A New Option in Valve Actuators Strengthens Control of Filtering Operations

Control Systems

Under pressure from the mandates of the Safe Drinking Water
Act, water works engineers and plant managers now are forced to scrutinize all
elements of their potable water treatment operations. None is more important
than filtering. Water leaving this point must fall within mandated turbidity
levels. Over the past decade, much attention has been directed at plant control
systems to achieve these levels. Like putting a dashboard from a new Cadillac
onto a Model T, harnessing these modern systems to antiquated valve actuators
yields little gain if precise valve control cannot be reproduced reliably.

In recognition of this need, water works engineers are now
turning to a new generation of pneumatic valve actuators that are capable of
executing the instructions of electronic control systems with the necessary
precision to accurately control effluent flow. Surprisingly simple but rugged
in construction, this new breed of actuators also is meeting the need to reduce
downtime, as some of the first ones to debut in 1981 are still in operation
without needing a spare (new) part. A cost-savings factor of up to 40 percent
when compared to electric actuators also helps to explain the widening
acceptance of these new pneumatic actuators by plant engineers and managers faced
with the responsibility of delivering potable water at a cost-effective rate.
Additionally, the 'fail safe' (fail-closed or fail-open) feature that
high-performance pneumatic actuators provide protects the plant during a
temporary or extended power outage.

Need for Accuracy

The search for simple, accurate and reliable valve actuation
has been prompted by the increasingly stringent mandates of the Safe Drinking
Water Act that calls for turbidity levels of 0.3 NTU (Nephelometric Turbidity
Unit) or less. Since filtration typically is the final step (before storage) in
most water treatment operations, any water leaving the filtration process
should be well within turbidity limits. Therefore, any efficiency gains in
filter operation will help plant operators not only to meet federal clean-water
requirements, but also to help
reduce plant-operating costs.

Plant managers know that a delicate balance must be struck
by maintaining the design flow rate of the filter. As pointed out in the
turbidity provisions of the EPA Guidance Manual (April 1999): "The goal of
maximizing water production may conflict with the objective of minimizing
treated water turbidity. The operator must use good judgment in establishing
operational goals and exercising process control to achieve optimal finished
water quality production."

With the introduction of modern plant programmable logic
controllers (PLCs) and supervisory control and data acquisition (SCADA)
systems, recommended process controls already are in place. Despite these
gains, archaic valve actuation remains the weakest link in filtering
operations.

There are only certain ways to achieve a lower turbidity,
and accurate valve control is one of the most important. Valves with actuators
receiving commands from the filter control system to properly execute the
backwash option are pivotal in the process. If the effluent valve does not shut
off, the backwash water containing the solids removed by the filter media will
flow into the clear well and the measured turbidity levels will exceed the
mandates.

Since mixed-media filter performance is affected
significantly by hydraulic characteristics, accurate valve control begins at
the point of bed loading. Typical loading rates range from 2?8 gallons
per minute (gpm) per square-foot of filter bed surface area. The effluent valve
must meter these rates carefully because as the filter bed becomes dirty and
clogged with solids, the resistance to flow rises.

When the filter media is fresh and clean it will pass more
water than the specified design. Therefore, the effluent valve must be closed
to the point where it allows only the flow rate that the filter media is
designed to pass at that time. A turbidity meter or headloss DP instrumentation
tracks the levels and determines the most appropriate time to trigger a
backwash. Accurate valve actuation allows the PLC or SCADA system to maintain
the correct flow rate until such time.

As the media becomes dirty near the end of the filter run
and the filter becomes clogged, the effluent valve needs to open more.
Ultimately flow will cease when the resistance to flow is greater than the
gravitational force compelling it. As the "head" (hydraulic pressure)
increases, solids particles are pushed further and further into the media bed.
Solids will be driven completely through the bed and appear in the filtered
water. Turbidity levels will increase and the filter controls will shut down
the process.

Performing a backwash prevents high turbidity levels.
However, it is an expensive and time-consuming process so it is not performed
until necessary. During backwashing that filter is out of commission. In
addition this process uses the potable water that you just spent money
cleaning. Therefore, the key to operational efficiency is to keep the flow at
exactly the right levels and backwash when determined by the filter control
system and carried out by the actuators.

The quality of the backwash process relies on proper valve
actuation. The inlet valve that feeds water from the clarifier to the filter is
closed. At the other end of the filter, the effluent valve that transfers water
to the clear well must be closed. When the backwash water and air is pumped
underneath the media, it must be diverted only through the drain valve and
returned to the recycle or holding pond.

If the filter effluent valve actuation fails during a
backwash, there is a leakage of the backwash into the potable water stream
resulting in non-compliance turbidity problems. A filter can be disrupted if
you open a rate of flow backwash valve too quickly. The valves must be ramped
up at the right speed to the right position and then held there during the
entire process.

Since valve control accuracy and reliability play such an
important role throughout all filtering operations, many older plants currently
are being upgraded. In most cases, the original pipe galleries and valves will
remain in place. However, one of the first steps implemented usually is a new
control system. This changeover immediately requires new actuators that
interface with modern control systems. Until recently, electric actuators were
the primary actuators used to interface with the electronic control systems.

