SUBJECT : Rules of thumb for pumps 2-7

If you want to know a pumps capabilities, the rules are simple. Look at the manufacturer's published pump curve. The problem is that you do not always have the curve available. Pump companies test their pump to determine its performance, they have no need for general guide lines or "rules of thumb."

Over the years I've accumulated many of these rules to help me estimate pump performance. Here are a few of them:

PUMP BASICS

• How to estimate the shut off head of a pump (inch sizes)
  • At 1750 rpm. Shut off head = Diameter of the impeller squared
  • At 3500 rpm. Shut off head = Diameter of the impeller squared x 4
  • For other speeds you can use the formula : Shut Off Head = D² x (new rpm / 1750)²
• Estimating metric head is a little bit more involved, but it still works:
  • Measure the shaft in mm. ( as an example: 250 mm )
  • Mark off two places. (2,5)
  • Square the number. (6,25)
  • For 1450 rpm, multiply by 3 (18,75)
  • Add 10 % for the answer in meters. (21 meters )
  • NOTE: For 3000 rpm, you'd multiply by 12 instead of 3. Although you can estimate shutoff head with these formulas you cannot estimate the pump capacity. you'll need the pump curves for that.
• The pumps best efficiency point (B.E.P.) is between 80% and 85% of the shut off head. At this point there is little to no radial thrust on the impeller. Also the "power in" is closest to the "power out".
• The L³/D⁴ ratio should be below 60 (2.0 in metric) to prevent excessive shaft bending. To calculate it for end suction centrifugal pumps :
  • L = length of the shaft from the center of the inboard bearing to the center of the impeller (inches or millimeters). Caution: do not use centimeters, the numbers will come out wrong.
  • D = diameter of the shaft (under the sleeve) in the stuffing box area (inches or millimeters) Do not use centimeters.
• Since most shaft materials have a similar modulus of elasticity, changing shaft materials will not prevent shaft bending when you operate off of the B.E.P. Lowering the L^3/D⁴ is the only logical and efficient solution. When pump manufacturers discuss operating off of the B.E.P. they relate problems to the heat that will build up in a minimum flow condition and ignore the problems with shaft bending.
• A double suction pump can run with 27% less N.P.S.H. or at a 40% faster speed without cavitating.
• If you double the speed of a pump you'll get twice the capacity, four times the head and it will take eight times the horsepower to do it.
• A stainless steel shaft has only a small portion of the conductivity of a carbon steel shaft. This is very important when you're pumping at elevated temperatures because we do not want to transmit the high temperature back to the bearing oil.
• If you double the speed of a pump you'll get almost four times the shaft whip, wobble or run out and eight times the wear.
• Multistage pumps reduce efficiency 2% to 4%.
• In many instances, an inducer can lower Net Positive Suction Head Required by as much as 50% .
• If you're pumping paper stock, modify the curves for head, capacity, and efficiency as follows:
  • 0.725 for 6% stock
  • 0.825 for 5.5% stock
  • 0.90 for 5%
  • 0.94 for 4.5%
  • 0.98 for 4%
  • 1.0 for 3.5% or less
• Open impeller clearance settings are determined by the pump manufacturer and normally run between 0.008" and 0.015" (0,2 to 0,5 mm) You lose 1% of the pumps capacity for each 0.002" (0,05 mm) you miss this setting.
• Wear ring clearances are very similar to impeller clearances, but you lose 1% pump capacity for each 0.001" (0,025 mm) of wear. A typical clearance would be 0.003 inch/inch diameter with 0.010 inches (0,3 mm) minimum clearance for wear rings less than two inches (50 mm.) in outside diameter.
• Bearing, grease or lip seals have a design life of less than 2000 hours. In a constantly running pump this would be only 83 days. These seals will also damage the expensive shaft and place a stress point at the maximum bending moment arm. Substitute non fretting labyrinth seals, or positive face seals in these locations. It is a good idea to install them in electric motors also to prevent moisture from entering and damaging the motor windings and bearings.
• Do not use a vent on the top of the bearing case. At shut down the outside moisture will enter the bearing housing through this vent. Let the moisture attempt to enter the case through the labyrinth seals instead, They will do a better job of directing the moisture to the external drain hole. If you install positive face seals you can forget about this problem.
• The axial clearance in a bearing is ten times the radial clearance. This is the reason proper installation is so critical. If the bearing is over compressed the bearing balls will distort and roll instead of spin causing excessive heat and premature failure. The temperature at the bearing race of a properly installed bearing is at least 10 degrees Fahrenheit (5° C) higher than the oil sump temperature.
• The life of bearing oil is directly related to its temperature. The rule of thumb used by the SKF Bearing Company, is that the service life of an oil is estimated to be 30 years at 30 degrees Centigrade (86° F) and it's life is cut in half for each 10 degree Centigrade (18 F) temperature increase. This corresponds to :
  • A life of 3 months at 100 C. (212 F.)
  • A life of 6 months at 90 C. (195 F.)
  • A life of 12 months at 80 C. ( 176 F.)

