 8. You notice vibrations or excessive of materials commonly used in pump
and out of different types of corrosion movement coming from the pump. construction to various chemicals. It
failures 70% are due to tribocorro- 9. The pump just stops working is important to refer to these when se-
sion. Tribocorrosion is corrosion due altogether. lecting a pump to ensure that all wetted
to combined mechanical action and 10. You see smoke or fire coming parts are chemically compatible with,
electrochemical reaction. Examples of from the pump. and resistant to, the pumped medium.
tribocorrosion are: erosion corrosion, It may be necessary to carry out tests
fretting corrosion, cavitative corro- How to prevent corrosion in pumps under field conditions and to consult
sion, and corrosion due to impact of Pump manufacturers spend a lot of with pump manufacturers before com-
fluid on the surface of the pump com- time and money on research and deve- mitting to a system.
ponent(3). lopment to create pump designs and
materials to improve the efficiency of the Cast iron (CI)
The major symptoms of pump pump, its performance and its durability. CI is commonly used to manufac-
failure Let’s have a look at the different ways ture pump housings because of its low
Instantaneous pump failures are to reduce corrosion, which will not only cost and the ease with which parts can
easy to detect. However, pumps start help save millions of dollars, but will be cast and machined. When used in ap-
showing particular symptoms over also, extend the lifetime of the pumps. plications with water or aqueous solu-

180 Chemical Weekly October 8, 2024

Contents Index to Advertisers Index to Products Advertised