So I might be on a side street
Or a stairway to the stars
I hear the high pitched cavitation
Of propellers from afar

("Wolfman's Brother," by Pfish, 1994)

Whew!

PTOA Readers and Students who are reading the PTOA Segments in the intended sequential order have slogged through an abundance of Centrifugal Pump operating theory!

Bummer Alert!

Up to this point, Process Operators cannot use one teeny tiny bit of information they have learned to  operate a Centrifugal Pump!

That's right! You read Your Mentor correctly!

 

Process Operators will not be around when the pump is selected for the specific process operating service ... that's done in an office cubicle during the design phase of the facility.

Nor will Process Operators be relied upon to perform the modifications that the Mechanic does to optimize pump performance after the purchased pump has been installed.

Nevertheless ...

The Outside Process Operator is totally responsible and accountable for making certain the pump s/he inherits works as long as it can without unscheduled downtime.

The major operational problem that can severely damage a Centrifugal Pump is Cavitation.

And gaining the awareness that an Outside Process Operator needs to prevent Cavitation really does begin with understanding how Performance Curves illustrate Centrifugal Pump Operating theory. For example:

Total Dynamic Head (TDH) is determined from the difference between Total Discharge Head and Total Suction Head (PTOA Segments #166 and #167).

And the TDH-Capacity Curve (aka the Characteristic Curve of the Pump) is determined by an impeller with a specified diameter spinning a speed measured in RPMS within a closely fit Volute (featured in (PTOA Segment #169 and PTOA #170).

The optimal condition to operate the Centrifugal Pump is at the Capacity that correlates within maximum Efficiency (PTOA Segment #168).

The TDH at maximum Pump Efficiency will be attained with a correlating, required Brake Horsepower..

 

.

After the Centrifugal Pump is purchased and installed, modifications to the pump's Impeller or Speed might be made so that the Real world pump operates in the maximum Efficiency range (PTOA Segments #169AFFINITY LAWS and #170). 

 

 

In summary ...

the focus on Cavitation ...

is why PTOA Readers and Students have been challenged to learn about Performance Curves!

This PTOA Segment #171 explains the Real World causes and observations of the Centrifugal Pump problem called Cavitation ...and what the Outside Process Operator can do to anticipate and prevent it!

 

 

 

 

 

 

DESCRIBING CAVITATION

Pumps Are For Liquids ... Not Gases or Vapors

Your Mentor expects every PTOA Reader and Student to know by now that the purpose of a pump is to add pressure to liquids ... not gases.

Trying to pump a gas or vapor just doesn't work!

In fact ... as PTOA Readers and Students will learn ...

A pump needs to be primed with the process liquid that is going to be pumped before it is started up.

Vapor bubbles that form in a liquid which is being pumped by a Centrifugal Pump eventually flow into an area of higher pressure where they collapse on the nearest metal surface ... the Impeller.

Each of the zillion collapsing bubbles implodes on the Impeller surface with the force of a zillion tiny explosions ... and in the process of collapsing, the bubbles erode away metal.

The below photo shows an Impeller before and after experiencing Cavitation.

The ability of the chewed up Impeller to infuse the PV Pressure into the liquid via Centrifugal Action is greatly reduced compared to the Impeller that is not eroded.

The Centrifugal Pump that has experienced Cavitation is going to need downtime for costly repairs ... exactly what Plant Managers want to avoid!

And THAT IS PRECISELY WHY the PTOA has devoted so many nouns and verbs to explaining Centrifugal Pump Performance!

 

The Sound and Symptoms of Cavitation

In the Real World, Cavitation has a distinct sound associated with it; those zillions of tiny bubbles collapsing on the Impeller of a Centrifugal Pump sound like a zillion marbles being poured onto a Caribbean steel drum.

What Your Mentor is trying to tell you is that the happenstance of Cavitation is unmistakable and heard throughout the processing facility; like making a loud fart that everybody instantly knows the Outside Process Operator is responsible for.

The Outside Process Operator can confirm a suspicion of Cavitation by observing the pump vibrating more severely than normal which will be accompanied by rapid and wide variations of the pump Discharge Pressure.

