Means of detecting Blockages in Centrifugal Slurry Pumps

Pump Blockages

Experience shows that slurry compacts in the slurry pump casing and impeller and totally clogs the intake and discharge branches. The motor torque is sufficient to keep the slurry pump impeller turning and shearing the slurry even though the slurry is largely compacted solids. In the worst case, the increase of temperature and pressure will not be measured at the intake or discharge pump flange connections. Instrument lines can block with slurry. Slurries can also be corrosive making direct fluid measurements difficult.

Zero flow can occur with any centrifugal water or slurry pump given the right set of circumstances

Visual

No obvious signs apart from the possibility of some steam escaping at the pump gland. Slurry pump casing maybe ‘hot’ to the touch.

Poor

Pressure

Blockage may prevent transmission of pressure to the pressure instrument. There are no direct or practical methods to measure the pressure inside the slurry pump. Burst discs in the pipeline, even though close to the slurry pump, may not activate.

Poor

Flow (or pipe velocity)

Slurry systems typically do not always incorporate means to measure flow. Low or zero flow measurement could be utilised as an alarm or to shut-down the slurry pump. Not useful during starting and stopping. Slurry Pumps should typically not be operated continuously below their safe minimum allowable thermal flow, which normally is 15 to 25% of the pumps Best Efficiency Flow at a particular speed.

Poor to Good. Stop pump as soon as possible or within first 10 to 15 minutes.

Temperature

Useful on metal lined slurry pumps to monitor temperature on outside of metal liners. Not directly applicable to elastomer lined slurry pumps due to poor thermal conductivity of elastomer. Can be utilised as a thermal cut-out device to stop the slurry pump at a preset temperature.

Good for metal lined pumps.

Motor Current

Tests have shown that motor amps are not a good indicator of low or zero flow. Typically one third to one half of the motor input power goes into heating the fluid inside the pump at zero flow condition.

Poor

Density

Density can vary widely in operation.

Poor

Pump starting and stopping

Flow can be ‘low’ for a short time depending on the starting method. Water hammer can occur during operation or shutdown in particular due to rapid valve closure. It is desirable to have both a local and a remote motor OFF control in case of emergency and to help ensure the safety of personnel.

Poor. Normally transient conditions.

Burst Discs (or pressure relief valves)

If installed in the pipe, may not by activated by high internal pump pressure, may not provide a high level of reliability. Water type pressure relief valves/ discs are not reliable for use with slurry due to wear or valve clogging and
corrosion.

Poor

Flow bypass pipeline

To prevent temperature rise at low flow, it is possible to bypass some of the liquid back to the source if the fluid is water. However, this strategy is normally avoided with slurry, as the low flow will cause blockages and consequently reduce the cooling effect of the re-circulated fluid.

Poor