{"id":8705,"date":"2024-06-12T08:50:00","date_gmt":"2024-06-12T06:50:00","guid":{"rendered":"https:\/\/www.idro-elettrica.it\/esperto-risponde\/minimum-flow-rate-for-centrifugal-pumps-a-7-point-quick-guide\/"},"modified":"2026-07-01T09:47:25","modified_gmt":"2026-07-01T07:47:25","slug":"minimum-flow-rate-for-centrifugal-pumps-a-7-point-quick-guide","status":"publish","type":"esperto-risponde","link":"https:\/\/www.idro-elettrica.it\/en\/esperto-risponde\/minimum-flow-rate-for-centrifugal-pumps-a-7-point-quick-guide\/","title":{"rendered":"Minimum Flow Rate for Centrifugal Pumps: A 7-Point Quick Guide"},"content":{"rendered":"\n<p class=\"wp-block-paragraph\"><em>Minimum flow rate of a centrifugal pump<\/em>: This is a topic that often comes up when a pump is operating at partial load or is at risk of approaching very low flow conditions. Here is a practical guide to estimating the minimum \u201csafety\u201d flow rate (Qric) and, above all, to understanding why operating at a flow rate (Q) close to zero for many hours is never a good idea.* <\/p>\n\n<div class=\"wp-block-rank-math-toc-block\" id=\"rank-math-toc\"><h2>Table of Contents<\/h2><nav><ul><li class=\"\"><a href=\"#portata-minima-pompa-centrifuga-a-cosa-serve-davvero\">Minimum Flow Rate of a Centrifugal Pump: What Is It Really For?<\/a><\/li><li class=\"\"><a href=\"#quali-dati-servono-per-stimare-qric\">What data is needed to estimate Qric?<\/a><\/li><li class=\"\"><a href=\"#come-scegliere-laumento-di-temperatura-consentito-te-ts\">How to Choose the Permissible Temperature Rise (te \u2013 ts)<\/a><\/li><li class=\"\"><a href=\"#esempio-pratico-pompa-da-75-k-w\">Practical example: 75-kW pump<\/a><\/li><li class=\"\"><a href=\"#perche-applicare-il-coefficiente-1-5-e-cosa-centra-il-bilanciamento-assiale\">Why Apply a Coefficient of 1.5 (and What Does Axial Balance Have to Do With It?)<\/a><\/li><li class=\"\"><a href=\"#attenzione-a-q-ridotta-i-problemi-non-sono-solo-termici\">PLEASE NOTE: At low Q, the problems are not just thermal<\/a><\/li><li class=\"\"><a href=\"#conclusioni-mai-vicino-a-q-0-per-molte-ore-limita-il-tempo-a-qric\">Conclusions: Never stay near Q = 0 for many hours (limit the time at Qric)<\/a><\/li><\/ul><\/nav><\/div>\n\n<h2 id=\"portata-minima-pompa-centrifuga-a-cosa-serve-davvero\" class=\"wp-block-heading\">Minimum Flow Rate of a Centrifugal Pump: What Is It Really For?<\/h2>\n\n<p class=\"wp-block-paragraph\">When the flow rate decreases too much, the energy transferred to the machine can result in an <strong>increase in the<\/strong> fluid <strong>temperature<\/strong> in the volute area. For small pumps operating at a temperature <strong>sufficiently far<\/strong> from the <strong>vapor pressure<\/strong>, it is possible to estimate a minimum flow rate using a simplified method based on energy parameters. <\/p>\n\n<p class=\"wp-block-paragraph\">This estimate yields a reference value (Qric) that is useful for preventing unwanted temperature increases. However, it is essential not to focus solely on temperature: at low flow rates, critical <strong>mechanical phenomena<\/strong> also come into play. <\/p>\n\n<h2 id=\"quali-dati-servono-per-stimare-qric\" class=\"wp-block-heading\">What data is needed to estimate Qric?<\/h2>\n\n<div class=\"wp-block-math\"><math display=\"block\"><semantics><mrow><msub><mi>Q<\/mi><mtext>ric<\/mtext><\/msub><mo>=<\/mo><mfrac><mrow><mi>P<\/mi><mo>\u22c5<\/mo><mn>3600<\/mn><\/mrow><mrow><mi>\u03c1<\/mi><mo>\u22c5<\/mo><mi>C<\/mi><mo>\u22c5<\/mo><mrow><mo fence=\"true\" form=\"prefix\">(<\/mo><msub><mi>T<\/mi><mi>and<\/mi><\/msub><mo>\u2212<\/mo><msub><mi>T<\/mi><mi>i<\/mi><\/msub><mo fence=\"true\" form=\"postfix\">)<\/mo><\/mrow><\/mrow><\/mfrac><mspace width=\"0.2778em\"><\/mspace><mo form=\"prefix\" stretchy=\"false\">[<\/mo><msup><mi>m<\/mi><mn>3<\/mn><\/msup><mi>\/<\/mi><mi>h<\/mi><mo form=\"postfix\" stretchy=\"false\">]<\/mo><\/mrow><annotation encoding=\"application\/x-tex\">Q_{\\text{ric}} = \\frac{P \\cdot 3600}{\\rho \\cdot C \\cdot \\left(T_e &#8211; T_i\\right)} [m^3\/h]<\/annotation><\/semantics><\/math><\/div>\n\n<p class=\"wp-block-paragraph\">To determine the minimum flow rate (Qric) using the approach described, at least the following data and parameters are required:<\/p>\n\n<ul class=\"wp-block-list\">\n<li><strong>P<\/strong> = power of the centrifugal pump motor <strong>[kW]<\/strong><\/li>\n\n\n\n<li><strong>\u03c1<\/strong> = density <strong>[kg\/m\u00b3]<\/strong><\/li>\n\n\n\n<li><strong>c<\/strong> = specific heat of the fluid<\/li>\n\n\n\n<li>An acceptable <strong>temperature rise<\/strong> value <math><mrow><mo form=\"prefix\" stretchy=\"false\">(<\/mo><msub><mi>t<\/mi><mi>and<\/mi><\/msub><mo>\u2212<\/mo><msub><mi>t<\/mi><mi>s<\/mi><\/msub><mo form=\"postfix\" stretchy=\"false\">)<\/mo><\/mrow><\/math><\/li>\n<\/ul>\n\n<p class=\"wp-block-paragraph\">The permitted temperature increase  <math><mrow><mo form=\"prefix\" stretchy=\"false\">(<\/mo><msub><mi>t<\/mi><mi>and<\/mi><\/msub><mo>\u2212<\/mo><msub><mi>t<\/mi><mi>s<\/mi><\/msub><mo form=\"postfix\" stretchy=\"false\">)<\/mo><\/mrow><\/math>  is related to:<\/p>\n\n<ul class=\"wp-block-list\">\n<li><strong>ts<\/strong>: temperatures at <strong>the inlet<\/strong><\/li>\n\n\n\n<li><strong>te<\/strong>: Permissible temperature <strong>behind the<\/strong> pump <strong>balancing device<\/strong> <\/li>\n<\/ul>\n\n<p class=\"wp-block-paragraph\">In other words: you\u2019re not choosing a \u201crandom\u201d number, but rather setting the temperature range you can \u201caccept\u201d between the inlet and the area downstream of the balance point.<\/p>\n\n<h2 id=\"come-scegliere-laumento-di-temperatura-consentito-te-ts\" class=\"wp-block-heading\">How to Choose the Permissible Temperature Rise (te \u2013 ts)<\/h2>\n\n<p class=\"wp-block-paragraph\">As a rough estimate, we can assume a temperature difference of <strong>about 20\u00b0C<\/strong>  <math><mrow><mo form=\"prefix\" stretchy=\"false\">(<\/mo><msub><mi>t<\/mi><mi>and<\/mi><\/msub><mo>\u2212<\/mo><msub><mi>t<\/mi><mi>s<\/mi><\/msub><mo form=\"postfix\" stretchy=\"false\">)<\/mo><\/mrow><\/math>. This choice is intended to provide a quick estimate, consistent with the practical goal of preventing the machine from operating under thermally critical conditions when the flow rate is low.<\/p>\n\n<p class=\"wp-block-paragraph\">For water, the text uses:<\/p>\n\n<ul class=\"wp-block-list\">\n<li><strong>c = 4.18 kJ\/kg\u00b7K<\/strong><\/li>\n<\/ul>\n\n<p class=\"wp-block-paragraph\">This information is important because, given the same power and density, specific heat affects the minimum flow rate required to limit the temperature rise.<\/p>\n\n<h2 id=\"esempio-pratico-pompa-da-75-k-w\" class=\"wp-block-heading\">Practical example: 75-kW pump<\/h2>\n\n<p class=\"wp-block-paragraph\">Applying the proposed configuration (with the parameters listed above and a reference temperature increment), the following results are obtained for a <strong>75-kW pump<\/strong>:<\/p>\n\n<ul class=\"wp-block-list\">\n<li><strong>Qric=3.23 m\u00b3\/h<\/strong><\/li>\n<\/ul>\n\n<p class=\"wp-block-paragraph\">This is the \u201cbase\u201d minimum recirculation\/guaranteed flow rate value shown in the example. It is a useful figure for orientation, but it does not yet represent the final recommended value for operation, because an additional criterion related to the pump\u2019s internal mechanics comes into play. <\/p>\n\n<h2 id=\"perche-applicare-il-coefficiente-1-5-e-cosa-centra-il-bilanciamento-assiale\" class=\"wp-block-heading\">Why Apply a Coefficient of 1.5 (and What Does Axial Balance Have to Do With It)?<\/h2>\n\n<p class=\"wp-block-paragraph\">A <strong>coefficient of 1.5<\/strong> must be applied to the calculated value, with the following practical objective: <strong>to prevent Qric from falling below the flow rate required to achieve axial balance of the impeller<\/strong>.