SUBJECT: Some common misconceptions about centrifugal pumps. 11-2

• If you're going to use mechanical seals, large diameter stuffing boxes are good, and the tapered version is the best.

  ans. Tapered stuffing boxes direct solids to the narrow end of the stuffing box, where the lapped seal faces are located. This will damage the lapped faces. In hot condensate applications, the increasing velocity can cause cavitation problems. One major pump manufacturer actually put "speed bumps" into the stuffing box to slow down the rotation of the fluid.

• It makes sense to vent a running pump.

  ans. You cannot vent a running pump because centrifugal force throws the heavier liquid out the vent leaving the lighter air inside.

• If the ball bearings are getting too hot you should cool down the bearing case.

  ans. When you cool the outside diameter of a bearing, the metal shrinks, increasing the load and adding more heat to the bearing. People keep trying anyway. Water hoses on the bearing case are popular.

• If you're using packing in the pump, and there are solids in the fluid, flushing packing with a clean, compatible liquid is sensible.

  ans. The higher pressure flushing liquid will always follow the path of least resistance, and that path is to atmosphere, not the inside of the pressurized pump.

• A centrifugal pump cannot pump air.

  ans. Sure it can, but it cannot pump the air high enough to overcome atmospheric pressure. That's the reason you have to prime a centrifugal pump. Centrifugal pumps pump everything to the same height, only the power required varies.

• If you purchase a magnetic drive pump you can eliminate the need for mechanical seals.

  ans. It some instances it is true, but if it were true in every case we would use them 100% of the time. These pumps have several limitations and the most important one is that because of the close clearances, and non precision bearings, you're limited to pumping clean liquids that will lubricate the bearings. If you ever pump the tank dry, you'll probably trash the pump.

• The repeller pump is another way to eliminate the mechanical seal and all of its limitations.

  ans. Repeller pumps are notorious for pulling air into the system. In some applications this can cause a foaming problem with the product. The design usually has some type of a seal that opens when the pump is running and closes when the pump stops. Unfortunately the seal faces frequently close on a dirty fluid that will cause the pump to leak when it's stopped.

• It is alright to use packing in a condensate pump. You don't need an expensive mechanical seal for warm or hot water.

  ans. There never has been a packing that will seal a vacuum, and that's the problem with condensate pumps. They normally run with a negative pressure on the suction that the manufacturer tries to eliminate with a discharge recirculation line that still allows air to get in, and lets some of the valuable condensate leak out.

• Shop water is a sensible choice for the flushing of pump stuffing boxes.

  ans. Shop water should never be used to flush a stuffing box unless there's an air gap in the system. The fluctuating pressure in the stuffing box of a pump could back up through the flushing lines and contaminate the shop water system..

• Shop water is a good choice to circulate coolant through the cooling jacket on a high temperature pump.

  ans. There is too much calcium and other minerals in this type of water. Condensate or low pressure steam would be a much better choice.

• You can use the same shaft/sleeve arrangement when you convert a packed pump to a mechanical seal.

  ans. Mechanical seals should be placed on solid, not sleeved shafts. In most sleeve combinations the number is much too high. Remember that the packing was acting as part of the pumps shaft support system. You need the solid shaft for its added stiffness. Be sure to use a mechanical seal that will not frett the shaft. Pump shafts should not be a spare part. If they 're built from corrosion resistant materials they should last a very long time.

• Discharge recirculation is a good alternative to avoid excessive throttling the discharge side of a centrifugal pump.

  ans. Not if you're going to recirculate to the suction side of the pump. It'll raise the inlet fluid temperature, and increase the chances of cavitation. Discharge recirculation will also decrease the efficiency of your pump, and you did consider the efficiency when you it.

• ANSI and DIN pump standards are a good guide for the purchase of a centrifugal pump.

  ans. If you're going to use conventional packing in the pump you really have no choice, but if you want to use a mechanical seal, the standards call for an impeller location that is too far away from the bearings. The overall length of the pumps is too long. A shorter pump should cost less money and be a lot more reliable,

• A high efficiency pump is desirable.

  ans. Maybe it's desirable, but it'll be a maintenance nightmare. High efficiency means tight tolerances and smooth passages that will not interfere with fluid flow. You'll spend a lot of down time and money trying to maintain those two requirements.

• Starting a centrifugal pump with the discharge valve shut is desirable because it'll save power.

  ans. Maybe it will save power, but it will also put an excessive radial thrust on the impeller that could fail the mechanical seal or fracyure the shaft.

• Because of the time involved, you can justify not doing an alignment between the centrifugal pump and its motor.

  ans. If you use a "C" or "D" frame adapter, there is no excessive time involved. The additional cost of one of these adapters is not significant enough to justify skipping the alignment process.

• A variable speed motor is a sensible alternative to throttling the discharge of a pump, to regulate flow.

  ans. This is true if the head is mainly system or friction head. If the main head is static or pressure head a variable speed motor is not a wise choice.

• It doesn't make any difference if you fill a tank from the bottom or the top.

  ans. It all depends upon which type of pump you have. If it's a centrifugal pump you must fill the tank from the top to avoid changing the head as the tank fills. If you're using a positive displacement pump, filling from the bottom would make sense, because you'd be saving power.

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