Gillian Chew

Feedwater Regulator for Drum Boilers

Coal Fired Boilers, Combined Cycle, Pulp & Paper /

Feedwater Regulator for Drum Boilers Keeping accurate and stable drum level control is vital to optimal plant operation, but there are many challenges that are inherent with this process. Innate to level control on a drum style boiler is a dynamic called “shrink and swell”, which is caused by the formation of vapor bubbles in the boiler evaporation tubes as steam demand changes. On increase of steam demand, the drum pressure decreases which, in turn, causes additional steam to be created through water evaporation, and causes expansion of the vapor bubbles below the surface of the water. This phenomenon causes the drum level to rise initially, instead of the drop that would be expected upon more steam leaving the drum (swell). Likewise, on a decrease of steam demand, the pressure in the drum increases and the drum level initially drops (shrink). In order to control drum level accurately, the effects of “shrink and swell” are typically compensated for in the control system with a cascade/feed-forward control strategy that utilizes steam flow and feedwater flow transmitters in conjunction with the drum level and drum pressure transmitter measurements.  Literature Download the Feedwater Regulators Application Spotlight! Download

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Filter Flow Control

Water Treatment /

Filter Flow Control The filtration process removes suspended particles from water by passing the water through a medium. After a period of run-time, a filter needs to be “backwashed” to remove the captured particles from that run-period. Particle capture rate is important in ensuring filtered water contains no micro-organisms that could be harmful to people. Micro-organisms can be difficult to disinfect/remove from water, so operators measure the turbidity of water to gauge the capture rate of a filter. Ideally, an efficient filter runs the longest and captures the most particles (has the lowest turbidity).  In a traditional gravity fed filtration system, the most critical valve application is the effluent control valve (commonly a butterfly valve). This valve typically sits at the bottom of the filter where it controls the rate of water flow moving through the filter. Commonly, accurate modulating control of filter flows with this valve is imperative to maintaining a constant, stable flow in a filter. The more stable the flow, the greater the capture rate. Surges in flow can occur when flow rates are not stable, which can allow suspended matter to pass through the filter – effectively increasing the turbidity of the filter effluent.  REXA  Linear  and  Rotary  Actuators have a history of success in the water treatment industry with controlling filter effluent valves. Maintenance-free operation of these modulating valves over decade-long periods is common. More importantly, the position accuracy of REXA  Linear  and  Rotary  Actuators (dead-band as tight as 0.05) can modulate a filter effluent butterfly valve disc in very finite increments that eliminate hunting. Precise position accuracy will stable flows through a filter, allowing for more efficient particle capture and lower turbidity.  Literature Download the Filtration Filter Effluent Control Application Spotlight! Download

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Water Storage

Water Treatment /

Water Storage REXA has a history of providing solutions for water storage applications. Some of our customers have sought the modulating capabilities of REXA to accurately meter flows into and out of their water storage infrastructure. Others have required the reliability and fail-safe capabilities of REXA for applications used to protect the stored water during emergency conditions. In either case, REXA has the product and experience to help you

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Ozone Control

Water Treatment /

Ozone Control Controlling a gaseous service via a butterfly valve with any degree of accuracy is 100% dependent on the positioning of that butterfly disc. On small valves sizes common in the ozone treatment process, the position accuracy is even further emphasized, and the slightest movement of a butterfly disc can result in significant flow changes. The positioning accuracy of REXA  Linear and  Rotary  Actuators truly shines on this service, as the dead-band accuracy (as tight as 0.05%) can position butterfly valve discs in small enough increments to promote accurate flow control, improving the ozone treatment process efficiency.

