Fluke 707Ex Application Note

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Application Note

Tracking down problems within

a process loop can be a difficult

challenge in the best of environ-

ments. Doing so in an area that

has the potential for explosion

takes the degree of difficulty to

another level – one where the

technician needs proper training

and equipment. This article will

demonstrate the practical applica-

tion of loop calibrators designed

to troubleshoot process loops in

intrinsically safe environments.

What is “intrinsically

safe?”

Intrinsic safety is a protection

standard employed in potentially

explosive atmospheres. Devices

certified as “intrinsically safe”

are designed to be unable to

release sufficient energy, by

either thermal or electrical

means, to cause ignition of

flammable material (gas, dust/

particulates).

Intrinsically safe standards

apply to all equipment that can

create one or more of a range

of defined potential explosion

sources:

• Electrical sparks

• Electrical arcs

• Flames

• Hot surfaces

• Static electricity

• Electromagnetic radiation

• Chemical reactions

• Mechanical impact

• Mechanical friction

• Compression ignition

• Acoustic energy

• Ionizing radiation

Intrinsic safety is particularly

important for technicians

working in industries like petro-

chemical and pharmaceutical,

around bulk materials such as

grain, mining, or any environ-

ment where explosive gases are

present.

The importance of safety in

these environments can’t be

stressed enough. It takes a very

small amount of energy to cause

an ignition; e.g., a mixture of

hydrogen in air requires only

2O uJ of energy. The proper

practices and tools will minimize

the inherent risk involved in

working around these hazards.

Intrinsically safe loop

calibration

To conduct loop calibrations in

potentially explosive environ-

ments, you need a loop calibrator

that is certified as intrinsically

safe. Intrinsically safe loop cali-

brators, such as the Fluke 707Ex,

must be certified in accordance

Troubleshooting process

loops in potentially

explosive atmospheres

From the Fluke Digital Library @ www.fluke.com/library

with the European ATEX (“Atmo-

sphères Explosibles,” French for

explosive atmospheres) directive

(Ex II 2 G Ex ia IIC T4) in Zones 1

and 2 for use in Europe and NEC

500; N.I. Class 1, Division 2 areas

Group A-D for use in the U.S.

In addition to an intrinsically

safe loop calibrator, strict adher-

ence to calibration procedures is

recommended, including:

Lock out: Make sure the system

is shut down and other work-

ers are notified that a potentially

dangerous operation will be tak-

ing place.

Tape off area: Tape the work

area off to prevent workers from

entering with potentially dan-

gerous electrical devices (cell

phones, handheld computers,

non-intrinsically safe tools).

Purge or vent the systems:

Safely purge or vent the system

to remove any gases that may

remain.

Use a gas detector: In an envi-

ronment where explosive gas

may be present, the use of a gas

detector is a prudent step before

starting a loop calibration. Gas

“sniffers” are available for a wide

variety of applications and from

handheld to larger, carted models.

Calibrate: Perform your calibra-

tion using an intrinsically safe

loop calibrator.

Clean up and reactivate: At the

conclusion of calibration, reverse

the process and reactivate the

system.

Loop calibration

The 707Ex Loop Calibrator has

the ability to replace the power

source in a current loop so you

can power and read a transmitter

at the same time without carry-

ing a digital multimeter (DMM).

2 Fluke Corporation Troubleshooting process loops in potentially explosive atmospheres

With the calibrator in control of

the current, you can accurately

set the current between 4 and

20 mA, controlling the loop and

the device connected to it. This

makes it possible to test valve

positioners (see below), mechani-

cal position indicators, flow

indicators and mA signal condi-

tioners.

For testing devices requir-

ing a voltage input, like a signal

conditioner or PLC, you can use

the source mode of a calibrator

in conjunction with a precision

resistor to generate accurate volt-

ages.

Simply taking a precision

resistor and placing it across the

output leads of a loop calibra-

tor creates a voltage across the

resistor that is directly controlled

by the source current from the

calibrator. For example, placing a

250 Ω resistor across the source

output jacks and driving it with a

4 to 20 mA current will produce

1 to 5 V across the resistor. Place

this voltage on the input of a sig-

nal conditioner and we have cre-

ated a test system to set linearity

as well as the zero and span

points of the conditioner. With a

DMM or ProcessMeter™ instru-

ment like the Fluke 789,

you can

measure the output of the signal

conditioner and ensure the proper

levels are coming out of the con-

ditioner with a corresponding

input voltage.

