is barrier

8/3/2019 Is Barrier

http://slidepdf.com/reader/full/is-barrier 1/8

CHAPTER 31

669

UNDERSTANDING AND

APPLYING INTRINSIC SAFETY

INTRODUCTION

Intrinsic safety is a commonly used, but often misunderstood term in the instrumentationand sensor industry. Intrinsic safety prevents instruments and other low-voltage circuits inhazardous areas from releasing sufficient energy to ignite volatile gases. Intrinsic safetywas invented around 1915 in Great Britain as a result of a series of mine explosions. A bellsignaling circuit in the mine shaft created sparks that ignited the volatile gases in the mine.Intrinsically safe barriers were invented to prevent excess energy from reaching the hazard-

ous area while still allowing the low voltage to operate properly.Although intrinsic safety has been used in North America for many years, it has only beenpart of the National Electric Code (NEC) since 1990. Since that time, there have been moreand more intrinsically safe products introduced into the market, making the proper selectionof the barrier and instrument seem very difficult. The purpose of this chapter is to explainintrinsic safety technology so that sensors in hazardous areas operate safely and properly.

WHERE CAN INTRINSIC SAFETY BE USED?

When deciding where intrinsic safety can be used, one must first define the hazardous areawhere the instruments or sensors are located. In North America, these areas are defined byclasses, divisions, and groups.

CLASS I, II, III: The class defines the type of materials in the hazardous area:

Class I Flammable gases and vapors

Class II Combustible dusts

Class III Fibers and flyings

DIVISION 1 or 2: Hazardous areas are further broken down into two divisions:Division 1 Normally hazardous

Division 2 Not normally hazardous

GROUPS A to G: The group designates the type of vapor or dust in the area:

Group A Acetylene

Group B Hydrogen

Downloaded from Digital Engineering Library @ McGraw-Hill (www.accessengineeringlibrary.com)Copyright © 2010 The McGraw-Hill Companies. All rights reserved.

Any use is subject to the Terms of Use as given at the website.

Source: SENSORS HANDBOOK

8/3/2019 Is Barrier


670 CHAPTER THIRTY-ONE

Group C Ethylene

Group D Propane

Group E Metal dust

Group F Coal dust

Group G Grain dust

The class and group ratings are relatively simple to define. However, the division ratingis more subjective and will influence the equipment requirements needed to protect thesensors in the hazardous area. If the area is classified as Division 1, more stringent safetymeasures are usually required than for Division 2 locations.

METHODS TO PREVENT EXPLOSIONS

Three elements must exist for an explosion to occur: oxygen, fuel, and ignition (Fig. 31.1).This is known as the explosion triangle. If one of these three elements is missing, the explo-sion cannot occur.

In North America, there are three primary means to prevent explosions: purging, explosion-proof enclosures, and intrinsic safety.

Purging removes the fuel from the area by forcing air or inert nitrogen through aninterlocked enclosure. Explosion-proof enclosures contain the explosion. The resulting hotgases cool as they escape through specially designed threaded or machined flat joints.

These cooled gases are not hot enough to ignite the gases in the hazardous area. Intrinsicsafety is preferred by many in the sensor industry because it allows the user to maintain,calibrate, and work on the circuits while they are live without any danger of the circuitcreating a spark large enough to ignite the gases or dusts in the hazardous area. This canreduce the amount of downtime in the process industry.

Intrinsic safety removes the ignition source of excessive heating or sparking by keep-ing the energy levels of voltage and current below the ignition points of the material. (SeeFig. 31.1 showing the ignition curve.) As an example, a combination of 30 V and 150 mAwould fall on the ignition curve for hydrogen. Under the right conditions, this could createa spark large enough to ignite the gases. For intrinsically safe circuits, the maximum volt-

age and current produced under a fault condition on the safe side must always be belowthe curve. In this case, a combination of 24 V and 100 mA would be considered safe.Intrinsically safe applications always stay below these curves where the operating level of energy for sensors is about 1 W or less. Capacitance and inductance curves must also beexamined in intrinsically safe circuits.

LIMITING THE ENERGY TO THE

HAZARDOUS AREA

The intrinsically safe barrier is an energy-limiting device. Under normal conditions, thebarrier is passive and allows the field device to function properly. Under fault conditions,the barrier will limit excess voltage and current from reaching the field device and ignitingthe gases in the hazardous area. There are three components to a barrier that limit currentand voltage: a resistor, at least two zener diodes, and a fuse. (Fig. 31.2 displays the barrier.)The resistor limits the current to a specific known value, known as short circuit current , Isc.The zener diode limits the voltage to a value referred to as the open circuit voltage, Voc.



UNDERSTANDING AND APPLYING INTRINSIC SAFETY

8/3/2019 Is Barrier


UNDERSTANDING AND APPLYING INTRINSIC SAFETY 671

The fuse will blow when the diode conducts. This interrupts the circuit, which prevents thediode from burning and allowing excess voltage to reach the hazardous area. At least twozener diodes are always in parallel in each intrinsically safe barrier. If one diode should fail,the other will operate instead, providing complete protection.

A simple analogy is a restriction in a water pipe with an over-pressure shutoff and relief valve. The restriction prevents too much water from flowing through, just as the resistor in the

FIGURE 31.1 The explosion triangle.




8/3/2019 Is Barrier



barrier limits current. If too much pressure builds up behind the restriction, the over-pressureshutoff valve turns off all the flow in the pipe. This is similar to the zener diode and fuse withexcess voltage. If the input voltage exceeds the allowable limit, the diode shorts the input volt-age to ground and the fuse blows, shutting off electrical power to the hazardous area.

