Flowserve I90 Series - Manual (Page 2)

2 Pulsair Loop-Powered Positioner Modular Accessory System (Series I90/L90) WCAIM2053
I. INTRODUCTION
Pulsair Loop-Powered Positioner Modular Accessory System Series
I90/L90 (or I90/L90 M.A.S.) is designed to provide accurate control
over valve position, while providing feedback information from
pneumatically actuated valves. This version of the M.A.S. uses “loop-
powered” technology; the 4 to 20 milliamp signal loop provides all the
power that is required to operate both the electronic positioner
circuitry and solenoid valving.
The concept behind the unit is flexibility, allowing the user to
customize an I90/L90 M.A.S. for a particular application. For this
reason, these instructions have been arranged as a series of individual
chapters, each dealing with separate options or components available
in the I90/L90 M.A.S. Regardless, this unit is a complex device and
not all options can be accommodated simultaneously. A table
summarizing the available options follows on the bottom of page 3.
PLEASE READ (Regarding Stability of Position)
The following is to explain how the Pulsair operates from loop power,
and why, with some current sources, the Pulsair may seem jittery at
current input levels between 4 and 6 mA. One point that should be
made clear before starting is the fact that the loop driving voltage is
to be between 18 and 30 volts for the intrinsically safe board and
between 18 and 35 volts for the non-intrinsically safe board.
The Pulsair is loop-powered. Loop-powered means that the 4-20 mA
current signal provides both the signal to position the valve as well
as providing all the power required by the circuit board electronics
and the piezo valves. The piezo valve is a three-way pneumatic valve
wherein the operator is a piezo crystal (a form of quartz crystal). This
crystal has an unusual property. When an electrical field is placed
across the crystal, the crystal distorts (bends). The crystal in the
valve is shaped like a thin flat bar where the top and bottom surfaces
are metallized (a metallic coating is applied to the crystal surface).
The wires carrying the operating voltage are attached to these
surfaces, one wire to each surface. Quartz is an insulator and, for all
practical purposes, there is no current flow between the two
surfaces. The crystal, therefore, electrically simulates a capacitor in
operation. As with a capacitor, there is an inrush current associated
with charging the piezo crystal. This can be as much as 2 mA. The
valves and the circuit board must be capable of operating with a loop
current as low as 4 mA and, in fact, they will operate with the loop
current as low as 3.6 mA.
At this low current level it becomes very important that the current
source be capable of not only maintaining the average set loop
current, such as 5 mA, but also quickly responding to the power
demands of the piezo valves as they operate. It is critical that the
current source be capable of outputting an adjustable current that is
independent of loop resistance (impedance). This means that for a set
current, let’s say 5 mA, when the loop resistance changes from 50
ohms to 600 ohms (as an example) the loop current does not vary.
This requires an active circuit that will quickly adjust its output voltage
in order to maintain a constant set current in the loop as the loop
resistance or impedance changes. The only way to increase the power
available in the loop is to increase the loop driving voltage. In order
for the loop driving voltage to increase and yet maintain a constant
loop current, the input resistance of the circuit board must increase.
This takes place within the electronics of the circuit board, which is
why it is impossible to give an input impedance for the board. The
input impedance changes as the power requirements of the board and
piezo valves change.
This capability of the current source to respond quickly to the
changing power requirements of the loop is very important. This is
why, with some current sources, the Pulsair acts jittery at the lower
current end of signal range. These current sources are not capable of
maintaining a constant output current while responding to the
changing input impedance of the Pulsair circuit board. There are
some calibrators on the market, for example, that will maintain a set
output current only as long as the load impedance doesn’t change, or
they are incapable of responding quickly enough or not at all at the
lower end of the current range.
There are two other factors that can contribute to instability in the
Pulsair positioner. The first of these is that it is recommended that the
Pulsair not be set to an actuation time of less than the greater
actuation time displayed during self-calibration. This is the value that
“Auto” selects in the menu choices. A cycle time faster than this can
potentially lead to overshooting and hunting. This parameter is called
“tS” on the Pulsair setup menu. The other factor that can affect the
stability of the positioner is the dead band setting. Setting the dead
band too low can also lead to overshooting and hunting of the
positioner. The recommended setting of the dead band is “AUto” in
the “dEbA” parameter of the setup menu. This permits the
microprocessor to determine the best dead band setting. This is a
dynamic adjustment and will automatically change as loop conditions
change if this parameter is kept set to “AUto”. If you should choose to
set this parameter to a manual setting that is too low, be aware that
this can lead to instability of the positioner.
WARNING: The PULSAIR Series I90/L90 M.A.S. is an electro-
mechanical device subject to normal wear and tear. Its life is
dependent upon application and environmental conditions.
Breather/drain fittings are recommended for humid environments
when moisture may condense inside the sealed M.A.S. Housing.
Supply Air Quality – See page 16; it is very important that clean,
dry, oil-free air be supplied.
CAUTION: Flowserve recommends that all product which must be
stored prior to installation be stored indoors, in an environment
suitable for human occupancy. Do not store product in areas where
exposure to relative humidity above 85%, acid or alkali fumes,
radiation above normal background, ultraviolet light, or
temperature above 120°F or below 40°F may occur. Do not store
within 50 feet of any source of ozone.
The Intrinsically Safe (I90) version of the Pulsair will be intrinsically safe
when connected through CSA certified zener barriers or CSA certified
galvanic isolators rated 28 volts DC maximum, 300 ohms minimum
(Potentiometer – rated 28 volts DC maximum, 300 ohms minimum and
28 Volts DC-diode return) as shown in the wiring diagrams.
IMPORTANT: Shielded cable must be used for each intrinsically safe
circuit and, for zener barriers, the shield must be connected to a
zener barrier ground.
Flow Control Division
Worcester Controls