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Frequently Asked Questions : Water Pumps

Please click on the corresponding question below.


What is an open centrifugal pump?

Centrifugal water pumps use centrifugal force to pressurize and move water from the inlet to the outlet. A rotating set of vanes (called an 'impeller') is spun by the pump shaft. As water is forced through the impeller, rotational energy is transferred from the impeller to the water, which gains velocity and pressure through the centrifugal force applied and is flung from the impeller. The volute (a spiral-shaped case) funnels the now-pressurized water to the outlet. Image below shows clockwise rotation as viewed from the motor mound.

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Pressure and Flow, and what causes water pressure.

Pressure and flow are inversely related–that is, if pressure is increased, then flow decreases, and vice versa. "Pressure" is the amount of force per area, and "flow" is the volume of material moving through a given area per second (not the same as velocity, which is the distance traveled per time).

Water pressure is caused by resistance to flow. If a force tries to move water and there is resistance to that movement, then the water becomes pressurized. Consider a standard garden hose; when the end of the hose is unrestricted, water flows out of the hose in large quantities, but without much force. If you press your thumb over the opening, however, the stream of water that shoots out from around your thumb will be much more forceful and will travel further, but there will also be less water per second leaving the hose.

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What happens when an open centrifugal pump is "Dead-headed"?

A centrifugal pump is 'dead-headed' when it is operated without an open discharge outlet. Contrary to what one might assume, centrifugal pumps cannot be stalled if water flow is cut off. Very little horsepower input is required in order to spin the impeller with no water flow, so if the pump is driven by a hydraulic motor, the input hydraulic pressure will be low and the pump will not stall, instead continuing to spin the same volume of water as it would during normal operation. With no outlet, the rotational energy of the shaft and impeller is converted into heat; eventually, the water will boil inside the volute, causing severe damage over time.


When should I replace gaskets?

Any time that the water pump is serviced and the gaskets are removed, replace the gaskets. Gaskets are inexpensive, and a faulty gasket will cause the pump to leak.

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How do I adjust the packing, and when should I adjust it?

Different types of packing are designed to leak at certain rates. If the packing begins to leak at a greater rate than the allowed amount, tighten the packing gland with a simple end wrench until the leakage is reduced to lie within the packing parameters. Additionally, the stuffing box temperature should be checked regularly. If the temperature becomes too high, loosen the packing gland until the temperature drops to a consistently safe level.

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When should I replace the packing?

Over time as the packing is subjected to continual heat and pressure through normal use, lubricant will be forced out of the packing material. The life of the packing material can be extended by periodically tightening the packing gland whenever there is packing leakage greater than the specified amount (Mega's rope packing seals are designed to leak approximately 60 drops per minute). Eventually, however, all of the lubricant will have been squeezed out of the packing material, and tightening the packing gland will no longer prevent leakage. When this happens, it is time to replace the packing.


What causes pump cavitation?

When the inlet pressure of a water pump falls below pump design specifications, tiny vapor bubbles can form in the water around the eye of the impeller. When the water containing these bubbles is forced into a high pressure environment on the other side of the impeller, these bubbles collapse, thereby creating tiny shock waves and points of high temperature. These shock waves can actually corrode the surface of the impeller. To prevent cavitation, always be sure to operate your water pump within its pressure and flow specifications.

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Is it bad to run the pump dry?

Absolutely. The water pump shaft and impeller are spinning at extremely fast rates. With no water to transfer their rotational energy to, that energy is released as heat instead. If the pump is run dry, its moving parts will become extremely hot, causing severe damage to the pump over time and greatly limiting its service life.

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Will this pump pull water up vertically?

Mega Corp. offers one self-priming centrifugal pump for suction lifting applications: the B4-Z. This pump can lift water a vertical distance of 7-10 feet.

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Why is my spray performance low?

Several factors can affect water pump performance. If low water output is noticed, one of the first things to check is the water pump shaft rpm to ensure it is spinning within specifications. Also check that the water inlet of the pump is not obstructed. Also check to see if the impeller is damaged or if the shaft bearings are worn or damaged. Consult your Mega Field Service Manual or call Mega Corp. Product Support for complete details.

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Why isn't my pump pumping?

