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Centrifugal or progressive-cavity sludge pump?

Centrifugal or progressive-cavity sludge pump?

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    “Easy” applications = pumping primary sludge, return activated sludge (RAS), and waste activated sludge (WAS). “Difficult” applications = imported sludge and thickened sludge. “Hard” = transporting dewatered sludge. Images: Xylem Inc.

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Pumping is the second-largest expense, after labor, for most treatment plants, representing in some cases almost half the facility's annual operating costs. As communities urge you to increase energy-efficiency, knowing how a pump's design affects operations is key to specifying a unit that appropriately balances resources with demands.

Centrifugal pumps are the most common design on the market but are available with at least five different types of impellers, each of which functions a bit differently. The progressive-cavity (PC) pump is a new twist on an ancient design: the Archimedean screw.

Understanding the relative strengths and weaknesses of each will help you decide which is right for your application.

General recommendations: centrifugal pumps

During treatment the densest sludge is pumped first, followed by sludge with the consistency and hydraulic characteristics of water. Changes in sludge consistency have a greater impact on a centrifugal than a progressive-cavity (PC) pump.

Because power uptake typically increases with increasing flow, the motor must be large enough to handle the maximum anticipated additional load. When this requirement shifts the pump's head-capacity curve past the point of maximum efficiency, a variable-frequency drive (VFD) can be used to manage variations in load and capacity.

To determine how much power the motor needs, plot pump curves and system curves on a graph with maximum anticipated sludge density (i.e., total solids, or TS) on one axis and water on the other. Compute pump curves for the maximum speed under consideration. Power consumption should then be evaluated at the point of intersection between pump curve and system curve as well as at the corresponding intersection between sludge pump curve and sludge system curve. Size the motor to operate at the highest power level of the two.

Use the same formula to determine an operating speed range for VFD, calculating system curves for water and for the densest sludge. Plot the curves on a graph of the water pump curves for a range of speeds. Required minimum and maximum speeds are determined from the intersection between the pump curves and the system curves at the desired capacity.

VFD causes pumps to run at reduced speeds for long periods of time. While this reduces energy requirements — and thus costs — several factors may override potential benefits.

  • The lack of start/stop cycling means the pump doesn't benefit from the cleansing back flush that occurs whenever pumping stops.
  • Low speeds increase the rate of solids collection at the impeller and thus the risk for pipe fouling.
  • Efficiency decreases when speed decreases. If the VFD is set to output a voltage proportional to the square of the frequency, the available torque decreases with speed under nearly sustained motor efficiency. When sludge is diluted the resulting lower torque doesn't matter because the torque uptake decreases proportional to impeller speed raised to the power of 3. But when sludge density requires the pump to slow down, the output voltage must be proportional to the speed to sustain torque.

Finally, most centrifugal pumps are available in a range of impeller diameters. To provide room for maneuver if sludge behavior is outside expected bounds, don't choose the largest.

BASIC FACTS:Centrifugal

How it works: Sludge enters an impeller and is accelerated in circumferential direction along the impeller's vanes. Reduction of velocity as fluid moves through the pump's spiral-shaped casing converts kinetic energy to pressure.

Types of centrifugal pump

  • Channel impeller (most common)
  • Self-priming
  • Screw impeller
  • Semi-open impeller
  • Vortex (with and without a recessed impeller)
General recommendations: PC pumps

Unlike a centrifugal pump, a PC pump can maintain flow rate regardless of pressure. But as sludge density increases, maximum allowable speed decreases.

To reduce the wear rate on expensive components like the rotor and stator, pump speed should be lowered depending on how much abrasive solids like sand or grits are in the sludge. If, say, total solids (TS) concentration is less than 4%, the pump can run as high as 400 rpm; but if concentration approaches 45%, speed typically has to be lowered to 60 – 80 rpm. A variable-frequency drive (VFD) is required for the pump to maintain pressure when operating at slower speeds.

Depending on concentration levels, a larger pump may be necessary. Instead of going up in size, consider adding a pump stage to relax the stress on and potential for leakage at the contact line between the pump's stator and rotor.

Lubricant is necessary because of the metallic rotor's direct contact with a stationary elastomeric stator. To minimize the potential for damaging these components, consider installing a pressure guard that shuts off the motor if pressure drops to dry-run levels. A guard also protects against accidental operation against closed valves, which would damage either the pump or parts of the system.

If stopped, a PC pump acts as a closed valve: no return flow from high-pressure side can occur. However, don't classify the pump as a check value in terms of system design. Due to inevitable wear, leakage from high to low pressure can occur between the stator and the rotor.

—Magnus Fahlgren (magnus.fahlgren@xyleminc.com) is an application engineer for Xylem Inc. , which was spun off from ITT Corp. in October 2011. The company's N-pump is a semi-open self-cleaning design that prevents clogging by continually scraping material from the impeller. This article is based on information from the company's Handbook of Sludge Pumping, which will be available summer 2012. For more information, visit www.flygt.com.

CENTRIFUGAL VS. PC PUMPS

Primary attribute: Efficient
Why that's important: Energy savings
TS (total solids) concentration: 0% – 8%
Primary drawback: Flow rate may vary with changing sludge properties
Other general characteristics

  • Small footprint
  • Submersible or dry well
  • Can run dry for limited periods
  • Can't dry-prime (except for self-priming designs)
  • May require check valve to prevent backflow
  • Electric powered
  • Variable-frequency drive optional to control flow rates
  • Fairly simple design = low maintenance $

Progressive-cavity (PC)

How it works: An external motor turns a rotating internal screw (the rotor) that draws and moves sludge through the pump (the stator). The unit's two joints — between the rotor and coupling rod; and between the coupling rod and drive shaft — are the most critical design elements. Joint types:

  • Flexible shaft
  • Gear
  • Oldham
  • Pin

Primary attribute: Maintains the flow rate nearly independent of pressure
Why that's important: Flow rate is sustained even if sludge viscosity changes
TS (total solids) concentration: 4% – 45%
Primary drawback: Variable-frequency drive required to control flow
Other operating considerations

  • Power consumption increases in proportion to head at any given speed
  • Start torque is high and gets higher if the liquid possesses a yield stress

Other general characteristics

  • Large footprint
  • Self-priming possible
  • Can't run dry
  • Smaller sizes susceptible to clogging from large solids or rags
  • Grit wears down expensive components = high maintenance $