{"id":5541,"date":"2023-01-16T08:32:00","date_gmt":"2023-01-16T07:32:00","guid":{"rendered":"https:\/\/www.idro-elettrica.it\/esperto-risponde\/methods-of-regulating-a-pumping-system-with-centrifugal-machines\/"},"modified":"2026-07-01T09:47:37","modified_gmt":"2026-07-01T07:47:37","slug":"methods-of-regulating-a-pumping-system-with-centrifugal-machines","status":"publish","type":"esperto-risponde","link":"https:\/\/www.idro-elettrica.it\/en\/esperto-risponde\/methods-of-regulating-a-pumping-system-with-centrifugal-machines\/","title":{"rendered":"Centrifugal Pump Control: Methods and Q\/H Curves"},"content":{"rendered":"\n<p class=\"wp-block-paragraph\"><em>Adjusting centrifugal pumps<\/em> means modifying the actual operating point of a system by adjusting either the pump curve or the system curve. The practical goal is to adapt <strong>the flow rate (Q)<\/strong> and <strong>head (H)<\/strong> to the system\u2019s requirements, minimizing waste whenever possible. <\/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=\"#curva-q-h-e-punto-di-funzionamento-la-base\">Q\/H Curve and Operating Point: The Basics<\/a><\/li><li class=\"\"><a href=\"#curva-caratteristica-dellimpianto-cosa-descrive-davvero\">System characteristic curve: What does it really describe?<\/a><\/li><li class=\"\"><a href=\"#parte-statica-e-parte-dinamica-\ufffc-e-\ufffc\">Static part and dynamic part: \ufffc and \ufffc<\/a><\/li><li class=\"\"><a href=\"#il-principio-chiave-per-cambiare-il-punto-di-lavoro-devi-cambiare-una-curva\">The key principle: To change the working point, you must change a curve<\/a><\/li><li class=\"\"><a href=\"#regolazione-in-mandata-con-valvola-modificare-\ufffc-strozzando\">Outlet flow control using a valve: adjust \ufffc by \u201cthrottling\u201d<\/a><\/li><li class=\"\"><a href=\"#regolazione-con-bypass-modificare-\ufffc-ricircolando-la-portata-in-eccesso\">Bypass control: adjust \ufffc by recirculating the excess flow rate<\/a><\/li><li class=\"\"><a href=\"#variare-la-velocita-di-rotazione-spostare-la-curva-della-pompa-leggi-di-similitudine\">Varying the rotational speed: shifting the pump curve (laws of similarity)<\/a><\/li><li class=\"\"><a href=\"#variare-il-diametro-della-girante-curva-diversa-ma-intervento-piu-rigido\">Changing the impeller diameter: different curve, but stiffer response<\/a><\/li><li class=\"\"><a href=\"#pompe-in-parallelo-o-in-serie-modificare-la-curva-di-sistema-pompa\">Pumps in Parallel or in Series: Modifying the \u201cPump System\u201d Curve<\/a><\/li><li class=\"\"><a href=\"#conclusione\">Conclusion<\/a><\/li><\/ul><\/nav><\/div>\n\n<h2 id=\"curva-q-h-e-punto-di-funzionamento-la-base\" class=\"wp-block-heading\">Q\/H Curve and Operating Point: The Basics<\/h2>\n\n<p class=\"wp-block-paragraph\">The hydraulic performance of a centrifugal pump is represented by a <strong>Q\/H curve (flow rate\/Head)<\/strong>: at <strong>a fixed rotational speed<\/strong>, each flow rate Q corresponds to a specific Head H. All points on the curve are, in theory, points at which the pump can operate.<\/p>\n\n<p class=\"wp-block-paragraph\">The <strong>actual operating point<\/strong> is not determined \u201con paper\u201d simply by looking at the pump: it is given<strong>by the intersection<\/strong> of the pump\u2019s characteristic curve and that of the system.<\/p>\n\n<h2 id=\"curva-caratteristica-dellimpianto-cosa-descrive-davvero\" class=\"wp-block-heading\">System characteristic curve: What does it really describe?<\/h2>\n\n<p class=\"wp-block-paragraph\">The system characteristic describes <strong>how the required head H depends on the flow rate Q<\/strong>.