Pumps in a biogas plant: positive displacement, centrifugal and chopper — how to choose
Pumping slurry, digester slurry and digestate is a job for several types of pump. We compare features, applications, wear and typical failures.
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In a biogas plant, pumps transfer slurry, slurry between digesters, digestate and process liquids. For thick, fibrous media positive displacement pumps (rotary lobe, progressive cavity) are usually used, and for thinner media and large flows — centrifugal pumps with a chopper; the choice depends on dry matter, fibre and sand content, pressure and the required dosing accuracy. More important than the type is redundancy and protection against foreign bodies.
Pump types and applications
| Type | Application | Strengths | Weaknesses |
|---|---|---|---|
| Rotary lobe | transport of slurry, dosing of liquid substrates | high capacity, reversible direction, shredded fibres | wear of lobes and casing by sand, sensitivity to foreign bodies |
| Progressive cavity (eccentric screw) | thick slurries, high pressure, dosing | accurate dosing, high pressure | wear of rotor and stator, dry running destroys |
| Centrifugal with chopper | slurries with low dry matter, transfer pumping | high capacity, simple construction | drop in efficiency with thick media, wear from sand |
| Submersible | pre-tanks, pits | simple installation | limited choice, servicing requires removal |
| Diaphragm, piston | special applications | high pressure | higher cost and complexity |
Selection factors
- Dry matter — slurry 6–10% TS, slurry in the digester roughly 6–12% depending on the plant; higher values require positive displacement pumps.
- Fibres — straw, grasses, manure require choppers or cutting pumps.
- Foreign bodies and sand — stones, metals, sand destroy impellers; use stone separators, magnets, desludging.
- Required pressure and flow — lift height, length of pipelines.
- Dosing accuracy — progressive cavity and rotary lobe pumps have a linear relationship between flow and speed.
- Temperature and aggressiveness — corrosion-resistant materials (H₂S, acids).
- Availability of parts and servicing.
Selection example
The task: pumping 40 m³/h of slurry with TS of approx. 9% over a distance of 80 m with a height difference of 5 m. A rotary lobe pump of suitable capacity with a variable-speed drive will provide a stable flow and the possibility of reversing direction in the event of clogging. For the same task at TS 4% a centrifugal pump with a chopper would be cheaper to operate. The selection must be verified by calculating resistances (pipes, bends, valves) and the pump curve.
Energy use
Pumps use energy in proportion to flow and resistances. It is limited by: the right pipeline diameter (ones that are too small increase resistances), shortening routes, speed control (inverters) and avoiding operation on a throttling valve.
Typical failures
| Failure | Cause | Prevention |
|---|---|---|
| Clogging | fibres, foreign bodies | chopper, separator, reversing direction |
| Wear of rotor/stator | sand, abrasion | sand removal, right material, inspections |
| Dry running | no medium | level sensors, protections |
| Seal leakage | wear, wrong selection | replacement schedule, reserve |
| Cavitation | too great a suction height | suction arrangement, diameters |
| Overheating | pumping against a closed valve | temperature protections |
Good practices
- Install a standby pump on critical lines (feeding, recirculation).
- Provide isolating valves, drains and the possibility of flushing.
- Keep an operating hours counter and an inspection schedule.
- Monitor motor current as an indicator of wear or clogging.
- On pipelines in explosion hazard zones use equipment with the appropriate marking.
- Keep a stock of the parts that wear out fastest.
Sensible pump selection and consistent servicing reduce the number of stoppages, which in a biogas plant mean lost revenue and disturbances of the process.
Choppers and separation of foreign bodies
A chopper upstream of the pump reduces the risk of clogging, but uses energy and its knives are themselves subject to wear. Upstream of positive displacement pumps, stone separators, magnets for metals and a screen or basket catching large items are recommended. The costs of cleaning them are small compared with repairing a pump or stopping the feeding.
Inspections and spare parts
Set a schedule: inspection of the condition of seals every few months, measurement of clearances and pressures every half year, replacement of working parts according to operating hours. Keep in stock a complete set of seals, one set of working parts for critical pumps and the contact details of the service provider. The failure of a dosing pump stops the whole plant, so a reserve is cheaper than downtime.
Control and inverters
The use of inverters makes it possible to match the flow to needs without throttling and reduces starting currents. In positive displacement pumps speed control is the basis of dosing, and reading the current and the discharge pressure is a simple diagnostic tool: a gradual rise in current at the same flow usually means wear of working parts or growing deposits.
Sources
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