Hydraulic Pump Suction Characterstics

248 Hydraulic System Design for Service Assurance

Copyright 1999 by BarDyne, Inc. of Stillwater, Okalhoma. All Rights Reserved.

too much of the available atmospheric pressure to overcome frictional resis-tances in the suction line at the rated flow of the pump, insufficient pressure willbe available to accelerate the liquid to the velocity needed to fill the pumpingchambersthus the pump becomes starved and a serious vacuum condition willpersist in the pump chambers causing both gaseous and vaporous cavitation tooccur.

4.12 Pump Filling Characteristicsanufacturers of positive displacement hydraulic pumps generally specifythe filling characteristics as the minimum suction pressure which must ex-

ist at the pump inlet in order to induce sufficient flow at the pump suction port tofill the pumping chambers at a given maximum pump speed for a specific fluidviscosity and density. This pump speed characteristic is a necessary function ofthe design configuration of the flow duct or passage between the suction port ofthe pump and the actual pumping mechanism (see Fig. 4-23). Each pump typehas a different flow passage configuration and exhibits a specific internal pres-sure loss from the suction port to the suction chamber which varies with the ductsize, severity and number of flow diversions, and the nature of the constrictionswithin the duct.

Figure 4-23. Flow Path Between Pump Suction Port and Pumping Mechanism.

During the operation of a fixed displacement pump, the volume of itsworking chambers periodically increase and decrease while communication be-tween the suction line and the pumping chambers takes place through the suctionducts of the pump. A complete change in fluid volume occurs within the pumpduring the rotation of the shaft and this change for one rotation is called the dis-placement rating of the pump. The theoretical flow capacity of a pump is equal tothe displacement rating times the rotational speed of the shaft. The actual flow ofthe pump is always less than the theoretical capacity due to two factors:

M

Chapter 4 Hydraulic Reservoir and Suction Line Dynamics 249


Slippage or internal leakagethat is the flow that occurs be-tween the discharge and suction sides of the pump. Obviously,it is a function of the discharge pressure of the pump and theclearances associated with the pumping chambers.

Incomplete filling of the pumping chambers with liquidnor-mally a function of excessively flow restriction in the flow pathto the pump at a specific shaft speed of the pump.

The pumps volumetric efficiencya function of the pumps speed andpressure drop across the pumpreflects the internal leakage and any lack offilling of the pumps working chambers. At low speeds, more time is availablefor leakage to occur and, at high speeds, excessive suction losses cause incom-plete filling of the pumping chambers. Internal leakage is a function of the dis-charge pressure of the pump because the pressure differential between the dis-charge and suction pressures is one of the primary driving forces creating internalleakagethe other being the drag forces of moving surfaces or Couette flow.

Internal leakage is inversely proportional to fluid viscosity; hence, volu-metric efficiency is directly proportional to the viscosity of the fluid in the regionwhere internal leakage dominatesthat is, at low speed. However, increasing theviscosity of the fluid to minimize leakage will have an unfavorable effect on suc-tion losses. An increase in viscosity will result in higher suction line pressurelosses that will produce incomplete filling of the pumping chambers and be adominant factor in the efficiency of the suction line system. Thus an optimumvalue of fluid viscosity exists for any given pump and operating condition. Toolow a viscosity will cause excessively high internal leakage, whereas too high aviscosity will cause excessively high suction lossesthat is, higher pressuredrops between the reservoir and the suction chamber of a pump.

The major causes of incomplete filling of the working chambers of a pumpwhich occur when it is communicating with the suction port are

Too low a pressure existing at the pump intake port Too high a resistance to fluid flow through the suction ducts of

the pump at the operating speed The undesired presence of an excessive amount of entrained air

in the suction fluidThese factors can lead to the incomplete filling of the working chambers duringthe suction process. Incomplete filling of the pumping chambers will ultimatelyresult in dissolved air coming out of solution and/or vaporous cavitation occur-ring as the pressure falls below the vapor pressure of the fluid. The probability ofthis condition occurring increases proportionally with higher rotational speeds ofthe pump.

When the rotational speed of the pump increases, the amount of fluidpassing through the feed ducts and distribution (intake valves) of the system in-crease proportionally. Consequently, the flow resistance (suction head losses)increases correspondingly. However, for a constant fluid pressure at the pumpintake, a certain critical speed exists where the amount of fluid required to fill the



working chambers cannot enter the pump at the intake); by further increasing thespeed above this critical value, a proportional increase in pump delivery will notoccurindeed, the pump capacity may even decrease (a condition known aspump starvation) as Figs. 4-24 and 4-25 show.

Figure 4-24. Pump Filling Characteristics for Various Inlet Pressures and Speeds.

