Tutorial, troubleshooting the positive displacement rotary pump


TUTORIAL, TROUBLESHOOTING THE POSITIVE DISPLACEMENT ROTARY PUMP PT016

No liquid discharge.

  • The pump is not primed. Prime it from the outlet side by keeping the outlet air vent open until liquid comes out the vent.
  • The rotating unit is turning in the wrong direction.
  • Valves are closed or there is an obstruction in the inlet or outlet line. Check that the flange gaskets have their center cut out.
  • The end of the inlet pipe is not submerged. You can either increase the length of the inlet pipe into the liquid level or raise the level in the tank.
  • The foot valve is stuck.
  • A strainer or filter is clogged.
  • The net inlet pressure is too low.
  • A bypass valve is open.
  • There is an air leak some where in the inlet line. Air can come in through gaskets or valves above the fluid line.
  • The stuffing box is under negative pressure. Packing is allowing air to get into the system. You should convert the packing to a mechanical seal
  • The pump is worn. The critical clearances have increased. Something is broken. Check the shaft, coupling, internal parts, etc.
  • There is no power to the pump.

The pump is putting out a low capacity.

  • The pump’s internal clearances have increased. It is time to change some parts.
  • The net inlet pressure is too low; the pump is cavitating.
  • A strainer or filter is partially clogged.
  • The speed is too low. Check the voltage.
  • The tank vent is partially frozen shut.
  • A bypass line is partially open.
  • A relief valve is stuck partially open.
  • The inlet piping is damaged. Something ran over it
  • A corrosion resistant liner has collapsed in the inlet piping.
  • Air is leaking through the packing. You should go to a mechanical seal.

The pump looses its prime after it has been running for a while.

  • The liquid supply is exhausted. Check the tank level; sometimes the float is stuck, giving an incorrect level reading.
  • The liquid velocity has increased dramatically.
  • The liquid is vaporizing at the pump inlet because a bypass line is heating the incoming fluid.
  • An air leak has developed in the suction piping.

The pump is using too much power

  • The speed is too high.
  • The liquid viscosity is higher than expected.
  • The discharge pressure is higher than calculated
  • The packing has been over tightened. You should convert to a mechanical seal.
  • A rotating element is binding. Misalignment could be the problem or something is stuck in a close clearance and binding the rotating element.

Excessive noise and vibration.

  • Relief valve chatter.
  • Foundation or anchor bolts have come loose.
  • The pump and driver are misaligned.
  • The piping is not supported properly.
  • The liquid viscosity is too high. The pump is starving. Check the temperature of the incoming liquid to see if the supply tank heater has failed.

Excessive noise or a loss of capacity is frequently caused by cavitation. Here is how the NPSH required was determined initially:

With the pump operating at a 0 psig inlet pressure and a constant differential pressure, temperature, speed and viscosity; a valve in the inlet line is gradually closed until cavitation noise is clearly audible, there is a sudden drop off in capacity, or there is a 5% overall reduction in output flow. Cavitation occurs with:

  • A loss of suction pressure.
  • An increase in fluid velocity.
  • An increase in inlet temperature.

Here are some common causes of vibration problems:

  • A foot valve or any valve in the suction piping is sticking.
  • Something is occasionally plugging up the suction piping. If the pump suction is coming from a river, pond or the ocean, grass is a strong possibility.
    • A loose rag is another common cause.
  • A collapsed pipe liner.
  • A filter or strainer is gradually clogging up.
  • The tank vent partially freezes in cold weather.
  • The sun is heating the suction piping, raising the product temperature close to its vapor point.
  • The level in the open suction tank decreases causing vortex problems that allow air into the pump suction.
  • Several pumps in the same sump are running, decreasing the level too much.
  • The storage tank floa levelt is stuck. It will sometimes show a higher level than you really have.
  • discharge recirculation line, piped to the pump suction, opens and heats the incoming liquid.
  • Sometimes the suction lift is too high. The increase in pipe friction will reduce the suction head.
  • The vapor pressure of the product is very close to atmospheric pressure. The pump cavitates every time it rains because of a drop in atmospheric pressure.
  • The tank is being heated to de-aerate the fluid. Sometimes it is being heated too much.
  • The process fluid specific gravity is changing. This can happen with a change in product operating temperature or if a cleaner or solvent is being flushed through the lines.
  • The source tank is changing from a positive pressure to a vacuum due to the process.
  • A packed valve in the suction piping is at a negative pressure and air is leaking in through the packing.
  • The tank is being pumped dry.
  • The inlet piping has been moved or altered in some way. Has a foot valve, strainer, elbow, or some other type of hardware been installed in the suction piping?
  • Has a layer of hard water calcium or some other type of solid formed on the inside of the suction piping reducing its inside diameter over some period of time?

You are experiencing rapid pump wear.

  • There are abrasives in the liquid you are pumping causing erosion problems. You may have to go to a larger pump running at a slower speed.
  • There is some corrosion in one or more of the pump elements.
  • There is a lack of lubrication.
  • You have a severe pipe strain problem. It could have been caused by thermal growth of the hardware.
  • Too much misalignment.
  • The pump is running dry.

Posted

  • On February 18, 2018