Password please use the password
It opens the door to my heart
Password, love is the password
Just say the words of love and come on in

("Password," sung by Kitty Wells written by Bill Phillips, 1964)

THE LAST PERFORMANCE CURVE ON THE CHART

The time has come to learn what that last Performance Curve is ...

Net Positive Suction Head Required...

the yellow curve in the nearby graphic ...

and the role NPSH_r plays with respect to preventing Cavitation.

Truth be known ...

PTOA Readers and Students who are reading the PTOA Segments in the intended sequential order have already learned all the facts they really need to know about Cavitation in PTOA Segment #171. Including:

• Cavitation is the formation of bubbles in the Pump Suction.
• Liquids that are near their Boiling Point Temperature and/or have a high vapor pressure are prime candidates for Cavitation because ...
• The sudden Pressure Drop that occurs just beyond the eye of the Impeller is what causes the Close-To-The-Boiling-Point-Temperature liquid to begin forming bubbles that impact and erode the Impeller.
• The Outside Process Operator will hear the sound of Cavitation and can confirm the presence of Cavitation by observing the corresponding wild fluctuations in the Pump Discharge PI.
• The Outside Process Operator should start closing the Discharge Valve OR Open Up on the Suction Valve ... whichever is less disruptive to downstream units.

Yup! That's really all you need to know about Cavitation.

However, becoming a member of the elite Cavitation Prevention Club requires knowing the Cavitation Prevention Club password ...

and understanding what the password means.

Fred the Stickman wants to join The Club!

Repeat these words after Your Mentor, Fred, because in spoken words the password is:

"The Net Positive Suction Head Available (NPSH_a) must be greater than the Net Positive Suction Head Required (NPSH_r)."

or .. in code:

NPSH_a  > NPSH_r

Say .... whaaat??

Don't stress, Fred!

This PTOA Segment #172 explains what the password to the Cavitation Prevention Club means in the Real World.

Once PTOA Readers and Students understand the password and its meaning, then they can also become card-carrying members of the Cavitation Prevention Club!

Your future Supervisor of Process Operations will be a  member of The Cavitation Prevention Club.

Out of the blue one fine day s/he'll quiz you while trying to assess whether or not you are worthy of club membership.

S/he will ask whether or not you know what Cavitation is and how it can be prevented ...

And that's  when you spring the Cavitation Prevention Club Password.

Blurt out:

"NPSH_a must be greater than NPSH_r" ...

and you will have passed the initiation test to get into The Club ...

AND gained the respect of your Supervisor!

 

 

NET POSITIVE SUCTION HEAD (NPSH) DEFINED

In PTOA Segment #163, PTOA Readers and Students learned the flow path of a pumped liquid as it moves through a Centrifugal Pump from the Pump Suction to the Pump Discharge.

PTOA Readers and Students learned that Pump Suction Pressure is read from a PI which is logically situated at the Pump Suction.

Heck!

PTOA Readers and Students know all about Pump Suction Pressure!

In PTOA Segment #166, PTOA Readers and Students learned how the Suction Pressure that is read on a Pump Suction PI is the same thing as the Total Suction Head which has contributions from the Pressure Head, Static Suction Head, and Friction Head.

PTOA Readers and Students who are reading the PTOA Segments in the intended sequential order will remember that ...

Once the pump is in operation, the Suction PI is no longer indicating Total Static Suction Head but rather the Total Dynamic Suction Head.

The Total Dynamic Suction Head can likewise be determined by adding up the Pressure Head, Dynamic Suction Head, and Friction Head. If none of that sounds familiar, this is the exactly right time to reread PTOA Segment #167.

Doesn't it just make sense that the best way to prevent bubbles from forming at the Centrifugal Pump Suction is to make certain that the  Suction Pressure surrounding the pumped liquid is greater than the Pressure that would allow bubbles to start forming in that liquid?

Like, Duh!

Well guess what?

The "Vapor Pressure of the Pumped Liquid" IS THE PRESSURE at which bubbles will start forming in a liquid!

All PTOA Readers and Students who are reading the PTOA Segments in the intended sequential order will recognize the phrase "Vapor Pressure of a Liquid" because the concept was most recently featured in PTOA Segment #162.

Here's a quickie review:

PTOA Readers and Students learned that the vapor space above a liquid surface is filled with vaporized components from the stored liquid ...

and these vaporized particles exert a pressure on the walls of the container... aka their Vapor Pressure!

So a high Vapor Pressure infers that the liquid has a lot of light components that cannot wait to vaporize.

The nearby graphic shows that Vapor Pressure increases with Temperature.

That makes perfect sense because more liquid particles will want to change from the liquid to the vapor phase as the temperature surrounding them increases!