Testing valve positioners

Electronic valve positioners

should receive periodic in-field

calibrations as part of preventa-

tive maintenance programs. Fluke

loop calibrators are the ideal pro-

cess tool for these checks. Valve

positioners vary in design and

valve type and should be cali-

brated using specific instructions

from the individual manufacturer.

Quick operational checks

can be performed using a field

calibrator as a signal source

while observing the valve stem

position, mechanical position

indicators, or flow indicators as

input changes are made. Fluke

loop calibrators provide a conve-

nient source for simulating

the controller output to a valve

positioner.

The following example (using

a Fluke 707Ex calibrator) shows

a general method for an in-field

operational check of a valve fit-

ted with an electronic valve

positioner. These methods may

be adapted to various types of

valves, however manufacturer’s

specific instructions should

always be consulted for proper

and appropriate techniques. In

the following example, valve

operation and movement is

checked either by feel or by

observing valve stem movement.

Step 1: Basic set-up. Setting

the Fluke 707Ex Loop

Calibrator current output.

Place the calibrator in the 4-20

mA output current mode. Connect

the 707Ex to the input terminals

of the valve positioner.

Step 2: Zero adjustment

Set the 707Ex to an output of

4 mA and allow some time for

valve stem movement to stabilize.

Quickly decrease the current from

4 mA to 3.9 mA by depres-sing

and turning the vernier knob

in a counter-clockwise direc-

tion. You can operate the 707Ex

with one hand while feeling the

valve stem with your free hand

to check for any sign of move-

ment. Adjust for zero movement

between these two current set-

tings by using the Zero adjust-

ment on the positioner.

Increase and decrease current

from 4 mA to 4.1 mA using the

vernier knob in the depressed

position. Insure that the valve

stem just begins movement

above the 4.1 mA setting and

fully closed at 4 mA.

Step 3: Span (full open) posi-

tion check

Using the 25 % button, step the

valve input to 20 mA and allow

the valve to stabilize. Step the

input to 24 mA while watching

or feeling for movement of the

valve stem. Minimize this move-

ment using the Span adjustment

on the valve positioner.

Using the vernier knob in the

depressed position, adjust current

up and down between 20.10 mA

and 19.9 mA. There should be

no movement of the valve stem

above 20 mA and a slight move-

ment below 20 mA.

Step 4: Check zero and span

again

Many positioners have interactive

Zero and Span controls. This step

will help ensure proper valve

position adjustment.

Repeat Step 2 and Step 3.

Step 5: Linearity check

For valves with linear action, lin-

earity can be checked by setting

the 707Ex to 4 mA and stepping

current to 12 mA (50 %) while

observing valve travel. If your

valve is of a nonlinear type, refer

to the valve manual for proper

operational checks.

Step 6: Stroking the valve

Checking for smooth valve opera-

tion is easy to accomplish using

the Slow Ramp function of the

707Ex.

• Set the calibrator to mA source

mode and select the Slow

Ramp function ( ) by depress-

ing the 25 % and 0-100 %

buttons simultaneously.

• Allow the calibrator to ramp

through several cycles while

watching or feeling for any

abnormal operation of the

valve such as sticking in one

position momentarily or erratic

movement.

Summary

Calibrating process loops is an

essential part of regular plant

maintenance made all the more

difficult in potentially explosive

environments. Using intrinsically

safe tools and the proper proce-

dures to safeguard the working

area minimizes the risks involved.

Fluke Corporation

PO Box 9090, Everett, WA 98206 U.S.A.

Fluke Europe B.V.

PO Box 1186, 5602 BD

Eindhoven, The Netherlands

For more information call:

In the U.S.A. (800) 443-5853 or

Fax (425) 446-5116

In Europe/M-East/Africa +31 (0) 40 2675 200 or

Fax +31 (0) 40 2675 222

In Canada (800)-36-FLUKE or

Fax (905) 890-6866

From other countries +1 (425) 446-5500 or

Fax +1 (425) 446-5116

Web access: http://www.fluke.com

©2003-2010 Fluke Corporation.

Specifications subject to change without notice.

Printed in U.S.A. 7/2010 2098403B A-EN-N

Modification of this document is not permitted

without written permission from Fluke Corporation.

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