Fundamentals of Instrumentation (cover-to-cover)

Instrumentation

branch of physics which deals with the measuring, monitoring, and controlling variables in a process industry

Instrument

device that measures, monitors and control process variable

Instrumentation based on Industrial application

a collection of instruments, devices, hardware or function or their application for the purpose of measuring, monitoring or controlling an industrial process or machine, or any combination of those

PICS

Philippine Instrumentation and Control Society

SAMA

Scientific Apparatus Manufacturers Association

Critical, Non-critical, reference only

Classification of Instruments

Critical

An instrument which, if not conforming to specification, could potentially compromise product or process quality

Non-critical

An instrument whose function is not critical to product or process quality, but whose function is more of an operational significance

reference only

function is not critical to product quality, not significant to equipment operations, and not used for making quality decisions

Control

refers to regulation, command, or direction of all aspects of the process

Manual Control and Automatic Control

2 types of control

Process

any operations or sequence of operations involving a change of energy, state, composition, dimension or other properties that may be defined with respect to a datum/data. It also refers to the method of changing or refining raw materials to create end products.

Process control

the regulation or manipulation of variables influencing the conduct of a process in such a way as to obtain a product of desired quality and quantity in an efficient manner

System

an arrangement, set, or collection of physical components connected or related in such a manner as to form and/ or act as an entire unit.

Control system

an arrangement of physical components connected or related in such a manner as to command, direct, or regulate itself or another system

Instrumentation system

an arrangement of two or more instrument connected together to perform a unified task.

Control Loop

A process management system designed to maintain a process variable at a desired set point

Open control loop and Closed control loop

2 types of Control loop

Open control loop

Exists when the process variable is not compared, and action is taken not in response to on the condition of the process variable.

Closed control loop

exists when a process variable is measured, compared to a set point and action is taken to correct any deviation from setpoint.

Setpoint

a value for a process variable that is desired to be maintained or any input variable that sets the desired value of the measured variable

Process, measuring element, receiving element, and final control element

Elements of the instrument loop

process variable

any variable property of a process

Controller output

outputs signal generated in response to a change of a process variable

manipulated variable

a process variable linked to the controller output that is used to influence the measured variable

discreet field mounted

Computer function field mounted

shared display field mounted

PLC field mounted

Discreet located in main control panel

Shared display located in rear of main control panel

PLC located in local control panel

CF located in rear of local control panel

Pressure, level, flow, temperature

Major Process Variable

Instrumentation and control loop or instrument loop

A combination of two or more instrument or control functions arranged so that signal pass from one to another for the purpose of measurement and/ or control of a process variable

Measurement

The determination of the magnitude of a variable (value)

Detection

the determination of the existence or the presence of a variable

output represent the input faithfully

linear input/output relationship

no effect of temperature and others

Basic characteristics of a measuring element

Range

The limit or extent of variation. The lower limit is called LRV and the upper limit is call URV

Span

The number of units between two limits, the LRV and URV.

(Span = URV - LRV)

Analog signal and Digital signal

Standardized signals

Analog signals

A signal that has no discrete positions or states and changes value

Digital Signal

a signal that generates or uses binary digit signals to represent continuous values or discrete states

3 to 15 psi

Pneumatic or air pressure (English)

0\.2 to 1.0 bars or kg/cm^2

Pneumatic air pressure (Metric)

4 to 20 mA dc

Electrical or electronic (current)

1 to 5 V dc

electrical or electronic (voltage)

Sensor, transmitter, transducer, process switch

Types of measuring elements

Sensor

An integral part of instrument loop or instrument that first senses the value of a process variable, and that assumes a corresponding, predetermined, and intelligible state or output indictive or proportional to the process variable.

Transmitter

A device that senses a process variable through the medium of a sensor and has an output whose steady-state value varies only as a predetermined function of the process variable.

Transducer

Translate mechanical signal into an electric signal. A general form of a device that receives information in the form of one or more physical quantities, modifies the information and/or its form, if required and produces a resultant output signal.

Volts or millivolts

A signal sent by a transducer

Milliamps

A signal sent by a transmitter

Expensive, very accurate, very rugged, repairable, adjustable range, standard signal output

Characteristics of a Transmitter

Low cost, not so accurate, not so rugged, not repairable, fixed range, different outputs by the manufacturers

Characteristics of Transducer

Converter

A device that converts one type of signal into another type of signal. (current into voltage, analog to digital

Direct method and inferential method

Methods of measurement

direct method

The process variable is measured in units that represent the basic nature of that variable itself

inferential method

the measurement of a process variable using another variable

Single point type and continuous type

Types of measurement

Single point type

the measurement depends on a fixed value of the process variable and reads either as high or low

Continuous type

A type of measurement of a process variable in which the process variable is measured indirectly or using another variable.

