I can't believe we're giving up
(Love under pressure)
We only need the two of us
(Love under pressure)
I feel your pain from my mistakes
It's how it breaks
This love's under pressure

("Love Under Pressure," by James Blunt, 2021)

OBSERVING THE SENSED, MEASURED AND INDICATED PV PRESSURE

In the Real World, the Outside Process Operator will glance at a PI (Pressure Indicator) to gain vital process information such as:

• What is the PV Pressure of the process stream where the PI is situated?
• What is the PV Discharge Pressure of the Pump or Compressor?
• A Heat Exchanger (HEx) is being commissioned back in operation after a plant maintenance interval (aka "Turnaround"). Are the PV Pressures of the Tube-Side and Shell Side balanced so that the bundle does not get damaged?

 

In the Real World, the Control Board Operator will glance at a PI or PIC (Pressure Indicating Controller) represented on a CRT screen to ascertain information like:

• Is the Reactor Inlet PV Pressure ideal for the desired chemical reactions?
• Is the PV Pressure of a Separating Tower optimized for production of the most desired product?
• Does the trend of the Process PV Pressure PIC indicate that the post Turnaround Pressure Test is completed since the target PV Pressure has been sustained throughout the duration of the test?
• Is the PV Pressure indicated on a PI sufficient to keep the process stream in the liquid phase? gas phase? a mixture of the two?

 

Pressure Indicators (PIs) and Pressure Indicating Controllers (PICs) make it possible for Outside Process Operators and Control Board Operators to make important decisions related to operating the process safely and efficiently.

Ergo, modern Process Operators and Control Board Operators must possess a basic understanding about how the PV Pressure is sensed/detected and measured and indicated.

The modern Outside Process Operator and Control Board Operator are expected to be able to recognize faulty instrument readings and be sufficiently core competent to write work orders which concisely and accurately describe where a fault lies in PV Pressure Control Loop.

Brilliant "Instrument Techie" PTOA Readers and Students … meaning those who are reading the PTOA in the intended, sequential order … will identify many similarities between the common instruments used to detect, measure, and indicate the PV Pressure and the common instruments used to detect, measure, and indicate the PV Temperature which were featured in PTOA Segments #101 through PTOA Segment #117.

 

SENSING AND MEASURING THE PV PRESSURE

"Wet" PV Pressure Sensors Versus "Dry" PV Pressure Sensors

The crucial part of any PV Pressure sensing and measuring instrument is the part that is solely impacted by changes in Pressure and nothing else (for example, the PV Temperature).

This crucial part of the PV Pressure Sensor must also be able to stretch, bend, change position ... or has some other characteristic that changes linearly with a change in the applied PV Pressure.

The change in stretching, bending, position, or other characteristic must then be mechanically or electrically converted into a representation of PV Pressure that human beings can read on a PI or PIC.

PV Pressure Sensors can be categorized as "Wet" Sensors and "Dry" Sensors:

"Wet" PV Pressure Sensors contain a liquid that directly responds to changes in Pressure, like a U-Tube or Inclined Manometer.

"Dry" PV Pressure Sensors must feature an element that can expand and contract, ergo responding to a change in the sensed PV Pressure.

Common "Dry" PV Pressure Sensors include:

• C-Shaped, Spiral-Shaped, and Helix-Shaped (aka Helical-Shaped) Bourdon Tubes.
• Diaphragm.
• Bellows.

 

This PTOA Segment features the form and function of Manometers, the "Wet" PV Pressure Sensor.  The next PTOA Segment features the form and function of "Dry" PV Pressure Sensors.

 

Manometers, the "Wet" PV Pressure Sensor

The liquid inside a U-Tube Manometer is typically water for a short range of low-PV Pressure changes or mercury for more expansive changes in the PV Pressure. Thus, the units of PV Pressure sensed by a Manometer are typically "inches of water" or "millimeters of mercury" (mm Hg). A few Manometers are filled with gauge oil.

The Low-Pressure leg of the Manometer shown nearby is on the right side and is open to atmospheric Pressure (1 ATM=14.7 psia).  The High-Pressure leg of the Manometer is on the left side which is exposed to the PV Pressure that is to be measured.

When no PV Pressure is sensed, the level of the liquid is the same in both legs of the manometer.

When a PV Pressure is sensed, the force component of that PV Pressure pushes down on the liquid contained within the high-Pressure leg of the U-Tube Manometer.  Thus, the level of the liquid on the High-Pressure, left side of the Manometer is pushed downward and the level on the Low-Pressure right-side rises.

The difference between the levels is a height of a column of liquid, "h."

Note that h = 11.5 inches in the above schematic of a U-Tube Manometer.

Assume the liquid in the U Tube Manometer were water.

Using any conversion tool (e.g., the conversion tool found on Google) the 11.5 in height of the water converts into 0.415 psi.

Thus 0.415 psi is the sensed difference in Pressure between the High-Pressure and Low-Pressure legs of the U-Tube Manometer.

If the liquid in the U-Tube Manometer were mercury, the 11.5-inch level difference between the two Manometer legs converts into 5.64 psi. Otherwise stated, there is a 5.64 psi difference in PV Pressure between the High-Pressure and Low-Pressure sides of the Manometer.

A couple noteworthy things about Manometers include:

Manometers measure a difference in Pressure, aka Differential Pressure, aka ΔP.

