Rules Of Thumb For Pumps

RULES OF THUMB FOR PUMPS R023

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 guidelines or “rules of thumb.” Over the years I have accumulated many of these guide lines 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 (rpm / 1750)2
    • Estimating metric head is a little bit more involved, but it still works:
      • Measure the shaft in mm. (250)
      • Mark off two places. (2,5)
      • Square the number. (6,25)
      • For 1450 rpm, multiply by 3 (18,75)
      • Add 10 % (21 meters )

      NOTE: For 3000 rpm, you would multiply by 12 instead of 3. Although you can estimate shutoff head with these formulas you cannot estimate the pump capacity. You will need the pump curves for that.

    • The pumps best efficiency point (BEP) 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 the ratio 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 best efficiency point (BEP). Lowering the L3/D4 is the only logical and efficient solution. When pump manufacturers discuss operating off of the best efficiency point (BEP) they relate problems to the heat that will build up in a minimum flow condition and ignore the problems with shaft bending.
    • 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 will get twice the capacity, four times the head, and it will take eight times the horsepower to do it.
    • If you double the speed of a pump you will get almost four times the shaft whip, wobble or run out and eight times the wear.
    • A stainless steel shaft has only a small portion of the conductivity of a carbon steel shaft. This is very important when you are pumping fluids at elevated temperatures because we do not want to transmit the high temperature back to the bearing oil.
    • A good rule of thumb to remember is that each inch of stainless steel shaft will grow both axially and radially about 0.001 inches / inch of shaft for each 100°F. rise in Temperature, or 0.001 mm./mm. of shaft for each 50°C rise in temperature. This can have a profound affect on impeller to volute clearance.
    • 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 are 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 pump’s 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, orpositive 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 specified as 30 years at 30 degrees Centigrade (86 F.) and is cut in half for each 10 degree Centigrade (10 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 life 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)

 

  • Increasing the impeller speed increases the efficiency of centrifugal pumps.
    • About 15% for an increase from 1500 to 3600 rpm.
    • Less dramatic at lower speeds.
    • Maximum efficiency is obtained in the specific speed range of 2000 to <3000
  • If the wear ring clearance is too large:
    • The pump will take on excessive vibration caused by internal recirculation. This can cause seal and bearing component damage.
    • The pump will not meet its designed capacity because of internal recirculation.
    • Wear rings should be replaced when their clearance doubles. This additional clearance will increase the pump power requirements with the amount varying according to the specific speed ( NS ) of the impeller
      • NS 200 14% increase
      • NS 500 7% increase
      • NS 2500 Insignificant increase
  • Pumps are normally throttled with a discharge valve but in rare cases it can be done with a suction valve.
    • You must have sufficient NPSH to prevent cavitation.
    • Suction throttling prevents the over heating caused by discharge regulation. This can be important with fluids like jet fuel where the additional heat could vaporize the fluid.
  • Because an overhung impeller does not require the extension of a shaft into the impeller suction eye, single stage impellers are preferred for pumps handling suspended matter such as sewage.
  • Do not hydrostatically test a high temperature pump with water. Water trapped in small recesses and gaskets will flash to steam in high temperature applications, expand and then break something.
  • Operating off the BEP can break the pump shaft because the force is always in the same direction while the shaft is turning. This has the affect of flexing the shaft twice per revolution. In many cases you can easily exceed the endurance limit of the shaft material.
    • The stresses imposed in reverse bending are cumulative.
    • Most fatigue failure occurs in one million cycles or less. At 1750 rpm you get 2,520,000 cycles per day.
  • If a 300 series stainless steel shaft is running in a fluid containing chlorides, the shaft is subject to chloride stress corrosion problems that can be another cause of shaft cracking and breakage.
    • Do not let the welder use the pump as an electrical ground. You can ruin the seal or bearings in the process.
    • Pumping off of the best efficiency point will not excessively deflect the shaft with the following centrifugal pump designs:
    • Double volute casings.
    • Multi stage designs.
    • Diffuser or turbine pump designs.
    • Be sure to level the pump when you do an alignment.
    • If you trim the impeller, file the tips and re balance the assembly.
    • The next time that you look at the pump discharge gauge, remember that the pump pumps the difference between the suction and discharge heads. You must subtract a positive suction head to determine what head the pump is really creating.
    • Bearing lip or grease seals have a useful life of less than 90 days and will cut and score the shaft because of fretting corrosion.
    • Never cool a bearing housing because it will shrink and over compress the bearing. Cool only the bearing oil.

    APPLICATION

    • 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)
    • Pumps piped in parallel must have the same head (impeller diameter)
    • 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.
    • vortex pump is 10% to 15% less efficient than a comparable size end suction centrifugal pump.
    • The API (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.
    • Flushing the system with steam or a cleaner seldom flushes out the stuffing box of the pump.
    • Do not circulate shop water through the cooling jacket on a high temperature pump. Condensate or low pressure steam is a better choice. Be sure to install a thermal bushing in the end of the stuffing box to get effective temperature control in the seal area. Make sure you come into the bottom of the jacket and out the top to vent any air that might be trapped in the jacket.

    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 8500 (5200 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 net positive suction head required (NPSHR).
    • Cavitation damage just beyond the cut water on the casing and tip of the impeller blade indicates the impeller blade is too close to the cut water. 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 cavitation problem. A repaired or substituted impeller is often the cause of the problem in a non self-priming pump.
    • 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.
      • Self-priming pumps.
    • 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 best efficiency point (BEP). This is a very common problem because of the great number of oversized pumps in existence.

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  • On February 16, 2018