These numbers assume that the lubricating oil is not being contaminated by water from one or all of the following sources:

• Packing leakage
• The water hose used to wash the packing leakage away from the pump area.
• Aspiration, as moisture laden air enters the bearing case.

An automobile engine running at 1750 rpm. would cover about 100,000 miles (160,000 kilometers) every 2000 hours (83 days in the life of a constantly running pump ). Auto manufacturers recommend changing their automatic transmission oil every 25,000 miles ( 40,000 kilometers)

APPLICATION

• Use Centerline pump designs when the pumping temperature exceeds 200 degrees Fahrenheit (100° C). This design will allow the wet end of the pump to expand in two directions instead of from the feet up, destroying the wear rings..
• Try to buy pumps with a Suction Specific Speed (SSS) below 8,500 (10,000 metric) Do not buy pumps with a SSS over 12,000 ( metric 16,500) unless you're pumping hot water or mixed hydrocarbons. If you have a double suction pump you can divide the SSS number by 2
• Do not specify a pump with the largest impeller available . Give yourself an additional 5% or 10% you might need it.
• The maximum viscosity a centrifugal pump can handle would be a product similar to 30 weight oil at room temperature.
• Use a variable speed pump if your head is mainly system head. Circulating hot or cold water would be typical applications. If you have a high static or pressure head, as is the case with a boiler feed pump, the variable speed will not be of much help in keeping you on or near the best efficiency point.
• Pumps piped in series must have the same capacity (impeller width and speed)
• Pumps piped in parallel must have the same head (impeller diameter and speed )
• Use a rotary positive displacement pump if your capacity is going to be less than 20 gpm.(4,5 cubic meters per hour)
• A centrifugal pump can handle 0.5% air by volume. At 6% it will probably become air bound and stop pumping. Cavitation can occur with any amount of air.
• Use double volute pumps any time your impeller diameter is 14 inches (355 mm) or greater. They should also be used on long shaft vertical pumps to prevent excessive shaft movement that will cause problems with the packing, seals, bearings and critical dimensions.
• A Vortex pump is 10% to 15% less efficient than a comparable size end suction centrifugal pump.
• The A.P.I. (American Petroleum Institute). sixth edition states : High energy pumps, defined as pumping to a head greater than 650 feet (198 meters) and more than 300 horsepower (224 KW) per stage, require special consideration to avoid blade passing frequency vibrations and low frequency vibrations at reduced flow rates.

PIPING ETC..

