WO1997008530A1 - Method for determining the real cavitation point of material - Google Patents

Method for determining the real cavitation point of material Download PDF

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Publication number
WO1997008530A1
WO1997008530A1 PCT/FI1996/000459 FI9600459W WO9708530A1 WO 1997008530 A1 WO1997008530 A1 WO 1997008530A1 FI 9600459 W FI9600459 W FI 9600459W WO 9708530 A1 WO9708530 A1 WO 9708530A1
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WO
WIPO (PCT)
Prior art keywords
value
npsh
cavitation
calculated
pump
Prior art date
Application number
PCT/FI1996/000459
Other languages
French (fr)
Inventor
Teemu Paavola
Original Assignee
Teemu Paavola
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Teemu Paavola filed Critical Teemu Paavola
Priority to AU67433/96A priority Critical patent/AU6743396A/en
Publication of WO1997008530A1 publication Critical patent/WO1997008530A1/en

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Classifications

    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F04—POSITIVE - DISPLACEMENT MACHINES FOR LIQUIDS; PUMPS FOR LIQUIDS OR ELASTIC FLUIDS
    • F04D—NON-POSITIVE-DISPLACEMENT PUMPS
    • F04D29/00—Details, component parts, or accessories
    • F04D29/66—Combating cavitation, whirls, noise, vibration or the like; Balancing
    • F04D29/669—Combating cavitation, whirls, noise, vibration or the like; Balancing especially adapted for liquid pumps

Definitions

  • the subject of the invention is a method for determining the real cavitation point of material.
  • the cavitation point of a material can be estimated in different circumstances and therefore the result is proportional to the test drive result which usually is performed with water of +20 °C.
  • Known test results can be seen in Fig. 1. Used values for the model development are from references “Cavitation and NPSH requirements of Various liquids.” V. Salesman Transaction of the ASME. Journal of Basic Engineer ⁇ ing, June 1959 (1) and "Cavitation in Centrifugal Pumps with Liquids Other than Water.” Stepanoff, A.J., Trans. ASME. J. of Eng. for Power, Jan. -61. Liquid properties are from the reference “Liquid property data from DIPPR library”.
  • the cavitation sensitivity forces of the liquid's physical properties are calculated here from some known test drives. The factors do not base on the mass- or energy balance.
  • the formula is very like the ones that describe the heat or mass transfer through a resisting surface.
  • the property ratios of the pumped liquid and water influences as a factor to the comparative cavitation value.
  • the point where the cavitation starts when pumping another liquid can be estimated when the measured NPSH,- value is known.
  • the first methods to describe the suction performance of a centrifugal pump were the Suction head- and -lift -values. This calculation based on the density of the material and it didn't take care of the vapour pressure of the pumped liquid.
  • NPSH- method was developed to take care of the vapour pressure. The definitions and the formulas for the calculation of NPSH are described in the literature. This application compares the relation between the NPSH number and the real cavitation point.
  • NPSH r value of a system is calculated by using the density, viscosity and vapour pressure properties.
  • the NPSH number and the NPSH r values are compared directly.
  • the method according to the invention suggests also that the material properties of app. 1 have an effect on the cavitation sensibili ⁇ ty.
  • a normally used method to calculate the NPSH a of the system gives the result as "meters (or feet) of liquid column”. This value is compared to the NPSH-value given by the pump manufacturer and which is "meters of water column”. This means that we scale the result with density ratio of the two liquids. Without this scaling the result would be worse as can be seen on Fig. 2. In Fig. 2 the NPSH-value is scaled back with the density ratio to verify the same amount of pressure energy available at the suction flange of the pump.
  • the invention is characterised by the facts that are stated in the characterising part of the claim.
  • the calculated NPSH r number is modified to be comparable to the NPSH value given by the pump manufacturer by multiplying it at first by the density ratio of water and material. Resulting value is pressure proportionality value NPSH0.
  • NPSH0 number is modified with parameters obtained from material properties.
  • the model presents the effect of material properties to cavitation sensitivity as resistive. This means that the effect of different material properties either each separately or all combined can move the real cavitation point to critical area.
  • the calculation formula is a bit similar to the formula of calculating currents in resistors that are parallelly connected.
  • this mathematical model of the invention can be calculated by using after the density scaling any developed equation that depends on the material properties.
  • the resulting ANN a and ANN b numbers can be directly compared to the NPSH value measured in the test drive.
  • This invention concerns the use of this method in determining cavitation point of regulating valves and propellers for a particular material.
  • the main difference of the ANN a method to the present NPSH method is the density scaling of the NPSH number, and thus formed NPSH0 value is modified with the material property parameters.
  • the present method compares the NPSH number of the pump and the NPSH r number of the system by length criteria, i.e. the height of a water column is compared to the height of the liquid column.
  • length criteria i.e. the height of a water column is compared to the height of the liquid column.
  • the comparison is based on pressure criterion that is obtained by multiplying by the density ratio of water and material.
  • the relation between NPSH, NPSH0 and the suction height is presented in Fig. 3. It presents that the cavitation sensitivity depends on material properties. The material properties that can be measured have a cavitation effect to the formed vapour bubbles in the material. These bubbles and their collapsing in different pressure zones of a hydraulic apparatus cause the cavitation.
  • the real cavitation point of a material can be determined by calculating from the material properties.
  • ANN a (NPSH r - ANN b ) / (Dens (w) /Dens (1) ) (2)
  • ANN b ANN C - ANN d * (a+b+c+d+e) (3)
  • NPSH0 NPSH r * (Dens (w) /Dens (1) ) (4)
  • NPSH r The specified NPSH for water
  • Tr (T + 273,15) / T(critical)
  • Tr (T + 273,15) / T(critical)
  • Dens. Density kg/m 3
  • H.o.V. Heat of evaporation kJ/kg
  • Fig. 4 is a simplified picture of fig 3, wherein
  • cases 1) and 2) are compared.
  • cases 1) and 3) are compared, in which case 3) is modified by calculating to take notice of the effect of differences in the material properties.

