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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)o At 1750 rpm. Shut off head = Diameter of the impeller squaredo At 3500 rpm. Shut off head = Diameter of the impeller squared x 4o 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:o Measure the shaft in mm. ( as an example: 250mm )o Mark off two places. (2,5)o Square the number. (6,25)o For 1450 rpm, multiply by 3 (18,75)o Add 10 % for the answer in meters. (21 meters )o 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" isclosest to the "power out".

The L3/D4 ratio should be below 60 (2.0 in metric) to prevent excessive shaft bending. Tocalculate it for end suction centrifugal pumps :

o L = length of the shaft from the center of the inboard bearing to the center of theimpeller (inches or millimeters). Caution: do not use centimeters, the numbers

will come out wrong.

o D = diameter of the shaft (under the sleeve) in the stuffing box area (inches ormillimeters) Do not use centimeters.

Since most shaft materials have a similar modulus of elasticity, changing shaft materialswill not prevent shaft bending when you operate off of the B.E.P. Lowering the L

3/D

4is

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 withoutcavitating.

If you double the speed of a pump you'll get twice the capacity, four times the head and itwill take eight times the horsepower to do it.

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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 notwant 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 orrun 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 muchas 50% .

If you're pumping paper stock, modify the curves for head, capacity, and efficiency asfollows:

o 0.725 for 6% stocko 0.825 for 5.5% stocko 0.90 for 5%o 0.94 for 4.5%o 0.98 for 4%o 1.0 for 3.5% or less

Open impeller clearance settings are determined by the pump manufacturer and normallyrun 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% pumpcapacity 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 constantlyrunning 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 nonfretting 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 willenter 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 thisproblem.

The axial clearance in a bearing is ten times the radial clearance. This is the reason properinstallation is so critical. If the bearing is over compressed the bearing balls will distortand 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 theSKF 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 :

o A life of 3 months at 100 C. (212 F.)o A life of 6 months at 90 C. (195 F.)o A life of 12 months at 80 C. ( 176 F.)

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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 (160,000kilometers) 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 degreesFahrenheit (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 SSSnumber by 2

Do not specify a pump with the largest impeller available . Give yourself an additional5% or 10% you might need it.

The maximum viscosity a centrifugal pump can handle would be a product similar to 30weight oil at room temperature.

Use a variable speed pump if your head is mainly system head. Circulating hot or coldwater 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 youon 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 airbound 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) orgreater. 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 criticaldimensions.

A Vortex pump is 10% to 15% less efficient than a comparable size end suctioncentrifugal 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.

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PIPING ETC..

There should be at least 10 diameters of pipe between the suction of the pump and thefirst 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 another100 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 sideof the pump it will probably cause Cavitation.

After the pump and motor have been aligned, dowel both the pump and the motor to thebase plate. Be sure to dowel only the feet closest to the coupling, allowing the outboardends to expand with temperature changes.

Check impeller rotation after installing the pump. Do not assume it will turn in the correctdirection. 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 notthe 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:

o Low liquid levelso Liquid level falling greater than 3 Ft./sec. (1 Meter/ sec.)o There is a large concentration of dissolved gases in the liquid.o High outlet velocities in pipes leaving vessels. Generally greater than 10 feet/sec.

(3 meters/sec.)

o Liquids near their vapor point.o High circulation caused by asymmetrical inlet or outlet conditions.o Inlet piping too close to the wall or bottom of the tank. Consult the Hydraulic

Institute Manual or a similar publication for recommended clearances.

o In a mixer, the liquid level must be at least one and one half diameters of theblade, above the blade.

TROUBLESHOOTING

Cavitation damage on the trailing edge of the impeller blade means :o The N.P.S.H. available is too low.o Air is entering at the pump suction.o There is liquid turbulence at the pump suction.

Cavitation damage on the leading edge of the impeller blade indicates internalrecirculation. 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

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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 of1/8" (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 packingleakage, wash down hoses, and aspiration caused by the temperature cooling down in thebearing 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, baseplate, 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 thanthe base plate all around. Above 500 horsepower (375 KW) the foundation should be aminimum of 6 inches (150 mm.) wider.

Imaginary lines extended downward 30 degrees to either side of a vertical through thepump 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 stationarybearing. The preferred choice for bearing lubrication would be an oil mist system withpositive 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 systemthrough the pump stuffing box.. These applications include :

o Pumps that lift liquid.o Pumps that take their suction from a condenser or evaporator.o 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 thepump 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 isa 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 oversizedpumps in existence.