Figure 4-25. Pump Filling Characteristics of Gear and Piston Pumps.

Internal flow and pressure losses vary depending on the pump design. Inthe case of axial piston pumps, the internal head losses basically depend on theresistances offered by the distribution unit feeding the individual pistons of thepumpincluding the suction port restriction and also the resistances in the ductsconducting the fluid to the cylinder chambers. For gear and vane pumps, besidesthe losses resulting from flow constrictions in the intake, the centrifugal force of



the fluid during the rotation of the pumping chambers can also cause a significantresistance to flow.

From the inlet port of the pump to the pumping chamber, significant lossescan occur as follows:

Pressure drop due to the inertia of the fluid Pressure drop due to centrifugal forces Pressure drop due to axial velocity Pressure drop due to tangential velocity

The influence of each of these types of pressure losses varies according to thetype of pump and its suction passage configuration. The design of this suctionpassage imposes limits on the pressure that must be available at the inlet port ofthe pump and the maximum speed the pump can efficiently operate. High speedscan reduce the pressure at the suction chamber not only below the air-fluid satu-ration pressure where dissolved air is released but also below the vapor pressureof the liquid where vaporous cavitation can occur.

All pumps have limitations on their suction capabilities. The filling char-acteristics curves of the pump reflect these limitations. As the following suggests,there are at least two ways of presenting the filling characteristics curves of fixeddisplacement pumps:

Output flow versus pump speed as shown in Fig. 4-24 for vari-ous inlet pressures.

Output flow versus inlet pressure for a specific pump, pumpspeed and oil temperature, as Fig. 4-25 shows.

Figure 4-24 shows that for a given pump, fluid, temperature, and inlet pressure, aparticular pump speed cannot be exceededotherwise, the flow will be chokedoff and the pump starved. Similarly, for a given pump, fluid, temperature, andspeed, the system must maintain a specific inlet pressure to have cavitation sta-bility as Fig. 4-25 shows.

In any given application, it is essential to know the filling characteristics ofa fixed displacement pump for the operating conditionsfluid type, temperature,and operating speed. In addition, for variable displacement pumps, the technolo-gist must also know these filling characteristics under maximum stroking speedconditions. These conditions always increase the magnitude of the inlet pressurerequirements that is needed to avoid cavitating conditions.

The overall significance of knowing the filling characteristics curves of aspecific pump is that the curves establish the minimum inlet port pressure for agiven set of reference conditions. The system that does not maintain this mini-mum pressure at all times will lose cavitation stability and will ultimately destroyitself. It is important to realize that monitoring the inlet pump pressure is often anecessary operation because this monitoring alone can reveal physical changes inthe pump as well as critical suction line hydraulic parameters essential in main-taining cavitation stability.

In summary, in order to avoid pump cavitation, the inlet pressure of thepump should be higher than the pressure required to accelerate the fluid to the



flow rate required by the pump in order to equal or exceed the speed of the pumpdisplacement elements. For details in terminology, see The Conduit Systemchapter in the companion Hydraulic Component Design and Selection book. Inthe terminology commonly used in the fluid power industry, this means that theNet Positive Suction Head Available (NPSHA) must be greater than or equal tothe Net Positive Suction Head Required (NPSHR). NPSHA is the actual fluidenergy available at the inlet of the pump. According to Bernoullis equation, theNPSHA is described as:

NPSHA h h h h hh h h

a s f m vp

pi vi vp

= +

= + (4-38)

where ha = tank pressure head, normally is atmospheric pressurehs = static suction head, i.e., the vertical distance above the

centerline of pump inlet to the free level of the fluid inthe tank

hf = friction head losshm = minor head losshvp = fluid vapor pressure headhpi = pressure head at pump inlethvi = velocity head at pump inlet

Eq. (4-38) is more commonly expressed as:

NPSHA p vg

pi i vp= +

2

2(4-39)

where pi = pump inlet pressure = fluid specific weightg = acceleration of gravityvi = fluid inlet velocity

pvp = vapor pressure

The NPSHR is dependent on the pump structure, fluid properties, and op-erating conditions. It is normally determined experimentally. From publishedresearch literature of the pump manufacturer, for a specific pump design, theNPSHR is a function of pump speed, dynamic viscosity of fluid, and gas separa-tion pressure. Note that ideally, if there is no air in the fluid, the onset point ofvaporous cavitation depends on the fluid vapor pressure. However, if the fluidcontains air and the air separation pressure happens to be greater than the fluidvapor pressure, air will be released before the fluid evaporates. In other words,pump filling characteristics are determined by the effect of cavitation (vaporpressure, pvf) and aeration (gas separation pressure, pgs).

