Cen­trifu­gal Water Pump Design

by Johannes Weber

Cen­trifu­gal pumps are commonly used in indus­trial and res­i­den­tial appli­ca­tions due to their rel­a­tively simple design, man­u­fac­tur­ing, and ease-of-main­te­nance. They are also rather effi­cient and easily applic­a­ble in dif­fer­ent sizes. Thus, in the scope of his intern­ship project at FRIEND­SHIP SYSTEMS, Johannes Weber, a BSc student in Trans­port Systems Engi­neer­ing at the Tech­ni­cal Uni­ver­sity of Berlin, was tasked with the imple­men­ta­tion of a design process for a cen­trifu­gal water pump, making full use of the capa­bil­i­ties of CAESES.

Within this over­ar­ch­ing aim, the indi­vid­ual project objec­tives were:

• Pre­lim­i­nary design para­me­ter cal­cu­la­tion for the radial impeller, based on the selected pump characteristics.
• Para­met­ric modeling of the impeller.
• Flow domain modeling for CFD analysis.
• Dimen­sion­ing and modeling of the spiral casing, includ­ing the volute and the exit diffuser.

STEP 1 | Pump Type and Para­me­ter Selection

A radial type impeller was selected and the B‑NM-50/200A/C pump from Linn-Pumpen GmbH [1] was chosen as a ref­er­ence. The fun­da­men­tal para­me­ters of this pump are pre­sented in the fol­low­ing table.

With the help of the Buck­ing­ham-Pi-Theorem, the specific speed is defined as a non-dimen­sional para­me­ter for tur­bo­ma­chines. However, in most cases in the industry the dimen­sional form of the specific speed is used. Con­se­quently, the dimen­sional form was used in this project and cal­cu­lated as Ns= 1091 (US) [2].

STEP 2 | Pre­lim­i­nary Pump Design

While CAESES has an exten­sive set of ded­i­cated modeling func­tion­al­ity for tur­bo­ma­chin­ery, it does not provide pre­lim­i­nary design method­ol­ogy out-of-the-box. However, the so-called feature script­ing func­tion­al­ity provides the user with a complete pro­gram­ming envi­ron­ment that allows them to write their own custom routines. This approach was used here to imple­ment the pre­lim­i­nary dimen­sion­ing of the pump based on selected empir­i­cal formulae and cor­re­la­tion diagrams.

The cal­cu­la­tion of the hub diameter was set up using the Overall Effi­ciency – Specific Speed [2] graph and the result­ing shaft torque. The suction diameter, which is highly impor­tant for avoiding cav­i­ta­tion at the leading edge of the blades and accom­mo­dat­ing the minimum-flow require­ment, was deter­mined keeping the cav­i­ta­tion cri­te­rion (NPSHR) in mind. For the cal­cu­la­tion of both suction and impeller diam­e­ters, the Flow Coef­fi­cient – Specific Speed [3] graph was employed and imple­mented in CAESES.

Another impor­tant geo­met­ric design value is the minimum outlet width, which was cal­cu­lated through the graph of Relative Outlet Width — Specific Speed [3] and applied in CAESES. Due to possible cav­i­ta­tion and flow sep­a­ra­tion at the blade, the minimum outlet width should be increased by a certain margin to guar­an­tee the desired vol­u­met­ric flow.

The cal­cu­lated merid­ional dimen­sions of the impeller are given below.

Further, deter­mi­na­tion of the leading and trailing edge blade angles is impor­tant to achieve the least amount of resis­tance. To cal­cu­late the appro­pri­ate values, the velocity tri­an­gles were used as a basis. The deter­mined angles are pre­sented in the fol­low­ing table, together with the cal­cu­lated minimum number of blades [3, 4]:

STEP 3 | Impeller Modeling

Using the pre­vi­ously deter­mined main dimen­sions and geo­met­ric prop­er­ties as input para­me­ters, a full geo­met­ric model of the impeller was created. Obvi­ously, through­out this process, the def­i­n­i­tion of several addi­tional para­me­ters that describe more detailed attrib­utes of the geometry will be nec­es­sary. These para­me­ters will intro­duce a sizable design space that can later be used to fine-tune and optimize the pump for its specific appli­ca­tion using CFD, depart­ing from the basic pre­lim­i­nary design.

The first step in the modeling process of a radial impeller is to generate the 2D merid­ional contours. For this impeller, this was done by first defining a shroud contour that connects D1 and D2, taking into account the outlet thick­ness. The hub contour was then derived as an offset curve from the shroud with a given width dis­tri­b­u­tion between the inlet and outlet width.

The leading edge, which serves as the stacking curve, was ini­tial­ized as a simple straight con­nec­tion of two points, located on hub and shroud contour, respec­tively. These points were each defined by the inter­sec­tion of a given diameter and the cor­re­spond­ing contour and can be varied by two design vari­ables. Sub­se­quently, the pos­si­bil­ity to create a curved leading edge was intro­duced using an inter­me­di­ate inter­po­la­tion point posi­tioned at a para­me­ter­ized normal distance to the initial straight leading edge.

