Well, I think I'm goin' out of my head
Yes, I think I'm goin' out of my head

("Goin' Out of My Head," made famous by Little Anthony and the Imperials but written by T. Randazzo &  B,Weinstein, 1964)

 

PTOA Readers and Students who are reading the PTOA Segments in the intended sequential order were recently introduced to Performance Curves.

Performance Curves can be used to determine the optimal  Capacity of a Centrifugal Pump.

 

PTOA Readers and Students already know that the X-axis of a Performance Curve shows the Capacity range of the pump ... aka the amount of fluid that the pump can handle ... from 0 gpm to a maximum design flowrate.

And PTOA Readers and Students already know that the Y-axis is the Total Head of the pump, expressed in feet.

This is a good place to mention that Performance Curves do not apply to Positive Displacement type pumps because the operating principle of a Positive Displacement Pump is not related to centrifugal action.

Centrifugal action is illustrated in the nearby graphic; the pumped liquid is fed into the Suction-side eye of the spinning impeller and is thence flung outwards.

Centrifugal action will be explored soon in a future PTOA Segment so don't stress about understanding it now.

PTOA Readers and Students will soon learn exactly how the fabricated casing of the Centrifugal Pump ... aka the Volute ... changes the direction and velocity of the pumped fluid that has been spun out from the center of the impeller.

This PTOA Segment #167 continues the focus on Centrifugal Pump Performance Curves. PTOA Readers and Students will learn the difference between Total Dynamic Head and Total Static Head ... which will involve defining a myriad of other head types.

Whoa Pardner! Stop right here if you are confused by the phrase "Total Head!" The rest of us got knowed-up about Total Head just recently  in PTOA Segment #166 and you need to do yourself a favor and catch up.

 

Alert!

This PTOA Segment is gonna get kind of wonky explaining and differentiating Pump Head types.

By the end of this PTOA Segment everyone's mind will be blown and everyone's head will be spinning!

 

HEAD DISSECTION

First and foremost, PTOA Readers and Students must figuratively understand what the Centrifugal Pump lingo "head" means.

The nearby graphic exhibits many features found in the Typical Pump Set Up which was described in PTOA Segment #164.

This graphic also shows a "Real" versus "Imagined" pumping situation.

In real life, the pumped-up liquid flows out the Pump Discharge into the Discharge Line.

In real life, the Discharge Line is initially vertical but soon makes a turn to the horizontal plane via an elbow in the pipe.  The jagged line on the Discharge Line infers that the pipe is too long to show any more of the pipe in the diagram.

The blue vertical line that is shown extending from the pump's Discharge is the figurative expression of "Total Discharge Head."

The height of the figurative head gives a visual idea as to the amount of PV Pressure energy that has been infused into the liquid so that it has sufficient energy to flow where it is needed.

Many thanks to what was once identified as Pumpfundamentals.com for the nearby graphics that illustrated what is visualized as "Total Discharge Head."

So ... in YOUR HEAD ...when you hear the word "head" in pump lingo ...

visually Imagine a vertical stack of liquid ... and remember:

The taller the stack, the more PV Pressure energy the liquid has within it so that it can flow to where it is needed to be.

The Suction-side PV Pressure of the Centrifugal Pump can also be figuratively expressed as a vertical line.

As the nearby photo shows, the vertical blue line that figuratively illustrates the Total Suction Head is not as tall as the vertical blue line that figuratively represents the Total Discharge Head.

Who amongst the brilliant PTOA Readers and Students is asking themselves:

"Since all the fuss is about Pressure ... Why isn't the Y-axis of the Performance Curve plotted in units of Pressure (psi) instead of feet of head?"

An excellent question that can be  answered by another question:

Who amongst the brilliant PTOA Readers and Students has noticed that two different types of forces are creating Pressure on the Suction and Discharge sides of the pump?

 

The Suction-side Pressure is mainly caused by the Gravity force of hydrostatic head.

When the pump is operating, Centrifugal force creates the PV Pressure on the Discharge-side of the pump.

Righteeo!

Once the pump is turned on and whirling liquid around...