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  • Engineering & Computer Science (AREA)
  • Mechanical Engineering (AREA)
  • General Engineering & Computer Science (AREA)
  • Structures Of Non-Positive Displacement Pumps (AREA)

Abstract

The invention subject is a method for determining the real cavitation point of a material, in which method the NPSHa-value is calculated for the system and the NPSHr-value is then determinated therefrom. According to the method the NPSHr-value calculated for the system and the NPSH-value given by the pump manufacturer are changed comparable by multiplying by the density ratio to get the NPSHØ-value, and then the NPSHØ-value is handled with the factor calculated from the liquid physical properties by use of any equation that depends on the properties of the material after the density scaling. The resulting ANNa- and ANNb-values can be straightly compared to the calculated NPSH-value from the test drive.

Description

METHOD FOR DETERMINING THE REAL CAVITATION POINT OF MATERIAL
The subject of the invention is a method for determining the real cavitation point of material.
It has been εaid, that it is impossible to estimate the influence of the fluid properties to the cavitation in centrifugal pump suction. A model development has been seen difficult, because always more than one property is different when verifying two different liquids. With mathematical study it is possible to separate the influence of an individual property to the studied phenomenon. This is possible if there exists some relationship between the properties and real cavitation point and if there is enough representative comparison pars available.
According to this method the cavitation point of a material can be estimated in different circumstances and therefore the result is proportional to the test drive result which usually is performed with water of +20 °C. For example for a pump this means the comparability of the real cavitation point of a material and the NPSH-value from the pump manufacturer. Known test results can be seen in Fig. 1. Used values for the model development are from references "Cavitation and NPSH requirements of Various liquids." V. Salesman Transaction of the ASME. Journal of Basic Engineer¬ ing, June 1959 (1) and "Cavitation in Centrifugal Pumps with Liquids Other than Water." Stepanoff, A.J., Trans. ASME. J. of Eng. for Power, Jan. -61. Liquid properties are from the reference "Liquid property data from DIPPR library".
The cavitation sensitivity forces of the liquid's physical properties are calculated here from some known test drives. The factors do not base on the mass- or energy balance.
The formula is very like the ones that describe the heat or mass transfer through a resisting surface. In this model the property ratios of the pumped liquid and water influences as a factor to the comparative cavitation value. When using cold water as a reference liquid, the point where the cavitation starts when pumping another liquid, can be estimated when the measured NPSH,- value is known.
The first methods to describe the suction performance of a centrifugal pump were the Suction head- and -lift -values. This calculation based on the density of the material and it didn't take care of the vapour pressure of the pumped liquid. NPSH- method was developed to take care of the vapour pressure. The definitions and the formulas for the calculation of NPSH are described in the literature. This application compares the relation between the NPSH number and the real cavitation point.
In the present NPSH method NPSHr value of a system is calculated by using the density, viscosity and vapour pressure properties. The NPSH number and the NPSHr values are compared directly. The method according to the invention suggests also that the material properties of app. 1 have an effect on the cavitation sensibili¬ ty.
A normally used method to calculate the NPSHa of the system gives the result as "meters (or feet) of liquid column". This value is compared to the NPSH-value given by the pump manufacturer and which is "meters of water column". This means that we scale the result with density ratio of the two liquids. Without this scaling the result would be worse as can be seen on Fig. 2. In Fig. 2 the NPSH-value is scaled back with the density ratio to verify the same amount of pressure energy available at the suction flange of the pump.
