Coatings can Make a Difference in Wastewater

Those realities were behind a three-year field experiment conducted using wastewater air valves to evaluate the effectiveness of various types of internal coatings, an experiment that showed how those coatings can reduce downtime.

For this experiment, the valves were installed on the outlet of raw sewage lift pumps. This meant they were subjected to raw sewage, which provided quantitative data about the clogging and functionality of equipment in severe wastewater service. Included in the evaluation were bare cast iron, fusion-bonded epoxy, two-part liquid epoxy and fused polytetrafluoroethylene (PTFE). The results demonstrated the relative effectiveness of various coatings in preventing clogging.

THE SITUATION

As Mike Rowe learned the hard way, wastewater can be nasty stuff. It can contain liquids and liquid-carried waste from residences, commercial buildings, industrial plants and institutions as well as inflow from groundwater, surface water and stormwater. The solids in wastewater can pack into the tight places of flow control equipment and prevent it from proper operation. One of the most challenging locations for this flow is tourist areas that have numerous restaurants where greases and oils float and collect in the upper areas of equipment to form a thick crust of grease. Without good flushing action across the entire flow way of a valve, that valve will soon be clogged and require cleaning or maintenance.

One device prone to wastewater clogging is the air valve. This is because they are mounted on top of pipes with no natural flow-through flushing action.

Air valves are essential in keeping a pipe full of fluid. But if air and wastewater gases are allowed to collect in the pipeline, the results are additional head loss, check valve slam, surges and pipe corrosion. The air valves help promote efficient system operation by automatically removing trapped gas from the pipe and by providing vacuum protection.

Air valves are automatic devices that have orifices that open to expel air when air and gases collect in the valve body. As fluid enters, a float lifts because of buoyancy and closes the orifice in the outlet of the valve so fluid is not expelled. As the pressure in wastewater systems rises and falls, the wastewater must enter and exit the air valve. The challenge here is to prevent the wastewater sludge material from adhering to the internal surfaces of the air valve and clogging the inner mechanism or outlet orifice. The U.S. wastewater industry commonly installs iron air valves with no internal coating and relies on regular cleaning or backwash operations to keep the valves in working order.

With advances in valve coating systems, an opportunity to greatly reduce or eliminate the need for system shutdowns and regular maintenance of air valves has been born. This same opportunity can apply to other types of valves and related equipment such as pumps in these systems. To evaluate the effectiveness of these coatings, a field test was conducted using air valves in a raw sewage lift station over a period of three years.

For the test, three alternate interior coating systems were applied to 2-inch combination wastewater air valves mounted on the 10-inch discharge pipe of a submersible lift pump that was upstream of check valves. The function of the air valve was to release air and wastewater gases on pump startup and to allow air to reenter the pump column on pump shutdown. The air valves are normally open to allow the free flow of gas in and out of the valve. When sewage enters the valve, buoyancy makes the float rise, which operates a lever mechanism that in turn presses a stainless-steel plug against an elastomeric seat to close the valve. This prevents the escape of the fluid (Figure 1). Additionally, a small orifice in the center of the plug releases pressurized air and wastewater gases while the system is pressurized and in operation. This smaller orifice is sealed with a resilient orifice button when the gas is released, which allows the float to rise.

As an industry standard, these types of air valves receive no internal coating. In some applications, they may require regular backwashing or cleaning when leakage occurs. The valve bodies are elongated to prevent sewage from fouling the upper mechanism, and the bottom of the bodies are sloped toward the inlet to reduce debris buildup in the valve. The float and internal mechanisms are constructed of Type 316 stainless-steel trim and Buna-N elastomeric seals to resist corrosion and degradation over the valve’s life.

Three wastewater coating systems were evaluated, along with a valve that had no coating:

Valve 1—No coating, bare iron
Valve 2—Light-blue fusion-bonded epoxy (FBE)
Valve 3—Black-fused PTFE
Valve 4—Black, two-part liquid epoxy coating
Valve 1 was manufactured with no coating so it had a smooth internal cast iron surface and Type 316 stainless-steel float and trim. This configuration is common among many wastewater service valve manufacturers.