<\/p>\n\n<p class=\"wp-block-paragraph\">For a non-branched pipeline, the required head is calculated by applying<strong>Bernoulli\u2019s equation<\/strong> between two cross-sections (1 and 2), taking into account factors such as:<\/p>\n\n<ul class=\"wp-block-list\">\n<li>pressures in the upstream and downstream tanks (<math><msub><mi>p<\/mi><mn>1<\/mn><\/msub><\/math>,  <math><msub><mi>p<\/mi><mn>2<\/mn><\/msub><\/math>)<\/li>\n\n\n\n<li>density of the liquid (<math><mi>\u03c1<\/mi><\/math>)<\/li>\n\n\n\n<li>gravitational acceleration (<math><mrow><mi>g<\/mi><mo>=<\/mo><\/mrow><mrow><mn>9,81<\/mn><mspace width=\"0.1667em\"><\/mspace><mi>m<\/mi><mi>\/<\/mi><msup><mi>s<\/mi><mn class=\"tml-sml-pad\">2<\/mn><\/msup><\/mrow><\/math>)<\/li>\n\n\n\n<li>static head (<math><msub><mi>H<\/mi><mrow><mi>g<\/mi><mi>and<\/mi><mi>o<\/mi><\/mrow><\/msub><\/math>)<\/li>\n\n\n\n<li>total friction losses (<math><mrow><mpadded lspace=\"0\"><mi mathvariant=\"normal\">\u0394<\/mi><\/mpadded><msub><mi>H<\/mi><mrow><mi>t<\/mi><mi>o<\/mi><mi>t<\/mi><\/mrow><\/msub><\/mrow><\/math>)<\/li>\n<\/ul>\n\n<p class=\"wp-block-paragraph\">When the surface areas of the tanks are large compared to the pipes, the average velocities in the tanks are often negligible; this leads to a simplified form of the relationship.<\/p>\n\n<h2 id=\"parte-statica-e-parte-dinamica-&#xFFFC;-e-&#xFFFC;\" class=\"wp-block-heading\">Static and dynamic parts:  <math><msub><mi>H<\/mi><mi>s<\/mi><\/msub><\/math>  and  <math><msub><mi>H<\/mi><mi>d<\/mi><\/msub><\/math><\/h2>\n\n<p class=\"wp-block-paragraph\">The system curve can be viewed as the sum of:<\/p>\n\n<ul class=\"wp-block-list\">\n<li>a <strong>static part<\/strong> <math><msub><mi>H<\/mi><mi>s<\/mi><\/msub><\/math>, which does not depend on the flow rate<\/li>\n\n\n\n<li>a <strong>dynamic part<\/strong> <math><msub><mi>H<\/mi><mi>d<\/mi><\/msub><\/math>, related to pressure drops that increase with flow rate<\/li>\n<\/ul>\n\n<p class=\"wp-block-paragraph\">In <strong>closed systems<\/strong>, the static value is <strong>zero<\/strong>.<\/p>\n\n<p class=\"wp-block-paragraph\">Total pressure drops  <math><mrow><mpadded lspace=\"0\"><mi mathvariant=\"normal\">\u0394<\/mi><\/mpadded><msub><mi>H<\/mi><mrow><mi>t<\/mi><mi>o<\/mi><mi>t<\/mi><\/mrow><\/msub><\/mrow><\/math>  include inlet and outlet. For sufficiently high Reynolds numbers, they are <strong>proportional to the square of the flow rate<\/strong>. In simplified form, this results in a parabolic system characteristic curve, often expressed as:  <math display=\"block\"><mrow><msub><mi>H<\/mi><mrow><mi>i<\/mi><mi>p.m.<\/mi><mi>p<\/mi><\/mrow><\/msub><mo>=<\/mo><msub><mi>H<\/mi><mi>s<\/mi><\/msub><mo>+<\/mo><msub><mi>H<\/mi><mi>d<\/mi><\/msub><mspace width=\"1em\"><\/mspace><mtext>with<\/mtext><mspace width=\"1em\"><\/mspace><msub><mi>H<\/mi><mi>d<\/mi><\/msub><mo>\u221d<\/mo><msup><mi>Q<\/mi><mn class=\"tml-med-pad\">2<\/mn><\/msup><\/mrow><\/math><\/p>\n\n<p class=\"wp-block-paragraph\">Therefore, as Q increases, the head required by the system increases <strong>quadratically<\/strong>.<\/p>\n\n<h2 id=\"il-principio-chiave-per-cambiare-il-punto-di-lavoro-devi-cambiare-una-curva\" class=\"wp-block-heading\">The key principle: To change the working point, you must change a curve<\/h2>\n\n<p class=\"wp-block-paragraph\">The point where the pump curve intersects with the piping curve (system curve) is the <strong>characteristic operating point<\/strong>.<\/p>\n\n<p class=\"wp-block-paragraph\">To \u201ctruly\u201d change it, it\u2019s not enough to simply want a different flow rate: you have to <strong>modify one of the two curves<\/strong>:<\/p>\n\n<ul class=\"wp-block-list\">\n<li>the system curve (by adjusting  <math><msub><mi>H<\/mi><mi>s<\/mi><\/msub><\/math>, on  <math><msub><mi>H<\/mi><mi>d<\/mi><\/msub><\/math>, or on both)<\/li>\n\n\n\n<li>the pump curve (by adjusting speed, configuration, and impeller)<\/li>\n<\/ul>\n\n<p class=\"wp-block-paragraph\">Below you will find the methods mentioned and how they relate to this principle.