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By Annabelle Powell, Journalist, Pump Industry Magazine

Pump systems are critical to keeping our world in motion. When selecting a wastewater pump, there are key factors to consider to ensure maximum efficiency and longevity, and to reduce long-term costs.

Wastewater pumps are used in the collection o sewage, effluent, drainage and seepage water. When selecting a wastewater pump, whether for a new system or replacing an old one, there are selection criteria to consider that can make big differences in performance, reliability and cost-saving energy efficiencies.

This checklist includes liquid, flow rate, temperature, altitude and lifecycle costs. Industrial applications come with many demands, such as resisting corrosive chemicals and handling high temperatures.

Considering such demands as well as the application environment is crucial when selecting the most suitable pump for your application.

Determining these factors and selecting the right wastewater pump result in less maintenance, higher efficiency and long-term cost savings.

Different types of pumps

There are two main types pumps used for wastewater:

Conventional wastewater pumps: These are also known as non-clog, solids handling or dry-well pumps. Their main purpose is to pump liquids that contain soft solids and other stringy material. They don’t require frequent servicing and cleaning, and operate without plugging. They can either be installed with horizontal or vertical mounting.

Submersible pumps: These are non-clog pumps that are immersed in the wastewater for operation. They come equipped with powerful motors and are installed directly in the wet well. This means wastewater pumping stations that use submersible pumps don’t require a dry well.

The pumping system construction above the ground is minimal, and all that’s visible above the surface is a concrete slab and a small housing construction for the control centre.

Pump capacity

A good first step when selecting a wastewater pump is to examine the application’s hydraulic characteristics. In choosing the right pump it’s key to accurately estimate the capacity that is required.

Reviewing the required flow rate and pipe size determination will make it a simple matter to estimate and graphically display these characteristics with a system curve specific for the application.

In order to create the system curve, two basic elements are required: static head and friction loss. Once the system curve is known, it is easier to determine the best pump for that application.

Fluid characteristics

The type of material in the fluid is a key consideration when selecting a wastewater pump. Fluid streams in wastewater treatment plants are characterised by their properties – such as large solids, grit, sludge, scum and flocculated materials, or a mix – as these will create different considerations that need to be addressed when selecting a pump for the intended service.

Wastewater pumps are generally required to handle solids such as personal wipes, towels, and cleaning cloths, which have recently become more prevalent in wastewater flows.

Selecting a pump that is capable of dealing with these solids is important, as these materials can bind together to create a large mass that can lead to clogging issues in the pump and associated piping.

There are rotodynamic pumps on the market that are specifically designed to deal with solids and minimise clogging are referred to as solids-handling pumps.

Additional pump considerations include accounting for sludge, which can contain a significant volume of entrained gases, such as hydrogen sulfide, due to organic bacterial action.

The pumps should address these characteristics and minimise the potential for clogging and gas binding. Pumps also need to be able to handle flocculated materials.

Pumps that transfer flocculated materials need to keep the material in the flocculated state and not shear it. Fouling, corrosion, and erosion of pumps and pipes over time can be attributed to biological, chemical, and abrasive factors, so understanding fluid properties can be critical to avoid failure or the need for continuous and costly maintenance.

Altitude

When considering altitude, it is important to know that the site evaluation for the pump installation can affect its operation as, generally, the higher the elevation of the installation, the less suction lift there is available for the pump.

For pumping systems with atmospheric suction pressure, the net positive suction head available (NPSHa) calculation should be checked to include the actual atmospheric pressure at the application site.

Altitude will also affect the selection of the pump driver and, when applicable, the variable frequency drive (VFD) because higher altitudes will result in the air providing less cooling, therefore potentially requiring the driver and VFD to be derated.

Temperature

Heat can significantly impact a multitude of motor components, create deterioration and negatively impact pump performance. Consequently, temperature is a critical consideration when selecting a wastewater pump.

Many pump companies offer pumps with high-temperature winding options to address the maximum water temperature that can be pumped and the duration.

If high-temperature liquids are not properly accounted for when selecting a pump system, the pump may cavitate, which can reduce performance, and cause physical damage to the pump components and increased vibration.

Flow rate

A pump needs to be able to generate fluid flow, the output of which is represented in litres per minute (L/min). Flow rates are influenced by distance to the access points, pipework and elevation. ‘Maximum’ and ‘rated’ flow rates are helpful to understanding whether a pump is adequate for the function it needs to perform.

• Maximum flow rate: The maximum flow represents the number of litres that a pump can pressure immediately from itself without any need to travel up and through pipework

• Rated and normal flow rate: Rated flow is the operating condition that the pump is designed for. Another commonly used term is ‘normal flow’, which represents the conditions the pump is expected to operate at most of the time.

Obviously, the pump flow rate must be greater than, or equal to, the maximum expected effluent flow. In wastewater piping and pumping systems in particular, the flow rate needs to exceed certain limits in order to ensure trouble- free operation and avoid solid settling and sedimentation.

Conversely, the flow rate should also not exceed certain limits to avoid potential wear and tear from erosion and abrasion. It’s a good idea to consult the pump manufacturer if you are concerned about the operational flow rates expected, to avoid ending up with a pump that doesn’t perform its required function.

Pump manufacturers often provide a line graph displaying the expected maximum flow rates. Taking this into consideration when selecting a wastewater pump often requires some understanding of where your pump will be located and the network of pipes it will be attached to.

Rotative-speed limitations

Rotational speed can be quantified as the number of revolutions a rotating system makes within a defined period of time. Current wastewater pump design technology allows reliable operation of pumps with values of suction specific speed (Nss) through approximately 250 for metric units, depending on eye peripheral velocity, materials of construction, range of operation, pumped liquid properties, and other factors.

Higher Nss values result in pumps designed with lower NPSH requirements at the same or higher operating speeds. The maximum speed for a pump (n) due to NPSHa can be calculated from the Nss formula by expressing the rotative speed as a function of NPSHa, pump rate of flow (Q), and Nss.

The selection of pump rotational speed is closely related to the characteristics of the pump hydraulic system (circumferential speed, impeller, specific speed), as the overall strength and economic efficiency of the pump and drive system need to be taken into account.

Lifecycle costs

When investing in a pumping system, focusing on the initial capital investment gives an inaccurate depiction of the true costs of the pump over its life. One way to avoid this mistake and get a more realistic picture of the true cost is to use a Lifecycle Cost (LCC) calculation.

The purchase price for pumps is typically less than ten per cent of the total lifecycle cost, so an LCC calculation assists in gaining a better understanding of the total cost over the pump’s life.

The Hydraulic Institute has defined the following LCC formula for pumping systems, which has become the industry standard:

Cic = Initial costs

Cin = Installation and commissioning costs

Ce = Energy costs

Co = Operational costs

Cm = Maintenance and repair costs

Cs = Downtime costs

Cenv = Environmental costs

Cd = Decommissioning costs

For small stations pumping raw sewage, maintenance can be a major component in the total LCC calculation. This is especially true if the pump is poorly matched to the pump system’s maintenance requirements.

Find Submersible Pumps Related Companies In The Pump Industry Capability Guide



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