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Fail Safe Spring Design & Safety Spring Removal System |
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Hytork® springs are designed to the Spring
Research And Manufacturers Association (SRAMA) criteria for
reliability and have sufficient safety margins for their use as
a safety device. The designs are backed up by a tight
manufacturing Quality Control System, certified to ISO 9001
which is supplemented by a continuous random testing programme.
- Spring Return Actuators are "Fail Safe"
- "Fail Safe" devices are used only for
safety concerns:
- Any design with a predictable Spring
life less than the plant life is not an acceptable "Fail Safe"
design
- Hytork® Springs are designed and
guaranteed never to break!
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Sectional Diagram
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The load is distributed
evenly around the whole circumference of the nd Cap, preventing
distortion and point loading stresses.
This SafeKey
system also prevents the possibility of
overstressing bolt hole threads during assembly.
Click here to see how the
SafeKey
system works.
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Fail Safe Function |
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The Fail
Safe function of springs is the
prime design specification and Hytork® springs are designed
accordingly.
Hytork® engineers believe that aesthetic
compactness of design with associated increases in
spring stresses is secondary to the prime
requirement of continual safety:
this is why Hytork® springs are large, strong and reliable.
Corrosion protection:
Hytork® springs are also epoxy coated to
protect against corrosion.
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Corrosion Protection |
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Hytork®
springs are epoxy coated to protect against
corrosion.
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Confidence And Guarantee |
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Hytork® does not offer springs as a
spare part, as they are guaranteed for the working life of the
actuator with no cycle limitations. To offer spare springs would
be a contradiction in our design criteria for maximum safety.
We are so confident in our product that we
will replace or repair any actuator that has a spring failure in
normal service, free of any charge, irrespective of the date of
manufacture.
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Flexibility |
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Hytork® springs can be selected to match a wide range of air
pressures to optimise actuator efficiency. All Hytork® springs are
designed to the same high standards.
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Spring Design Constraints And Compromises |
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A Spring Return Actuator uses
air pressure to move the internal pistons and also
compress the spring to store energy for the return stroke.
As the springs compress there is an increase in the power
consumed by the springs and a consequent decrease in the
torque available for operation of the valve or damper.
The ideal solution to overcome this
would be to make the power from the Springs equal to the
power from the air in all positions throughout the
actuator's stroke (the 50/50
relationship).
For a spring to produce the same power
at the end of its working stroke (WS), as it does at the
beginning, the spring must be so long that the working
stoke is insignificant when compared with the total free
length of the spring. (See Fig. 2).
In other words, the spring would need
to be of infinite length. This is impracticable, so a
compromise must be made.
For example, two currently marketed
solutions have been:
a) Go for a very long spring In order
to make the drop off in power from the spring as small as
possible.
b) Go for a very short spring which
will give a very rapid drop off in spring power output,
but a very short actuator.
Long Springs:
The problem with a very long spring is
that although it is closer to satisfying the 50/50 ideal,
it produces a heavy, cumbersome actuator. Some designs
using this approach are asymmetric, having springs at one
end only. This creates problems for handling and
installation and often
interferes with other equipment or
passage ways.
Equally important, the offset loading
on the mounting can lead to pipes twisting, premature
valve stem leakage and a requirement for support brackets.
(See Fig. 3).
Short Springs:
A very short spring appears to solve
the weight and space problem. However, as very short
springs mean that the output power decreases rapidly, it
is sometimes necessary to increase the size of the actuator to compensate for the power drop off. Obviously,
if it becomes necessary to increase the size of the actuator, the apparent benefit of a short
actuator is
completely lost.
High stress levels in short Springs:
in order to gain the maximum
power from the short Springs, some designs use stress
levels above those recommended. The effect of this is that
the Springs can be overstressed and will predictably fail
in service.
