What type of grinder pump do you need?

Wastewater from a building’s fixtures is supposed to be carried to a public sewer main, then on to a treatment plant and ultimately returned to the environment, where it can lead a disinfected and productive second life. Sometimes a home or business’ sewer line is at a lower elevation than the sewer main. That’s where the sewage pump or a sewage grinder pump comes in. These pumps are traditionally put in some sort of tank and buried; either in the basement or outdoors. When water is used, wastewater is collected and stored in a holding tank. When it reaches a pre-set level, the pump is activated, pumping the waste out into the sewer system.

But not surprisingly, pumps can be susceptible to certain problems. Regular “solids handling” sewage pumps simply try to pass solids along. What goes in, goes out… or at least it tries. Certain waste items like diapers or articles of clothing or towels will clog regular pumps. Even if the pump can pass along some of these things, if the solid is larger than the pipe diameter, then the items can clog the pipes.
Grinder pumps are designed to grind waste into a fine slurry. But many grinder pumps can also be susceptible to problems. Failure of these pumps can come in different forms, but they all cause the same outcome. Wastewater from the residence or building stops flowing, an alarm may go off, and back up can occur if the alarm is ignored. For obvious reasons, this is not desirable.

There are three distinct grinder pump products:

• A single stage centrifugal grinder pump uses a single impeller, spinning on a shaft, powered by a motor to create the pumping action. Wastewater is pumped through the cutters that grind, shred or cut the material that is being pumped out of the basin. Single stage centrifugal grinders are typically used in applications that require lower discharge heights (less than 90 feet).

• A two stage centrifugal grinder pump is basically the same as the single stage unit but it has two impellers instead of one. Two-stage centrifugal grinders are typically used in applications that require more discharge height, typically in the 140 feet of lift range.

• A semi-positive displacement grinder is powered by a motor turning a stainless steel screw-type rotor that spins inside a rubber stator that creates a pumping action. This type of grinder pump allows for higher pumping heights as the material is squeezed inside the rubber stator pumping the wastewater through the pump and out of the basin. Semi-positive displacement grinders are typically used in applications that require higher discharge heights, typically in the 140 feet of lift range, sometime higher.

What are common problems of Grinder pumps?

Different types of grinder pumps can fail for different reasons. Here's the top 6 list:

Wadding — Material can wad up on the cutting mechanism, blocking the flow of water, overheating the pump and even burning out the motor.

Jamming — Material can actually get impacted inside the cutter mechanism to the point where it overcomes the torque of the motor and stops it.

Clogging — Ground material that gets past the cutting mechanism can clog up inside the pump housing blocking water or stopping the impeller.

Rubber stator failure — Macerated waste and pressure can wear on a rubber stator causing it to fail and render the pump useless.

Motor burn out — Some pumps must be applied to the right pressure and flow. Improperly applied pumps can overload and burn out the motor.

Seal failure — A common pump problem where the main seal fails, allowing water to get into the motor. This problem can be aggravated in a grinder pump because the ground material can actually wrap around the seal and cause it to fail.

How can I avoid these problems?

One of the newest cutter technologies on the market is the patented V-Slice® Cutter Technology by Liberty Pumps.

It is designed to eliminate the issues of wadding, clogging, and jamming that traditional cutters have. This new innovation provides superior shredding performance in demanding sewage applications. The hardened stainless steel cutting system provides in excess of 367,000 cuts per minute and aggressively shreds jeans, shop rags, diapers, sanitary napkins, and other difficult solids into fine slurry. Its special features allow it to do the shredding with minimum motor torque, and it has self clearing features, so it won't easily wad, jamb, or clog.

Seals: Traditionally a double mechanical seal arrangement is used because it is common for the first seal to fail since it is vulnerable to foreign/stringy material. This material gets wound around the shaft and can split the seal faces open. The Omnivore uses a Viton double lip seal for the lower seal in conjunction with a silicon carbide mechanical upper lip seal.