Camber lines at the hub, mid and shroud were created using a so-called Stream Section curve in CAESES, which is offered as pre-defined feature with the option to create the camber curve in dif­fer­ent modes. The blade angle dis­tri­b­u­tions used as an input to the camber line gen­er­a­tion are defined in a separate diagram. Finally, the three camber lines are aligned at the trailing edge by rotating them with the dif­fer­ence between their indi­vid­ual wrap angle and the wrap angle from the hub camber line, result­ing in a per­pen­dic­u­lar trailing edge.

Then, the mean camber surface was modeled as a lofted surface inter­po­lat­ing the dif­fer­ent camber lines, followed by the blade surface, which was created by applying thick­ness normal to the camber surface, again using a pre-defined section def­i­n­i­tion provided in CAESES and a spec­i­fied thick­ness distribution.

Finally, the solid blade is the result of inter­sect­ing dif­fer­ent surfaces to the desired closed boundary rep­re­sen­ta­tion (Brep). The solid hub is defined in a similar way, con­nected to the blade with a Boolean oper­a­tion an the root fillet is gen­er­ated at the intersection.

As the pre­lim­i­nary design pro­ce­dure is built directly into the para­met­ric model of the pump, the complete model can simply be updated by changing one or more of the pump’s para­me­ters. The impact of all three fun­da­men­tal para­me­ters (H, Q, and N) on the impeller geometry is visu­al­ized in the ani­ma­tions below, obtained by varying each para­me­ter with a linear increase.

STEP 4 | Flow Domain Modeling

The flow domain is the actual geometry that will be exported to an external grid gen­er­a­tor for meshing and can either be modeled manually or using a selec­tion of pre-defined feature def­i­n­i­tions provided in CAESES. In this study, manual modeling was used to generate a periodic single blade passage domain, result­ing in a robust def­i­n­i­tion that conforms to the respec­tive changes of the impeller geometry.

STEP 5 | Spiral Casing Modeling

The dimen­sions of the volute and the exit velocity were cal­cu­lated by the method and graphs pre­sented in the book ​„Fun­da­men­tals of Tur­bo­ma­chin­ery“ [2].

With these starting values at the volute exit, an algo­rithm can be started, which con­tin­u­ously deter­mines the geo­met­ric values at each cross-sec­tional position. The result was an A/R ratio of ∼16.07 at the exit, whereas the starting A/R from this cal­cu­la­tion was 0. [2]

Due to the dis­ad­van­tages of man­u­fac­tur­ing a volute with a starting A/R ratio of 0, it is common in the industry to maintain a constant A/R ratio in last part of the volute. In this project, a constant A/R ratio with a value of ∼1.49 was applied to the initial 5% of the volute’s total arc length.

After defining the dimen­sions, the volute together with the diffuser and the tongue surface in between the volute and the diffusor could be modeled. This was simply based on the CAESES tutorial ​„Volute Modeling“ and the included Feature Def­i­n­i­tions. The final result of the spiral casing together with the radial impeller can be seen in the fol­low­ing image.

Con­clu­sion

By using CAESES, the eval­u­a­tion of dif­fer­ent values of the pump’s fun­da­men­tal para­me­ters is fast and prac­ti­cal. The cal­cu­la­tion of pre­lim­i­nary design para­me­ters, as well as empir­i­cal graphs, can be imple­mented directly into CAESES with the help of feature def­i­n­i­tions, auto­mat­i­cally result­ing in a correct dimen­sion­ing of the radial impeller for the current para­me­ter require­ments. This makes it easy to use the project for pre­lim­i­nary design development.

Another aspect is the pos­si­bil­ity to inte­grate a vir­tu­ally unlim­ited — and fully cus­tomized — selec­tion of addi­tional para­me­ters that allows to fine-tune every aspect of the geometry, creating an ideal set up for a quick vari­a­tion of the shape after the pre­lim­i­nary design in order to optimize the pump on the basis of flow analysis.

About the Author

Johannes Weber is a Trans­port Systems Engi­neer­ing student at the Tech­ni­cal Uni­ver­sity of Berlin. He con­ducted his intern­ship at FRIEND­SHIP SYSTEMS in 2022.

Ref­er­ences

[1] LINN PUMPEN, publ., 2015: Close Coupled Cen­trifu­gal Pumps with flanged con­nec­tions. [Last checked on: 30.05.2022]. Avail­able via: https://www.linn-pumpen.de/com/centrifugal-pumps/NM-MNS-DIN-pumps.html

[2] PENG, W. W., 2008: Fun­da­men­tals of Tur­bo­ma­chin­ery. 1st Edition. Hoboken, John Wiley & Sons, ISBN 978 – 94-017‑9627‑9

[3] GÜLICH, J. F., 2020: Kreiselpumpen – Handbuch für Entwick­lung, Anlage­pla­nung und Betrieb., 5th Edition. Berlin, Springer Viewig, ISBN 978 – 3‑662 – 59784

[4] CFturbo, publ., 2021: User manual for CFTurbo Software, Dresden [Last checked on: 30.05.2022]. Avail­able via: https://manual.cfturbo.com/CFturbo_en.pdf

More Infor­ma­tion

See this overview to learn more about the capa­bil­i­ties of CAESES for tur­bo­ma­chin­ery design and contact us to discuss your indi­vid­ual require­ments and our solu­tions in more detail.