The Centrifugal force that creates the PV Discharge Pressure is not the same thing as the gravity force that creates the PV Pressure on the Suction-side of the Centrifugal pump.

By now every dedicated PTOA Reader and Student who is reading the PTOA Segments in the intended sequential order can recite in their sleep that the magnitude of the Suction-side Pressure is created by the Suction-side Static Head and depends only upon the Specific Gravity and height of the liquid ... ergo ..

The greater the density of the liquid the more Pressure is exerted at the bottom of the hydrostatic head.

Alternatively stated, a liquid with a S.G less than 1.0 (the S.G. of water) will require a greater head to exert the same pressure as water would.

And vice versa, a liquid with a S.G. greater than 1.0 will require less head to exert the same pressure as water.

Yeah. We all know that by now.

HOWEVER ...

On the Discharge-side of the operating pump, Centrifugal action rules in the process of creating PV Pressure energy.

Once the Centrifugal Pump is operating, the liquid being pumped is no longer "static" ... it is moving and is therefore described as "dynamic."

Your Mentor will never be convinced that Process Operators need to know the particulars of math-modelling dynamic fluid flow to perform their jobs well.

Just file it in the back of your head that the PV Pressure created by Centrifugal force depends upon the square of the velocity of the liquid ... but not the density of the liquid.

And accept that the two noteworthy outcomes of the PV Pressure created by Centrifugal Force are:

"A centrifugal pump with a given impeller diameter that is rotating at a specified speed will develop the SAME FEET OF TOTAL DYNAMIC DISCHARGE HEAD no matter what the liquid is."

Yes Indeedo!

Be it water or gasoline or whatever ... the impeller will spin the fluid to the same imaginary height.

However ...

 

Process Operators must be aware ...

The Discharge Pressure that the Process Operator reads on the Discharge PI DOES INCREASE with increasing liquid Specific Gravity!

The two Centrifugal action phenomena described above explain why the manufacturers of Centrifugal Pumps prefer to plot the Total Head on the Y-axis of Performance Curves in feet of liquid instead of Pressure in psig.

Ergo ...

PTOA Readers and Students must slog through learning what Total Head is ... which they did in PTOA Segment #166.

Incidentally, the Pump Manufacturing Industry is not nearly as anal retentive as the Instrumentation Industry; they haven't convened for hours and days of meetings to settle on common terminology for Performance Curves.

So don't be confused when Total Head on the Y-axis is described as "Total Discharge Head" or "Total Dynamic Head" or "Characteristic Curve" ... yadda yadda yadda.

 

HEAD TYPES AND PERFORMANCE CURVES

One chart that attempts to define the different types of Pump Head is shown below.

The phrases that appear in this Pump Head chart will be shown in red writing when mentioned in the below text.

Total Static Head = Total Head at 0 gpm

Total Static Head in the above chart is almost the same thing PTOA Readers and Students learned to determine in PTOA Segment #166.

"Static" means not moving ... thus at rest and standing still.

Imagine how the Total Static Head relates to the Centrifugal Pump Set Up shown below ... before power to the Centrifugal Pump's driver is engaged.

Imagine:

1. The Suction Line to the Pump Suction has been primed and the pump is liquid-full.
2. No flow has yet commenced.  The impeller of the Centrifugal Pump is not spinning.

Since the pump is not spinning, no Friction Losses have been generated.

Thus, only the Static Head and Pressure Head on both sides of the pump contribute to Total Head ... not Friction Losses.

On any chart of Performance Curves, the static situation is the leftmost vertical line ... where Capacity = 0 gpm.

So what Your Mentor is trying to tell ya is that the maximum Total Head of a Centrifugal Pump is also known as the Total Static Head when the Capacity = 0 gpm.

On the nearby chart of Performance Curves, the "0 gpm flow" is where Total Static Head and Total Head both equal 105 feet.

PTOA Readers and Students who are reading the PTOA Segments in the intended sequential order will not be surprised to learn that:

Total Static Head =
(Total Static Discharge Head) - (Total Static Suction Head)

and in the case of lift:

(Total Static Discharge Head) - (Total Static Suction Lift).