The lip seal keeps debris or stringy material away from the upper mechanical seal, and it is not hindered by the foreign material. The upper seal is a high quality unitized silicon carbide seal which provides hard seal faces that will not wear out. Silicon carbide seals are better and more reliable than ceramic-carbon seals used by other competitors.

Starting circuit: In order to provide the most power at startup, grinder pumps typically have a separate starting circuit, which is assisted by its own capacitor. When the pump comes up to speed, the start circuit shuts off by means of a switch. The best pump-starting switch is one that won’t wear out. Some grinders have relays with coils and contacts that can wear out. The Omnivore has a solid state starting circuit without a relay. There are no contacts or coils to burn up after many cycles.

Some pumps have a specific pumping zone, and they can only operate within a specific pressure range.
This is referred to as a pump with a minimum head requirement. If it operates without enough backpressure, (referred to as head, or feet of lift) then it can end up "over-pumping" and can burn out the motor. The pump manufacturer always provides a performance curve. Look for a pump that can operate anywhere within its performance curve.

When Liberty set out to design a new grinder pump, many considerations were taken into account.

• What are the current problems within the industry?

• What product features are desirable in a new grinder?

• How could a product be designed that would cover the most criteria in a single unit and still meet market needs?

The Liberty Pumps marketing ?and engineering team asked themselves the following questions. The answers pointed the way.

• What would be the best type of motor?

The Omnivore features a top quality US made motor. The motor was designed specifically for tough grinding applications. It has oversized windings which enable cool operation for maximum endurance. When the going gets tough the solid state starting circuit will kick in to assist a brutal grinding job. It also has a tough hermetically sealed thermal overload for optimum motor protection.

• How should the cutter system work?

A good cutting system will make the finest slurry possible, without clogging or jamming. The concept was smaller and more cuts per revolution, taking less torque to do it. The V-Slice® Technology was invented to do this. Each revolution potentially cuts 108 times. This is all made possible by the V-shape working in conjunction with a specifically designed rotary cutter.

The 440 SS Cutter Plate is hardened to approximately 58 Rockwell C. Its single piece construction features nine V-Slice® arrangements, and nine exclusion slots designed to self clean the cutter, It also has 4 spirals which remove debris from underneath the impeller. All of these features reduce the potential for problems that can occur with typical cutter designs.

The cutter’s sleek design has a recess for the shaft bolt, which reduces a known problem of waste materials wrapping around the cutter. This eliminates problems of flow reduction and motor overloading causing possible pump failure. The bottom of the rotary cutter is open at the back so that debris does not impact under the cutter. Its long arc design reduces the required cutting torque force.

• What would be the best impeller?

Non-corroding 316 stainless steel is the material of the impeller. It has back pump-out vanes for solids clearing while most competitive grinders do not have a stainless steel impeller. The impeller design is efficient enough to produce the required head-flow and overcome known clogging issues within the volute.

• What problems are currently known with grinders in the field?

Volute clogging: Other grinder pumps have a cutwater (a sharp corner at the discharge to improve impeller performance) inside the volute which can cause solids to build up and clog the discharge. The Omnivore does not have a cutwater, but instead has a smooth, wide-open discharge. Extensive testing has proven this to allow slurry to flow freely out of the volute, eliminating all clogging.

Proven performance

With literally tens of thousands of Liberty grinders now installed and operating all across North America, the Omnivore® series has proven to be a leader in performance and quality. In one particularly unique application, the Omnivore® grinder was known to have saved a school’s very unusual tradition in Ohio. The high school’s football team celebrated their victories by flushing their gym socks down the toilets after each winning game. Unfortunately, this caused the existing grinder pump to jam and thus required the school’s maintenance team to pull the pump to unjam it. Eventually, this lead to the school banning this time honored tradition due to maintenance costs. That is, until they saw an actual Omnivore® grinder demonstration at an industry trade show. They installed the Liberty grinder with patented V-Slice® cutter technology and with no jamming problems to date, the tradition continues! Information provided by Liberty Pumps.

For more info, www.libertypumps.com.


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