Beware the pitfalls of a centrifugal booster pump
CDC Technical Paper

If you are one of those lucky mines that can use a centrifugal pump to boost your water pressure, a pump developed recently by CDC is approximately two-thirds of the price of the competitor’s and more robust.

The conventional centrifugal pump currently offered to coal mines is not only rather costly but uses a bent axis hydraulic motor, which is susceptible to wear from dirty oil. The rig developed by CDC, in addition to being considerably less expensive, is much more accommodating to dirty oil supply.

However, before you go ahead and invest in a centrifugal pump, be aware of the pitfalls.

A major requirement in good dust suppression is an adequate supply of water of 120 lit/min at a nozzle pressure of 20 bar. Whilst machine cut-outs are set at 80 lit/min and 15 bar, the best results are achieved with the higher water flows and the higher pressures.

Taking into account that even with good piping and clean filters, the pressure drop in the system will be at least 7 to 8 bar, rising to as much as 15 bar during normal operation (as filters start to clog) it can be seen that a pump delivering at least 35 to 40 bar at 120 lit/min is an essential part of the spray fan system.

Traditionally, booster pumps have been positive displacement piston pumps with the Myers D35 being the most popular unit in the market, which can be set to deliver the volume required at a steady 40 bar regardless of operating conditions, since the pressure is regulated by the relief valve and is to a large extent independent of hydraulic pump and motor performance.

In order to keep costs down, there has been a move towards the installation of a centrifugal pump to meet the requirements. On the face of it, this has some positive attraction in that generally the centrifugal pump is cheaper to purchase and cheaper to maintain. However, centrifugal pumps need to be applied with caution and with an understanding of their weaknesses.

The usual rationale behind the selection of a centrifugal pump is, we have 12 bar available at the machine together with a 12 bar boost from the pump, giving us 24 bar, which is sufficient to drive the nozzles in the spray fan system.

However, it often turns out that the 12 bar at the machine is in fact a static pressure which soon drops to 3 bar or less when the water flow rises to 120 lit/min. furthermore, 12 bar is usually the maximum static head that the pump can deliver and this falls to 10 bar or less at the full flow rate.

So, taking into account the pressure loss through the system and the filters, the pressure at the spray block can drop to less than 5 bar, which is totally inadequate for effective operation.

The correct way to establish whether a centrifugal pump will meet the requirements is first of all to plot the supply pressure volume curve – CDC can quickly do this for you with its combination flow meter pressure gauge and ball valve rig, which fits to the end of the supply hose. Then the centrifugal pump curve is added to establish the actual supply pressure volume curve. Superimposed on this is the system resistance curve. This is a y=x2 ((squared)) curve with a fixed point at 120 lit/min flow of 40 bar.

The operational point is where the system resistance curve crosses the supply curve.

Our experience has shown that unless the water supply to the machine can deliver 120 lit/min at a pressure of at least 12 to 15 bar, an eight-stage centrifugal pump will be insufficient to meet the requirements.

In addition it must be remembered that a centrifugal pump is very sensitive to speed. A running speed of 3000 rpm is required to achieve a flow of 120 lit/min at a pressure of 10 bar, so a drop in speed from 3000 to 2500 rpm will mean a drop in pressure to 7 bar or below.

Also, unlike a positive displacement pump, which will continue to produce the water flow at the required pressure right through its life, in the case of the centrifugal pump the hydraulic motor and pump start to wear, with efficiency falling off fairly rapidly and manifesting in a sharp decline in output pressure.

As there is no relief valve in the centrifugal pump system, it is not possible to adjust for any decline in performance by readjusting the RV.

CDC customer support engineers can survey your water supply parameters and offer you advice on the best system to fit to ensure that your dust suppression systems meet the DME’s requirements.