<\/p>\n\n<h2 id=\"regolazione-in-mandata-con-valvola-modificare-&#xFFFC;-strozzando\" class=\"wp-block-heading\">Outlet flow control with valve: adjust <math><msub><mi>H<\/mi><mi>d<\/mi><\/msub><\/math> by \u201cthrottling\u201d<\/h2>\n\n<p class=\"wp-block-paragraph\">This is the simplest and most cost-effective method: it involves adjusting the opening of a <strong>control valve<\/strong> installed immediately downstream of the pump\u2019s outlet flange.<\/p>\n\n<p class=\"wp-block-paragraph\">This introduces a <strong>variable localized pressure drop<\/strong>, which increases the slope of the system\u2019s characteristic curve and shifts its intersection with the pump curve: at a constant speed, <strong>the flow rate<\/strong> can <strong>be reduced<\/strong>.<\/p>\n\n<p class=\"wp-block-paragraph\">Be aware of the energy impact: the pump may generate a head greater than that required by the system, and the excess is \u201cconsumed\u201d by the valve. This energy is <strong>dissipated as heat<\/strong> and is lost. This loss is acceptable if the control range is small or adjustments are infrequent.  <\/p>\n\n<h2 id=\"regolazione-con-bypass-modificare-&#xFFFC;-ricircolando-la-portata-in-eccesso\" class=\"wp-block-heading\">Bypass Control: Modify <math><msub><mi>H<\/mi><mi>d<\/mi><\/msub><\/math> by recirculating the excess flow rate<\/h2>\n\n<p class=\"wp-block-paragraph\">In this diagram, a valve is installed downstream of the pump to <strong>divert excess flow rate into the inlet tank<\/strong>.<\/p>\n\n<p class=\"wp-block-paragraph\">This method is also <strong>energy-wasting<\/strong>: the pump delivers a flow rate greater than that required by the system. The text explicitly states that bypass losses are <strong>greater<\/strong> than those resulting from regulation using an outlet flow control valve. <\/p>\n\n<p class=\"wp-block-paragraph\">When using the bypass, to identify the operating points, we move from the concept of:<\/p>\n\n<ul class=\"wp-block-list\">\n<li><strong>equivalent circuit<\/strong> (system and bypass in parallel)<\/li>\n\n\n\n<li><strong>equivalent pump<\/strong>, determined using graphical constructions based on points on the characteristic curves<br\/>, leading to the graphical determination of the operating points of the pump and the system.<\/li>\n<\/ul>\n\n<h2 id=\"variare-la-velocita-di-rotazione-spostare-la-curva-della-pompa-leggi-di-similitudine\" class=\"wp-block-heading\">Varying the rotational speed: shifting the pump curve (laws of similarity)<\/h2>\n\n<p class=\"wp-block-paragraph\">Another method is to change the operating point by varying the machine\u2019s <strong>rotational speed<\/strong>. First, we introduce the <strong>laws of similarity<\/strong>, which are based on the idea that machines that are geometrically and kinematically similar will have the same hydraulic efficiency, and which allow us to establish relationships between <strong>head H<\/strong>, <strong>flow rate Q<\/strong>, and <strong>rotational speed n<\/strong>. <\/p>\n\n<p class=\"wp-block-paragraph\">In practical terms, changing the speed results in a <strong>different pump characteristic curve<\/strong> for each speed. The system curve will intersect each pump curve at a different point; this allows the machine to adapt its performance to changes in demand. <\/p>\n\n<p class=\"wp-block-paragraph\">The text cites the classic example of pressurizing a drinking water distribution pipeline, where hourly fluctuations in demand are managed using electronic control systems (sensors and inverters), resulting <strong>in significant energy savings<\/strong> because energy loss is avoided.