PRT 140: Lesson 2 Pressure

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)
• Local or Remote reading

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. Pressure Gauge, showing the Bourdon Tube [Image 140-02-2]

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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?

Units – Mercury vs. Water – 1 psi Mercury vs. water – 1 psi[image 140-2-09]

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:

1 of (Column Heading) = (table value) of (Row Heading)
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