Instrument range

refers to the capability of the instrument to measure a variable

Calibration range

refers to the set of values within the instrument measuring range where the scaled output; 4-20mA, 3-15 psi or 1-5 V is set during calibration.

Instrument span

It is the distance (or difference) between the upper range value (URV) and lower range value (LRV).

Upper Range Value

the highest value of the measured process variable that the output of a transmitter is currently configured to measure.

Lower range value

The lowest value of the measured process variable that the analog output of a transmitter is currently configured to measure

Discreet sensors

Typically take the form of switches, built to trip when the measured quantity either exceeds or falls below a specific value. These devices are less sophisticated than so called continuous sensors.

Discrete process measurement

Measurement process in which sensors are only able to indicate a true or false condition.

Normal status of a switch

The status in which electrical contacts are in under a condition for minimum physical stimulus.

Normally-open status

The lamp will energize only if someone pressed the switch holding it in a closed position. It is also referred to as Form-A contacts.

Normally-closed status

A lamp will energize only if the switch was left alone, but it would turn off if anyone pressed the switch. It is also referred to as form-B contacts

Hand switches

An electrical switch actuated by a person’s hand motion. This may take a form of toggle, pushbutton or rotary.

Limit switches

detects the physical motion of an object by direct contact with that object. This switch will be in its “normal” status when it is not in contact with anything.

Proximity Switches

detects the closeness of an object. These switches are non-contact sensors, using magnetic, electric, or optical means to sense the (closeness) of an object

Pressure switches

It often use diaphragms or bellows as the sensing elements, the motion of which actuates one or more switch contacts.

Level switches

These switches usually use floats as sensing element, the motion of which actuates one or more switch contacts

Temperature switches

These switches often use bimetallic strips as sensing element.

Flow switches

A switch that uses paddles as sensing element, the motion of which actuates one or more switch contacts

Discrete Control Element

A device that either allows unimpeded flow or acts to prevent flow altogether

Controlling element

known as the brain of the control system. It performs appropriate functions for maintaining the desired level (set point) of parameters to restore quality and rate production

Controller

A device that receives data from a measurement instrument, compares that data to a programmed setpoint, and, if necessary, signals a control element to take corrective action.

Programmable logic controller and Distributed control system

Common examples of controller

Programmable Logic Controller

Usually computers connected to a set of Input/output devices which are programmed to respond to inputs by sending outputs to maintain all processes at setpoint.

Distributed Control System

controllers that, in addition to performing control functions, provide reading of the status of the process, maintain databases, and advance man-machine interface.

Feed forward

Correcting an action before the process

Feed back

Correcting an action after the process

Basic Process Control System

BPCS

Final control element

the part of the control system that acts to physically change the manipulated variable. Typically used to increase or decrease fluid flow.

Actuator

The part of the final control element that causes a physical change in the final control device when signaled to do so.

Control Valves

Manipulate the flow rate of gas or liquid; whereas control switches manipulate the electrical energy entering a system.

Globe valves

A linear motion valve used to stop, start, and regulate fluid flow. The three primary body designs for this valve are Z-body, Y-body, and Angle.

Recommended for throttling application, Good for frequent operation, easy to repair

Advantages of Globe valve

Flow path causes a significant pressure drop, more costly than alternative valves

Disadvantages of Globe Valves

tee, angle, wye

Types of globe valves

Tee

most common body type for a globe valve. The horizontal setting of the seat allows the stem and disk to travel perpendicular to the horizontal line. This design has the lowest coefficient of flow and highest pressure drop. They're used in severe throttling services, such as in bypass lines around a control valve. When pressure drop is not a concern and throttling is required, this globe valve design is a good solution.

Angle

The ends of this globe valve are at a 90° angle allowing fluid to flow in a single 90° turn. This valve is used for applications that have periods of pulsating flow because of their ability to handle "the slugging effect" which naturally occurs with a pulsating flow.

Wye

This type is an alternative for the high pressure drop commonly found in globe valves. The seat and stem are at a 45° angle which provides a straighter flowpath when fully open and offers the least resistance of flow. This design is commonly used for throttling during seasonal or start-up operations.

Gate Valves

Flow is controlled by raising or lowering the valving element. They are not usually used to regulate flow because the valving element can be damaged when in the partially open position

**Solid Wedge Gate Valve**

the most common & widely used disk type because of its simplicity and strength. May be installed in any position, and it is suitable for almost all fluids.