Electric Actuators

When comparing these electric actuators to the old hydraulic
or pneumatic cylinder power actuators they replaced, they seemed to be the only
solution at the time. The first actuators were water-actuated cylinders fixed
to the back end of a mounting plate. They had a lever on the cylinder shaft to
push and pull the valve open and closed. However, it was impractical to mount
input controls and feedback mechanisms onto this crude device to interface with
the new control systems. Thus, the progression from cylinders to electric
actuators.

The shortcomings of electric actuators quickly became
apparent to water-treatment plant operators (especially the repair and
maintenance staff). The easily understood piston-actuator problems could be
diagnosed and field-repaired by in-house maintenance personnel, but not the
more complex electric actuators.

Electric actuators generally have to be serviced by a
factory representative. The representative's time and the actual parts are
expensive. However, the real loss results from having the filter down. Factory
technicians must be scheduled for a maintenance visit to the plant. This could
take days, and water would not be flowing until the problem is fixed.

Another significant disadvantage with many electric valve
actuators is that they do not offer a fail-safe condition. A power loss can
cause filter galleries to flood if the electric actuator is frozen in its last
position. While most electric actuators have a hand wheel override mechanism,
once the power goes out, water will tend to overflow and flood the filter pipe
gallery unless the valves move to a fail-safe position.

Additionally, when the filter galleries overflow, electric
actuators are submerged in water and many times end up damaged. Replacement is
an expensive process, as electric actuators cost up to 40 percent more than
their pneumatic counterparts.

Reciprocating Pneumatic Actuators

Today's pneumatic actuators offer simple and reliable
performance at a cost-effective price, yet certain mechanical insufficiencies
inherent in the design of all reciprocating-cylinder actuators prevent them
from meeting the precise control needs of today's water treatment plants. For
example, rack and pinion designs suffer from a common leak path at the O-ring
shaft seals that are subject to wear. The high-friction O-ring of the piston
also is subject to wear. Side-load compensation pads also wear over time.
Collectively, these items dictate regular maintenance and, hence, more plant
downtime.

Typical piston actuator designs also are subject to heavy
side load on the valve shaft. There are no travel adjustments, and the design
introduces unnecessary hysteresis that greatly influences accuracy and
accelerates wear. These actuators also require the periodic replacement of
their high-friction, piston O-ring seals. Additionally, it is difficult to
mount the control components that interface with the PLC or SCADA control
system. (See Figure 1.)

The scotch yoke actuator design features more working parts,
and typically is too large to fit into the cramped quarters of the filter pipe
gallery. These actuators also require maintenance of their piston-ring seals.

Traditional piston-style actuators convert linear motion to
rotary motion through cranks, gears or levers. This conversion creates unwanted
hysteresis, side-loads, pinch points, high friction and torque loss. As a
result, control-accuracy suffers and a higher level of maintenance is required.

Pneumatic Rotary Actuators

Rotary actuators first were introduced to the United States
from Europe in the early 1970s. These rotary actuators meet American Water Works
Association standards and are finding themselves as a good option for new
facilities as well as plant upgrades.

The actuator's simple design utilizes only one moving part.
By scribing an arc, all torque forces directed to the valve remain constant from
fully open to fully closed. Absent the need to convert linear motion to rotary,
"pinch points" are avoided. Given a smaller torque-to-size ratio,
compact vane actuators can fit into the tight quarters of filter galleries and
still exert a tremendous amount of force.

The vane design also ensures accurate control and no
hysteresis. Because no O-ring seals are needed, vane actuators can provide
years of service in demanding, high-cycle, fast-operation and critical
modulating applications. (See Figure 2.)

One vane actuator manufacturer even designs-in a
"default" setting to protect treated water in the event that a power
grid goes out. The actuator can be set to a plant operator's specification as
to whether the valve should be held in the open or closed position. The rotary
vane of the actuator then automatically holds that position until power is
restored.

Field Installation

Given the advantages inherent with rotary actuators, coupled
with the fact that they are generally less expensive than electric actuators,
facility engineers are installing them in water works plants more frequently.
Some vane actuator designs come ready equipped for mounting into existing
plants.

One problem is that the valve manufacturers do not
necessarily make the mounting hardware for retrofit. Therefore, a facility
engineer or a supplier must sketch something out on a piece of paper and take
it to a local machine shop for drilling.

For challenging retrofits, some rotary-actuator
manufacturers send their experts out into the field to help facilitate the
installation process. Factory personnel or the qualified representative does
the survey on the valve and returns to the factory with the dimensions and
recommended actuator sizing. Given seven sizes to work with (torque outputs up
to 150,000 inch/pound) and adjustable rotations from 80 to 100 degrees, the
proper actuator for any given application can be found. Integral limit switches
and positioners are installed. The mounting plate then is fabricated, set up
and tested with the actuator. This process also includes the correct control
module to interface with the plant's existing PLC, SCADA or even older
pneumatic systems. The actuator then is shipped to the plant with the correct
valve mounting kit.

Such turnkey installation procedures make it easy for plant
managers on a tight budget to initiate plant upgrades from maintenance money.

The Bottom Line

While the accurate control made possible by rotary actuators
helps lower turbidity levels, it is their reliability and low cost of ownership
that really matters to most plant managers. Vane actuators are allowing water
works management to increase their water production while decreasing
maintenance costs.

Information for this story was provided by K-Tork, Dallas, Texas.

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