How to choose the right valve for wastewater treatment applications
Valves for wastewater treatment plants must operate in a safe, efficient and consistent manner to avoid unnecessary maintenance. In addition, the selection should focus on the accuracy, repeatability and feedback requirements of the automatic control valves. The following criteria should always be considered.
▸The purpose of the valve: isolation, or throttling/regulation.
▸Process parameters: flow, pressure, and temperature.
▸Chemical compatibility: concentration, solids percentage, media, and density.
▸Process requirements: operating frequency, allowable leakage rate, cleanliness, available space
Gate valves have a wedge or disc that moves up and down to stop or allow water flow. They are primarily used for isolating (shutting off) wastewater applications. Gate valves have a stem that can be either rising or non-rising. A rising stem requires clearance above the valve to move up when the valve is open. This will provide a visual indication of the open/closed position. If space is limited or in areas where safety is a concern, a non-rising stem may be used.
In general, gate valves are cheaper for smaller valve sizes (<300 mm). However, larger valves can be expensive and require adequate valve clearance.
Ball valves have a spherical plug with a hole in the center. They are typically used for chemical applications and to isolate air and water in smaller diameters (<50 mm). They are available for service to high performance valves and are easily available in a variety of configurations such as top inlet, end inlet, and three piece. In general, available sizes range from 25 mm to 300 mm.
Some of the characteristics associated with ball valves include ease of operation, high pressure and temperature capability, high flow rates, and the ability to handle severe chemicals. Ball valves are also considered to be high recovery valves (relatively high Cv,) similar to butterfly valves.
Ball valves are not suitable for slurry applications. Similarly, the weight of larger size ball valves is much higher than other similar size valve types.
Butterfly valves have a disc mounted on a shaft in the center of the valve. They are typically used in air supply systems for wastewater treatment. Butterfly valves are an economical alternative to larger valve sizes (>200 mm) because of their compact design and lighter mass compared to other valves. In addition, they have relatively high flow coefficients (Cv), standard face-to-face dimensions, and can be manufactured with chemically resistant materials. In general, butterfly valves are available in sizes ranging from 50 mm to 1200 mm.
The butterfly valve is considered a high recovery valve because only the valve flap obstructs the flow path of the valve. the Cv is relatively high and the pressure drop at both ends of the valve is relatively low.
Although butterfly valves are economical, they tend to scale when used for sludge containing solid/viscous materials and therefore do not handle slurry applications well. The cavity around the disc stalk is particularly vulnerable, and it may retain fluids and slurries. As a result, unwanted contamination may occur in mud service. Although some high-performance butterfly valves may meet ASME leak ratings, these valves are not typically gas-tight rated.
Plug valves are similar to ball valves. The moving parts of a plug valve consist of a conical plug rather than a ball. Plug valves are used primarily in pipelines that convey untreated sewage, sludge, and gravel. They are also used in biogas systems.
Plug valves can seal well and they do have a tight shutoff. However, some plug valves have smaller ports, which means that the flow path through the valve is smaller than the cross-sectional area of the adjacent pipeline. This results in a higher pressure drop. Therefore, if needed, look for a full bore plug valve. Plug valves are heavy and require more space, but are reliable and durable.
In some cases, plug valves are used for throttling purposes.
Knife Gate Valves
Knife gate valves are similar to gate valves; however, the knife edge of the gate cuts through the solids that have built up. Knife gate valves are used in wastewater systems for handling abrasive slurries or sludge applications. They are available in standard cast configurations as small as 50 mm and in specialty valves as large as 1,800 mm. Knife gates for sludge applications should be specified with bi-directional pressure ratings, making them suitable for pulling out of the pipeline from either direction.
Knife gate valves cut off slurry, scale and surface buildup. They provide high flow rates (Cv) because they have an unobstructed flow path. They also have a small face-to-face size, which helps reduce the weight of the valve and facilitates piping design.
Knife gate valves are not suitable for relatively low pressure applications. Two disadvantages are the inability to provide gas-tight shutoff and cavity formation. Therefore, they are not recommended for high purity applications.
Globe valves have a round body with two cavities and a circular opening in the middle smaller than the pipe size. Generally, globe valves are available in sizes from 50 mm to 200 mm. The valve operation is a linear rising stem with a multi-turn handwheel.
Globe valves can be used for precise throttling and control services in wastewater treatment applications because they can be easily automated with positioners, limit switches and other accessories. Generally, control valve manufacturers provide software for control valve sizing because it is much more complex than isolation sizing.
Globe valves have a relatively low Cv and cannot handle slurries. They are relatively expensive. For these reasons, they are not typically specified for control in high-purity service or slurry service. In addition, the low Cv results in relatively high pressure drops at both ends of the valve.
Depending on the application, the following points should be kept in mind when selecting the correct type of valve.
▸For smaller valve sizes, gate valves are cheaper.
▸Ball valves allow for fast quarter switching operation, but have poor throttling characteristics.
▸Butterfly valves are high recovery valves, but are not gas tight.
▸Plug valves can seal well and have tight shutoff.
▸Knife gate valves can shut off slurries and have very low flow resistance. However, they are not suitable for high purity applications.
▸Globe valves are suitable for throttling purposes, but have relatively high head loss.