<\/p>\n\n<p class=\"wp-block-paragraph\">So, in the example:<\/p>\n\n<ul class=\"wp-block-list\">\n<li>base value: <strong>Qric=3.23 m\u00b3\/h<\/strong><\/li>\n\n\n\n<li>Applying a coefficient <strong>of 1.5<\/strong>: the operating value becomes <strong>approximately Qric = 5\u20136 m\u00b3\/h<\/strong><\/li>\n<\/ul>\n\n<p class=\"wp-block-paragraph\">This passage directly links the topic of \u201ctemperature\u201d to a \u201cmechanical\u201d issue: it is not enough to prevent overheating; one must also avoid conditions in which the machine is unable to maintain an acceptable axial balance.<\/p>\n\n<h2 id=\"attenzione-a-q-ridotta-i-problemi-non-sono-solo-termici\" class=\"wp-block-heading\">PLEASE NOTE: At low Q, the problems are not just thermal<\/h2>\n\n<p class=\"wp-block-paragraph\">The text clearly points out that when the flow rate decreases and the pump operates at minimum flow for long periods, mechanical risks increase. Specifically: <\/p>\n\n<ul class=\"wp-block-list\">\n<li><strong>Increasing axial thrust<\/strong>: This must be compensated for by <strong>suitable bearings<\/strong>.<\/li>\n\n\n\n<li><strong>Single-volute pumps under partial loads<\/strong>: They generate <strong>high radial thrust<\/strong>, which requires precautions such as:\n<ul class=\"wp-block-list\">\n<li><strong>shaft reinforcement<\/strong><\/li>\n\n\n\n<li>Pay attention to the <strong>bearings<\/strong><\/li>\n<\/ul>\n<\/li>\n<\/ul>\n\n<p class=\"wp-block-paragraph\">And there\u2019s another critical issue: operating at <strong>reduced Q<\/strong> can cause <strong>damage due to cavitation<\/strong>. So, even if \u201cthermally\u201d you might think you\u2019re in control, the machine can still be damaged. <\/p>\n\n<h2 id=\"conclusioni-mai-vicino-a-q-0-per-molte-ore-limita-il-tempo-a-qric\" class=\"wp-block-heading\">Conclusions: Never stay near Q = 0 for many hours (limit the time at Qric)<\/h2>\n\n<p class=\"wp-block-paragraph\">It is not advisable to operate centrifugal pumps at flow rates <strong>close to zero<\/strong> for extended periods of time: this type of operation causes <strong>serious mechanical damage<\/strong>. And this is a key point: such damage is <strong>unrelated<\/strong> to the increase in water temperature inside the volute. <\/p>\n\n<p class=\"wp-block-paragraph\">Consequently, even when you set the device to <strong>Qric<\/strong> mode, this should <strong>be limited to a few dozen minutes<\/strong> and not continued for long periods.<\/p>\n","protected":false},"excerpt":{"rendered":"<p>Minimum flow rate of a centrifugal pump: how to estimate it to prevent temperature increases and, above all, mechanical damage. Let\u2019s look at the parameters, a practical example, and why operation at reduced flow rate should be limited in duration. <\/p>\n","protected":false},"featured_media":8707,"parent":0,"template":"","meta":{"_acf_changed":false,"_angie_page":false,"footnotes":"","_members_access_role":[],"_members_access_error":""},"tags":[1148,1145,1147,640,1141,1140,1142,1146,1143,1144],"class_list":["post-8705","esperto-risponde","type-esperto-risponde","status-publish","has-post-thumbnail","hentry","tag-axial-balancing-of-the-impeller","tag-axial-thrust","tag-bearings","tag-cavitation","tag-centrifugal-pump","tag-minimum-flow-rate","tag-qric","tag-radial-thrust","tag-recirculation","tag-temperature-increase"],"acf":[],"_links":{"self":[{"href":"https:\/\/www.idro-elettrica.it\/en\/wp-json\/wp\/v2\/esperto-risponde\/8705","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.idro-elettrica.it\/en\/wp-json\/wp\/v2\/esperto-risponde"}],"about":[{"href":"https:\/\/www.idro-elettrica.it\/en\/wp-json\/wp\/v2\/types\/esperto-risponde"}],"version-history":[{"count":0,"href":"https:\/\/www.idro-elettrica.it\/en\/wp-json\/wp\/v2\/esperto-risponde\/8705\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.idro-elettrica.it\/en\/wp-json\/wp\/v2\/media\/8707"}],"wp:attachment":[{"href":"https:\/\/www.idro-elettrica.it\/en\/wp-json\/wp\/v2\/media?parent=8705"}],"wp:term":[{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.idro-elettrica.it\/en\/wp-json\/wp\/v2\/tags?post=8705"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}