Hidrostal Axial Diffuser Pumps

Axial Diffuser Pumps is Hidrostal's name for its pumps with an axial flow direction, and it is a group rather than a single machine. Hidrostal supplies it in two versions, the Axial Column Pump and the Vertical Turbine Pump, and both come in a wide range of sizes. If you have arrived here from a search, the version you want is almost certainly decided by head, not by flow.

Axial Column or Vertical Turbine

The turbine figures are Bedford Pumps' own, for the DA and DB range. That is the brochure Hidrostal publishes on its Vertical Turbine Pumps page, and the page links straight to Bedford. Worth knowing before you send the enquiry, because it tells you where the engineering answer will come from.

The numbers are an envelope, not a duty point

Hidrostal quotes the Axial Column Pump as 6 to 1500 l/s and up to 20 m. Both figures are true, and they are not true at the same time. This is the part that costs people money. The performance range chart in the brochure shows the top of the envelope reaching 20 m over the middle of the flow range, and by the right-hand end of the chart the available head has fallen to single figures. Ask for 1500 l/s and 20 m together and you are asking for a corner of the chart that has no pump in it.

So take the head first. If the total dynamic head is a few metres and the flow is large, you are on the Axial Column Pump and the full column range is where to look. If the head runs to tens of metres and the water sits well below floor level, you are on the Vertical Turbine Pump, and the flow figure will rarely be what decides it.

Where these fit, and where they do not

Hidrostal names land drainage, rainwater and flood pumping stations, watering and drainage systems, coastal and flood protection, water regulation, return activated sludge and oily water for the Axial Column Pump. Bedford names water abstraction and supply, flood alleviation, surface and storm water, dock dewatering and impounding, cooling water circulation and irrigation for the turbine range.

Neither drops into a wet well. The column version wants a tube chamber built to it, down to the counter flange it seats and seals against, and the turbine version wants a well or suction chamber with the inlet conditions right, which is why Bedford offers bellmouth and draft tube options at all. On a drainage station that is where most of the trouble starts. The mistake we see most often is not the pump at all. It is a sump geometry that was never checked against what the pump needs, or a flow figure that has collected a safety margin at every stage of the design and arrived far larger than the site will ever deliver. Send us the sump drawing with the flow figure and we will check the duty before anyone sizes anything.

An axial diffuser pump discharges along the axis of its own shaft instead of sideways through a volute. On the Axial Column Pump, Hidrostal replaces the volute of the low pressure version with a casing that discharges axially, which is what lets the whole unit sit inside a tube or concrete chamber rather than in a wet well. Water enters through a fixed suction cover, or an adjustable wear cone where the medium carries grit, and is driven up the column.

The screw centrifugal impeller

The impeller on the column version is Hidrostal's screw centrifugal impeller, the same hydraulic it uses across its range. Hidrostal's own claims for it are a large free ball passage, low-clogging and clog-free pumping, gentle delivery through low shear forces, low pulsation, a stable and steep pump curve, and flow proportional to speed. That combination is why the same pump is specified for return activated sludge, for oily water where an emulsion has to be avoided, and for passing live fish.

Hidrostal puts the hydraulic efficiency at up to over 82%, which with its own submersible motor gives the low energy consumption a drainage station is judged on over twenty years rather than on the day it is commissioned.

Sealing, motor and monitoring

The Axial Column Pump shaft is sealed by a double mechanical seal in a tandem arrangement with barrier fluid. The drive is a Hidrostal submersible motor cooled by the pumped liquid, from 1.5 kW to 250 kW at 375 to 1500 rpm, on 50 Hz, 60 Hz or through a frequency converter. Built-in circuit breakers monitor the winding temperature. On request Hidrostal adds a conductivity probe and float switch for mechanical seal leakage, bearing temperature sensors at both bearing locations, and screened cables where electromagnetic radiation is a concern.

Take the leakage detection. A mechanical seal is the weak point in almost any pump, and on a machine that sits at the bottom of a tube chamber you will not see a seal weeping until the motor has drunk it.

The turbine version

Bedford builds the Vertical Turbine Pump as a suspended bowl pump, single or multistage. The bowl assemblies carry integral diffuser options, and the impellers can be radial, mixed or axial flow, fully shrouded or semi-open. Multistage construction is what gives the higher efficiencies on high head and lower flow. Inlet conditions are handled with bellmouth and draft tube options, shafts are designed as stiff shaft construction operating below the first critical speed, and sealing runs from a soft packed gland through a controlled leakage device to split, single or double mechanical seals.

Hazardous areas

Both routes offer explosion protection as an option rather than as a property of the range. Hidrostal states its submersible motors for the Axial Column Pump are available optionally for applications in potentially explosive atmospheres; Bedford states ATEX compliant options are available for hazardous area applications such as storm water pumping. Certification covers a specific build, so send us the zone classification for the station and we will get the certificate for the exact pump and motor before anything is ordered.