Ergo ...

Light components can be identified by their correspondingly high Vapor Pressures. The greater the Vapor Pressure ... the lighter the liquid.

Gases won't even be a liquid at room temperature! Gases have much greater Vapor Pressure than liquids. And light liquids have much higher Vapor Pressures than heavy liquids.

So that's why ... to prevent Centrifugal Pump cavitation ...

The Suction Pressure of a Pumped Liquid must always be GREATER THAN the Vapor Pressure of the Pumped Liquid.

Hey that's easy to understand!

To prevent Centrifugal Pump cavitation ...

Suction Pressure > Vapor Pressure!

Guess what?

PTOA Readers and Students just learned the definition of Net Positive Suction Head which is abbreviated NPSH!

Net Positive Suction Head (NPSH) is the amount that the Suction Pressure exceeds the Vapor Pressure of the Liquid Being Pumped!

Otherwise defined as a mathematical expression:

NPSH = 

SUCTION PRESSURE (of the Pump)  -

VAPOR PRESSURE(of liquid being pumped)

Beware!

All of the components in the above NPSH definition/mathematical expression must be expressed in feet of head pressure!

 

There are two logical, noteworthy conclusions to be drawn from the above definition of NPSH:

• Once the Suction Pressure is equivalent to the Vapor Pressure of the Pumped Liquid, NPSH = 0 ... which means there is no more pressure to keep bubbles from forming in the liquid! Like, Duh!
• Once the Suction Pressure drops below the Vapor Pressure of the Pumped Liquid, NPSH is less than 0 ... and therefore negative ...so  bubbles start forming. Like, Duh again!

In conclusion ...

to prevent Cavitation ...

it is important to

STAY POSITIVE!

Yuk! Yuk! Yuk!

 

"NET POSITIVE SUCTION HEAD AVAILABLE" (NPSH_a)
and
"NET POSITIVE SUCTION HEAD REQUIRED" (NPSH_r)

NPSH_a

"NPSH Available" is the NPSH that is available in the actual, installed pumping system ...

as opposed to the theoretical NPSH that a design engineer calculated while sitting in a cubicle of a far away office building.

The short-hand for "NPSH Available" is NPSH_a.

Otherwise stated:

NPSH_a is the difference between the "Real World" Pump Suction Pressure and the "Real World" Vapor Pressure of the "Real World" liquid that is being pumped.

NPSH_a can be calculated after the Suction Pressure of the Centrifugal Pump and the Vapor Pressure of the liquid being pumped are converted from psi to feet of head pressure:

NPSH_a =

"Real World" Pump Suction Press.  -  "Real World" Vapor Press.

 

NPSH_r

The "Net Positive Suction Head Required" is the NPSH determined by the Pump Manufacturer during a shop test of the pump.

The short hand for Net Positive Suction Head Required is NPSH_r.

 

The Pump Manufacturer determines NPSH_r while varying the pump's Capacity.

The results of the NPSH_r shop tests are plotted as a Performance Curve and correlate with the Capacity of the Pump.

In the nearby graphic, the NPSH_r Performance Curve is the curve that appears toward the bottom of the graphic.

The NPSH_r Performance Curve illustrates the minimum NPSH that is needed to prevent the Centrifugal Pump from Cavitation.

As long as the "Real World" NPSH_a is greater than what the Pump Manufacturer has determined to be the NPSH_r ... the pump will not experience Cavitation.

That's Right!

Cavitation will not happen when ...

NPSH_a  > NPSH_r

Ta-Dah!

All PTOA Readers and Students now understand:

• The intent and meaning of the NPSH_r Curve on the Centrifugal Pump's chart of Performance Curves!
• The intent and meaning of the Cavitation Prevention Club password!

Welcome to the Club!

 

 

HOW TO ESTIMATE NPSH_a FOR VACUUM PUMP SERVICE

The PTOA Department of Redundancy Department repeats the following Cavitation Prevention rule:  NPSH_a  > NPSH_r.

By now all the brilliant PTOA Readers and Students know:

• NPSH_r is easy to find because it is plotted on the chart of Performance Curves that is supplied by the Pump Manufacturer.
•  IF the Centrifugal Pump is part of the Typical Pump Installation Set Up, THEN NPSH_a is easy to determine; just subtract the Vapor Pressure of the pumped liquid (expressed as a head pressure) from the PV Pressure read from the Suction PI (expressed as a head pressure) .
• The Typical Pump Installation Set Up is shown in the below graphic; note that the Centrifugal Pump is physically located below the Tank or Reservoir and therefore Dynamic Suction Head significantly contributes to the observed Suction Pressure when the pump is in operation.

