Check valves, which prevent reverse flow when the pumps are not in use in a lift station, are an essential component in wastewater systems. Shutoff valves come in many different designs and are critical to permit isolation of the wastewater system and allow for component maintenance. And air valves are an important component to maintain the flow efficiency in a wastewater system. With so many types of valves playing pivotal roles in wastewater systems, SWPA members John Holstrom, engineering project manager, and Diane Meyer, marketing manager, of Val-Matic Valve and Manufacturing Corporation, prepared the following overview that will be of interest for any pumping professional.
What are some of the essential characteristics that affect check valve performance and what should submersible pump users look for?
The types of check valves are broken into two categories: self-operated and power operated. Self-operated valves include swing check (metal and rubber hinged), silent check, dual disk check, tilted disc check, and nozzle check valves. Power operated check valves may be ball, plug, or rotary cone valves.
Self-operating check valves are generally less energy efficient than power operated check valves, but self-operating check valves have a lower initial cost over power operated check valves. Up-front versus long term operating expenses should be reviewed and balanced.
Check and shutoff valves should be located on the discharge line in a separate valve chamber. If a valve is integral to the pump, it may be located wet well. In this instance, if the check valve could be submerged it should not have external components such as levers, weights, springs, or actuators.
Self-operated check valves respond to system operating conditions and performance may change with respect to changing system conditions. Some valves have different weights, springs, or dampeners that offer some adjustment in the valves opening, closing, and response, but the system operating condition changes will affect changes to the valves response time, opening, and closing characteristics.
The various types of valves have different system response rates and energy losses (costs) associated with their performance. The type of valve affects the system transients as well as pump output and energy costs. Good valve selection is based on system design conditions such as system resistance, system current and future capacity needs, system length, transient pressure, transient wave period, piping size, and local fluid velocity. Consideration should also be given to deceleration of the liquid when a pump is stopped; deceleration is impacted by operation of multiple pumps, the amount of vertical (static) lift, and inertia of the pumps and pumped fluid.
Seal tightness may be another performance characteristic worth evaluating. Elastomeric seated valves tend to seal tighter at lower differentials than metal seated valves. Also the differential pressure necessary to affect a tight seal varies by valve type and seal material selections.
Self-operating check valves should be sized to fully open for lowest operating head loss and long term longevity. Operation at full open velocities also allows wastewater check valves to open sufficiently to pass a 3-inch sphere. Smaller than line size valves may be necessary for optimum long term performance.
Power operated check valves require a power source including an uninterruptable power source (e.g., spring, weights, hydraulic accumulator, air compressor and vessel, battery back-up, etc.) but offer much greater long term energy efficiency, more operational control adjustments and possibilities, as well as consistent operation regardless of system operating changes or differences. These valves are more consistent at shut-off and generally less dependent on reverse flow system pressure.
How do the different parameters of a wastewater system determine the right type of shutoff valve? What are the different needs for small and large diameter applications?
The various valve types also have different full open head loss characteristics that affect energy consumption and pumping capacity. The most energy efficient valves are the plug, ball, and rotary cone valve. Valves whose closure members (obturators) remain in the flow stream when open may collect larger debris or string-like solids and should be avoided in some system locations. The various valve types may have crude trap locations that will not properly flush solids downstream. Gate and globe type valves should be avoided where crude trapping is a concern.
As long as throttling and fluid flow control is not a concern, shut-off valves may be full line size to avoid higher energy costs. However, in larger line sizes the use of valve types capable of high line velocities and low flow losses, such as ball, plug, and rotary cone valves, may be reduced to less than line size and reduce installation cost at minimum operating cost increase. In larger line sizes, valves with a smaller foot print (like the butterfly valve) can reduce installation cost, volume, and space requirements.
Explain the challenges of proper air valve maintenance in the often dirty work of wastewater service.
Wastewater service air valves should have larger bodies and baffling that keep the operating mechanisms further away from the wastewater fluids. Wastewater air valves should be cleaned and flushed regularly to maintain effective operation. The ease of maintenance activity should be considered when selecting the installation location. The cleaning and venting ports of the valve may be plumbed to drain and flush valves to aid and simplify this operation.
Look for air valves whose operating mechanisms are simple, clean, and use strong and corrosion resistance materials that are easing to clean, rinse, flush, and will withstand abusive treatment during cleanings and in operation. Air release orifice sizes should be selected to be as large as possible to avoid early fouling. ◆
By: SWPA Executive Director Adam Stolberg and John Holstrom and Diane Meyer,
Val-Matic Valve and Manufacturing Corporation.
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