<\/p>\n\n<h2 id=\"variare-il-diametro-della-girante-curva-diversa-ma-intervento-piu-rigido\" class=\"wp-block-heading\">Changing the impeller diameter: different curve, but stiffer response<\/h2>\n\n<p class=\"wp-block-paragraph\">At a fixed speed, the performance curve can be adjusted by changing the <strong>impeller diameter<\/strong> (in pumps with radial impellers).<\/p>\n\n<p class=\"wp-block-paragraph\">Compared to adjusting the speed, this procedure is described as <strong>much more involved<\/strong>, because it requires a mechanical adjustment: reducing the diameter or replacing the original impeller with one of a larger or smaller diameter.<\/p>\n\n<p class=\"wp-block-paragraph\">Here, too, the principle remains the same: changing the pump curve alters its intersection with the system curve and, consequently, the operating point.<\/p>\n\n<h2 id=\"pompe-in-parallelo-o-in-serie-modificare-la-curva-di-sistema-pompa\" class=\"wp-block-heading\">Pumps in Parallel or in Series: Modifying the \u201cPump System\u201d Curve<\/h2>\n\n<p class=\"wp-block-paragraph\">The operating point can also be adjusted by connecting pumps:<\/p>\n\n<ul class=\"wp-block-list\">\n<li><strong>In parallel<\/strong>: multiple pumps each draw water independently and discharge into the same Outlet manifold; for any given head, the resulting flow rate is the <strong>sum<\/strong> of the flow rates of the individual pumps<\/li>\n\n\n\n<li><strong>In series<\/strong>: The same flow rate passes through all the pumps, while the system&#8217;s head is the <strong>sum<\/strong> of the heads of the individual pumps (at the same Q)<\/li>\n<\/ul>\n\n<p class=\"wp-block-paragraph\">These are two different configurations, but both affect the \u201cresulting curve\u201d as seen by the system.<\/p>\n\n<h2 id=\"conclusione\" class=\"wp-block-heading\">Conclusion<\/h2>\n\n<p class=\"wp-block-paragraph\">Adjustment is not an \u201cabstract\u201d operation: it means choosing how to shift the intersection between the pump curve and the system curve. You can do this by adjusting for leaks and resistance (valve or bypass) or by adjusting the pump (speed, impeller, series\/parallel configuration), keeping in mind that some methods result in energy loss while others better match the demand. <\/p>\n","protected":false},"excerpt":{"rendered":"<p>Regulating centrifugal pumps involves shifting the operating point by modifying either the pump curve or the system curve. In this answer, we\u2019ll look at the most commonly used methods and what they entail in terms of flow rate, Head, and power loss. <\/p>\n","protected":false},"featured_media":5542,"parent":0,"template":"","meta":{"_acf_changed":false,"_angie_page":false,"footnotes":"","_members_access_role":[],"_members_access_error":""},"tags":[1232,1239,1233,1236,1235,1237,1238,1240,736,1234],"class_list":["post-5541","esperto-risponde","type-esperto-risponde","status-publish","has-post-thumbnail","hentry","tag-centrifugal-pump-control","tag-impeller-diameter","tag-operating-point","tag-outlet-valve","tag-pressure-drops","tag-pump-bypass","tag-pump-inverters","tag-pumps-in-series-and-parallel","tag-q-h-curve","tag-system-characteristic-curve"],"acf":[],"_links":{"self":[{"href":"https:\/\/www.idro-elettrica.it\/en\/wp-json\/wp\/v2\/esperto-risponde\/5541","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\/5541\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.idro-elettrica.it\/en\/wp-json\/wp\/v2\/media\/5542"}],"wp:attachment":[{"href":"https:\/\/www.idro-elettrica.it\/en\/wp-json\/wp\/v2\/media?parent=5541"}],"wp:term":[{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.idro-elettrica.it\/en\/wp-json\/wp\/v2\/tags?post=5541"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}