BASIC INSTRUMENTATION

INSTRUMENTS COVERED

Flow measurement

Orifice Tube

Differential pressure meter

FLOW MEASUREMENTPurpose of Flow Measurement

Accurate measurement of flow is essential in many process control applications. These measurements are used to monitor and control flow rates. The flow measurement, along with measurements of temperature, pressure, and composition, are used to develop material and energy balances on processes. Also, in some applications, controlling the feed rate in a particular upstream process helps to sustain the efficiency and to minimize waste in downstream processing operations.

Flowmeter Definition

A flowmeter is defined as “A device that measures the rate of flow or quantity of a moving fluid in an open or closed conduit. It usually consists of a primary device and a secondary device.”

Primary Device Definition – A primary device is defined as “The device mounted internally or externally to the fluid conduit that produces a signal with a defined relationship to the fluid flow in accordance with known physical laws relating the interaction of the fluid to the presence of the primary device.

Secondary Device Definition – A secondary device is defined as “The device that responds to the signal from the primary device and converts it to a display or to an output signal that can be translated relative to flow rate or quantity. ”

Note: The secondary device may consist of one or more elements as needed to translate the primary device signal into standardized or nonstandardized display or transmitted.

Flowmeter

Orifice Tube

Differential PressureDifferential Pressure Transmitters

The differential pressure flowmeters output a nonlinear signal that corresponds to the square of the flow rate. A change in flow at a low rate produces a very small signal change. A change in a large flow rate produces a much larger signal change. For example, a small flow rate from 0 to10% outputs a signal from 0 to 1%; for a large flow rate, a 90 to 100% flow rate change outputs an 81 to 100 % signal. A transducer called a square root extractor is often used with transmitters. A transducer converts a square root signal to a linear signal. The differential pressure transmitter outputs a signal that is used for control, indication, or totalization.

LEVEL MEASUREMENT

Purpose of Level Measurement

The purpose of level measurement is to provide a measured variable representing the height or material presence within a vessel. Level measurement is essential to the effective control of a vessel’s process material level. The level control scheme for a vessel requires a level measurement value. The level measurement value provides the level control scheme either a quantity, such as a representation of height, or a logical value, such as the on/off condition of a limit switch that represents the detection of process material presence.

Continuous Level Process Measurements

In continuous level process measurements, a level measurement system can provide a numeric representation of the current position (height) of the process material’s surface. The numeric value, sometimes expressed in meters or feet, is based upon a proportion of material currently sensed by the level measurement system. The level is measured continuously between a lower reference level and an upper reference level.

Level Switch

Level Switch

A Level switch operates on the same principle as the float in a household flush toilet tank. Many level switches operate on this principle. Instead of activating a mechanical valve, floats activate switches. The switches, in turn indicate status, control equipment, such as a pump or control valve. The control equipment raises or lowers the level until the switch de-activates, which in turn causes the control equipment to go to the desired on or off state.

Displacer Level Meter Torque Tube Displacer

Displacement level measurement is the most common measurement within the process industry. A torque tube displacer is so named because torque due to buoyancy. Forces acting upon the displacement element are used to infer a level measurement. At the top of the torque tube chamber, the displacer’s buoyancy force is measured by a torque arm assembly.As the level in the tank changes, the level in the external chamber changes. As the level changes, the buoyant forces placed upon the displacer changes. Although the small movement on the displacer appears insignificant to an observer, the small movement produces a torque that can be measured and converted to a level measurement.

Pressure Measurement

Purpose of Pressure Measurement

In process measurement, the values of process variables (e.g., pressure, temperature, level, and flow) in a process operation are continually determined to permit the process operation to be monitored or, more specifically, to permit the process variables to be controlled (i.e., held at their setpoints or within their operating ranges).

Effective monitoring depends on accurate measurements of pressure forthe following reasons:

• Pressure values themselves are essential data for monitoring.• Often, the values of process variables other than pressure are derived

from(inferred from) the values that are measured for pressure.

As an example of inferral from pressure, the value for the level of a liquid in a storage tank can be derived from the value of the hydrostatic pressure that is exerted by the liquid. As another example, the value for the rate at which a fluid is flowing through a pipeline can be derived from a differential pressure value that is produced by an orifice plate.

Pressure Transmitter

The pressure transmitter has a diaphragm as pressure receiving part which converts pressure to strain, and the strain is detected by semi-conductor strain gage.

The detecting part has full bridge system which consists of four semi-conductor strain gages. Electrical signal is obtained from the bridge circuit that is proportional to the strain. The signal is converted to direct current output or DC voltage output by a signal conversion circuit.

This linear strain is then correlated to the corresponding pressure.

Pressure Gauge

Typically, pressure gauges are devices used for measuring the pressure of a gas or liquid. The value of pressure is then read manually at the point location within the field. This reading is essential from the point of view that outside operators can determine if process plant is operating as intended and providing a visual representation of the actual pressure in the system.

The pressure gauge functions by means of expansion of a Bourdon tube which moves proportionally to applied pressure.

Temperature Measurement

Purpose of Temperature Measurement

Accurate temperature measurement is essential to the process control of distillation,fractionation, storage, and transportation of intermediate and final products. Additionally,temperature measurement contributes to greater product yield and better quality

processing.Accurate temperature measurement is essential to maximize the efficiency or

throughputs ofprocessing operations. Measuring and controlling temperature also minimizes the

reprocessing of the product because it does not meet the specification requirements.

Thermowell

A thermowell helps protect a temperature sensor from a process fluid’s pressure, corrosive or erosive effects, or mechanical damage from the impact of flowing fluids. A thermowell is constructed as a sturdy, metallic, tube-shaped enclosure. Industry standards, such as ISA S1.2 and ISA S19.3, define thermowells that permit interchangeability among bimetallic thermometers, thermocouples, and RTDs. The thermowells constructed to these standards are engineered to give the best combination of strength and speed of response. The thermowell is often specified along with the temperature sensor and related assemblies to make sure that the sensor fits within the thermowell.

Thermocouple

Thermocouples are perhaps one of the most common devices used in process control for temperature measurement. A thermocouple consists of two dissimilar metal wires fastened together at one end to form a junction. The junction is called the “hot” or measuring junction. The opposite end of the wires can be joined to form a reference or cold junction.