WHICH SENSORS AND INSTRUMENTS

CAN BE MADE INTRINSICALLY SAFE?

When designing an intrinsically safe circuit, begin the analysis with the sensor or instru-ment that is referred to as the intrinsically safe apparatus or field device. This will deter-mine the type of barrier that can be used so that the circuit functions properly under normalconditions but is still safe under fault conditions. The types of field devices most commonlyused are shown in Table 31.1.

More than 85 percent of all intrinsically safe circuits involve commonly known instru-ments and sensors. These devices, also referred to as intrinsically safe apparatuses, mustbe further classified as either simple or complex devices. Simple apparatus is defined inparagraph 3.12 of the ANSI/ISA-RP 12.6-1987 as any device that will neither generate nor

store more than 1.2 V, 0.1 A, 25 mW, or 20 µJ. Examples are simple contacts, thermocou-ples, RTDs, LEDs, noninductive potentiometers, and resistors. These simple devices do not

FIGURE 31.2 Barrier circuits.

TABLE 31.1 Most Common Types of Field Devices

Intrinsically apparatus Intrinsically safe applications (%)

Switching 32.0

Two-wire transmitters 22.0

Thermocouples and RTDs 13.0

Load cells 8.5Solenoid valves 4.5

Potentiometers 2.5

LEDs 2.0

I/P transducers 2.0

Other devices 13.5

Total field devices 100.0




8/3/2019 Is Barrier



need to be approved as intrinsically safe. If they are connected to an approved intrinsicallysafe apparatus (barrier), the circuit is considered intrinsically safe.

A nonsimple or complex device can create or store levels of energy that exceed those pre-viously listed. Typical examples are transmitters, transducers, solenoid valves, and relays.When those devices are approved as intrinsically safe, under the entity concept, they havethe following entity parameters: Vmax (maximum voltage allowed); Imax (maximum currentallowed); Ci (internal capacitance); and Li (internal inductance). The Vmax and Imax valuesare straightforward.

Under a fault condition, excess voltage or current could be transferred to the intrinsi-cally safe apparatus (sensor). If the voltage or current exceeds the apparatus’ Vmax or Imax,the device can heat up or spark-ignite the gases in the hazardous area. The Ci and Li valuesdescribe the sensor’s ability to store energy in the form of internal capacitance and internalinductance. A comparison of the entity values of intrinsically safe apparatus and associatedapparatus is as follows:

Associated Apparatus (barrier) Apparatus (sensor)

Open-circuit voltage VO ≤Vmax

Short-circuit current Isc ≤ Imax

Allowed capacitance Ca ≥ Ci

Allowed inductance La ≥ Ii

MAKE SURE THE CIRCUIT WORKS

It is very important to make sure that the circuit functions properly under normal condi-tions. With a current-limiting resistor in the barrier, a voltage drop will be between the inputand output of the barrier. A voltage drop must be accounted for in the circuit design. Thepurpose of this chapter is to show how easy it is to make a circuit intrinsically safe.

Temperature Sensors: Thermocouples and RTDs

First, test your understanding of intrinsic safety by examining a thermocouple circuit. Considerthe ignition curves to demonstrate a point about thermocouples. A thermocouple is classifiedas a simple device. It will not create or store enough energy to ignite a mixture of volatilegases. However, if a thermocouple is installed in a hazardous area, it will not be consideredintrinsically safe without a barrier between it and the recorder in the safe side. Why?

The reason is that a fault of 110 V could be introduced into the recorder that could reachthe hazardous area and ignite the gases. A barrier must be installed to limit the energy thatcould be created by a fault on the safe side.

BARRIER TYPES

The most common barriers are classified as DC barriers, which are rated for a DC voltagewith a positive or negative potential, and AC barriers, which are rated for an AC voltage input.Since a thermocouple has both a positive and negative leg, one positive and one negative DCbarrier or one AC barrier could be used. To avoid polarity problems, select an AC barrier.




8/3/2019 Is Barrier



RATED VOLTAGE

A thermocouple will generate a millivolt signal. The rated voltage (Vn) of a barrier must be

equal to or larger than the voltage supplied to it. Choose an AC barrier with a rated voltage Vn of 1 volt or higher.

FIGURE 31.3 Typical values—thermocouple circuits.

FIGURE 31.4 Intrinsically safe three-wire RTD circuit.




8/3/2019 Is Barrier



INTERNAL RESISTANCE

Since the millivolt signal has a very small current and is going to a high-impedance voltme-

ter, the resistance of the barrier will not influence circuit function. A simple rule of thumbis that when a signal is going to a high-impedance voltmeter, an internal resistance of lessthan 1000Ωwill not affect the millivolt signal. It is usually good practice to select a barrierwith the lowest internal resistance in case the circuit is modified later.

For RTDs, the same analysis will draw similar conclusions. Selecting the correct barrierto make all thermocouples and RTDs intrinsically safe is not difficult. Use a double-channelAC barrier with a rated voltage of at least 1 V with the lowest internal resistance. (SeeFigs. 31.3 and 31.4.) All thermocouples and RTDs are classified as simple devices and donot need approval. If they are connected to an approved intrinsically safe barrier, the circuitsare intrinsically safe.




8/3/2019 Is Barrier



A i bj t t th T f U i t th b it


is barrier

Documents