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Raw Water Influent Flow Control

Water Treatment /

Raw Water Influent Flow Control Reliable control of  incoming flows is very important to starting the wastewater treatment process off on the right foot. Incoming flows are being modulated, any flow swings due to actuator hunting can cause upsets further along in the treatment processes. For this service, it is more important that the actuators are reliable, in order to avoid any possible disruption to the desired incoming flow rates. REXA has a history of offering both reliable and accurate flow control for this important service

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Collections

Wastewater Treatment /

Collections Gates are commonly used within a collection system to control flows. Automation of these gates requires an actuator that can reliably respond to changing conditions. During a wet weather event, the actuators on these gates are the critical aspect of the entire high flow management strategy. A failure of actuators to respond can cause big problems. Imagine a scenario where a collection system interceptor station has both influent control and diversion gates. The flow typically runs through the control gate that can be modulated to pinch off or shut off flows, perhaps using in-house storage upstream within the collection system. During very high flow events, the diversion gate would divert to a local water way or a storage area.  The worst case scenario occurs if the control gate is pinched off/closed, but the diversion gate fails to open on account of an actuator failure. During an extreme wet weather event, such a failure could cause flooding upstream of this interceptor, potentially resulting in public or private infrastructure damage, as well as a black eye for the wastewater operations.  Literature Download the Collections Wet Weather Management Application Spotlight! Download

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UV Disinfection

Wastewater Treatment /

UV Disinfection Background Wastewater treatment plants often employee ultraviolet (UV) disinfection as the last stage of treatment to meet strict bacterial limitations on plant discharges. A UV system’s reliability and efficiency is critical to mitigating the threat of permit violations and fines for the discharge of improperly disinfected wastewater effluent. During the UV disinfection process, treated wastewater is exposed to UV light, causing cellular damage to any microorganisms present. Therefore, these pathogens, such as viruses and bacteria, are incapable of reproduction or infection.  On open channel UV systems, either a controlling gate or weir structure regulate and maintain constant water levels over the UV lamps. For UV systems with horizontally-installed lamps, it is imperative to properly regulate wastewater levels to provide sufficient  UV exposure to contaminants. In addition, the UV system effluent gates serve as one of the last points of control before discharge.  Problem Poorly-controlled actuators on this application leads to a variety of problems. If the gate cannot accurately control the water level over the UV lamps and gets too high, there may not be enough UV light to treat the additional water flowing through the channel. Consequently, this leads to potential discharge permit violations. Conversely, if the water level gets too low, the UV lamps can overheat, leading to maintenance issues. From a fail-safe perspective, if the UV system is compromised for any reason, and the effluent gate does not close, the plant can discharge non-permit water.  Solution The reliability of REXA Linear and Rotary Actuators prevents downtime and maintenance costs on UV applications. More importantly, the superior control capabilities of REXA Linear and Rotary Actuators assures proper level control in plants’ UV channels. This allows them to meet disinfection standards for their effluent, reduce the threat of fines, and maximize UV bulb life. The easy addition of a fail-safe option to any REXA design gives the treatment plant peace of mind that a UV channel can be closed and isolated when an upset or emergency condition occurs. UV effluent control structures are tailor-made for the reliability, control, and fail-safe capabilities offered by REXA. Literature Download the UV Disinfection Effluent Level Control Application Spotlight! Download

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Headworks

Wastewater Treatment /

Headworks It is common for one or more cast iron sluice gates to be employed within the headworks of a wastewater treatment plant to control the flows entering the facility from the collection system. These gates will most often remain fully open, but can be throttled to regulate flows during high-flow events. Above all, reliability of these sluice gates is of great importance to the treatment plant.  Many facilities do not modulate the main influent gates in the headworks due to the inoperability of the existing actuators. In such instances, the gates may be permanently chained in the open position, preventing the plant from fully utilizing the control structures in the headworks. Regardless, if the actuator controlling a headworks gate becomes inoperable to the point that it impedes or disrupts the flows coming into the wastewater treatment plant, big problems can arise by allowing too much or too little flow into the treatment plant.  REXA  Linear  and  Rotary  Actuator’s are ideal for headworks gates, allowing a plant to reliably operate these gates, as well as control and modulate them (an added benefit many plants do not yet employ). 

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