• There should be at least 10 diameters of pipe between the suction of the pump and the first elbow. This is especially critical in double ended pump designs as the turbulent inlet flow can cause shaft thrusting, and subsequent bearing problems.
• Substituting a globe valve for a gate valve in a piping system is similar to adding another 100 feet (31 meters) of piping to the system. On the discharge side of the pump this will cause the pump to run off of its B.E.P. with a resultant shaft bending. On the suction side of the pump it will probably cause Cavitation.
• After the pump and motor have been aligned, dowel both the pump and the motor to the base plate. Be sure to dowel only the feet closest to the coupling, allowing the outboard ends to expand with temperature changes.
• Check impeller rotation after installing the pump. Do not assume it will turn in the correct direction. I've heard about two speed pumps with the second speed wired backwards. They will drive you crazy because the pump will often meet its head requirement but not the capacity when the second speed cuts in. You'll also notice excessive noise at this time.
• Use eccentric reducers rather than concentric reducers at the pump suction. Concentric reducers will trap air. Be sure the eccentric reducer is not installed up side down.
• Suction piping should be at least one size larger than the suction flange at the pump.
• Vortexing can occur if any of the following conditions are present:
  • Low liquid levels
  • Liquid level falling greater than 3 Ft./sec. (1 Meter/ sec.)
  • There is a large concentration of dissolved gases in the liquid.
  • High outlet velocities in pipes leaving vessels. Generally greater than 10 feet/sec. (3 meters/sec.)
  • Liquids near their vapor point.
  • High circulation caused by asymmetrical inlet or outlet conditions.
  • Inlet piping too close to the wall or bottom of the tank. Consult the Hydraulic Institute Manual or a similar publication for recommended clearances.
  • In a mixer, the liquid level must be at least one and one half diameters of the blade, above the blade.

TROUBLESHOOTING

• Cavitation damage on the trailing edge of the impeller blade means :
  • The N.P.S.H. available is too low.
  • Air is entering at the pump suction.
  • There is liquid turbulence at the pump suction.
• Cavitation damage on the leading edge of the impeller blade indicates internal recirculation. Check the Suction Specific Speed number to see if it is below 9000 (10,000 metric). Higher numbers mean that the problem is with the impeller shape or adjustment. The problem was created when the pump manufacture tried to come up with too low a N.P.S.H. Required.
• Cavitation damage just beyond the cutwater, on the casing and tip of the impeller blade, indicates the impeller blade is too close to the cutwater. This clearance should be at least 4% of the impeller diameter up to a 14 inch (356 mm.) impeller, and 6% greater than 14 inch ( 356 mm.). Some self priming pump manufacturers want a maximum clearance of 1/8" (3 mm) and, as a result, often experience this problem. A repaired or substituted impeller is often the cause of the problem in a non self priming pump.
• Water in the bearing oil will reduce bearing life 48%. The water enters from packing leakage, wash down hoses, and aspiration caused by the temperature cooling down in the bearing casing after shutdown and moisture laden air entering the bearing case. A 6% water content in the oil will reduce bearing life by as much as 83%
• The mass of the pump concrete foundation must be 5 times the mass of the pump, base plate, and other equipment that is being supported, or vibration will occur.
• Up to 500 horsepower (375 KW), the foundation must be 3 inches (76 mm.) wider than the base plate all around. Above 500 horsepower (375 KW) the foundation should be a minimum of 6 inches (150 mm.) wider.
• Imaginary lines extended downward 30 degrees to either side of a vertical through the pump shaft, should pass through the bottom of the foundation and not the sides.
• The bearing oil level should be at the center of the lowest most ball of a stationary bearing. The preferred choice for bearing lubrication would be an oil mist system with positive face sealing at the bearings, if you could solve the emission problem.
• Pipe from the pump suction flange to the pipe rack, not the other way around.
• Make sure eccentric reducers are not installed upside down at the pump suction. The top of the reducer should go straight into the suction flange.
• Valve stems, T Branches and elbows should be installed perpendicular to the pump shaft, not at right angles.
• Do not use packing in any pump that runs under a vacuum, as air will enter the system through the pump stuffing box.. These applications include :
  • Pumps that lift liquid.
  • Pumps that take their suction from a condenser or evaporator.
  • Any pump that takes its suction from a negative pressure. Heater drain pumps are a typical application.
• Be sure too vent the stuffing box of a sealed, vertical pump back to the suction side of the pump or air will become trapped in the stuffing box. The vent must be located above the lapped seal faces.
• If the specific gravity of the pumping liquid should increase, due to temperature, there is a danger of overloading the motor and therefore motors having sufficient power should be used. The same overloading power will occur if the pump is run too far to the right of its B.E.P.. This is a very common problem because of the great number of oversized pumps in existence.

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