In general, the term cavitation represents any bubble formation in the fluidthat is caused by either gas desorption or separation or by a suction pressure be-low the vapor pressure of the fluid. Hence, the critical pressure required to initi-



ate gaseous or vaporous cavitation is the maximum of the fluid vapor pressureand the gas diffusion or air separation pressure.

p p pvp vf gs= max( , ) (4-40)Furthermore, the gas separation pressure is a function of both the fluid tempera-ture and the amount of gas dissolved in the fluid. A set of the typical gas separa-tion pressure curves for air in oil is shown in Fig. 4-26. Using gas separationpressure as the reference parameter, Professor Tsuji at Tokyo Institute of Tech-nology, Japan, reported that the NPSHR required to avoid cavitation can be mod-eled by the following relationship:

NPSHR K v Kpm

+( )1 2 (4-41)where K1 = a constant

vp = velocity of pumping elementK2 = a constant = kinematic viscosity of fluid

m = an exponent

Figure 4-26. Air Separation Pressure Curves.



Typically, K1, K2, and m bear the following respective empirical values:for gear pumps (0.086, 2.0, 1/3), fixed displacement vane pumps (0.172 to 0.266,2.30 to 4.28, 1/8), variable displacement vane pumps (0.160, 0.35, 1/3), and pis-ton pumps (0.7, 4.17, 1/13) where vp is in m/sec and in cm2/sec. However, to bemore accurate, each design should be tested to obtain its specific K1, K2 and mvalues.

Considering the model representation of a pumping mechanism as shownin Fig. 4-27, the energy balance equation can be derived from Bernoullis equa-tion as follows:

p p vh f = 2

2(4-42)

where ph (p) = higher (lower) pressure acts on the fluid slug in the pumpingchamber

vf = velocity of fluid slug = fluid mass density (=/g)

Note that cavitation occurs whenever the fluid velocity cannot catch upwith the piston velocity. Hence, the onset pressure to introduce cavitation iswhen vf is equal to vp. In practice, it is more convenient to use a simplified ap-proach by assuming the NPSHR be solely proportional to the pump speed andintroducing a dimensionless cavitation factor, Kc, into the energy equation (Eq.4-42) to account for the energy loss due to the fluid entering from the inlet port tothe pumping chamber. In addition, the pumping flow rate into each chamber isequal to one half the product of the piston reciprocating velocity and the pistonarea. By substituting piston velocity, vp, with 2qa/Ap, into Eq. (4-42), and assum-ing the acceleration pressure (ph-p) is approximated by the measurable pressuredifferential, (pi-pvp), yields,

( )p p A K q vi vp p c a p = (4-43)where Ap = pumping element pressure sensing area

qa = actual flow delivery from a single pumping element (= Qa/np)Qa = total actual pump delivery flow ratenp = number of pumping element

Thus, NPSHR is given by:

NPSHR pK q v

Avpc a p

p= +

(4-44)



Figure 4-27. Pump Filling Characteristic Model.

Note that it is assumed that the pressure adjacent to the displacementpumping element is the critical cavitation pressure, pvp. The term at the righthand side of Eq. (4-42) is the acceleration force modified by the cavitation factor,Kc. It is the minimum pressure force required to pump the fluid from the inletport into the pumping chamber without cavitation. However, if the NPSHA (i.e.pi) is less than NPSHR, cavitation occurs. As a result, the actual pump deliverywill be less than the ideal (theoretical) flow rate. In this case, the actual flow ratedelivered by each individual pumping chamber can be derived by replacing vpwith vf and NPSHR with NPSHA in Eq. (4-44). Furthermore, substituting vf with2qa/Ap, yields,

q ANPSHA p

Ka pvp

c

=

2 (4-45)

Hence the ratio of the total actual flow, Qa, delivered by the pump to the idealflow rate, QT, must include cavitation flow as given as

QQ

n qND

a

T

p a

p= (4-46)

where QT = pump theoretical flow rate (= NDp)N = pump rotational speed

Dp = total pump theoretical displacement (= npdp)dp = theoretical displacement (capacity) of a single

pumping element

Eq. (4-46) is the Pump Filling Characteristic Model. It is only valid whenNPSHA is larger than pvp and is less than or equal to NPSHR. The ratio has avalue from 0 to 1. This equation describes the filling characteristic curves as



shown in Fig. 4-24. It also serves as the basis for deriving parameter adjustmentfactors that will be discussed in the following sections.

