Do you read me?
Well won't you tell me?
I've been hurting, but I've been learning, just the same.

("Do You Read Me," by Rory Gallagher, 1976)

 

THE ISA DECREES THAT "P" IS FOR "PRESSURE"

PTOA Readers and Students who are reading the PTOA Segments in the intended sequential order learned way back in PTOA Segment #8 that "T" is used by the ISA for "temperature-related" instrumentation tags. PTOA Segment #8 is entitled "Give Me A "T" For "Temperature"!"

And that all made perfect sense because PTOA Readers and Students had previously learned in PTOA Segment #6 what the ISA is and understood its mission to standardize the symbols used in PFDs and P&IDs.  PTOA Segment #6 is entitled "Every Picture Tells a Story ... Don't It?"

And PTOA Readers and Students had also learned in PTOA Segment #7 that ISA symbols are used to decode PFDs and P&IDs. PTOA Segment #7 is entitled "What Do P&IDs Have In Common With Crime Scenes?"

Ah yes ...

that stored knowledge in your brain will refresh once you glance at the below P&ID excerpt that shows the piping and instrumentation surrounding a pair of carbon filters:

So who amongst us is going to be shocked to learn that the ISA has declared ...

 "P" is for "Pressure"!

 

Common "Local" Pressure Instruments

All PTOA Readers and Students know that a "local" instrument is one that is situated out in the plant with the pumps and pipes and other processing hardware.

 

Common "local" pressure-related instruments include Pressure Indicators (PI), Pressure Transmitters and Delta Pressure Transmitters (PT), Pressure Controllers (PC), Pressure Safety Valves (PSV), Pressure Alarms High and Low (PAH and PAL).

Take another look at the carbon filter P&ID excerpt and verify the following statements to be accurate:

• PSV 1A is connected by piping to the top of Carbon Filter V-200A (aka "Vessel-200A, pictured on the left). PSV 1A will relieve excess pressure and protect the integrity of the vessel in the event the filter gets clogged up.
• PSV 1B is connected by piping to the top of Carbon Filter V-200B and protects this carbon filter from over pressuring.
• PI 2A is situated at the bottom of V200A and PI 2B is situated at the bottom of V200B.
• PSH 2A is activated by the same line pressure that would be indicated by PI 2A. PSH means "Pressure Switch High."
• PSH 2B is activated by the same line pressure that would be indicated by PI 2B.
• PAH 2 is a locally mounted high pressure alarm that receives standard electrical signal input from both PSH 2A and PSH 2B.

 

Common "Remote" Pressure Instruments

All PTOA Readers and Students know that the adjective "remote" infers that the instrument will be found in an area where the instrument can be easily viewed by more than one person ... like a Control Room.

 

PTOA Readers and Students already know that the ISA tag symbols reveal which pressure-related instruments on a P&ID are mounted locally versus found in the Control Room ...

because they have read the PTOA Segments in the intended sequential order!

Typical "remote" pressure instruments are Pressure Indicating Controllers (PICs) and also Pressure Differential Controllers (PSID).

PTOA Readers and Students just learned in PTOA Segment #140 what a FCC Reactor and FCC Catalyst Regenerator are and how controlling the ΔP between them is crucial for safe production of FCC products.

The below scan of a Human-Machine control Interface (aka "interactive computer screen graphic")  depicts the controls needed to safely operate a  FCC-Regenerator system.

Put your glasses on!

Can you locate the Pressure Differential Control (PSID)?

Hint: The current differential pressure on the PSID is indicated as "5.6."

The graphic also shows these additional relevant FCC-Regenerator pressures:

• The Reactor Pressure is 27.01 psig.
• The Regenerator Pressure is currently 32.83 psig.
• The pressure of the combustion air that is needed to burn the coke off the spent catalyst is currently 40.36 psig.

The above exercise shows that some PIs are digitally represented on the Human Machine Interface ... and likely digitally trended as a PR on a different "page" of the HMI.

 

WHAT DO COMMON PRESSURE-RELATED INSTRUMENTS LOOK LIKE?

The (Mechanical) Pressure Indicator

A "PI" is a Pressure Indicator(pronounced "pea-eye" ... not "pie").

"PIs" look a whole bunch like "TIs" ... Temperature Indicators.