All of these visual and audible observations are consistent for a pump that is choking on the vapors that are evolving from the pumped liquid.

 

After Cavitation

In the worst cases, the pump Impeller may be destroyed by Cavitation.

In the best case scenario,  the pump that has experienced Cavitation will limp along while operating very inefficiently because ...

in Centrifugal Pump Theory lingo ...

the Impeller is so eroded it cannot create the TDH requirement at the desired condition of maximum Efficiency!

 

CAVITATION IS A SUCTION SIDE PROBLEM

Which of the brilliant PTOA Readers and Students remembers the Typical Pump Installation Set Up described in PTOA Segment #164?

Those PTOA Readers and Students instantly understand what is meant by the following statement:

Cavitation is a Suction-Side pump problem

The graphic below shows the "Pressure Profile" of a pumped liquid as the liquid moves through the Centrifugal Pump shown in the bottom frame; The liquid's Pressure is plotted on the vertical Y axis and the location of the pumped liquid as it flows through the Centrifugal Pump is shown on the horizontal X axis.

PTOA Readers and Students should note that the Suction Pressure initially decreases gradually in the Suction Line ...

and then drastically decreases as the pumped liquid is drawn in and whirled around by the spinning Impeller.

Don't fret! The upcoming PTOA Segment on Centrifugal Action will explain why the PV Pressure of the pumped liquid is at a minimum just beyond the eye of the Impeller.

Then the PV Pressure starts to increase dramatically ... and that's when the bubbles start collapsing on the Impeller's surface.

 

CAUSES OF CAVITATION

So what makes the pumped liquid start forming bubbles (boiling) while being pumped?

A list of Cavitation Causes is below.

• The diameter of the Suction Line is too small.
• The length of the Suction Line is too long.
• There are too many fittings on the Suction Line.
• The liquid being pumped has a high vapor pressure and is somewhat near its Boiling Point Temperature.
• The pump is being operated too fast which means the pump is sucking more liquid into the Impeller than can be supplied by the Suction-side hardware.

Note that the Outside Process Operator can only impact the last cause of Cavitation on the list ... because that is the only cause of  Cavitation which involves operating the pump!

All the other Cavitation causes listed above should have been addressed back in that cubicle during the design phase of the process facility.

However, the most brilliant Process Operators will always be mentally aware when the service of a tank and pump has deviated from the original plan ... which definitely happens due to market conditions.

When the tank and pump service changes to a different liquid that has a significantly higher vapor pressure, then the operating conditions move closer to favoring Cavitation, especially if the pumped liquid is near its Boiling Point Temperature.

 

PTOA Readers and Students who are reading the PTOA Segments in the intended sequential order learned about the interrelationship between the PV Pressure and PV (Boiling Point) Temperature way back in PTOA Segment #157.

So all PTOA Readers and Students know that a decrease in the PV Pressure will cause a liquid to boil at a much lower Temperature!

The sudden decrease in the PV Pressure caused by Centrifugal Action will cause a liquid that is near its Boiling Point Temperature to Cavitate.

Here are some noteworthy pump service applications that have a history of Cavitation.

• Boiler Feed Water (BFW) pumps.
• Ammonia or other extremely low Boiling Point Temperature/high vapor pressure liquids used in refrigerant applications.

 

THANKS TO MOUNTAIN STATES ENGINEERING AND CONTROLS FOR THIS CAVITATION DEMONSTRATION

Thanks to Mountain States Engineering and Controls (MSEC) for the below You Tube that demonstrates Cavitation with see-through, acrylic piping.

Additional kudos go to Tony Kuphaldt at Bellingham Technical College for creating the demonstration.

MSEC CAVITATION DEMO WITH TONY K.

 

OOPS! This video is no longer available. Too bad Tony doesn't want to help others learn about Cavitation.  Here's a description of what the MSEC You Tube illustrated with water.

The instructor intentionally drops the PV Pressure by closing off the Suction Valve.

