mish Flashcards
Frequency is used to measure
Capacitance, reluctance, reactance and transformer effects e.g AC tachometer
Square Root extractors
Only used with flow with a differential pressure sensing element (Head meter)
High/Low Select Relays (High pressure selector relay)
High/Low Select Relays (Low pressure selector relay)
Integrators / Totalizers
Output: counts/time Input: 4-20mA
Purpose is to count a measured quantity
A transducer does what
converts one energy to another, such as converting pressure to current
A typical
instrument air system consist of the following components:
Compressor
Oil Filter
Local control panels
Air dryer and receiver
Particulate
Air compressors generally come as an integrated unit consisting of
the following components:
Control system and panel
Oil filters
Lubrication system
After coolers
Inlet air filter
Drive motor
. Various types
of compressors are available, including:
Diaphragm
Centrifugal
Rotary (vane, screw,liquid ring)
Reciprocating oiled & oil-less pistons
Solder composition
60/40 tin/lead with a resin core
Benefits of solder
permanent connection
prevents oxidation
electrically conductive connection
sealed from liquid or gas
stronger than wire
remain secure under stress from tension, vibration or expansion/contraction caused by temperature changes
Screw driver size colours
orange(00)
yellow(0)
Green(1)
Red(2)
Black(3)
Common pipe sizes
1/8, 1/4, 3/8, 1/2
Pipe is measured by:
Tube is measured by:
Pipe internal diameter
Tubing: external diameter
Common construction for pipes and fittings:
brass(non-corrosive, soft, expensive)
plastic (not very strong)
black iron and galvanized
Stainless steel (doesnt rust, strong, twice as expensive as brass)
Pipe schedule:
40
common Tube sizing
1/4, 3/8, 1/2, (1/8 less common)
Common materials of construction for tubing and fittings
copper(comes in rolls)
plastic
stainless steel (comes straight)
Pipe threads are tappered whereas tubing threads are straight
always check tubing connections with snoop or soapy water
Tube bundles
consists of many tubes in a protective jacket
plastic, copper or stainless
can be heat traced + insulated to protect against cold and can have protective coverings to protect from heat
often contains communication wire
Regulators are known as
PRV’s (pressure regulating valves)
Three main parts of a regulator
diaphragm
spring
Needle valve
regulators are used in industry to reduce supply from
60-80 psi to 20 psi and maintain a pressure at 20 psi regardless of flow
regulators are not the same as control valves
they are self contained pressure control loops with a
sensor
controller
valve
regulators are
simple
dependable
rugged and inexpensive
Regulator maintenance: when do you have a plugged filter
Decreased output pressure Compensates by opening supply port
A regulators weakest link
How do you know if you have a hole in your diaphragm
the air will flow out of the exhaust port to atm
Regulators work on what principle
Feedback principle, output air pressure is fed back to diaphragm so that output pressure balances spring tension
2 things can affect a regulators output pressure
in put pressure drop below out put pressure
output flow is greater than regulator capacity (it’ll open it’s supply port until it can no longer do so )
compressed air is commonly used as a source of power for process control e.g. as a means of postiioning a control valve in both pneumatic and electrical control loops because:
Adequate power to move a valve stem
positions valve stem accurately
inexpensive, rugged
accurate pneumatic controllers
2 types of air supplies in an industrial setting
plant air (may contain dirt and oil)
instrument air (must be clean and free of water vapour)
Operation of a pressure regulator (PRV) Pressure Regulating Valve
1) output pressure is applied under feedback por this decreases spring behind diaphragm opening supply port
3) o.p. increases diaphragm moves up closing supply valve and opening exhaust port allowing o.p. to escape to atm: through exhaust vent
4) pressure will remain constant when output pressure and spring tension are in balance
5) o.p. is increased or decreased by spring tension
6) regulators will maintain a set value regardless of flow rate until the capacity of the regulator is reached at which point, the needle valve is completely open and o.p. begins to drop
Reasons for compressed air as a “power” source
(C)an move the valve stem
(A)ccurately positions the valve stem
(R)ugged, inexpensive fool proof diaphragm control valves
(S)imple accurate pneumatic controllers
to supply air to an instrument air must be:
(C)ompressed and conditioned to use
(A)Final adj. must be made before reaching the instrument
Distributed to the instrument
Inlet Air Filters
Outside and in the coolest area
Removes grit and dust
After coolers
The compression of a fluid or gas will result in a rise in the temperature of the air. After coolers reduce air temperature below dew point
Atmospheric pressure (PISA)
Pressure at the earths surface caused by the weight of the air above it. (14.7 PSIA or 101.4 kPa)
Gauge pressure (PSIG):
Uses atmospheric pressure (14.7PSIA) as a zero point.
Absolute pressure (Pabs)
gauge pressure + atmospheric pressure
Differential pressure:
The difference in pressure between two measurement points in a process.
Pressure Units: “H2O, “WC, “Hg, mm Hg, or torrs (1 mm Hg absolute).