There could be several factors behind this:

  • Is there enough water? Check to ensure that there is enough water present in the tank to allow safe pumping; the Mega Digital Spray Control System shuts the water pump once a sufficiently low water level has been reached.
  • Beware of air locks. If there is too little water in the tank, the pump may have drawn in a large air pocket that is preventing the flow of water (see What is an air lock? question below).
  • Is the lift displacement too great? If you are operating a suction lift pump, be sure that the water reservoir you are drawing from is no more than 7 feet below the pump, otherwise the resistance to flow may be too great.
  • Are you using pipes that are too small or have too many elbows? Pumping water through small diameter pipes (and pipes with elbows and check valves) increases the water pressure, but as a result also increases flow resistance. If the resistance is too great, your pump will not perform.

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What is an air lock?

An air lock occurs when large pockets of air are drawn into the inlet side of the water pump. This essentially acts as a void, and can prevent the pump from drawing in water. Not only will this decrease (or even stop) pump performance, it can be very damaging as well. If the air pocket is drawn through the impeller and into an area of high water pressure, the bubbles will collapse and create large shock waves that will severely damage the pump.

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Why did the pump shaft break?

Water pump shaft failures can be caused by several factors:

  • Sudden start or stop of the pump.
  • Pump rpm should be ramped up and down over a minimum of 2 seconds.
  • Engaging a water pump suddenly can twist the impeller off the shaft and damage shaft bearings.
  • Water hammer caused by sudden shut off of water flow can cause the impeller to break the pump shaft and damage shaft bearings over time.
  • An unbalanced or damaged impeller can cause shaft failure and bearing damage over time.

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Does Mega Corp. offer a corrosion-resistant water pump?

Yes, we do! Our M-4B pump (part number 306201) is available in a corrosion-resistant form. In this version of the M-4B, all parts that come into contact with water are stainless steel.

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Does Mega Corp. offer a water pump with a mechanical shaft seal?

Yes! The B-4J water pump is available with a mechanical seal (part numbers 301908 and 304742).

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How do I identify what water pump I have?

Look for a metal identification tag on the frame of the pump. Compare your pump with the photos in this catalog or on our website: www.megacorpinc.com. Send us a photo of the water pump and request identification at 1-800-345-8889 and ask for either Product Support or Parts Sales.

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Which direction should my pump be spinning?

Water pumps rotate either clockwise or counterclockwise as viewed from the drive shaft end of the pump. To determine rotation direction position the pump with the shaft end facing yourself. If the outlet of the pump is facing 90 degrees to the right, straight down on the right side of the pump centerline, or straight up on the left side of the pump centerline, then it is a clockwise rotation pump.

If the outlet of the pump is facing 90 degrees to the left, straight down on the left of the pump centerline, or straight up on the right side of the pump centerline, it is a counterclockwise pump.

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Can Mega water pumps be damaged by freezing?

Yes. If a pump is allowed to freeze with water in it, cracking of the volute case as well as damage to the shaft and bearing set may occur. Thoroughly drain the water system of your pump and tank in freezing weather. If the pump has been frozen, inspect it for damage before returning it to service.

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How often should the water pump bearings be greased?

Water pump bearings should be greased with about 2 oz of grease every 250 hours of service. Most water pumps should only be greased by hand and it should be noted that over greasing can damage bearings as well as no grease. High pressure grease guns can blow out grease seals in the pump causing grease loss and bearing failure. Mega M-4B water pumps, (306200 and 306201) can be greased with an auto greaser or looped into a prime mover's auto greasing circuit. Refer to Mega maintenance manuals for complete details.

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The water pump shaft is spinning, but there is low/no water pressure. What is wrong?

  1. Ensure the shaft is spinning in the correct direction. The pump will flow a little bit of water when run backwards. Refer to how to determine whether your pump is clockwise or counterclockwise. All Mega hydraulically driven pumps are clockwise. Only some shaft driven water pumps are counterclockwise.
  2. Check to see if the impeller is still connected to the shaft.

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My pump is noisy. What is wrong?

While most water pumps are fairly quiet during operation, hydraulic drive motors can be quite noisy. Ensure that the noise you hear is in fact coming from the water pump. If the pump is making a growling, grinding, squealing, or popping noise, it should be immediately disassembled and inspected for damage.

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How much water pressure should my pump develop?

Find and refer to the water pump performance curve chart for the water pump you have, available in this manual and on our website (or by calling Mega Product Support). Water pressure should be checked with water in the tank and water pump, and with no spray heads open while the tractor engine's at high idle. To obtain acceptable spray performance, a minimum of 85 psi water pressure under static conditions (no water flowing) is required. A static water pressure exceeding 125 psi can cause damage to water lines, connections, and spray heads.

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What type of materials are Mega water pumps constructed?

Mega water pump are constructed of cast plain carbon steel and grade 316 stainless steels.

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