But how is NPSH_a determined for a Centrifugal Pump that is physically located above the Tank or Reservoir ... like the vacuum pump situated in a Suck Truck?

Otherwise stated ...

How is NPSH_a determined for the pump in a Dynamic Suction Lift service?

Good news!

PTOA Readers and Students already have the tools in their toolbox to figure this out!

First ...

Recall the definition of NPSH_a  ...

NPSH_a =

"Real World" Pump Suction Press.  -  "Real World" Vapor Press.

Next, look up the Vapor Pressure of the liquid being pumped at the temperature that the liquid is being pumped.

The liquid that is being pumped by the Suck Truck in the nearby photo is Water. Assume the local ambient temperature is 80 °F.

The Vapor Pressure of Water at 80 °F can be found after a  Google search with the phrase "Vapor Pressure of Water at 80 °F."

The nearby table indicates the Vapor Pressure of the Water at 80 °F is 0.507 psia; this data is shown in the small print in the middle part of the above data table.

A Vapor Pressure in psia must be converted to feet of head pressure, like this:

Head of Water Vapor Pressure =

0.507 psia Water * 2.31 foot / 1 psi Water = 1.17 feet

Great! The Vapor Pressure of the Water in terms of feet of head pressure is known!

To determine the Suction Pressure in a Dynamic Suction Lift service, more data about the Centrifugal pump's physical situation is needed.

PTOA Readers and Students may assume the  Suck Truck is situated exactly as it was last described in PTOA Segment #166:

• 

  PTOA Readers and Students already know that this graphic shows Suction Head on the left side and Suction Lift on the right side.

  Suction Lift Head = -10 feet.

• Suction Pressure Head = 0 psig =14.7 psia.  Atmospheric Pressure is converted to feet of head pressure like this: 14.7 psia * 2.31 feet/psi = 34.0 feet.
• Suction Friction Head = -1.3 feet

So ...

Since

NPSH_a = Suction Pressure - Vapor Pressure of Liquid Being Pumped

and

Suction (Vacuum) Pressure = Total Dynamic Lift =

(-10 ft) + (34 ft) + (-1.3 ft) = 22.7 feet

That must mean ...

NPSH_a = 22.7 feet - 1.17 feet = 21.53 feet!

And ya know what else that means, Fred?

Cavitation will be prevented IF the NPSH_r of the Suck Truck's vacuum pump is less than 21.53 feet of head pressure while operating at the desired Capacity.

Even better news, Fred!

This is the very last PTOA Segment devoted to understanding how to interpret Centrifugal Pump Performance Curves!

THUMBS UP to all PTOA Readers and Students for slogging through what is the most theoretical content of the PTOA!

Even though Process Operators do not consciously use Centrifugal Pump Performance Charts and theory on a daily basis ...

The theory does graphically explain how to optimally operate a Centrifugal Pump ...and that IS THE JOB of the Process Operator!

 

TAKE HOME MESSAGES: Cavitation of a Centrifugal Pump can be prevented by maintaining a Pump Suction Pressure that is greater than the Vapor Pressure of the liquid that is being pumped.

The term "Net Positive Suction Head" (aka NPSH) refers to the magnitude of the difference between the Suction Pressure of a Centrifugal Pump and the Vapor Pressure of the liquid that is being pumped.

NPSH = Suction Pressure (expressed in feet of head) - V.P. of Pumped Liquid (expressed in feet of head)

The weird terminology  ... "Net Positive" ... is to emphasize that the difference between the Suction Pressure (expressed as head pressure) and the Vapor Pressure of the liquid being pumped (expressed as a head pressure) must always be positive to prevent Cavitation.

NPSH_a is the Net Positive Suction Head Available in the Real World as opposed to the NPSH that would be estimated by a design engineer tasked with designing a pumping system.

NPSH_a is the difference between the Real World Dynamic Suction Head/Lift of a Centrifugal Pump and the Real World Vapor Pressure of the Real World liquid being pumped.

NPSH_r is the minimum Net Positive Suction Head needed for the pump to operate at various Capacities to avoid Cavitation. NPSH_r is determined by the pump's Manufacturer under shop testing conditions. 

NPSH_r appears as a separate Curve on the chart of Performance Curves that the Pump Manufacturer supplies with the Centrifugal Pump.

To prevent Cavitation, the NPSH Available in the Real World (NPSH_a) must always be greater than the NPSH Required (NPSH_r) by the Manufacturer. Otherwise stated:

NPSH_a > NPSH_r  will prevent Cavitation.

PTOA Readers and Students are now familiar with all the curves in a Centrifugal Pump Performance Curve chart.

 ©2017 PTOA Segment 0172
PTOA Process Variable Pressure Focus Study Area
PTOA PV Pressure Rotating Equipment Focus Study

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