The Pump Handbook Series 1
apor pressure, cavitation,and NPSH are subjectswidely discussed by engi-neers, pumps users, and
pumping equipment suppliers, butunderstood by too few. To graspthese subjects, a basic explanationis required.
VAPOR PRESSUREKnowledge of vapor pressure
is extremely important whenselecting pumps and theirmechanical seals. Vapor pressureis the pressure absolute at which aliquid, at a given temperature,starts to boil or flash to a gas.Absolute pressure (psia) equals thegauge pressure (psig) plus atmos-pheric pressure.
Lets compare boiling water atsea level in Rhode Island to boil-ing water at an elevation of 14,110feet on top of Pikes Peak inColorado. Water boils at a lowertemperature at altitude becausethe atmospheric pressure is lower.
Water and water containingdissolved air will boil at differenttemperatures. This is because oneis a liquid and the other is a solu-tion. A solution is a liquid with dis-solved air or other gases. Solutionshave a higher vapor pressure than
their parent liquid and boil at a lowertemperature. While vapor pressurecurves are readily available for liq-uids, they are not for solutions.Obtaining the correct vapor pressurefor a solution often requires actuallaboratory testing.
CAVITATIONCavitation can create havoc with
pumps and pumping systems in theform of vibration and noise. Bearingfailure, shaft breakage, pitting on theimpeller, and mechanical seal leak-age are some of the problems causedby cavitation.
When a liquid boils in the suc-tion line or suction nozzle of a pump,it is said to be flashing or cavitat-ing (forming cavities of gas in theliquid). This occurs when the pres-sure acting on the liquid is below thevapor pressure of the liquid. Thedamage occurs when these cavitiesor bubbles pass to a higher pressureregion of the pump, usually just pastthe vane tips at the impeller eye,and then collapse or implode.
NPSHNet Positive Suction Head is the
difference between suction pressureand vapor pressure. In pump designand application jargon, NPSHA is the
net positive suctionhead available to thepump, and NPSHR isthe net positive suc-tion head requiredby the pump.
The NPSHAmust be equal to orgreater than theNPSHR for a pumpto run properly. Oneway to determine theNPSHA is to mea-sure the suction pres-sure at the suctionnozzle, then applythe following formu-la:
NPSHA = PB VP Gr+ hv
where PB = barometric pres-sure in feet absolute, VP = vaporpressure of the liquid at maximumpumping temperature in feetabsolute, Gr = gauge reading atthe pump suction, in feet absolute(plus if the reading is above baro-metric pressure, minus if the read-ing is below the barometricpressure), and hv = velocity headin the suction pipe in feetabsolute.
NPSHR can only be deter-mined during pump testing. Todetermine it, the test engineermust reduce the NPSHA to thepump at a given capacity until thepump cavitates. At this point thevibration levels on the pump andsystem rise, and it sounds likegravel is being pumped. Morethan one engineer has run for theemergency shut-down switch thefirst time he heard cavitation onthe test floor. Its during thesetests that one gains a real apprecia-tion for the damage that will occurif a pump is allowed to cavitate fora prolonged period.
CENTRIFUGAL PUMPINGCentrifugal pumping terminol-
ogy can be confusing. The follow-ing section addresses these termsand how they are used:
Head is a term used toexpress pressure in both pumpdesign and system design whenanalyzing static or dynamic condi-tions. This relationship isexpressed as:
head in feet = (pressure in psi x 2.31)
Pressure in static systems isreferred to as static head and in adynamic system as dynamichead.
To explain static head, letsconsider three columns of anydiameter, one filled with water,one with gasoline, and one withsalt water (Figure 1). If thecolumns are 100 ft tall and you
Nomenclature and DefinitionsBY PAT FLACH
Static head using various liquids.
43 psi 52 psi32.5 psi
WaterSp. Gr. = 1.0
GasolineSp. Gr. = .75
SaltWaterSp. Gr. = 1.2
2 The Pump Handbook Series
measure the pressure at the bot-tom of each column, the pres-sures would be 43, 32.5, and 52psi, respectively. This is becauseof the different specific gravities,or weights, of the three liquids.Remember, we are measuringpounds per square inch at thebottom of the column, not thetotal weight of the liquid in thecolumn.
The following four terms areused in defining pumping systemsand are illustrated in Figure 2.
