Down, down, down, down
I'm going down, down, down, down
I'm going down, down, down, down
I'm going down, down, down, down

("Going Down," by Bruce Springsteen, 1982)

 

UNDERSTANDING VACUUM PRESSURE

PTOA Readers and Students who are reading the PTOA Segments in the intended sequential order recently learned that:

• A Vacuum Pressure is any pressure that is below Atmospheric Pressure (aka P_atm).
• The units in which a Vacuum Pressure are measured include millimeters of mercury (mm Hg), inches of mercury (in Hg), inches of water (in H2O or inWC), and millibars.

Vacuum Pressures are sometimes called "negative pressures" because they are less than P_atm.

 

HOW TO GET A FLUID AT P_atm TO FLOW

Who amongst the brilliant PTOA Readers and Students remembers the following statement?

A fluid will not flow unless a pressure differential exists between an area of higher pressure and an area of lower pressure.

PTOA Readers and Students who are reading the PTOA Segments in the intended sequential order were first introduced to this rule of fluid flow way back in PTOA Segment #57 (entitled "Where Do We Go From Here? #2").

Then PTOA Readers and Students revisited the subject in the more recent PTOA Segment #138 ("Where Do We Go From Here? #4, Part 1") wherein they committed the following to memory:

• A Delta P (aka "Pressure Differential" aka "Differential Pressure," aka "dp" or ΔP) is required to create a Flowrate!
• The greater the Delta P, the faster the Flowrate!

So how is it possible to get a fluid to flow when the area of higher pressure is the pressure of the atmosphere that surrounds us

... P_atm ?

In that situation, no flow will be created unless a vacuum pressure is created ... because a vacuum is a pressure that is lower than P_atm!

You yourself create a vacuum when you suck a beverage up through a straw ...

the inhaling effort creates a low pressure that encourages the beverage to flow upward through the straw ...

from the "higher pressure area" that is the local atmospheric pressure.

 

ENERGY IS REQUIRED TO CREATE A VACUUM

It takes a lot of energy to maintain "pulling a vacuum" because a "vacuum pressure" is not the normal, everyday pressure that the natural environment is used to maintaining.

All PTOA Readers and Students now know by heart that the normal, everyday pressure of the local environment is "atmospheric pressure" (aka P_atm).

Just think about how difficult it would be to maintain the inhaling effort that it takes to suck the carbonated drink continuously up a straw and you'll get an idea of the energy it takes to establish and maintain a vacuum pressure.

A "perfect vacuum" would be a theoretical pressure of 0 psia.

But it would be really, really hard to pull and continuously sustain a perfect vacuum.

 

USES OF VACUUM PRESSURES IN THE PROCESS INDUSTRIES

Creating Draft in Fired Heaters

PTOA Readers and Students were introduced to the creation of one type of vacuum pressure called "draft" way back in PTOA Segment #70 entitled "Flame Management 101."

PTOA Readers and Students learned that maintaining the upward flow (aka "draft") of excess air and combustion products through the firebox and chimney stack is essential for both optimal heat transfer and the elimination of the dangerous firebox situation known as "flame out."

Establishing and maintaining draft encourages the ambient air that surrounds the fired heater (and which is at P_atm ) to flow into the vents of the burners that are located on the floor of the firebox.

The "Draft Gauges" that are located above and below the damper are used to determine the magnitude of the vacuum pressure that is needed to maintain the desired draft.

Outside Process Operators control the draft of flue gases (aka combustion products and excess air) by adjusting the position of the damper that is located in the chimney stack.

PTOA Readers and Students learned in PTOA #70 that some draft gauges read in "inches of water column" (inWC) as shown in the below chart.

Any instrument that is measuring in units of "Inches of water column" (aka in WC) is measuring a vacuum pressure. 

 

Vacuum Trucks

The Maintenance Department of any processing plant will own at least one Vacuum Truck (aka "Suck Truck").

The Suck Truck is used to remove sludge and/or fluids that have accumulated in purposefully installed sewers and underground containers (aka "sumps").

A vacuum pump is installed on the Suck Truck.

The pump does the work that creates the low pressure area which thus encourages the gunk to flow into the hose that delivers the debris to the container on the Suck Truck.

So the vacuum pump in the Suck Truck performs the same function that you do when you sip a beverage through a straw.

The same work is performed by the vacuum-creating technology installed within a common household vacuum cleaner or shop vacuum while in use sucking up debris from home floors or carpets.

 

Making Heavy Hydrocarbons Boil Under Vacuum Pressures

PTOA Readers and Students are very familiar with the below chart that shows how the boiling point of water changes depending upon the P_atm.

PTOA Readers and Students learned in PTOA Segment #140  that the boiling point of water (212 °F aka 100 °C in the below chart) will decrease when the P_atm is less than the 14.7 psia (101.3 kPa in the below chart) ... and vice versa.

