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Pump damage can lead to costly downtime, repairs. Knowing the cause of damage can help in selecting pumping equipment to reduce the chance of failure. Here, we take a look at four common causes of pump damage and how to avoid them.
cavitation
Cavitation is the result of insufficient pressure on the pump suction or available net positive suction head (NPSHa) causing liquid in the pump to become vapor at low pressure.
At low pressure, this creates air bubbles that burst as the liquid moves from the suction side of the impeller to the delivery side.
The implosion of the bubbles creates a shock wave that puts pressure on the inner surfaces of the pump, causing vibration and mechanical damage, and eventually failure.
When this happens repeatedly, cavitation can cause pitting and fractures in the impeller, volute and casing, which weakens the metal, increases resistance to flow and reduces pumping efficiency. Shock loads from cavitation can also shorten shaft and motor life.
Cavitation and the associated problems it causes can drastically affect pump life, shortening it by 10-15 years, and in extreme cases even longer.
Cavitation is easiest to avoid during the design phase, ensuring that the selected pump has sufficient NPSHa to keep the liquid above its vapor pressure.
NPSH needs to be calculated for each application because different liquids have different vapor pressures and will vary with pressure and temperature.
This can be used in pump selection as manufacturers will be able to give any of their pumps the required Net Positive Suction Head (NPSHr) to match the specification.
corrosion
Corrosion in pumps is the result of a chemical reaction between the metal and the fluid being pumped.
This reaction can lead to uniform corrosion of wet surfaces - mainly in pumps made of non-stainless steel materials - or localized corrosion of a small number of components - most often on metal-attached and passivated surfaces that form an oxide layer.
When corrosion does occur, pump performance and efficiency can be affected, increasing the need for more frequent maintenance and more downtime, and if left untreated, can lead to failure.
The key to mitigating corrosion is choosing a pump made of the material best suited for the application.
Cast iron is one of the cheapest options for pump material and is often used for the casing.
It has good corrosion resistance to neutral and high pH liquids, making it a popular choice for general purpose, irrigation and mining pumps. However, it is not suitable for low pH applications which are more prone to corrosion.
Stainless steel is one of the most commonly used materials for pumps because of its good corrosion resistance in a variety of environments where other carbon and low alloy steels would corrode.
Under certain conditions, such as exposure to water in the presence of chlorides, pitting corrosion may occur on lower grades of stainless steel.
However, this is not a problem with higher grades of stainless steel which have high corrosion resistance.
Therefore, corrosion is the easiest to avoid when choosing a pump.
Material selection will determine corrosion resistance in a particular application as well as the overall cost of the pump, including initial cost, maintenance, replacement, downtime, life cycle and reliability.
dirt
Fouling occurs when particulate matter adheres to the internal surfaces of the pump, most commonly in the distribution lines connected to the inlet or outlet.
When this happens, pumping efficiency and flow are reduced and may eventually lead to failure. This is an unavoidable problem, but is more prevalent in applications where the pumped fluid contains particles.
However, various cleaning methods can be used to maintain the pump and increase efficiency and capacity.
Put on
Wear is unavoidable when running a pump, however, there are a few factors that can cause excessive wear that can accelerate pump aging.
Particulate matter in the pumped fluid not only causes fouling, but also increases the rate at which the pump wears.
These particles scour and roughen the pump's internal surfaces, reducing pumping efficiency over time as it works harder to move fluid.
Eventually, the pump will wear out to the point where it can no longer generate enough lift and can cause excessive vibration.
Wear can also be the result of an improperly sized pump, which can lead to pressure imbalances that place excessive stress on bearings and seals; turbulent flow; fluid velocity; wear ring degradation; and erosion and corrosion.
By ensuring that the pump is sized and made of the right material for the application, the rate of wear can be reduced, and regular maintenance is done to catch and fix any problems before they lead to unplanned downtime or pump failure.
As defined by MIL-STD-753C, a passivation process is a final treatment/cleaning process used to remove iron from the surface of corrosion-resistant steel components, resulting in a more uniform passivated surface formation for enhanced corrosion resistance.
Stainless steel differs from other metals in that the composition of the metal actually changes when you get close to the surface. During passivation, free iron is removed from the surface into solution, leaving behind a higher chromium content. A good chromium to iron ratio is generally considered to be 1.5 to 1 or higher.
What is passivation?
Passivation is a chemical treatment of stainless steel and other alloys that increases the corrosion resistance of the treated surface.
Passivating equipment and systems has many benefits:
Passivation removes surface contamination
Passivation improves corrosion resistance
Passivation reduces the risk of product contamination
Passivation allows you to extend system maintenance intervals
Composition of stainless steel
To understand the passivation of stainless steel, the key is to look at the stainless steel itself. All stainless steels are alloys of iron, nickel and chromium. Chromium makes up at least 10% of the metal. It is this element that makes stainless steel corrosion resistant. Steelmakers often add molybdenum to enhance the protective properties of chromium in highly corrosive or high temperature applications.
In addition to the chemical composition of the metal, the different layers that make up stainless steel also vary in composition. On the surface is a passivating layer, sometimes called a passivation film, which is responsible for providing corrosion resistance. It is a very thin layer of highly stable metal atoms that does not corrode or rust easily. Only a few atomic layers thick, the ratio of chromium to iron (Cr/Fe) is at least 1.5 to 1. Chromium combines with oxygen to form a chemically inert "passive" surface.
Below the passivation layer is a transition region with a higher nickel concentration. Like the passivation film, the thickness is only 3 to 4 atomic layers. The nickel in this section protects the passivation film by preventing a chemical reaction with the iron in the layer below. It also acts as a protective barrier for the base metals that make up the bulk of stainless steel. The percentages of chromium, nickel and iron vary depending on the intended use of the article.
Why do you need passivation?
Although passivation occurs naturally in corrosion-resistant and chromium-rich alloys, under the right conditions, new stainless steel vessels or components will require passivation before being placed in service. Fabrication, machining, and welding leave behind contaminants such as metal oxides, inclusions, fabrication debris, and tramp iron that impair the metal's natural resistance to corrosion.