When concerning cavitation the question is of the energy that is available at the suction side of the pump. At the pressure side of the pump it is OK to compare meters of liquid to meters of water, because there the centrifugal forces cause the pressure. The heavier the liquid is, the more energy it takes from the motor. In the suction side it is not possible to take in opinion only the density ratio. Fig. 2 shows, that the result is better when doing so, that the other properties of the liquid influence as much to the cavitation as the density. This patent application concerns method for dimensioning and regulating wherein the real cavitation point is calculated according to known material properties (surface tension, viscosity, etc.) as shown in app. 1. The present NPSH comparison is performed by comparing heights of water and liquid columns. This gives a wrong impact of the starting point of cavitation. Cavitation causes e.g. premature deterioration of pumps and damages that could be avoided by using the method of this invention for dimensioning and regulation.
The invention is characterised by the facts that are stated in the characterising part of the claim.
1. According to this method the calculated NPSHr number is modified to be comparable to the NPSH value given by the pump manufacturer by multiplying it at first by the density ratio of water and material. Resulting value is pressure proportionality value NPSH0.
2. Subsequently thus obtained NPSH0 number is modified with parameters obtained from material properties. The model presents the effect of material properties to cavitation sensitivity as resistive. This means that the effect of different material properties either each separately or all combined can move the real cavitation point to critical area. The calculation formula is a bit similar to the formula of calculating currents in resistors that are parallelly connected. On the other hand, this mathematical model of the invention can be calculated by using after the density scaling any developed equation that depends on the material properties.
3. The resulting ANNa and ANNb numbers can be directly compared to the NPSH value measured in the test drive.
This invention concerns the use of this method in determining cavitation point of regulating valves and propellers for a particular material. The main difference of the ANNa method to the present NPSH method is the density scaling of the NPSH number, and thus formed NPSH0 value is modified with the material property parameters.
The present method compares the NPSH number of the pump and the NPSHr number of the system by length criteria, i.e. the height of a water column is compared to the height of the liquid column. In the ANNa method, i.e. the method of the invention, the comparison is based on pressure criterion that is obtained by multiplying by the density ratio of water and material. The relation between NPSH, NPSH0 and the suction height is presented in Fig. 3. It presents that the cavitation sensitivity depends on material properties. The material properties that can be measured have a cavitation effect to the formed vapour bubbles in the material. These bubbles and their collapsing in different pressure zones of a hydraulic apparatus cause the cavitation.
If the cavitation sensitivity caused by material properties can be classified, the real cavitation point of a material can be determined by calculating from the material properties.
This method suggests that in the cavitation point the minimum energy amount of the liquid is lowered. This energy (=pressure) consists of two factors: NPSH = NPSH0 + ANNb.
The formulas to describe the real cavitation point are:
ANNa' *= NPSH0 + ANNb (1)
ANNa = (NPSHr - ANNb) / (Dens(w)/Dens(1)) (2)
ANNb = ANNC - ANNd * (a+b+c+d+e) (3)
Where:
NPSH0 = NPSHr * (Dens(w)/Dens(1)) (4)
NPSHr = The specified NPSH for water
ANNC = 58,201
ANNd « 1, *» a = (1 / (al * (Dens.(1)/Dens.(w)) + a2) (5) b = (1 / (bl * (Visc.(1)/Visc.(w)) + b2) (6) c = (1 / (cl * (Surf.tens.(1))/Surf.tens.(w)) + c2) (7) d = (1 / (dl * (H.o.V.(1)/ H.o.V.(w)) + d2) (8) e = (1 / (el * rPT(1)/rPT(w) + e2) (9) rPT - Pr/Tr (10)
Tr = (T + 273,15) / T(critical)
Tr = (T + 273,15) / T(critical)
Pr = P / P(critical)
T = Temperature °C
P = Pressure kPa
T(c) = Critical temperature K
P(c) - Critical pressure kPa
Dens. = Density kg/m3
Vise. **•= Viscosity cP
Surf.tens. = Surface tension N/m2
H.o.V. = Heat of evaporation kJ/kg
Subscripts: (1) - liquid
(w) = water
Coefficients: i: 1 2 ai 0,00092 0,02137 bi 0,23132 0,03519 ci 218,48023 -32,01193 di 0,83296 0,21725 ei 0,00780 0,10899
*) ANNd-variable takes care of the possible influence of the NPSH range to the cavitation sensitivity factors.
The difference can be presented by the pressure caused by liquid columns, as can be seen from Fig. 4, which is a simplified picture of fig 3, wherein
Column 1 — water, specific density = 1
Columns 2, 3 = unknown material, specific density = 0,5
Pi = 1 bar P2 = 0,5 bar P3 = 1 bar hj = 10 h2 = 10 m h3 = 20 m
In the present method cases 1) and 2) are compared. In the ANNa method of the invention cases 1) and 3) are compared, in which case 3) is modified by calculating to take notice of the effect of differences in the material properties.

