WHERE DO WE GO FROM HERE? (#4 … Part 2)
("Under Pressure," David Bowie & Queen, 1981-82)
PREVIEW OF THE PTOA'S
PROCESS VARIABLE PRESSURE FOCUS STUDY AREA
(CONTINUED)
This PTOA Segment #139 continues the preview of
PTOA's PV Pressure Focus Study Area ...
because the originally written preview was so long it had to be broken up into two parts!
The third and fourth PV Pressure Focus Studies
in the PTOA PV Pressure Focus Study Area are:
- 3. PV Pressure Process Industry Equipment which will include:
Pressure-Increasing Equipment (aka Rotating Equipment ... Compressors and Pumps)
Pressure-Reducing Equipment (for example, Pressure Safety Valves, Flares, and Steam Let Downs)
Vacuum-Creating Equipment (for example, Steam Jet Ejectors and Flue Gas Stacks)
Hydrostatic Head Equipment (for example, Tanks and Vessels)
Uses of Hydraulic Pressure and Pneumatic Pressure in Process Industry
- 4. Common Pressure Sensors found in the Process Industries
PREVIEW:
THE PTOA'S PV PRESSURE
PROCESS INDUSTRY EQUIPMENT FOCUS STUDY
Remember that a fired heater and a package boiler are two types of process industry equipment a Plant Owner could install to increase the process variable Temperature?
Well... Glory Be!
There is specially built Process Industry pressure-increasing equipment too!
Compressors increase the pressure of gases.
Pumps increase the pressure contained in liquids ...
and that's kind of a minor miracle, you see ...
because if you punch the water that is being held in a container ...
all's you will get is wet knuckles!
So just how is pressure built up in a substance that doesn't allow pressure to be built up in it?
Inquiring minds want to know!
(Big Hint: The Pressure-Velocity Swap IS NOT a miracle ... just ingeniously applied human-made technology!)
Pumps and Compressors have rotating parts and are therefore classified as "Rotating Equipment."
Ergo...
PTOA's PV Pressure Process Industry Equipment Focus Study
will include an introduction to "Rotating Equipment."
Pressure-Decreasing Process Industry Equipment
Likewise ...
No PTOA Reader or Student should be surprised to learn that pressure-decreasing equipment exists.
This Pressure Safety Valve is being tested. Normally the outlet would be flanged to another pipe.
Some pressure-decreasing process industry equipment is actually categorized as safety equipment ... because getting rid of a lot of pressure in a hurry is sometimes necessary to avoid an unplanned catastrophic event!
Even Negative Pressures (aka "Vacuum Pressures")
are Useful in the Process Industries
None of these sunbathers is worried about the atmospheric pressure they are feeling on their skin.
PTOA Readers and Students have grown so accustomed to the pressure on their skin that they don't even recognize it as pressure at all.
What your skin is feeling is called atmospheric pressure.
In English measuring units, atmospheric pressure is 14.7 pounds per square inch.
The Pressure Indicators (PIs) ...aka pressure gauges ... used in process industry don't feel that pressure either.
Unless otherwise stated, the pressure read from a PI is displayed as "gauge pressure" ...
which means that the pressure gauges that appear all over the plant (and which are calibrated for English measuring units) will display a pressure reading that is approximately 14.7 less pounds per square inch ...
when compared to the pressure reading displayed by an absolute pressure gauge situated at the same location.
Some process industry equipment is designed to create a pressure that is below atmospheric pressure.
A pressure that is below atmospheric pressure is below the pressure that you have grown so accustomed to feeling on your skin that you don't even feel it on a day to day basis.
Any pressure that is less than atmospheric pressure is classified as a "vacuum pressure."
PTOA Readers and Students who are reading the PTOA Segments in the intended order have already been introduced to an industrial use of vacuum pressure:
PTOA Readers and Students learned in PTOA Segment #70 (entitled "Flame Management 101") that Process Operators must maintain a "draft" (aka "vacuum pressure") so that the combustion gases will properly flow through the fired heater's firebox and stack.
Maintaining a draft is just one of the many uses of vacuum pressure in the process industries.
