Cavitation and net positive suction head (NPSH) issues are commonly misunderstood pumping problems that, if not attended to, can cause massive plant failures. When operators hear what sounds like gravel being pumped through pipes, accompanied by seal and pump failures, these symptoms may be attributable to cavitation caused by the pump’s inability to satisfy the NPSH demands of the pumping operation.
To illustrate the concept of NPSH, envision a company’s monetary budget. If withdrawals exceed deposits, the company may eventually default. In the context of a pumping operation, a pump can be destroyed if it is required to withdraw more NPSH (labeled as NPSHr) than is available in the pumping system’s NPSH budget (NPSH available, or NPSHa). On the other hand, if there is a surplus of NPSH, then vapor does not form, removing the threat of cavitation. But if there is a deficit, vapor may form and harmful cavitation can occur.
What is this destructive phenomenon? Cavitation is the implosion of vapor bubbles within the pump’s pressure stages. Vapor can form upstream of the pump, within the pump, or both. When vapor implodes, the liquid walls collapse and produce extremely destructive and noisy shock waves. These shock waves can cause damage to internal components, which can then cause decreased operational efficiency, increased maintenance time and costs and potentially catastrophic pump failure.
Cavitation Scenarios
Oftentimes, operators focus on keeping the NPSH at a level less than what the system requires. However, vapor may form upstream of the pump, especially in the transport of fluids with high vapor pressures that operate at a deficit from the start. Even low-vapor-pressure liquids may have a physical system layout that requires transfer conditions that are ripe for creating a deficit of NPSH.
Other scenarios that could create a dangerous NPSH deficit include excessive suction-lift requirements, long piping runs, flow restrictions at the inlet, a clogged strainer, partially closed valves and unnecessarily high flow velocities caused by undersized piping. These conditions all create the potential for expensive, destructive and dangerous cavitation.
Suppress the Threat
Simply put, if you eliminate the vapor, you eliminate the threat of cavitation. Blackmer®, Grand Rapids, MI, USA, a product brand of PSG®, a Dover company, Oakbrook Terrace, IL, USA, has developed a solution with the invention of the Cavitation Suppression Liner, which defuses destructive implosion of vapor bubbles. Internal recirculation jets break apart vapor bubbles, making them a fraction of their original size, and much smaller bubbles mean much smaller implosion reactions.
The Cavitation Suppression Liner also allows a controlled amount of fluid at discharge pressure back toward the suction of the pump. This design feature also drastically reduces vapor-bubble size, resulting in less noise, vibration and wear.
Blackmer offers its Cavitation Suppression Liner as a component on its CRL, LGL, SGL, XL and XLW Series Sliding Vane Pumps. These pumps can handle a wide array of fluid types in both stationary and transport applications.
Sure, system engineers could set an operational parameter so that the NPSHa (budget) could meet the NPSHr (consumption), but this has to be impeccably executed on a case-by-case basis, with little room for error. The Blackmer Cavitation Suppression Liner simplifies the process by significantly reducing the size of the vapor bubbles, and it can be used in many applications.
Left unchecked, cavitation can cause myriad problems in pump operation. From compromised internal components leading to increased downtime to creating dangerous operating conditions, cavitation is not something that can be overlooked. With unmatched reliability in difficult NPSH and high-cavitation applications, the tried-and-true solution is a Blackmer Sliding Vane Pump with the Cavitation Suppression Liner.