Total static head is the verti-cal distance between the surfaceof the suction source liquid andthe surface level of the dischargeliquid.
Static discharge head is thevertical distance from the center-line of the suction nozzle up tothe surface level of the dischargeliquid.
Static suction head applieswhen the supply is above thepump. It is the vertical distancefrom the centerline of the suctionnozzle up to the liquid surface ofthe suction supply.
Static suction lift applieswhen the supply is located belowthe pump. It is the vertical dis-tance from the centerline of thesuction nozzle down to the surfaceof the suction supply liquid.
Velocity, friction, and pressurehead are used in conjunction withstatic heads to define dynamicheads.
Velocity head is the energy ina liquid as a result of it traveling atsome velocity V. It can be thoughtof as the vertical distance a liquidwould need to fall to gain the samevelocity as a liquid traveling in apipe.This relationship is expressed as:
hv = V2/2g
where V = velocity of theliquid in feet per second and g =32.2 ft/sec2.
Friction head is the headneeded to overcome resistance toliquid flowing in a system. This
at a pump suction flange, convert-ing it to head and correcting to thepump centerline, then adding thevelocity head at the point of thegauge.
Total dynamic dischargehead is the static discharge headplus the velocity head at the pumpdischarge flange plus the total fric-tion head in the discharge system.This can be determined in the fieldby taking the discharge pressurereading, converting it to head, andcorrecting it to the pump center-line, then adding the velocityhead.
Total dynamic suction lift isthe static suction lift minus thevelocity head at the suction flangeplus the total friction head in thesuction line. To calculate totaldynamic suction lift, take suctionpressure at the pump suctionflange, convert it to head and cor-rect it to the pump centerline, thensubtract the velocity head at thepoint of the gauge.
Total dynamic head in asystem is the total dynamic dis-charge head minus the totaldynamic suction head when thesuction supply is above the pump.When the suction supply is belowthe pump, the total dynamic head
resistance can come from pipe fric-tion, valves, and fittings. Values infeet of liquid can be found in theHydraulic Institute Pipe FrictionManual.
Pressure head is the pressure infeet of liquid in a tank or vessel on thesuction or discharge side of a pump. Itis important to convert this pressureinto feet of liquid when analyzing sys-tems so that all units are the same. If avacuum exists and the value is knownin inches of mercury, the equivalentfeet of liquid can be calculated usingthe following formula:
vacuum in feet = in. of Hg x 1.13specific gravity
When discussing how a pumpperforms in service, we use termsdescribing dynamic head. In otherwords, when a pump is running it isdynamic. Pumping systems are alsodynamic when liquid is flowingthrough them, and they must be ana-lyzed as such. To do this, the follow-ing four dynamic terms are used.
Total dynamic suction head isthe static suction head plus the veloc-ity head at the suction flange minusthe total friction head in the suctionline. Total dynamic suction head iscalculated by taking suction pressure
Total static head, static discharge head, static suction head, and static suction lift.
The Pump Handbook Series 3
is the total dynamic discharge headplus the total dynamic suction lift.
Centrifugal pumps are dynamicmachines that impart energy to liq-uids. This energy is imparted bychanging the velocity of the liquid asit passes through the impeller. Mostof this velocity energy is then con-verted into pressure energy (totaldynamic head) as the liquid passesthrough the casing or diffuser.
To predict the approximate totaldynamic head of any centrifugalpump, we must go through two steps.First, the velocity at the outside diam-eter (o.d.) of the impeller is calculatedusing the following formula:
v = (rpm x D)/229
where v = velocity at the periph-ery of the impeller in ft per second, D= o.d. of the impeller in inches, rpm= revolutions per minute of theimpeller, and 229 = a constant.
Second, because the velocityenergy at the o.d. or periphery of theimpeller is approximately equal to thetotal dynamic head developed by thepump, we continue by substituting vfrom above into the following equa-tion:
H = v2/2g
where H = total dynamic headdeveloped in ft, v = velocity at theo.d. of the impeller in ft/sec, and g =32.2 ft/sec2.
A centrifugal pump operating ata given speed and impeller diameterwill raise liquid of any specific gravi-ty or weight to a given height.Therefore, we always think in termsof feet of liquid rather than pressurewhen analyzing centrifugal pumpsand their systems. n
Patrick M. Flach is the we