No Process Operator will ever be asked to calculate Total Static Head ... but the value of the knowledge is understanding how the term is incorporated into the Performance Curves.

For example PTOA Readers and Students will know that Total Static Head is the maximum value of Total Head and is plotted on the Y axis where Capacity = 0 gpm.

 

Total Dynamic Head 

Now Imagine that:

• The starter button on the Pump's driver is engaged and thus the impeller begins to spin.
• The Outside Process Operator gradually opens up the Discharge Valve until it is opened 100%.

Shazam!

The Centrifugal Pump is now operating in a dynamic mode ... doing what it was built to do ...

Adding PV Pressure energy to the liquid that enters the Pump Suction ...

which is expressed on the Performance Curve as Total Dynamic Head (TDH) at any Capacity above 0 gpm.

The change in TDH with the pump Capacity appears as the top curve on the vast majority of Centrifugal Pump Performance Curves.

This is the mathematical expression for the TDH Curve:

Total Dynamic Head (TDH) =
(Total Dynamic Discharge Head) - (Total Dynamic Suction Head)

and in the case of lift TDH =

(Total Dynamic Discharge Head) - (Total Dynamic Suction Lift).

No Process Operator will ever be asked to calculate Total Dynamic Head  ... but the value of the knowledge is understanding how the term is incorporated into the Performance Curves.

TDH is the Total Head curve that is plotted at all values of Capacity above 0 gpm.

 

APPLYING ABOVE TO THE TDH PERFORMANCE CURVE

Wow!

All Performance Curve charts show that ...

A relatively small increase in flow rate (aka Capacity) correlates to a rapid decrease in Total (Dynamic) Head.

A nearby Performance Curve graphic illustrates where the Capacity/Total Head combination falls into the cavitation range.

Notice that the Capacity point labelled "V" is considerably beyond and therefore greater in magnitude than the maximum Capacity which is labelled V_max.

In the real world, the left side of the cavitation range indicates where the pump begins sucking liquid into the eye of its impeller at a rate that is too great for the Suction-side to keep supplying sufficient Total Dynamic Suction Head.

Wait! I see Fred waving his hand!

What do you have to say, Fred?

"The Centrifugal Pump that is operating at V has done gone out of its Total Dynamic Head!

Yuk! Yuk! Yuk!"

 

 

TAKE HOME MESSAGES: Performance Curve charts only apply to Centrifugal Pumps.

Figuratively speaking, the phrase "head" should conjure up the vision of a vertical "stack" of liquid.

The Pressure that is built up by Centrifugal action is proportional to the square of the liquid velocity but is not at all impacted by the liquid's density or specific gravity.

Therefore the Total Dynamic Discharge Head of a Centrifugal Pump with a given impeller diameter rotating at a specified speed will be the same for any liquid, regardless of specific gravity.

However, the Discharge Pressure of a Centrifugal Pump is still impacted by the pumped liquid's specific gravity; the greater the specific gravity of the liquid, the greater the Discharge Pressure observed by the Process Operator.

"Static" means at rest or not moving. With respect to Centrifugal Pumps the phrase describes the state wheren the pump is primed liquid full but the power to the driver has not been supplied so the impeller is not spinning.

"Dynamic" means moving. With respect to Centrifugal Pumps the phrase describes the pump operating and the impeller spinning.

Total Static Head is the maximum value of Total Head and is plotted on the Y axis of a Performance Chart Curve where Capacity = 0 gpm.

The calculation of Total Static Head does not include any Friction Loss because the liquid is at rest.

Total Dynamic Head is the value of Total Head at all points on the Performance Chart Curve where Capacity is greater than 0 gpm.

Every Total (Dynamic) Head Curve for a Centrifugal Pump will indicated that a small increase in flow rate (aka Capacity) correlates to a rapid decrease in Total (Dynamic) Head.

©2017 PTOA Segment 0167
PTOA Process Variable Pressure Focus Study Area
PTOA PV Pressure Rotating Equipment Focus Study

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