Since the Impeller is still spinning at a constant RPM, closing off the Suction Valve creates a vacuum ... which can be mentally visualized as an invisible (yet noisy!)  horizontal vortex of water being sucked into the Impeller at a greater flowrate than it can be supplied.

Vapor bubbles start forming in the water because the created vacuum pressure decreases the boiling point of the water way below  of 212  °F (100 °C).

When the bubbles collapse on nearby surfaces, they make a sound which is much louder (and destructive) on metal surfaces than on the see-through acrylic pipes used in the test apparatus.

 

ACTIONS PROCESS OPERATORS CAN TAKE
TO PREVENT CAVITATION

Process Operators must always remember and never forget that the root cause of Cavitation is:

The Centrifugal Pump is sucking in the pumped liquid faster than it can be supplied.

The two options for remedy the Outside Process Operator can do are:

Open up on the Suction Valve

The MSEC You-Tube bench apparatus was purposely designed to create and correct Cavitation by respectively closing and opening a Suction Valve.

Throttling down (aka "closing more") the Suction Valve effectively created a vacuum pressure which resulted in vaporizing the flowing water.

A Real World pump installation is designed to discourage Cavitation; The steep decrease in the PV Pressure occurs at the Pump Suction Inlet.

In the Real World, the Suction Valve would be mostly open from the get-go.

 

Figuratively speaking, opening up 100% on the Suction Valve would make the left-side blue vertical line that represents Suction Head in the nearby photo taller.

The pressure differential experienced across the Suction side of the pump would thus decrease and this outcome will stop Cavitation.

PTOA Readers and Students who are reading the PTOA Segments in the intended sequential order will immediately understand how increasing pump Suction Head by opening up the Suction Valve will likewise increase pump Capacity.

The pump will be operating at a greater Capacity-less TDH condition point ... a much more inefficient operating condition than the pump was designed for.

Additionally, all of the processing units downstream of this pumping system will be impacted by this upset.

 

Throttle Down (aka "close a little") the Discharge Valve.

Pinching back ("throttling down") on the Discharge Valve will decrease the Discharge Head.

Figuratively speaking, throttling down on the Discharge Valve will make the blue vertical line that represents Discharge Head in the nearby photo shorter.

The resulting decrease in Total Discharge Head will likewise decrease the pressure drop experienced on the Suction side of the pump and halt Cavitation.

In the Real World, the Discharge Valve is mostly if not completely open and can thus always be throttled down.

Furthermore this plan-of-attack typically ... but not always ... has the least effect on downstream processing units.

The Outside Process Operator should always choose the method that has the least effect on plant production so that operational upsets are minimized.

TAKE HOME MESSAGES: Cavitation is a Suction-Side Centrifugal Pump problem that can result when:

• The pump installation is poorly designed.
• The pump is mismatched to the pumped liquid characteristics or ...
• The tank and pump service changes to a liquid with a lower Boiling Point Temperture/Higher Vapor Pressure and the PV Temperature of the pumped liquid is near its Boiling Point Temperature.
• The Process Operator is operating the pump too fast which means that more liquid is being sucked into the Impeller than can be supplied.

Cavitation happens when the pressure drop through the Centrifugal Pump causes bubbles to form at the Pump Suction; when these bubbles implode, Impeller erosion occurs.

The best Outside Process Operators will be aware when the tank and pump service changes to a lighter liquid than the pump has been designed for.

Outside Process Operators must be particularly aware of Cavitation with BFW pumps and pumps that are in refrigeration applications. 

Outside Process Operators will become aware of Cavitation by the sound akin to marbles being tossed onto metal.

Outside Process Operators can confirm Cavitation by noticing excessive pump vibration and wild swings in the Pump Discharge Pressure which are indicative of a pump that is choking while trying to "pump" vapor bubbles.

Outside Process Operators can stop Cavitation by either opening up on Pump Suction OR pinching back on Pump Discharge. Both of these options decrease the pressure drop across the pump. The latter option typically results in less upset to downstream units.

©2017 PTOA Segment 0171
PTOA Process Variable Pressure Focus Study Area
PTOA PV Pressure Rotating Equipment Focus Study

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