4 Common Pressure Scales
(D)ifferential Pressure
(A)bsolute
(V)acuum
(G)auge
DAVG
Barometer
Used to measure atmospheric
pressure.
The 3 types of range:
Suppressed: Higher than 0 LRV
Elevated: Lower than 0 LRV
Zero: has a 0 LRV
Relay
Boost output and valve volume
The purpose of a pressure element
To create mechanical motion
from an input pressure.
When Ordering a gauge
(P)ressure range scale
(F)ace size
(A)ccurary
(C)onnection location
(T)ype of mounting
PFACT
Incline- Tube Manometer Advantages
-Measures extremely low pressure, even in a vacuum (negative pressure)
-Extremely accurate
-Unlike mechanical or electronic manometers, inclined manometers don’t have any parts that could wear out or deteriorate.
-Sensitivity of inclined tube manometers is high
Gauge Accessory: Snubbers
(R)educes oscillation caused by fluctuations and process pressure
(U)ses porous filter disks, pistons, variable orifices or oil filled rubber bulbs
(S)low response time so pressure element can respond to pressure changes
Gauge Accessory: Chemical Seal
-Protects gauge from corrosive or hot services
-Isolates the internal parts from the process material steam, acid, slurries
-Filled with non compressible fluid
Diaphragm Element
- Stainless steel and filled with liquid
-Used in both pneumatic and electric transmitters - Movement dependent on metal thickness, diameter, and # of corrugations
Bellows Element
- Used in pneumatic transmitters, recorders, controllers
- brass or stainless steel
- more sensitive than bourdon tubes so used for low pressures
-Sensitivity increases with diameter
Helical Bourdon Tube
- Used for continuously fluctuating services
- Higher pressure span = more coils
Spiral Bourdon Tube
- More motion than C type
- Causes flapper to move closer to nozzle
- Used in pneumatic motion balance transmitters
C Bourdon Tube
-Used in direct indicating gauges
-Forces a bar in flapper assembly
-Linear indication with gears, cams, pinions
Flapper Nozzle Assembly
Converts Mechanical Motion into a standard pressure signal
Range
Region between limits within a measured quantity expressed by stating the lower and upper range values
Direct Acting
As input goes up, output goes up
when speaking relays inputs the back pressure output is the 3-15 psi
Reverse Acting
input up, output down
when speaking relays inputs the back pressure and output is the 3-15 psi
What is the purpose of a restriction before the nozzle
To reduce the volume of the flow
What would happen on a flapper nozzle if blocked the nozzle
The back pressure would rise up to the supply pressure (20psi)
Why is it necessary to have an exhaust valve and port on a relay
output air will escape to atm when backpressure decreases
Fixed Orifice Restriction
provide free flow in one direction and Restricted (metered) flow in the reverse direction.
Nozzle
pressurized air comes out of the nozzle through the gap between the nozzle and flapper.
Baffle or Flapper
increases the nozzle back pressure.
Define cavitation
liquid to gas and back again caused by change in pressure(vavle or restriciton)
A strain gauges measures what?
The deformation of an object when subjected to stress.
1”Hg=”H2O
13.61”H2O
1 atm=bar
1.0135bar
1 “H2O=mmHg
1.87
1 psi = ______kPa
6.895kpa
A strain gauges measures what?
The deformation of an object when subjected to stress.
3 Types of manometers and which one is used in HVAC
U-Tube
Well
Incline (HVAC, detects small pressures in gases)
What does a transducer do?
converts one form of energy to another, like pressure to current or vise versa
Resonance
Causes a wire to vibrate at it’s resonant or natural frequency
Changes in pressure = changes in frequency of an oscillating fan
Determined on 1)mass 2) square root of the tension 3) length
Tension is proportional to applied pressure
A strain gauges resistance changes when:
stretched, proportional to the pressure applied
A Mass flow meter measures what
The rate of flow but compensates for temp and pressure density resulting in a true mass flow reading
Positive displacement meter
Won’t allow flow if jammed and uses energy from process to operate
Variable area flow meter
(P)roportional to the rate of flow
(O)orifice creates a fixed differential pressure
(U)ses a device in response to flow rate
Head meter
creates a differential pressure related to velocity (head pressure)
Capillary action
Liquids cohesive and adhesive forces (meniscus in a manometer)
cohesion
allows liquid to resist tensile stress
viscosity
fluids resistance to deformation (water=low, oil=high)
A High select relay
with 2 or more inputs; a high select relay selects the high pressure value
compares two pressures and selects the higher of the 2
causes the flapper to close the lower input pressure port
A High select relay
with 2 or more inputs; a high select relay selects the high pressure value
compares two pressures and selects the higher of the 2
causes the flapper to close the lower input pressure port
FQ
Flow Integrator
LVDT
Linear variable differential transformer
What does a Integrator/totalizer do and it’s purpose
Output is in counts it’s purpose is to count a measured quantity
A Differential pressure transmitter is used to measure?
low pressure
negative pressure
pressure drop
flow (differential pressure)
level
density