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The Hytork® Approach |
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Fail Safe Actuators are significantly more expensive
than double acting models. However the more closely
an actuator's performance can match the valves
requirement, the more cost effective the actuator
will be.
Hytork® spring designs are
engineered to make the solution match the
application.
Hytork® is also fully aware that
the end users need flexibility. The Hytork® design
permits end users to alter the spring
characteristics safely and easily as required,
without having to return the actuator to the
manufacturer.
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Spring Adjustments For Different Valve Torque Requirements & Plant
Air Supply Pressures |
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Diverse types of quarter turn valves have significantly different operating torque characteristics. Plants have requirements for both fail open and fail close valves. The air supply pressure available at the actuator location also effects the actuator size.
Hytork® springs are designed to give the user flexibility for whatever application and operation is required. The actual selection of the correct spring combination can be achieved by either referring to the torque charts given in the literature on spring return actuators or by using the comprehensive Hytork® actuator sizing manual.
The below table displays the spring adjustments for different air pressures.
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For Actuator models
45,70,185,425,680,1370,2585,4580
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Left Spring
Assembly |
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Right Spring
Assembly |
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Spring Rating
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Inner
15 psi
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Outer
25 psi
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Inner
15 psi
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Outer
25 psi
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S30
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15 psi
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none
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15 psi
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none
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S40
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15 psi
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none
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none
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25 psi
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S50
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none
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25 psi
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none
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25 psi
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S55
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15 psi
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25 psi
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15 psi
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none
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S65
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15 psi
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25 psi
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none
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25 psi
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S80
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15 psi
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25 psi
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15 psi
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25 psi
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For Actuator models 130,280,1125
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Left Spring
Assembly |
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Right Spring
Assembly
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Inner
20 psi
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Outer
20 psi
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Inner
20 psi
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Outer
20 psi
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Spring rating
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30 psi
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30 psi
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S40
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none
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20 psi
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none
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20 psi
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S60
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none
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30 psi
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none
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30 psi
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S80
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20 psi
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20 psi
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20 psi
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20 psi
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Actuator Sizing Made Easy |
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Correct actuator sizing is critical in ensuring safe and effective
plant operation. As an aid to our customers, Hytork® recommends the
use of the Hytork® actuator sizing manual. The manual not only makes
selection more accurate, it is easy and quick to use.
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Hytork®'s
"Safety Spring Removal System" |
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Hytork® has made changing springs on site both simple and safe by the
combination of the End Cap
SafeKey
Retainer System and the
Spring Removal System.
These very important features protect and
facilitate the technician when springs have to be changed to adjust
for variations in air supply pressures to the actuator.
The force of the springs locks the SafeKey
in place and does not allow the end cap to
be removed until the spring is safely captured by the spring
retractor rod.
The retractor rod is inserted through the end cap
and screwed into the retractor cap.
The nut on the rod is then screwed against the
spring end cap to draw the spring away from the piston. This action
contains the spring force within the end cap and releases the
locking force on the SafeKey.
Once the locking force has been released, the
SafeKey
is free to be removed and the spring return end ap assembly can be
safely removed as a module, from the actuator body.
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Normal Maintenance Requirements |
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For normal maintenance, when spring changes are not required, it is
not necessary to completely relax the springs. The spring end cap
module can be left with the retracted rod holding it together ready
for re-assembly.
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Spring Power Adjustments |
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If it is required to alter the spring rating, the nut on the
retractor rod is gradually unscrewed until the spring is in its
completely relaxed position.
Once the spring is fully relaxed the retractor
rod can be removed and the springs changed as required.
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Unique Hytork®
Features |
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- Springs are "Fail Safe" by design and guaranteed for life.
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Springs are epoxy coated for corrosion protection.
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Springs are easy and safe to change on site.
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Springs can be easily adjusted to match
valve torque and site air pressures.
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Actuators are balanced around the
center of the valve stem preventing pipe twisting.
- Springs are produced to suit high and low
temperature environments.
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