Valve 2 received an FBE coating in the factory. During the FBE process, the surfaces that were to be coated were first grit-blasted to a near-white metal surface finish and cleaned further with an etching solution. The parts were then preheated in an oven to 350°F (177°C), spray coated with epoxy powder using an electrostatic process and post cured in an oven at the same temperature until a hard glass-like surface was achieved. Valve 3 went through a similar 650°F (343°C) fusion process except the coating material was PTFE, resulting in a smooth, matte finish. Finally, Valve 4 received a two-part liquid epoxy system applied at ambient temperature in the factory to provide a smooth, glossy surface.

The four air valves were installed in June 2006 on the discharge of parallel, 10-inch primary submersible lift pumps at the wastewater treatment plant. The plant serves 22,500 residents and has a design average flow of 3.3 million gallons per day (MGD) and an 8 MGD maximum flow. The valves were placed upstream of check valves and had to be able to rapidly exhaust air and wastewater gases every time a pump was started, and then allow air back into the pipe when a pump was stopped. During pump operation, the air valves also had to release entrained air in the piping system through a small orifice drilled through the plug. The valves were installed in a valve vault opposite the submersible pump wet well as shown in Figure 2. The pumps run regularly every few minutes in lead-lag fashion (except during a rainy period when all four pumps can run continuously). The air valves are continuously subjected to raw sewage—the only pre-treatment was mechanical screens.

After one month, the four valves were tested in place to make sure they were still functioning properly. All functioned and released air as required without expelling wastewater fluid when closed. The valves were then isolated one at a time to view the internals. After three months, none of the valves showed any measureable build-up of sludge, and there was no damage or wear to any valve part.

In July 2009, about three years after installation, the valves were inspected again. Plant personnel reported the valves saw about equal usage over that three-year period. Plant personnel performed no backwashing or maintenance of the valves over the duration of the evaluation, and there were no problems noted with the operation of the valves during this period.

During inspection, Pump 4 started, a burst of air was released from the air valve for two to three seconds, and the air valve closed without leakage. The four valves were isolated from the piping by closing the isolation ball valves located beneath each air valve. With the air valves isolated, Pump 4 stopped a few minutes later and several repetitive slams of the check valve were heard.

Plant personnel reported air valves are needed for this system to vent pockets of air and prevent check valve slam from momentary vacuum conditions in the pipe after pump shutdown. All the valve exteriors still displayed the factory epoxy or blue primer coating and did not show excessive corrosion. The plated cover bolts were easily removed with a socket wrench. The covers were pried loose with a tapered scraper without damaging the fiber gaskets.

It was a dirty job, but each cover and float mechanism was lifted from the valve, and the body and float mechanisms were photographed. There was no damage or wear to any of the valve mechanisms. All the valves were operational and no clogging in the valve mechanism or bottom of the body occurred. The average thickness and percentage of the areas coated were also recorded. A summary of the observations from the test are given in Table 1 and illustrated in Figures 3-6.

CONCLUSIONS

After three years of continuous usage, the 2-inch air valves were effective in exhausting and admitting air in 10-inch raw sewage pump discharge lines and assisted in quiet check valve operation. During the test period, backwashing or cleaning was not required. The valves with the coated interiors were more resistant to buildup of sludge or debris than the valve with no coating. Therefore, the expectation is that, over the life of the valve, shutdown and cleaning of the valves will be required if buildup becomes excessive.

Sometimes it’s the small factors that can eventually make the difference in system performance. Taking the time to specify an engineered interior coating like fusion-bonded epoxy can save time and money, and most importantly, reduce the number of dirty jobs.

John V. Ballun, P.E. is executive vice president and COO of Val-Matic Valve & Mfg. Corp. (www.valmatic.com) headquartered in Elmhurst, IL. Ballun has been a prominent contributor to valve standards development work for American Society Safety Engineer, American Water Works Association and the Manufacturers Standardization Society. Reach him at jvb@valmatic.com.