# PRT 140: Lesson 2 Pressure

Contents

## Objectives

• Define pressure and formula P=F/A
• Define terms associated with pressure and pressure instruments, per textbook
• Identify common types of pressure-sensing/measuring instruments used in the process industry:
• manometers
• pressure gauges
• differential pressure (d/p) cells
• strain gauge transducers (‘piezoelectric effect’)
• capacitance transducers
• Describe the purpose and operation of pressure instruments
• Discuss and perform pressure unit conversion calculations
• Describe and identify P&ID symbols for Pressure instrumentation
• Connect and read a pressure gauge, describe Bourdon tube operation

### Pressure

P = F/A

Pressure = Force / Area

Measurements are in pounds/square inch

### Parameters Affecting Force

• FORCE = push/pull that causes change in direction
• SPEED = temperature, how fast molecules move
• MASS/Weight = amount of matter
• Larger molecules weigh more — Hg vs. H2O
• DENSITY = molecules/volume

### Specific Gravity

#### Liquids

• Specific gravity = density of x/density of water
• Density water = 1.0 at 39 deg F

QUESTION — If SG <1, is material lighter or heavier than water?

#### Gases

• Specific gravity = weight of x/weight of air
• Air at standard conditions for reference

### Pressure Instruments

• Gauge (PI) or Transmitter (PE-PT or PT)

### Manometers

• Essentially, open-ended tubes filled with liquid
• Applying pressure to one end of the tube will cause the liquid to rise on the other side

### Pressure Gauge – Bourdon tube

Principle of Operation: The Bourdon Tube flexes in response to pressure changes — mechanical response

The flexing tube is connected to the indicator needle with gears.

[/panel]

### dP Gauge

Principle of Operation:

This example is also a mechanical device, similar to a Bourdon tube

### Strain Gauge Transducer

Principle of Operation:

Group of wires stretch when they are exposed to pressure. Current flows through the wires, and resistance changes as the wires are stressed. Emits an electrical signal.

### Capacitance Transducer

Principle of Operation:

Two metal capacitor plates are pushed closer together as they are exposed to pressure; distance between the plates changes the amount of electrical charge that the two plates can hold (electrical capacitance). Emits an electrical signal.

### Pressure transmitters — How to identify them in the field?

• Read the nameplate tag on the instrument
• Should show calibration range
• Will give model number — can look it up
• May include facility tag number
• Look at how the instrument connects to the process
• Sensors have an isolation valve at the process connection
• If a dP cell, then there will be two sensing leads (one may be to atmosphere) — look for H/L stencil on the instrument body (High/Low)
• If a P cell, then there will be one sensing lead
• Be aware that many pressure transmitters are used as level or flow instruments

### Differential Pressure

• dP, DP
• P2 — P1
• Difference in pressure between 2 distinct points.
• dP can be used to calculate other process variables, specifically FLOW or LEVEL (we’ll learn more later)
• dP around equipment used for monitoring the fouling or plugging of the equipment (filters).

### Absolute Pressure — Pressure scale where 0 = full vacuum

• Pounds/square inch ABSOLUTE
• Total Vacuum is 0 psia
• Normal atmospheric P (sea level) = 14.7 psia
• Most gauges read pressure above atmospheric pressure, called GAUGE pressure — psig
• Normal atmospheric P (sea level) = 0 psig
• Atmospheric pressure changes depending on elevation, conditions, but:
• For practical purposes, psia = psig + 14.7

### Pressure Units — many many

• Need for different scales
• Water, mercury, etc. can measure smaller variations in pressure more clearly
• How is ‘in. H2O’ a pressure unit? Isn’t pressure force/area?
• Look at columns of water, pressure

### Pressure Units — In. W.C.

• The unit “in. w.c.’ or “in. H2O’ means:
• The pressure is equivalent to the pressure exerted by a column of water that high.
• If you had a column of water that was 1 square inch in cross-sectional area, 27.7 inches high, the weight of that water would be 1 pound. 1 pound/sq. inch = 1 psi
• The pressure exerted by a column of liquid is independent of the diameter of the column.
• WHY is that?

### Conversions — Unit Equivalency

• psia = psig + 14.7
• psig = psia — 14.7
• 1 psi = 2.04 in. Hg = 27.7 in. H2O = 0.069 bar
• It goes on and on — how to convert between units?
• Learn or look up conversion factors for every change — see Table 2.3….
• Or learn the main ‘equivalencies’ and how to convert that way

#### Table 2-3, Pressure Unit Conversion Chart

psibarmbarIn. HgIn. H2OmmHgmmH2O
psi114.5040.0145040.491180.0361270.0193370.0014223
bar0.06894610.0010.0338650.00249080.00133329.8068 x 10-5
mbar68.9461000133.8652.49081.33320.098068
In. Hg2.035929.5290.02952910.0735520.0393680.0028959
In. H2O27.68401.470.4014713.59610.535250.039372
mmHg51.714750.060.7500625.4011.868310.073558
mmH2O703.050.1019710.197345.3225.33913.5951
atm0.0680450.986920.000986920.0334220.00245830.00131589.6788 x 10-5

In this type of table, always best to confirm that you’re reading it right.

I know that 27.7 in. H2O = 1 psi, so I can figure out how to read this particular table:

Example: 1 mmHg = 0.53525 in. H2O

#### Conversion Table 2-3

You can use each column in the table as a string of ‘equivalent units’

Note: Not every table is the same — verify before you calculate. You know that 1 psi = 27.7 in H2O

psibarmbarIn. HgIn. H2OmmHgmmH2O
psi114.5040.0145040.491180.0361270.0193370.0014223
bar0.06894610.0010.0338650.00249080.00133329.8068 x 10-5
mbar68.9461000133.8652.49081.33320.098068
In. Hg2.035929.5290.02952910.0735520.0393680.0028959
In. H2O27.68401.470.4014713.59610.535250.039372
mmHg51.714750.060.7500625.4011.868310.073558
mmH2O703.050.1019710.197345.3225.33913.5951
atm0.0680450.986920.000986920.0334220.00245830.00131589.6788 x 10-5

### Equivalency Calcs

1 psi = 2.04 in. Hg = 27.7 in. H2O = 0.069 bar

New units         =                      (known units)                 x                     (new equivalent units)

(known equivalent units)

Ex:   Convert 50 in. water to psi

New psi = (50 in H2O)/(27.7 in H2))     x (1 psi)

New psi = 1.8 psi