Third of Six Parts When a system is not working as expected, users and engineers must see the complete picture, especially in wastewater transport systems. While the increasing use of wipes is a challenge, as discussed in Parts 1 and 2 of this series (Pumps & Systems, November, read it here, and December 2015, read it here), different systems will handle this problem differently, and different fluid content will influence performance. Wastewater is not all the same. Depending on the operation or the system's design, the sensitivity can vary for the particular kind of wastewater. Several international initiatives attempt to clarify and define different wastewater. Engineering and planning of wastewater facilities requires knowing what kind of wastewater or sewage must be pumped for trouble-free performance and energy efficiency.
Shifting trends in water use and changing sewage composition cause complex problems for the world's sewer systems.
01/27/2016
Figures 1 (left) and 2 (right). Two general designs of a submersible pumping station (Images and graphics courtesy of KSB)
The content and mix of the pumped fluid matters greatly. Often, "sewage" is the only information application engineers receive when asked to specify pumps. But providing a detailed definition of the particular fluid (i.e. unscreened raw sewage, pre-screened sewage, stormwater, sludge) is vital for selecting the hydraulic/impeller. Especially in raw sewage, increased wipe use compounds the challenge for pump selection, system engineering and operation.
This article will exclude pre-screened wastewater and focus on the transport of raw sewage to treatment plants.
Figure 3. Example of a large combined stormwater and sewage station
For a combination of stormwater and sewage pumping station, engineers must consider aspects of both (see Figure 3 and Image 1). Even if a pumping station is flushed in a storm event, the sewage's settling solids can pose problems. Another disadvantage arises if the same pump size handles dry-weather flows and high flows during storms.
Image 1. Example of a combined station
For these cases, a jockey pump is often selected. Even if this pump is available, it may seem better to use variable frequency drives (VFDs) for speed reduction of the larger pumps for dry-weather flow, but this causes trouble because of low velocity.
Even if a pump can handle the mixture's fluidity portion, the pump's tip speed and the pipe's velocity may not be sufficient to properly transport solids and rags. The pump should operate in the Hydraulic Institute's recommended Preferred Operating Range.
Combined stations must be able to pump very high flows during certain events as well as low flows that can carry a lot of solids and wipes during dry weather.
Image 2. A poor example for trouble-free operation
Another mistake is the diameter of the discharge pipe increasing sometimes three times in the vertical line to reduce friction and save energy. In contrast to water pumping where diameter and flow velocity can be calculated according to economical targets (low friction), higher flows/velocities are required in wastewater transport. If the diameter is too large to have a velocity able to move the load, solids may remain in the pipe (even if the liquid comes through), and when the pump is shut off, everything will fall back on the back side of the check valve or into pump and/or sump. If variable speed is used, the lowest possible speed should be considered in regard to diameter and velocity.
The risk for horizontal pipes is that solids settle in the pipe if the flow velocity does not overcome the sink velocity, which depends on the solids' specific weight, size and shape. Engineers should consider recommended minimum velocities to avoid the risk.
Clogging is a risk in check valves, which are normally designed for a discharge diameter in accordance with the flow. The zeta value depends on the velocity and opening angle, but with the same velocity, the opening angle will be smaller if the nominal diameter is larger. For wastewater, the best operation can be when the check valve is fully open in a minimum system diameter of 4 inches. If the check valve is too large or the velocity is too small, the valve will not open fully. Manufacturers give different nominal diameters as a function of opening angle and velocity.
Figure 4. Example of an optimized and proper collecting pipe piece
Figure 5. If the selected size of the valve is too large or the variable speed is causing a too low velocity, the check valve will not open fully. Rags and fibers cannot pass through the opening.
The pumps should run up to full speed during startup, run a short time at nominal speed and be adjusted to the required (lower) operating speed after a short time. In cases that require long periods of operation in lower speeds, an increase of pump speed to produce flushing periods must be provided. A good option is to return to full speed shortly before the pump's switch-off. This operation can be achieved using programmable controllers.
Part 4 of this series will discuss in detail de-ragging flushing and VFD use. Read it here.
References
KSB Know How Brochure, Volume 7, KRT Planning Information
To read other articles in this series, click here.