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

Reading

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

A diagram of a dP gauge
dP Gauge
[image 140-2-05]

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.

A diagra of a strain gauge transducer
Strain Gauge Transducer
[image 140-2-06]

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.

A diagram of a capacitance transducer
Capacitance Transducer
[image 140-2-07]
A diagram od a detail of a capacitance transducer
Capacitance Transducer Detail
[image 140-2-08]

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

A diagram of a pressure gauge
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

PID, PFD, Symbols Information

A chart showing PID and PFD symbols
PID, PFD, Symbols
[image 140-2-11]

P&ID Detail – Pressure Instruments

A diagram of a pressure instrument
P&ID Detail
[image 140-2-12]

Reading a P&ID – Instrumentation

  • To interpret instrumentation on a P&ID, you apply your own logic to the information given. Some rules of thumb:
    • The signal begins at the process and moves outward.
    • Signals move in one direction only, through each signal line shown.
    • Follow the signal path to describe what the instruments are doing.
  • Balloons used for instrumentation provide info on remote/local, mechanical/electrical, etc.