The PI shown in the nearby photo can measure in pounds per square inch (red scale) and kiloPascals (black scale).

"Instrument Techie" PTOA Readers and Students know exactly what is going on behind that dial face because they learned how C-shaped bourdon tubes transduce a sensed pressure into mechanical motion back in  PTOA Segment #103.

 

Pressure Indicating Transmitters

Nowadays the modern PIT is a real multitasker that performs several functions:

• PE ... the pressure element that detects and measures a process pressure. Modern P&IDs don't typically illustrate this duo function of a PT because the strain gauge element is mostly associated with the transmitter.
• PT ... the device that transmits the measurement in a standard signal format to a remote controller which is part of the DCS.
• PI ... a local pressure indicator which digitally indicates the currently detected pressure to the Outside Process Operator.

The PTOA does not endorse any particular kind of equipment; that being said ... in the good ole USA the term "Pressure Transmitter" and PT are pretty much just called "Rosemonts" for the smart, extremely accurate and reliable devices manufactured by Rosemont (now part of  Emerson™).

The signature blue housings of Rosemont PTs, PITs, DPSIs will be present all over the USA processing facility.

 

See the antenna on the Rosemont PIT shown above? That antenna means that this PIT is wireless. The standard signal output from this PIT is in the form of microwaves.

Over the years, Rosemonts have replaced Ye old Foxboro 43AP, a 1940 Foxboro™ PIC technology that is still in use to detect, measure, and locally indicate and control process pressures and delta pressures.

The picture to the left shows a pneumatic Foxboro™ pressure indicating controller that would be situated locally ... out with the pumps and the pipes and  the processing hardware.

 

This type of PIC instrumentation was hot stuff in 1940 and is still useful in some applications. However note the use of a bellows and all the linkages (the bellows is the black thing behind the red pointer on the middle of the left side).

The hardware most definitely is capable of measuring, indicating, and controlling a local pneumatic pressure. And a pneumatic controller would be a good choice in an environment that cannot tolerate the potential of a spark.

But otherwise it doesn't take a genius to figure out that truly critical pressure measurement and control applications will be performed better with modern, smart instruments.

 

Local Pressure Recorders (PRs)

The local Pressure Recorder (PR) shown at the right would be used to record the history of a pressure that is expected to be constant ... meaning not dynamically changing.

This kind of PR might be used to record the line pressure of a gas distribution pipeline.

 

Pressure Safety Valves (PSVs)

Folks who are unfamiliar to the world of industrial processing might think the below photo is showing something going wrong ...

but it is really a picture showing many things going right:

 

The big flame extending from the flare indicates that the flare is doing its job burning hydrocarbons that have been intentionally sent to it via a Pressure Safety Valve release.

The intentional burning of gases and light hydrocarbons avoids a potential catastrophic event occurring in one of the processing units.

Pressure Safety Valves (PSVs) will be situated on any vessel that experiences a significant pressure and the vast majority of the PSVs will release to a flare header.

The photo to the right shows a PSV being tested at a certified testing facility. The outlet is not connected to a flare header so you can see what the release would look like.

Note that the diameter of the inlet pipe flanged to the bottom of the PSV is smaller than the diameter of the outlet flange connection of the PSV.

The outlet is much bigger than the inlet because of the need to get rid of pressure in a hurry!

 

Take Home Messages: The ISA tag for all pressure-related instruments will begin with a "P." PTOA Readers and Students are able to decode local and remote pressure-related instruments on P&IDs and PFDs.

Common local pressure-related instruments are:

• PIs- Pressure Indicators that can be mechanical or digital.
• PT - Pressure transmitters that can be "smart" or pneumatic and some detect and measure Delta P.
• PSV - Pressure Safety Valves.
• PAH - Pressure Alarm High
• PAL - Pressure Alarm Low
• PIC - Pressure Indicating Controllers like the Foxboro™ 43AP in pressure-measuring service.
• PR - Pressure recorders like a chart recorder.

Common remote pressure-related instruments that will be part of the DCS system and accessed via a Human-Machine Interface are:

PI, PIC, PR, PSID, PAH, PAL

©2016 PTOA Segment 0141
PTOA Process Variable Pressure Focus Study Area
PTOA Introduction to PV Pressure Focus Study
PTOA Industrial Automation Study

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