Example 4-3The theoretical flow rate of a 9-piston pump is 60 gpm at 1800rpm. The piston diameter is 0.5 inches. The NPSHA at the pumpinlet measured at 150F is 0.4 atm. Assume the cavitation factoris 0.7 with no air in the fluid, find the actual pump delivery if theworking fluid isa. MIL-H-5606b. Oil-Water Emulsion

Solution:The NPSHA at the inlet is 0.4 atm which is 5.879 psia. If theNPSHR is larger than 0.4 atm, cavitation occurs. The NPSHRcan be determined using Eq. (4-44). From the data given, wehave the following parametric values for calculation:

Kc = 0.7

Ap = 0.25d2 = 0.25(0.5)2 = 0.196 in2

qa = Qa / np = 60 gpm / 9 = 25.667 in3/secw = 1 g/cm3 (mass density of water)

[ ][ ]

sec

ft787.21sec

in439.261

rpm1800/2)in196.09rpm1800/(gpm602

N/2)AnN/(Q2

revolutionpertime)strokepiston(2

v

2

ppaa

==

=

==

a. MIL-H-5606The vapor pressure, pvp, of MIL-H-5606 at 150F can befound from Fig. 4-34 which is approximately 0.8 mm Hg.The specific gravity is 0.86. Thus,



psia941.1in196.0

sec

ft787.21sec

in667.25cm

g186.07.0mmHg8.0

AvqK

pNPSHR

2

3

3

p

pacvp

=

+=

+=

Because the NPSHA (5.879 psia) is larger than the NPSHR(1.941 psia), there is no cavitation. Hence, the flow rate willbe the same as the theoretical flow that is 60 gpm.

b. Oil-Water EmulsionFrom Fig. 4-24, it is found that the vapor pressure, pv, of oil-water emulsion at 150F is around 210 mm Hg. The specificgravity can be assumed to be the same as water. Thus,

NPSHR pK q v

A

mmHg

gcm

in ft

inpsia

vpc a p

p= +

= +

=

2100 7 1 25667 21787

01966 3

3

3

2

. ( ) ( .sec

) ( .sec

).

.

Because the NPSHA (5.879 psia) is smaller than the NPSHR(6.3 psia), cavitation occurs. Hence, use Eq. (4-45) to findthe actual delivery.

Q n q n A NPSHA pK

in psi mmHgg cm

gpm

a p a p pvp

c

= =

=

=

2

9 0196 5879 2102 0 7 1

54 058

23

( )( . ) . ( )( . )( / ).

The actual pump delivery is only 90.1 percent of the idealflow rate. There is about 10% flow loss due to the cavitationeffect.

Example 4-4Find the minimum NPSHR if the MIL-H-5606 fluid contains air.Use the same parameters and data as those in Example 4-3 forcalculation.



Solution:From Fig. 4-26(a), the gas separation pressure at 150F is around7 psia which is greater than the fluid vapor pressure (= 0.8 mmHg or 0.015 psia). Thus, the gas separation pressure must be usedto determine the minimum NPSHR.

NPSHR pK q v

A

psia

gmcm

in ft

inpsia

vpc a p

p= +

= +

=

70 7 0 86 1 25 667 21787

01968 925

3

3

2

. ( . ) ( .sec

) ( .sec

).

.

Hence, the NPSHR is 8.925 psia or 0.607 atm. It is much higherthan the NPSHA given (= 0.4 atm). Thus, the pump will cavitateif air exists in the fluid at the above stated operating conditions.

Example 4-5Set up a HyPneu circuit to generate the filling characteristiccurves for a pump similar to those shown in Fig. 4-22 (inlet pres-sure effect) and Fig. 4-21 (pumping speed effect). Use data de-scribed in Ex. 4-2.

Solution:Construct a HyPneu circuit as shown in Fig. 4-28 below:

Figure 4-28. HyPneu Circuit for Simulating Pump Cavitation.

The inlet pressure (NPSHA) effect on the cavitation can besimulated by using a variable pressure source varying from 1 atm(14.7 psia) down to 0 atm (0 psia). Using the same circuit, wecan simulate the pumping speed effect by varying the electrical



motor speed from 0 to 5000 rpm at various inlet pressure set-tings. The simulation results are shown in Fig. 4-29 below.

Figure 4-29. HyPneu Simulation Results of Pump Cavitation.

4.13 Pump Suction Pressure Adjustmenthe filling characteristics of a pump establish the pressure that must exist atthe pumps intake (suction) port when a given fluid, temperature, and rota-

tional speed exists. The question that must be answered is what pressure isneeded at the reservoir pump outlet port in order to ensure that the required pumpintake port pressure will exist. The following conditions establish the necessarypressure adjustments that must be added to the reference atmospheric pressure inthe reservoir:

When the fluid density is not the same as that of the referencefluid used at the time the filling characteristics test was con-ducted

T

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Hydraulic Pump Suction Characterstics