instrumentation engineering

Instrumentation is the art of measuring the value of some plant parameter, pressure, flow, level or temperature to name a few and supplying a signal that is proportional to the measured parameter. The output signals are standard signal and can then be processed by other equipment to provide indication, alarms or automatic control. There are a number of standard signals; however, those most common in a CANDU plant are the 4-20 mA electronic signal and the 20-100 kPa pneumatic signal.
This section of the course is going to deal with the instrumentation equipment normal used to measure and provide signals. We will look at the measurement of five parameters: pressure, flow, level, temperature, and neutron flux.
2.1 PRESSURE MEASUREMENT
This module will examine the theory and operation of pressure detectors (bourdon tubes, diaphragms, bellows, forced balance and variable capacitance). It also covers the variables of an operating environment (pressure, temperature) and the possible modes of failure.
2.1.1 General Theory
Pressure is probably one of the most commonly measured variables in the power plant. It includes the measurement of steam pressure; feed water pressure, condenser pressure, lubricating oil pressure and many more. Pressure is actually the measurement of force acting on area of surface. We could represent this as:
or
The units of measurement are either in pounds per square inch (PSI) in British units or Pascals (Pa) in metric. As one PSI is approximately 7000 Pa, we often use kPa and MPa as units of pressure.
2.1.2 Pressure Scales

Before we go into how pressure is sensed and measured, we have to establish a set of ground rules. Pressure varies depending on altitude above sea level, weather pressure fronts and other conditions.
The measure of pressure is, therefore, relative and pressure measurements are stated as either gauge or absolute.

Revision 1 ñ January 2003

Science and Reactor Fundamentals ñ Instrumentation & Control 8 CNSC Technical Training Group
Gauge pressure is the unit we encounter in everyday work (e.g., tire ratings are in gauge pressure).
A gauge pressure device will indicate zero pressure when bled down to atmospheric pressure (i.e., gauge pressure is referenced to atmospheric pressure). Gauge pressure is denoted by a (g) at the end of the pressure unit [e.g., kPa (g)].
Absolute pressure includes the effect of atmospheric pressure with the gauge pressure. It is denoted by an (a) at the end of the pressure unit [e.g., kPa (a)]. An absolute pressure indicator would indicate atmospheric pressure when completely vented down to atmosphere - it would not indicate scale zero.
Absolute Pressure = Gauge Pressure + Atmospheric Pressure
Figure 1 illustrates the relationship between absolute and gauge. Note that the base point for gauge scale is [0 kPa (g)] or standard atmospheric pressure 101.3 kPa (a).

The majority of pressure measurements in a plant are gauge. Absolute measurements tend to be used where pressures are below atmosphere. Typically this is around the condenser and vacuum building
Atmospheric Pressure

Perfect Vacuum

101.3 kPa(a)
0 kPa(a)

0 kPa(g)
-101.3 kPa(g)

Figure 1
Relationship between Absolute and Gauge Pressures
2.1.3 Pressure Measurement
The object of pressure sensing is to produce a dial indication, control operation or a standard (4 - 20 mA) electronic signal that represents the pressure in a process.
To accomplish this, most pressure sensors translate pressure into physical motion that is in proportion to the applied pressure. The most common pressure sensors or primary pressure elements are described below.

Revision 1 ñ January 2003

Note

Science and Reactor Fundamentals ñ Instrumentation & Control 9 CNSC Technical Training Group
They include diaphragms, pressure bellows, bourdon tubes and pressure capsules. With these pressure sensors, physical motion is proportional to the applied pressure within the operating range.
You will notice that the term differential pressure is often used. This term refers to the difference in pressure between two quantities, systems or devices
2.1.4 Common Pressure Detectors Bourdon Tubes
Bourdon tubes are circular-shaped tubes with oval cross sections (refer to Figure 2). The pressure of the medium acts on the inside of the tube. The outward pressure on the oval cross section forces it to become rounded. Because of the curvature of the tube ring, the bourdon tube then bends as indicated in the direction of the arrow.

Pressure

Figure 2 Bourdon Tube

Due to their robust construction, bourdon are often used in harsh environments and high pressures, but can also be used for very low pressures; the response time however, is slower than the bellows or diaphragm.
Bellows
Bellows type elements are constructed of tubular membranes that are convoluted around the circumference (see Figure 3). The membrane is attached at one end to the source and at the other end to an indicating device or instrument. The bellows element can provide a long range of motion (stroke) in the direction of the arrow when input pressure is applied. 

construction safety

General Requirements {Division 3/Subdivision P 29 CFR 1926.651}

Underground Installations [29 CFR 1926.651(b)]

• 􏰁  Determine the estimated locations
  
• 􏰁  Contact the utility or owner
  
• 􏰁  Proceed cautiously
  
• 􏰁  Find the exact location
  
• 􏰁  Support, protect, or appropriately remove the installation in open excavations
  Surface Encumbrances [29 CFR 1926.651(a)]
  

􏰁 Remove or support if creating a hazard to workers
What are some examples of surface encumbrances?
poles rocks trees
__________ __________ __________
Access and Egress [29 CFR 1926.651(c)]

1. (1)  Structural ramps used for access or egress of equipment must be designed by a competent person qualified in structural design.
   