This directly corresponding relationship between the pressure above a liquid's surface and boiling point applies to all liquids, not just water.

The purpose of installing a Vacuum Distillation Tower at a fuels refinery is to make it possible for heavy hydrocarbons to boil at lower temperatures and thus upgrade the less valuable tower feed into valuable products.

The Vacuum Distillation Tower is operated at a vacuum pressure ... and this "lower-than-P_atm-pressure" lowers the boiling point of heavy hydrocarbons to a temperature range that is more economically attainable with heat exchange and fired heaters.

But how is it possible to "pull a vacuum" on a tall tower?

Industrial equipment called "steam jet ejectors" can create absolute pressures (i.e. vacuum pressures) of 10 to 40 mm Hg (aka Torr).

A series of ejectors is installed at the top of a vacuum tower to create the "sucking action" needed to create a vacuum pressure.

The nearby photo shows three steam jet ejectors; they look like three vertically mounted "pipes" that have a diameter that swages down to a smaller diameter before increasing once again.

In summary, the below are just a few of the beneficial uses of vacuum pressures in processing industries:

• Creating upwardly flowing drafts inside a fired heater box.
• Removing sumped or sewer fluids with a Suck Truck.
• Making it possible to separate heavy liquids into much more valuable hydrocarbon products defined by their respective boiling point ranges.

 

Uses of Strong Vacuum Pressures in Manufacturing Industries

At the time that this PTOA Segment was written, "advanced manufacturing industries" created 40% of all vacuum pressures.

The manufacture of circuit boards and solar cell wafers requires "strong vacuum pressures" in the range of 10 to 3 millibar (almost a perfect vacuum).

 

 

UNWANTED VACUUM PRESSURES CAUSE IMPLOSIONS!

All Process Operators must be aware of conditions that can create non-desirable vacuum pressures because they can cause equipment implosion.

The nearby photo depicts a vessel that has suffered an implosion. The vessel is now useless and will have to be replaced.

 

The implosion of the vessel in the nearby photo might have been cause by a vortex.

A vortex is that thing that you see when water swirls downward into a drain. A tornado is a vortex, too.

The below graphic shows a Filled Tank on the left hand side.

The liquid inside a full tank exerts outward "Pascal forces" on the internal walls of the tank at every level.

But when the vessel is emptied (the Empty Tank in the nearby graphic), the Pascal forces do not exist to counteract the atmospheric pressure surrounding the tank.

A downward pull on the interior walls of the vessel ... perhaps caused by a vortex or condensation ...will cause the vessel to implode.

Most tall columns and towers will have a mechanical device that interrupts the formation of a vortex.  These devices are logically called vortex breakers.

 

BEWARE THE CREATION OF AN UNWANTED VACUUM!

The conditions that are ripe for an implosion occur sporadically ... like during a turnaround of the processing facility in which towers are cleaned in preparation for inspection and repair. 

One way to prepare a vessel or tower for inspection and subsequent repairs is to steam it out.

This step will logically be called "Steam-Out" in the turnaround procedures.

"Steam Out" literally means to connect a steam hose to the bottom of the tower and allow the steam to clean the internals and exit through an open vent at the top of the tower.

In the event the vent is erroneously closed (aka "blocked in") after "Steam Out" is completed, the condensing steam will 'pull a vacuum' on the interior walls of the tower ... and the tower will implode.

The below You Tube link demonstrates how cooling and condensing of a liquid can implode a drum:

Demo of Condensing Steam Imploding a Drum

Just like the imploding drum, a tower that does not have a vent open to equalize internal tower pressure to P_atm will implode while steam condenses on the interior walls.

Implosion due to unwanted vacuum will also occur after loading fuel trucks or rail cars with hot fluids that cool and condense if they likewise are not vented to atmosphere.

TAKE HOME MESSAGES: Fluids will not flow unless there is a Pressure Differential between an area of high pressure and low pressure. When the area of high pressure is Patm, the only way to make a fluid flow is to create a vacuum pressure.

Some of the important uses of vacuum pressure in the process industries include:

• Creating and maintaining draft in fired heaters.
• Lowering the boiling points of heavy hydrocarbons so that they can be converted into much more valuable products.
• Vacuum pumps in Suck Trucks (aka Vacuum Trucks) are used to suck the debris and liquids from sumps and sewers.

The advanced-manufacturing industries use "strong vacuums" to fabricate solar cell wafers and circuit boards.

Undesired vacuum pressures will implode towers and vessels, fuel trucks, and rail cars.

Undesirable vacuum pressures can be caused by vortexes and condensation which pull downward on the interior walls of vessels and towers that are being emptied and/or which are emptied and experiencing internal condensation.

Outside Operators must be aware of the conditions that can result in the creation of an unwanted vacuum pressure.

©2017 PTOA Segment 0151
PTOA Process Variable Pressure Focus Study Area
PTOA Introduction to PV Pressure Focus Study

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