SEEPEX Progressive Cavity Pumps for accurate product metering and control

Many industries need pumps to add small amounts of additives into process lines. These can include aggressive and corrosive chemicals in waste water treatment and in paper production, viscous products such as polymer for water treatment, abrasive lime slurries in sewage sludge treatment and valuable minor ingredients into beverage and food products. Although there are many different types of pumps used for metering, progressive cavity pumps offer many advantages over alternative positive displacement type pumps.

  • Linear accuracy
  • Low pulsation
  • No Vapour Locking
  • Non-return valves unnecessary
  • Easy calibration
  • Simple maintenance
  • Handles solids without blocking
  • Ideal for abrasive or corrosive products

Further advantages of the progressive cavity pump is that the flow rates are minimally affected by changes in pressure or product viscosity, meaning that a wide range of products can be metered by the same pump design.

Products are subjected to a low shear action when compared to other positive displacement pumps and solids within the product are not crushed by the action of the pump, neither do they cause blockages within the pump.

The action of pumping is the rotation of a single helical rotor inside a double internal helical stator which progresses a cavity of product through the pump; hence the name progressive cavity pump. This action produces more cavities per minute than a piston action associated with diaphragm pumps and is why pulsation is minimised. The virtual lack of pulsation means that a constant product stream is produced and a stable flow rate achieved up to +/-1% accuracy.

Because the rotor has a compression fit within the stator, both high and low viscosity products, even water, can be accurately metered, with no necessity for non-return valves. The constant dose and linear accuracy to +/- 0.5% of a progressive cavity pump means that calibration is simple. Ball valves and pulsation dampeners are not needed so vapour locking and sticking valves are eliminated.

The flow rate is directly proportional to pump speed so using a variable speed drive (VSD) means that flow rates can be varied and adjusted in real time using a closed loop feedback control system. Drive selection can offer a wide range of flow rates of a single pump with turndowns of 100:1 possible using VSDs.

Flow control valves are not needed either as the constant action provides flow control in real time without the need for sampling or averaging commonly needed on piston diaphragm pumps.

Other advantages of the progressive cavity pump are that the flow rates are minimally affected by changes in pressure or product viscosity thus a wide range of products can be metered by the same pump design.   Products are subjected to a low shear action when compared to other positive displacement pumps and solids within the product are not crushed by the action of the pump.

Progressive cavity pumps can also be installed in a variety of flexible configurations including vertically where space may be an issue.   A single pc pump can replace a system which may need flow control, pulsation dampeners and non return valves in addition to a pump so capital costs can be reduced.

One example of this is the use in SEEPEX ferric compounds in the waste water treatment industry. Iron compounds are commonly used within the waste water treatment and anaerobic digestion processes to reduce phosphate content in settled sludge, reduce hydrogen sulphide content in biogas and sometimes to augment flocculation.

There are four iron compounds used within the waste water treatment process. These are:

Ferric Chloride (FeCl3)

Ferrous Chloride (FeCl2).

Ferric Sulphate (Fe2(SO4)3)

Ferrous Sulphate (FeSO4)

Although they are different compounds, they are used for broadly the same purposes and often referred to by the generic term ‘ferric’. All ferric compounds used are very aggressive, but Ferric Chloride is the worst and quickly corrodes all but very exotic metals, so dosing pump materials need very careful selection. Many progressive cavity pumps can handle abrasive and corrosive chemicals at a range of temperatures. Experienced engineers ensure pump selection using the correct materials for reliable and repeatable dosing without corrosion of pump parts.

The compounds are added preferably at the works inlet, prior to the works primary settlement system. However it can also be added to the Return Activated Sludge (RAS) before the RAS itself is aerated or even directly into the aeration lanes. Where it is being added for H2S control in digesters it is usually added in the digester feed lines. The key is to add it at a point of greatest turbulence as it mixes better and is far more effective due to its very quick reaction time when in contact with the sewage so a pulsation free uninterrupted dose is always beneficial for the most effective reaction.

The metering rates are quite critical as if too little is added, it is ineffective and if too much is added, some remains free in the process increasing corrosion to tanks, pipes and pumps. Pulsation caused by the use of piston diaphragm or peristaltic pumps can damage pipe-work and don’t give a smooth constant dose. Due to the complex series of reactions that take place, it is difficult to calculate the correct dose making trial and error a necessary part of the tuning process. For this reason accurate control and easy calibration is vital.

Progressive cavity pumps can provide the best method of dosing for these aggressive products and for many other additives due to their inherent characteristics of accurate linear flow rates with minimal pulsation.

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