2. (2)  A safe means of entering and leaving excavations must be provided for workers. A stairway, ladder, ramp, or other means of egress must be located in trench excavations which are:
   􏰂 four feet or more in depth, and
   􏰂 require no more than 25 feet of lateral travel.
   OR-OSHA 302 Excavation Safety
   

Speed Shore Corp.

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General Requirements {Division 3/Subdivision P 29 CFR 1926.651}

Exposure To Vehicular Traffic [29 CFR 1926.651(d)]
Employees must be provided and wear warning vests or other suitable garments marked with or made of reflectrorized or high visibility material.
Exposure To Falling Loads [29 CFR 1926.651(e)]
Employees are not allowed under loads handled by lifting or digging equipment. Workers must either stand away or otherwise be protected from any vehicle being loaded or unloaded to avoid spilling or falling material.
Warning System for Mobile Equipment [29 CFR 1926.651(f)]
A warning system must be utilized when mobile equipment is operated adjacent to an excavation, or when such equipment is required to approach the edge of an excavation, and the operator does not have clear and direct view of the edge. Barricades, hand or mechanical signals, or stop logs can be used.
OR-OSHA 302 Excavation Safety

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Hazardous Atmospheres

Purpose

Prevent exposure to harmful levels of air contaminants such as:

• 􏰂  Oxygen deficiency,
  
• 􏰂  Explosives/Flammables,
  
• 􏰂  T oxins;
  and to assure acceptable atmospheric conditions through:
  

• 􏰂  Atmospheric testing,
  
• 􏰂  Removal of the substance,
  
• 􏰂  Proper ventilation,
  
• 􏰂  Respiratory protection,
  
• 􏰂  etc.
  Testing and Controls
  

[29 CFR 1926.651(g)]

Testing is required where oxygen deficiency (less than 19.5 percent oxygen), or a hazardous atmosphere exists or could reasonably be expected to exist.
Examples include excavations in landfill areas or in locations where hazardous substances exist (i.e. utilities, tanks, contaminated soil, etc.)
When controls such as ventilation are used to reduce the level of atmospheric contaminants to an acceptable level, testing must be conducted as often as necessary to ensure continuing safety.
Emergency Rescue Equipment
Emergency rescue equipment must be readily available where hazardous atmospheric conditions exist or can reasonably be expected to develop.
OR-OSHA 302 Excavation Safety

NOTE: Division 3/Subdivisions D & E provide additional requirements. Additionally, workers entering bell- bottom piers or other similar deep and confined footing excavations must utilize a harness and lifeline system.

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General Requirements {Division 3/Subdivision P 29 CFR 1926.651}

Water Accumulation [29 CFR 1926.651(h)]
Employees must be properly protected when working in excavations where water has accumulated or is accumulating. Precautions will vary with each situation but may include diversion, dewatering (well pointing) systems, special supporting systems, or water removal equipment. The competent person must monitor water removal equipment.
Water is one of the major concerns during excavation operations. The action of water in excavations can cause undermining and cave-ins.
Stability of Adjacent Structures [29 CFR 1926.651(i)]
Where the stability of adjacent buildings, walls, or other structures is endangered by excavation operations, support systems such as shoring, bracing, or underpinning must be provided to ensure stability.
Excavation below the level of the base or footing that could pose a hazard is not permitted except when:
• the excavation is in stable rock, or
• support system (underpinning) is provided, or • Registered Professional Engineer approves.
Sidewalks, pavements, and appurtenant structure must not be undermined unless a support system or another method of protection is provided to protect employees from collapse.
OR-OSHA 302 Excavation Safety

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This material is for training use only

General Requirements {Division 3/Subdivision P 29 CFR 1926.651}

Daily Inspections [29 CFR 1926.651(k)]
Daily inspections of excavations, adjacent areas, and protective systems must be made by a Competent Person for evidence of a situation that could result in possible cave-ins, failure of protective systems, hazardous atmospheres, or other hazardous conditions.
When? 1._______________ 2._______________ 3._______________ What are we inspecting?
If evidence of a possible cave-in, failure in the protective system, hazardous atmosphere, or other significant concerns are found, all affected workers must be removed from the hazardous exposure until rendered safe.
Protection from Loose Rock or Soil [29 CFR 1926.651(j)] Scale back to remove loose material or install protective barricades and place all
material and equipment at least two feet from the edge.
Fall Protection [29 CFR 1926.651(l)]
Walkways must be provided where employees or equipment are
permitted to cross over excavations.

Adequate barrier physical protection must be provided
at all remotely located excavations. All wells, pits, shafts,
etc., must be barricaded or covered. Backfill as soon as possibl