L.3 - Pressure

Question 1

Define Hydrostatic Pressure:

Answer 1

pressure exerted by a column of liquid

Question 2

Define Differential Pressure w/ units:

Answer 2

the difference between two pressures. Unit: kPad or Psid

Question 3

Define Absolute Pressure w/ units:

Answer 3

the pressure with respect to absolute zero pressure (a perfect vacuum). Unit: kPaa or Psia

Question 4

Define Gauge Pressure w/ units:

Answer 4

pressure with respect to atmospheric or barometric pressure. Unit: kPag or Psig

Question 5

Define Vacuum Pressure w/ units:

Answer 5

pressure less than atmospheric pressure (negative gauge pressure). Unit: kPag or Psig (negative sign)

Question 6

What is a Manometer?

Answer 6

a pressure measuring device that uses the height of a column of liquid if known density to determine magnitude of applied pressure

Question 7

What is a U-Tube Manometer?

Answer 7

it is a pressure measuring device that measure the difference in pressure between two ports (labelled P1 and P2)

Question 8

What does the accuracy of a manometer depend on?

Answer 8

1) the accuracy of the measuring scale
2) the density of manometer fluid (temp. of, pressure of)
3) Ability of operator to accurately read the scale

Question 9

What are some requirement for a manometer fluid to be usable?

Answer 9

1) it needs to be clean with few contaminants
2) it needs to have a precisely know density
3) it needs to have relatively low viscosity
4) little tendency to wet manometer tubes
5) in some cases low vapour pressure

Question 10

What are pressure transducers?

Answer 10

sensors that measure pressures ranging from really low absolute pressures to really high gauge pressures

Question 11

How do mechanical pressure transducers work?

Answer 11

they rely on elastic deformation of a material in response to a change in an applied pressure

Question 12

List some mechanical pressure transducers:

Answer 12

1) Bourdon Tubes (C-type, Spiral, Helical)
2) Diaphragm

Question 13

How do Bourdon Tubes Work?

Answer 13

> when the pressure applied to the inside of the tube is greater than the outside pressure, the tube tends to straighten because area of the outer circumference is > area of inside circumference
resultant force creates an outward movement/deflection at the tip of the tube

Question 14

What material are bourdon tubes made of, and why?

Answer 14

who cares

> can be made up of stainless steel for ex.
material selection is based on the instrument service requirements, such as measurement range and material compatibility

Question 15

What is “drift” in a bourdon type?

Answer 15

it is permanent deformation resulting from long-term use of a bourdon type

Question 16

What affects the deflection of the bourdon tube tip?

Answer 16

1) Material Properties
2) Length of Tube
3) Thickness
4) Cross-Sectional Area

Question 17

What is a Diaphragm, how does it work?

Answer 17

> a thin plate made up of metal/silicon supported along it’s outer edge by rigid housing

> diaphragm produces (small) linear deflection relative to applied differential pressure

Question 18

What does a safety blowout do?

Answer 18

it vents process fluid away from the front of the gauge to prevent gauge glass from breaking (away from glass and blowout from back)

Question 19

What does an oil filled case do?

Answer 19

filling the gauge with oil dampens mechanical vibrations, making the gauge easier to read and extending its service life

Question 19

What does a snubber do?

Answer 19

used to dampen/restrict pressure fluctuations which could wear out the gauge and make it difficult to read

Question 20

What is a chemical seal/Diaphragm seal used for?

Answer 20

it is placed between the gauge and the process fluid to help extend the gauge service life in case where the fluid is corrosive or to prevent fluid from plugging the sensing element

Question 21

What does a DP cell do?

Answer 21

a differential pressure cell can be used to measure pressure, level, flow etc.