Cavitation & NPSH

Introduction:

Pump cavitation is a phenomenon that occurs when the pressure of a liquid drops below its vapor pressure. This can cause the liquid to vaporize, forming bubbles. When these bubbles collapse, they can create shock waves that can damage the pump. One of the most common causes of pump cavitation is poor suction conditions. This can happen if the liquid level in the suction tank is too low, or if there is too much friction in the suction piping. Another common cause of cavitation is operating the pump at too high of a flow rate. This can cause the liquid to lose pressure too quickly as it enters the pump.

Cavitation can cause a number of problems, including:

• Reduced pump performance

• Increased noise and vibration

• Damage to the pump's mechanical seal

• Fires and explosions

There are a number of things that can be done to prevent pump cavitation, including:

• Ensure that the liquid level in the suction tank is high enough.

• Minimize friction in the suction piping.

• Operate the pump at a flow rate that is below the pump's cavitation limit.

• Use an inducer on the pump inlet.

• Choose a pump with a higher NPSH requirement.

How to Identify Pump Cavitation

• Pump discharge pressure fluctuates, ranging from normal to low, with no cavitation indicated by a normal to high variation.

• Pump flow decreases unpredictably.

• Pump suction pressure gradually drops and then abruptly rises.

• Motor amps exhibit irregularly low values.

• The pump may occasionally produce a rattling sound. 

However, larger high-head pumps under marginal suction conditions may defy conventional cavitation symptoms, pumping less with a consistently reduced discharge pressure. This contradicts some literature but reflects the operational reality. Increasing suction pressure from 1 to 3 psig raises discharge pressure from 80 to 150 psig, observed primarily in large pumps with low suction pressure.

Pumps experiencing this malfunction don't damage their mechanical seals as conventionally cavitating pumps with erratic behavior do.

What causes less NPSH?

Frictional loss occurs through the pump's suction piping and fittings, along with upstream vessel nozzle exit loss, incorporating the impact of vortex breakers, which are often unnecessary.

If the design maintains a typical suction line velocity of 1 to 2 ft/sec, these losses are negligible, unless the outlet nozzle from the vessel is partly obstructed. The vessel's internal bottom outlet screen frequently becomes fouled, or fractionation trays collapse on the outlet nozzle. In Texas City, maintenance once removed a completely blocked contractor from a xylene splitter pump suction nozzle.

To distinguish between a restricted outlet nozzle and frictional loss in the suction line, follow these steps:

• Place a pressure gauge on the pump's suction.

• Increase the flow until the pump just begins to cavitate.

• Reduce the flow by 1 or 2% until cavitation ceases.

• If the suction pressure rises by 1 or 2 psi, the issue is frictional loss in the suction line.

• If the suction pressure rises by 5 or 10 psi, the problem is a partly blocked vessel outlet nozzle.

Malfunctions are often caused by vessel outlet nozzles. Pump suction "Y" strainer screens commonly restrict flow, necessitating spare pumps for strainer cleaning.

The concept is that when a pump initiates, the liquid in the suction line is stationary. Energizing the mass of liquid requires energy, which cannot be sourced from the pump but relies on the available NPSH. This initial NPSH subtracts from the NPSH required to operate the pump. Hence, to prevent pump cavitation during startup, a proficient operator gradually opens the discharge valve—an acceptable practice, but with limitations. Prolonged operation of the pump at a very low rate results in the barrier fluid or seal flush between seal faces drying out, leading to damage in the seal faces. 

Following actions can be considered to increase NPSH:

Maximize the upstream vessel level:

• Increasing the level in the upstream vessel can enhance NPSH availability during pump startup.

Spray water on the bare suction piping and pump case:

• Introducing water spray on the suction piping and pump case can provide subcooling, significantly reducing fluid vapor pressure, especially for lighter components.

Raise pressure in the upstream vessel by 5 or 10 psig:

• Elevating the pressure in the upstream vessel instantly increases suction pressure at the pump. This results in additional available NPSH for a brief period, as lighter liquid from the vapor-liquid interface takes some time to migrate down to the pump suction.

Utilize spill-back line cautiously:

• If a spill-back line is available, using it sparingly can facilitate pump startup. However, excessive use may lead to pump overheating and cavitation.

Avoid the misconception of starting with the discharge shut:

• Contrary to a common misconception, starting a centrifugal pump with the discharge shut is not advisable. Instead, the discharge valve should be opened one-quarter to one-half turn during startup to prevent overheating of the liquid inside the pump case.

Leaking Suction Causes Problem

Water entering the suction of a hot hydrocarbon pump can vaporize and flash to steam, leading to choked flow and cavitation. In vacuum service pumps or those at an elevation above the liquid level, such as sump pumps or tank field pumps removing the heel from a tank, even minor flange leaks in the suction piping can induce cavitation by drawing air into the pump suction flow. Mechanical seals under subatmospheric pressure, especially common in packing leaks on isolation gate valves, can also promote cavitation by drawing air into the pump case.

To identify leaks, a water hose or appropriate heavy oil can be poured over suspected areas. The fluid displaces air temporarily, restoring pump discharge pressure briefly. This method was used successfully by Jerry to find a leaking flange on the suction of a vacuum gas oil pump.

Anything causing partial vaporization of the liquid in the pump's suction line results in NPSH loss and cavitation. However, malfunctions like these are often more complex than the described problems, as illustrated by the following incident.