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Rotary-action, pneumatic valve actuators for waterworks plants strengthen control of filtering operations

Fred R. Underwood, P.E.
CEO and Founder
K-TORK Actuators and Controls
Dallas, Texas

Under pressure from the mandates of the Safe Drinking Water Act, waterworks engineers and plant managers are now forced to scrutinize all elements of their potable water treatment operations, and 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, waterworks engineers are now turning to a new generation of pneumatic valve actuators that are up to the task 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 is also 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.

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

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

There are only certain ways you can achieve a lower turbidity, and the one that is important is accurate valve control. 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. Actuators must operate the valves precisely closed. There's no point in having the best PLC system if the actuators it controls aren't executing those instructions accurately and reliably.

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. These rates must be carefully metered by the effluent valve 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 GPM, so you must close the effluent valve to the point where it only allows the flow rate that the filter media is designed to pass at that time. 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 gets 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, but it is an expensive and time-consuming process. That filter is out of commission during the backwash process, and must use clean potable water that you just spent money cleaning. The key to operational efficiency is to keep the flow at exactly the right levels and backwash when necessary, determined by the filter control system and carried out by the actuators.

The quality of the backwash process itself 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 are pumped underneath the media, they must only be diverted through the drain valve and returned to the recycle or holding pond.

Reliability is extremely important, for if the filter effluent valve actuation fails during a backwash, then you end up with a leakage of the backwash into the potable water stream and this results in non-compliance turbidity problems. You can even disrupt a filter if you open a rate of flow backwash valve too quickly. The valves must be ramped up at the right speeds, to the right position, and then held there during the entire process.

Because valve control accuracy and reliability play such an important role throughout all filtering operations and in meeting federally-mandated turbidity levels, many older plants are currently involved in plant improvements and controls upgrading. In most instances, the original pipe galleries and valves will remain in place. However, a new control system is usually the first step to be implemented. This changeover immediately requires new actuators that interface with modern control systems. Yet, until recently, electric actuators were the primary actuators that came equipped to interface with the first electronic control systems.

The shortcomings of electric actuators
In comparison to the old hydraulic or pneumatic cylinder power actuators they replaced, electric actuators 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, and had a lever on the cylinder shaft to push and pull the valve opened and closed. But it wasn't easy to mount input controls and feedback mechanisms onto this crude device to interface with the new control systems. Hence the progression from cylinders to electric actuators.

Yet, it didn't take long for the shortcomings of electric actuators to become 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 so for the more complex electric actuators.

Another significant problem with many electric valve actuators is that they do not offer a fail-safe condition. The primary reason filter galleries flood occurs during a power loss, which causes the electric actuator to hold its last position. But water treatment—being the dynamic process that it is—cannot tolerate a steady state for long.

Additionally, when the filter galleries overflow, electric actuators are submerged in water and subsequently end up damaged. Replacement is an expensive process, as electric actuators cost up to 40 percent more than their pneumatic counterparts. Not to mention, electric actuators are generally powered by 460 Volt, three-phase power, which can be lethal.

Reciprocating pneumatics also fall short
Completely sidestepping the inherent problems of electric 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, which are subject to wear. The high-friction O-ring of the piston is also 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 are also subject to heavy side load on the valve shaft. There are no travel adjustments, and the design introduces unnecessary hysteresis that greatly affects 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.

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

A simple, new option in the form of pneumatic rotary actuators
Rotary actuators were first introduced to the United States from Europe in the early 1970s and are quickly becoming the new option of choice for new facilities as well as plant upgrades.

The basis of this newfound success stems from the actuator's simple design, which 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 on quarter-turn valves, for instance.

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—thus allowing water treatment engineers to carefully control turbidity levels.

Vane actuators can even be designed with 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. This prevents the flooding of filter galleries.

Much appreciated by maintenance staff, the simple structure of pneumatic rotary-vane actuators allows "in-the-field" repair if necessary.

Simple field installation helps fuel the changeover to pneumatic rotary actuators
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 waterworks plants with intensifying frequency.

However, most manufacturers of valves don't necessarily make the mounting hardware for retrofit. That leaves a facility engineer or a supplier with the responsibility of sketching something out on a piece of paper and taking it to a local machine shop. So, for challenging retrofits, it's important to find one of the few actuator manufacturers that is willing to send experts out into the field to help facilitate the installation process. Factory personnel or the qualified representative perform a survey on the valve and return to the factory with the dimensions and recommended actuator sizing. Integral limit switches and positioners are installed. The mounting plate is then 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 is then 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 proven reliability of the pneumatic rotary design also factors significantly in the decision to upgrade.

The bottom line
Given the lowered costs, operational advantages, and ease-of-installation afforded by pneumatic vane actuators, water treatment managers are awakening to this new option for optimizing their water production, avoiding downtime due to maintenance problems and remaining within federally-mandated turbidity levels.

In a not-so-roundabout way, vane actuators are allowing waterworks management to up their water production while decreasing maintenance costs. Whether private or municipal, these kinds of operational efficiencies can be taken straight to the bank.

To reach Fred R. Underwood, call (214) 341-1099 or send e-mail to info@ktork.com.