Pressure Caused by Liquid Levels
The fact is that the Process Variable Pressure shows up all over the place in a processing facility ...
and that's because any container of liquid that has any liquid level in it at all has a pressure profile that increases from the top of the liquid surface to the deepest level at the bottom of the tank.
The deeper and deeper Fred swims, the more pressure his ear drums will feel.
Any swimmer in a swimming pool has felt that increase of pressure on their ears while swimming deeper and deeper into the deep end of a swimming pool.
Understanding how any liquid level in a closed container exerts pressure is fundamental to understanding "hydraulic pressure".
PTOA Readers and Students have already been introduced to the concept of hydraulic pressure:
Way back in PTOA Segment #14 (entitled "I Just Gotta Get a Message to U") ...
the above graphic illustrated how hydraulic pressure is used to actuate the valves that interface between raw crude oil that lies below the ground and that which has just been produced and is flowing in a pipeline, destined for further clean up and processing.
The example of a "Christmas Tree" oil-production valve device was the only example Your Mentor could think of that would infer where the ISA standard signal for hydraulic signal transmission would apply.
Most definitely there are many more uses of hydraulic pressure actuating components of auxiliary support systems throughout the process industries.
Hydraulic pressure is needed to create large forces used to move stuff.
However ...
The PTOA is going to spend time on smaller magnitudes of pressure generated by liquid levels.
For example ...
PTOA Readers and Students will perform the easy exercise of calculating the pressure caused by a single cubic foot of water at its base.
hey! that's the all important "0.433" fudge factor Instrument Techs use in many calculations! How about that!!!
Then PTOA Readers and Students will then learn the short cuts of determining the pressure created by any liquid level in any kind of tank ... no matter how weirdly shaped the tank is!
And the end purpose of all this "pressure created by a liquid level" mumbo jumbo is because ...
Understanding how to convert the pressure caused by a liquid level into a "head pressure" is crucial to understanding the terminology related to Rotating Equipment.
And guess what?
Liquid is not the only tool in the "Uses of Fluid Pressure Tool Box":
Pressurized gas is used all over the processing facility to actuate valves.
The pressurized gas is usually pressurized air.
The high-dollar fancy word for pressurized gas used in the service of operating another piece of instrumentation or equipment is "pneumatic pressure."
PREVIEW:
THE PTOA PV PRESSURE-SENSOR FOCUS STUDY
The PTOA PV Pressure Focus Study Area will conclude with a focus on common pressure sensors used in industry.
PTOA Readers and Students who are reading the PTOA Segments in the intended sequential order will cruise through this last PTOA PV Pressure Focus Study because ...
they already know about how the C-Shaped bourdon tube detects and measures pressure changes that are displayed on a local PI (Pressure Indicator).
and...
"Instrument-Techie" PTOA Readers and Students already know how a Wheatstone Bridge works in an RTD.
The same technology is used in the popular pressure measuring device called a strain gauge that can generate a pressure measurement output in a standard signal form that can be sent to a control room.
Yes, indeedo!
PTOA Readers and Students who are reading the PTOA Segments in the intended sequential order are going to have a much easier time integrating the PV Pressure information into their growing list of Process Technology core competencies.
Say, if you are a PTOA Reader or Student who didn't quite start at the beginning ...
CLICK HERE ...
to start at the beginning so that you will be a Know-It-All!
HEY FRED?
ARE YOU READY TO FOCUS ON THE PROCESS VARIABLE PRESSURE?
THEN LET'S DO THIS!
Take Home Messages: The PTOA PV Pressure Focus Study Area will include four separate focus studies:
- Introduction to the PV Pressure
- Interrelationship of the PV Pressure with PV Temperature, PV Flow, and PV Level
- Pressure-Related Process Industry Equipment
- Common Pressure Detectors and Transducers
PTOA Readers and Students who have not started at the beginning of the PTOA will be at a disadvantage understanding the integrated concepts in the PTOA PV Pressure Focus Study Area and should invest the time to catch up.
©2016 PTOA Segment 0139
PTOA Process Variable Pressure Focus Study Area
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