The measurement of ΔP is extremely useful in process monitoring and control. ΔP can be used to infer a PV Flowrate as well as determine a PV Level.

The relationship between ΔP and the PV Flowrate was the subject matter featured in PTOA Segment #158 and pops up in the PTOA as the PV Pressure ↔ Fluid Velocity Swap in PTOA Segment #159 as well as "The Venturi Effect" featured in PTOA Segment #207.  PTOA Readers and Students will learn more about this important relationship in the upcoming PTOA PV Flowrate Focus Study Area.

The relationship between ΔP and the PV Level was the subject of PTOA Segment #160 and featured in the Submarine Lessons of PTOA Segment #146.  PTOA Readers and Students will learn more about this relationship in the upcoming PTOA PV Level Focus Study Area.

Brilliant PTOA Readers and Students ... meaning those who read the PTOA Segments in the intended, sequential order ... can discern from the above paragraphs that an accurate measurement of ΔP will yield accurate PV Flowrate and PV Level information ... and vice versa. When a ΔP measurement is inaccurate, so is the PV Flowrate and PV Level inferred from the measurement.

The U-Tube Manometer can also measure Vacuum Pressures.  In this application, the High-Pressure leg of the U-Tube Manometer is exposed to Atmospheric Pressure (1 ATM). The level in the Low-Pressure leg of the U-Tube Manometer can only rise if it is exposed to a Vacuum Pressure.

Once again, locate the graphic that includes 3 U-Tube Manometers. Figure 2-3 on this graphic is measuring a Vacuum Pressure.

Atmospheric Pressure (1 ATM) is sensed on the High-Pressure right-hand leg of the Figure 2-3 Manometer. Don't be confused by the arrow that points downward labelling a Vacuum on the left side of this Manometer. The level on the left side of this U-Tube Manometer is rising, not falling.

Some Manometers are flat ...not U-Tube shaped ...and slant slightly downward like the one shown in the nearby photo.

The resulting Inclined Manometer can measure very small changes in the PV Pressure.

Inclined Manometers may be employed as draft gauges on Fired Heaters. Draft gauges measure the amount of flue gas (aka, draft) flowing from the Fired Heater.  The magnitude of the draft is determined from the ΔP between the Atmospheric Pressure (1 ATM) and the Vacuum Pressure created by the draft.

In the nearby schematic of a Fired Heater on the left, note the Draft Gauge label and arrow at the top of the heater. The arrow points to the position where the Low-Pressure, Vacuum Pressure-sensing side of the Inclined Manometer would be located. The High-Pressure leg of the Inclined Manometer would be exposed to local Atmospheric Pressure.

Note the "Damper" label and arrow pointing where it would be located on the heater stack. A more detailed photo of the stack and damper on a Fired Heater is on the right.

Establishing the optimal amount of draft is essential to operating a Fired Heater. Draft is an example of Vacuum Pressure found in process industries and was featured in PTOA Segment #151. The magnitude of the draft/flue gas velocity is impacted by the how much open or closed the Damper is.

Controlling the amount of draft from a Fired Heater helps to ensure that the amount of Combustion oxygen is optimized (see PTOA Segment #73).

Optimizing the amount of combustion oxygen in a Fired Heater is one way a Process Operator can effectively mitigate global warming.

 

TAKE HOME MESSAGES: Pressure Indicators and Pressure Indicating Controllers give Outside Process Operators and Control Board Operators information from which important process control decisions are made.

The validity of the PV Pressure that is indicated on a Pressure Indicator or Pressure Indicating Controller greatly depends upon the accuracy of the PV Pressure sensing and measuring device(s).

One way to classify PV Pressure sensors is "Wet PV Pressure Sensors" and "Dry PV Pressure Sensors." There are many similarities between the "Dry Sensors" used to measure the PV Pressure and the PV Temperature.

The U-Tube Manometer and Inclined Manometer are "Wet PV Pressure Sensors."  Like its name describes, the U-Tube Manometer is shaped like the letter "U."  One of the legs of the U is exposed to a higher PV Pressure than the other leg.

When both legs of the U-Tube Manometer are exposed to the same PV Pressure, the levels of liquid in both legs are at the same height. When there is a difference in PV Pressure sensed between the two legs of the U-Tube Manometer, the level in the Low-Pressure leg will be higher than the level in the High-Pressure Leg. The difference in the level can be translated into a difference in PV Pressure using a conversion tool.

When the Low-Pressure leg of the U Tube Manometer is exposed to Atmospheric Pressure (1 ATM), the U-Tube Manometer is effectively sensing and measuring a Gauge Pressure.

When the High-Pressure leg of the U-Tube Manometer is exposed to Atmospheric Pressure (1 ATM), the U-Tube is sensing and measuring a Vacuum Pressure (less than 1 ATM).

An Inclined Manometer is flat and slanted. Inclined Manometers can sense and measure very small changes in vacuum PV Pressures. Inclined Manometers are used to measure draft in Fired Heaters.

Manometers measure a difference in Pressure, aka Differential Pressure, aka ΔP. Measuring ΔP can be used to determine a PV Flowrate or a PV Level.

©2022 PTOA Segment 0226

PTOA PV PRESSURE STUDY AREA
PV PRESSURE INSTRUMENTATION

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