Claims

1. A method for producing a pumping system, a control valve system or a propeller system, c h a r a c t e r i s e d by determining the real cavitation point by a method in which the NPSHa-value of a pump system is calculated in an ordinary way and then the NPSHr-value is determinated, and that the NPSHr-value for the system and the NPSH-value from the pump manufacturer are transformed to a comparability form with multiplying them with the density ratio of the reference liquid and the specified liquid to get the NPSHø-value, and the NPSHø-value is then scaled with a factor which is determinated by the liquid properties to get the ANNa- and ANNb-values. The ANNa- and ANNb-values are used to compare the pump system cavitation point to the NPSH-value given by the pump manufacturer.
2. A method according to claim 1 , c h a r a c t e r i s e d by that the calculated values ANNa and ANNb are used for determining or regulating the cavitation point of a pump system so that the required pressure level is not lowered.
3. A method according to claim 1 , c h a r a c t e r i s e d by that the NPSHr-value is changed to be comparable with the NPSH- value from the pump manufacturer by multiplying it by the density ratio of water and material to be pumped.
PCT/FI1996/000459 1995-08-28 1996-08-28 Method for determining the real cavitation point of material WO1997008530A1 (en)

Priority Applications (1)

Application Number Priority Date Filing Date Title
AU67433/96A AU6743396A (en) 1995-08-28 1996-08-28 Method for determining the real cavitation point of material

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
FI954040A FI98651C (en) 1995-08-28 1995-08-28 Method for manufacturing a pumping, regulating valve or propeller system
FI954040 1995-08-28

Publications (1)

Publication Number Publication Date
WO1997008530A1 true WO1997008530A1 (en) 1997-03-06

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Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN102072143A (en) * 2010-12-02 2011-05-25 中国人民解放军空军工程大学 Method for coupling computation of pressure, flow rate and temperature of constant-pressure plunger pump

Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3942372A (en) * 1973-03-14 1976-03-09 Astilleros Espanoles, W.A. Procedure for making tests of propeller cavitation
SU672531A1 (en) * 1977-11-25 1979-07-05 Предприятие П/Я В-8534 Stand for cavitation testing of pumps
SU823640A1 (en) * 1979-07-05 1981-04-23 Институт Ядерной Энергетики Ан Белорусскойсср Stand for cavitation testing of compressors
SU983315A1 (en) * 1981-04-09 1982-12-23 Институт ядерной энергетики АН БССР Stand for cavitation testing of pumps
SU1054573A2 (en) * 1982-05-11 1983-11-15 Институт ядерной энергетики АН БССР Method of cavitation tests of pumps

Patent Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3942372A (en) * 1973-03-14 1976-03-09 Astilleros Espanoles, W.A. Procedure for making tests of propeller cavitation
SU672531A1 (en) * 1977-11-25 1979-07-05 Предприятие П/Я В-8534 Stand for cavitation testing of pumps
SU823640A1 (en) * 1979-07-05 1981-04-23 Институт Ядерной Энергетики Ан Белорусскойсср Stand for cavitation testing of compressors
SU983315A1 (en) * 1981-04-09 1982-12-23 Институт ядерной энергетики АН БССР Stand for cavitation testing of pumps
SU1054573A2 (en) * 1982-05-11 1983-11-15 Институт ядерной энергетики АН БССР Method of cavitation tests of pumps

Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN102072143A (en) * 2010-12-02 2011-05-25 中国人民解放军空军工程大学 Method for coupling computation of pressure, flow rate and temperature of constant-pressure plunger pump

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Publication number Publication date
FI954040A0 (en) 1995-08-28
FI98651C (en) 1997-07-25
FI98651B (en) 1997-04-15
AU6743396A (en) 1997-03-19

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