From The Editor | April 30, 2014

How To Select The Right Valve And Valve Control System For Your WWTP

Laura Martin

By Laura Martin


When an abundance of solids collect around a valve used for wastewater treatment, it can result in a reduced flow. This is often overcome by increasing the size of the pump, so that the speed of the water can break through the solids, or keep them in suspension.

But installing a bigger pump is not always the most cost-efficient method when the long-term financial picture is considered, said Timothy Fallon, a product manager at Henry Pratt Company.

“The bigger size of a pump you need the more power you are going to consume and the more money you are going to have to pay,” said Fallon.

Instead of replacing the pump, Fallon suggests looking at the type of valve used.  For wastewater treatment plants using swing check valves, plug valves may be a more effective alternative.

A plug valve is hidden from the flow stream, so no debris builds up on the plug. Its port offers less resistance to slurry, sludge, and fluid flow, which results in less erosion, longer service life, and reduced pumping costs.

But the higher upfront cost of the plug valve discourages some from considering it over the swing check valve.  That initial investment could be worth future savings, said Fallon, whose company offers both swing check and plug valves.

“It could be argued that the cost difference between the two valves could be recouped two to three times over in electrical pumping costs, over the lifetime of the valve,” he said.  “So, you can go cheap by using the swing check in terms of capital expenditure and pay more in the long run with higher electrical pumping costs, or spend the higher dollar up front with a plug valve and save money in reduced electrical pumping costs.”

However, plug valves aren’t ideal for every application. For facilities that use small pipelines or deal with wastewater that isn’t heavy with solids, swing check valves might make more sense if they are already working effectively, explained Fallon.

Valve Control Systems

Once the valve type has been selected, it is equally important to select the best control system for the job. There are several factors to consider when determining which system to use with valves in wastewater applications. Fallon shared the pros and cons for the three most commonly used systems.

 Pneumatic Actuators

A pneumatic actuator can be a cost-effective approach to both open/close and modulating applications. The technology works with varying rates of operation and can provide independently adjustable rates of open and close. It also provides a fail-safe capability if needed during an emergency. However, when working with larger valve sizes, such as those over 36”, the pneumatic actuator can sometimes struggle to maintain valve position. It also requires a larger operating cylinder when used with large diameter valves, so it may not be best for space-constrained applications.

Electric Actuators

Electric actuators can produce very high torque output.  Thanks to recent technology advancements, many now also provide a greater degree of process monitoring, using intelligent actuation. Electric actuation provides a fixed rate of torque and speed of operation output.  But despite updates, there are disadvantages to the technology. It is more expensive than pneumatic actuation, especially for plants located in remote locations where it is difficult to source a high amount of outside power.  Relying solely on available electrical supply can also present some challenges.

“If the power fails, and if there is no external backup power supply, the valve will also fail,” said Fallon. “When an electric operator fails, it just sits open, and if this outage occurs during a heavy rainstorm event, it can cause severe  flooding and in some applications, cause a combined sewer overflow.”  

High Pressure Oil Hydraulic Actuator

This technology is more reliable during a power loss than an electric operating system. The actuator stores pressurized oil — which is used to fuel the device — so if a pump fails and the valves open, the system has enough energy stored to close all valves immediately.

“This approach takes the high torque output that an electric operator can produce and combines it with the variable rate of open/close and fail close the pneumatic actuator can provide,” said Fallon. “This is built into a compact cylinder operator that does not require the electric demand that a similar  sized electric actuator requires.”

Fallon describes the high pressure oil hydraulic actuator as approximately the same size as a lunch box, so it is ideal for plants with space constraints. The drawback to the technology is its increased level of complexity compared to the electric or pneumatic actuators.

Operator Viewpoint

In addition to understanding which technologies will work best with your application, it is important to consider the viewpoints of those who will be working directly with the equipment.

“Listen to your operators,” said Fallon. “Understanding the level of acceptability of certain valves and valve control systems before installing them will make the process go smoother. These are people who are actually going to be working with the equipment, and they need to feel comfortable as they grow and improve their infrastructure.”

Image credit: "Wastewater treatment plant," Sustainable sanitation © 2007, used under an Attribution 2.0 Generic license: