One of the most common questions surrounding the use of centrifugal pumps is whether or not a suction strainer is necessary. When placed ahead of the pump, these devices help prevent sediment and solids from entering the pump. This helps reduce wear and tear on the pump impeller, which in turn increases overall efficiency and longevity. However, many see suction strainers as an unnecessary add-on, especially given the advances in impeller technology.
Is it Needed? Not in Most Cases
Before multi-stage centrifugal pumps became a popular choice for a number of operations, including water and wastewater facilities, turbine pumps were the go-to solution for these applications. In contrast to most modern centrifugal pumps, turbine pump impellers featured very close tolerances, making their design problematic when it came to handling sediment and other foreign particles. For this reason, engineers always included suction strainers in the designs of wastewater handling applications that relied on turbine pumps.
Most centrifugal pumps lack the close tolerances needed for turbine pumps to function, making them fundamentally more robust and resistant to accelerated wear. Centrifugal pumps are better suited for passing grit and sediment-filled liquids, as these particles are less likely to come into contact with the impeller and other vital moving parts due to their looser tolerances.
For this reason alone, having a suction strainer isn't necessary for the average centrifugal pump. Instead, the use of wye-pattern strainers and inlet baskets are usually more than enough for the majority of pump applications.
Another Reason against the Use of Suction Strainers
Another reason engineers often avoid suction strainers involves a potential decrease in net positive suction head (NPSH), which could eventually lead to cavitation and damage to the pump. As the strainer collects more particles, it also causes greater pressure drops throughout the suction line. This has several effects on the pumped water inside the line:
- The temperature of the pumped water increases dramatically, especially if it's being pumped from a deaerator.
- The water eventually reaches its flash point, causing the water to suddenly flash into vapor
- Vapor bubbles formed by the flashing water eventually collapse, causing destructive cavitation to pump components.
Notable Exceptions and Common Practices
While most applications make do without the need for a suction strainer, there are a few notable exceptions to consider:
- While grit and sediment are no problem for a centrifugal pump, larger solids found in wastewater can impact its overall efficiency. This is especially true for pumps that were not specifically designed with handling solids in mind.
- Concerns about pump longevity and efficiency may be alleviated with the use of suction strainers as a precautionary measure against accelerated wear and tear. Users should be careful to factor NPSH losses when using suction strainers for added pump protection.
Keep in mind that suction strainers come in a variety of mesh sizes to meet a variety of needs. If you do decide to install a suction strainer, only to find that it's getting clogged too often, decreasing the size of the mesh may help alleviate that problem. Mesh numbers are often the inverse of the sieve size – smaller sieve openings use higher mesh numbers, while larger sieves are denoted by their lower numbers.
Experts also note that it's common practice for installers to use suction strainers with a minimum surface area that's 2.5-times the pipe flow area. Differential pressure gauges are also employed to monitor differences in pipe line pressure before and after the strainer, which can drop if the strainer becomes clogged with sediment and solids. Extra vigilance is usually necessary for applications where a suction strainer is needed.
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