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

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

 

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dP Gauge

Strain Gauge Transducer

Capacitance Transducer

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

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

	psi	bar	mbar	In. Hg	In. H2O	mmHg	mmH2O
psi	1	14.504	0.014504	0.49118	0.036127	0.019337	0.0014223
bar	0.068946	1	0.001	0.033865	0.0024908	0.0013332	9.8068 x 10-5
mbar	68.946	1000	1	33.865	2.4908	1.3332	0.098068
In. Hg	2.0359	29.529	0.029529	1	0.073552	0.039368	0.0028959
In. H2O	27.68	401.47	0.40147	13.596	1	0.53525	0.039372
mmHg	51.714	750.06	0.75006	25.401	1.8683	1	0.073558
mmH2O	703.05	0.10197	10.197	345.32	25.339	13.595	1
atm	0.068045	0.98692	0.00098692	0.033422	0.0024583	0.0013158	9.6788 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

	psi	bar	mbar	In. Hg	In. H2O	mmHg	mmH2O
psi	1	14.504	0.014504	0.49118	0.036127	0.019337	0.0014223
bar	0.068946	1	0.001	0.033865	0.0024908	0.0013332	9.8068 x 10-5
mbar	68.946	1000	1	33.865	2.4908	1.3332	0.098068
In. Hg	2.0359	29.529	0.029529	1	0.073552	0.039368	0.0028959
In. H2O	27.68	401.47	0.40147	13.596	1	0.53525	0.039372
mmHg	51.714	750.06	0.75006	25.401	1.8683	1	0.073558
mmH2O	703.05	0.10197	10.197	345.32	25.339	13.595	1
atm	0.068045	0.98692	0.00098692	0.033422	0.0024583	0.0013158	9.6788 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

P&ID Detail — Pressure Instruments

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.