3570 Series
8
2-inch (50 mm) maximum travel. If the actuator maxi-
mum travel is greater than 2 inches (50 mm), an addi-
tional retainer spacer (key 235, see figure 3) is re-
quired. Refer to the parts list for the additional spring
retainer spacer (key 235) part number.
D Codes for valve travels greater than 2 inches
(50 mm), up to and including 4-1/8 inches
(105 mm), are used for actuators that have a
4-1/8 inch (105 mm) maximum travel, except the
Types 480-12 or 480-15 Size 20 actuators which have
a 2-1/8 inch (54 mm) maximum travel. If the actuator
maximum travel is greater than 4-1/8 inches (105
mm), two an additional retainer spacer are required.
Refer to the parts list for the additional spring retainer
spacer (key 235) part numbers.
D Codes for valve travels greater than 4-1/8 inch-
es (105 mm), up to and including 8-1/8 inches (206
mm), are for actuators that have a 8-1/8 inch (206 mm)
maximum travel. If the valve travel is 4-1/8 inches (105
mm) or less, two additional spring retainer spacers are
required. Refer to the parts list for the additional spring
retainer spacer (key 235) part numbers.
If the input signal range and travel do not match any of
the selections in table 4, consult your Fisher sales of-
fice or sales representative for information. To change
the springs, refer to the range and bias spring proce-
dures in the Maintenance section.
From table 4, select the signal range that matches
your application. Find the travel length for the applica-
tion under the signal range selected, then use the
code (from the direct or reverse column) that matches
the direct or reverse operation of the positioner.
The first number in the code is used to identify the
range spring, the letter in the code is used to identify
the bias spring, and the second number (after the let-
ter) is used to identify the spring retainer. For exam-
ple, from table 4, for a signal range of 0 to 15 psig
(0 to 1.0 bar), an actuator travel of 9/16-inch (14.3
mm), and direct action, the signal range code from
table 4 is 6G3. The ‘‘6’’ indicates the range spring. The
‘‘G’’ indicates the bias spring. The ‘‘3’’ indicates the
spring retainer.
Note
It is necessary to add the bias spring
seat (key 8) to a unit when changing
from an extension type spring (key 9) to
a compression type spring (key 48).
When planning to change the bias spring in an existing
unit, inspect the unit first to determine if the current
bias spring is an extension type spring (key 9) or a
compression type spring (key 48). To change from an
extension type spring to a compression type spring, it
is necessary to add the bias spring seat (key 8). Refer
to the appropriate procedures in the Maintenance sec-
tion of this manual.
In some cases, it is necessary to add an additional
spring retainer spacer or change from the standard
bellows to the optional high pressure bellows. Table 4
footnotes indicate the use of an additional spring re-
tainer spacer or high pressure bellows.
Use the code from table 4 while referring to tables 5,
6, and 7 to determine the part numbers for the range
spring, bias spring, and spring retainer. Unless other-
wise specified, use the standard bellows. Also, no
spring retainer spacer is required unless the spacer is
specified in the footnotes of table 4.
Frequency Response
Figure 5 shows how a Type 3570 or Type 3570C posi-
tioner with Type 470 or Type 480 actuator responds
when the input signal pressure is cycled at a small am-
plitude (3 to 5 percent), and at an increasing frequen-
cy. Assume the cycling input signal and the movement
of the actuator rod are represented by sine waves. As
the actuator rod is forced to move faster, its motion
begins to fall behind the input signal in both time
(shown as phase lag) and amplitude (shown as
normalized gain).
Adjustment Procedures
Refer to figure 3. The 3570 Series positioners have
three adjustments:
D The bias spring. It is the zero adjustment which
determines the starting point of the valve plug travel.
D The range spring. It is the span adjustment
which determines the full valve plug travel for a given
input signal range.
D The relay nozzle adjustment. This adjustment
determines the steady-state positioner output pressure.
To illustrate the use of the various adjustments, as-
sume that the positioner has been repaired or has be-
come completely out of adjustment. Assume also that
the input signal range is 3 to 15 psig (0.2 to 1 bar).
Proceed as follows:
1. Make sure the input signal range and the valve
travel stamped on the nameplate agree with the pres-
ent operating conditions.
2. Loosen the four thumb screws on the underside of
the positioner base and remove the cover.
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