{"id":7894,"date":"2020-07-14T15:49:00","date_gmt":"2020-07-14T13:49:00","guid":{"rendered":"https:\/\/www.idro-elettrica.it\/esperto-risponde\/vtp-convergence-meaning-and-why-it-matters\/"},"modified":"2026-07-01T09:46:00","modified_gmt":"2026-07-01T07:46:00","slug":"vtp-convergence-meaning-and-why-it-matters","status":"publish","type":"esperto-risponde","link":"https:\/\/www.idro-elettrica.it\/en\/esperto-risponde\/vtp-convergence-meaning-and-why-it-matters\/","title":{"rendered":"VTP Convergence: Meaning and Why It Matters"},"content":{"rendered":"\n<p class=\"wp-block-paragraph\"><em>VTP Submergence<\/em> is the minimum immersion depth required for a Vertical Turbine Pump (VTP) to operate correctly, preventing undesirable phenomena in the inlet. In this answer, I\u2019ll explain what it means, why it\u2019s required, and how it affects the effective volume of the tank. <\/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=\"#cose-la-sommergenza-vtp\">What Is VTP Convergence?<\/a><\/li><li class=\"\"><a href=\"#perche-la-sommergenza-vtp-e-importante-vortici-e-aria-in-aspirazione\">Why VTP Convergence Is Important: Vortices and Inlet Air<\/a><\/li><li class=\"\"><a href=\"#le-cause-dei-vortici-cosa-li-genera-secondo-il-testo\">The causes of vortices: what generates them, according to the text<\/a><\/li><li class=\"\"><a href=\"#da-cosa-dipende-la-sommergenza-vtp\">What Causes VTP Sommergenza?<\/a><\/li><li class=\"\"><a href=\"#indicazione-pratica-sommergenza-vtp-e-diametro-d\">Practical Note: VTP Overlap and Diameter D<\/a><\/li><li class=\"\"><a href=\"#collegamento-con-l-npsh-richiesto-coerenza-tecnica-del-parametro\">Link to the required NPSH (technical consistency of the parameter)<\/a><\/li><li class=\"\"><a href=\"#impatto-sul-volume-utile-della-vasca-uni-en-12845-9-3-5\">Impact on the usable volume of the tank (UNI EN 12845, 9.3.5)<\/a><\/li><li class=\"\"><a href=\"#come-usare-la-sommergenza-vtp-in-progettazione-senza-forzature\">How to Use VTP Overlap in Design (Without Forcing It)<\/a><\/li><li class=\"\"><a href=\"#conclusione\">Conclusion<\/a><\/li><\/ul><\/nav><\/div>\n\n<h2 id=\"cose-la-sommergenza-vtp\" class=\"wp-block-heading\">What Is VTP Convergence?<\/h2>\n\n<p class=\"wp-block-paragraph\">The <strong>submergence<\/strong> (denoted as <strong>S<\/strong>) of <strong>Vertical Turbine Pumps (VTP)<\/strong> is the <strong>minimum depth to which the pump<\/strong> must <strong>be submerged<\/strong> to <strong>prevent the formation of vortices at the free surface of the tank<\/strong>.<\/p>\n\n<p class=\"wp-block-paragraph\">As part of the requirements, it is specified that <strong>the minimum submersion depth<\/strong> must be indicated <strong>for submersible pumps<\/strong>. In other words, this is not \u201coptional\u201d information, but a parameter that must be explicitly stated. <\/p>\n\n<h2 id=\"perche-la-sommergenza-vtp-e-importante-vortici-e-aria-in-aspirazione\" class=\"wp-block-heading\">Why VTP Convergence Is Important: Vortices and Inlet Air<\/h2>\n\n<p class=\"wp-block-paragraph\">The primary objective of VTP backflow <strong>prevention<\/strong> is <strong>to prevent air from being drawn<\/strong> into <strong>the inlet<\/strong> or, in the case of VTPs, <strong>directly into the pump<\/strong>.<\/p>\n\n<p class=\"wp-block-paragraph\">The problem stems from <strong>the vortexes<\/strong> that can form <strong>on the surface of the storage tank<\/strong>. When vortices form on the free surface, they can create conditions that allow air to enter the Inlet\u2014a phenomenon that designers explicitly seek to prevent. <\/p>\n\n<h2 id=\"le-cause-dei-vortici-cosa-li-genera-secondo-il-testo\" class=\"wp-block-heading\">The causes of vortices: what generates them, according to the text<\/h2>\n\n<p class=\"wp-block-paragraph\">Vortices on the free edge (which we want to avoid) are attributed to two main causes:<\/p>\n\n<ul class=\"wp-block-list\">\n<li><strong>insufficient water depth<\/strong> at the starting end of the pipe (i.e., not enough water \u201ccover\u201d over the inlet);<\/li>\n\n\n\n<li><strong>speeding<\/strong>.<\/li>\n<\/ul>\n\n<p class=\"wp-block-paragraph\">The text also highlights a very clear practical implication: <strong>if the speed limits specified by the rules are observed<\/strong>, then the primary cause of air ingress is <strong>an insufficient hydraulic head<\/strong>.<\/p>\n\n<p class=\"wp-block-paragraph\">This logic directly links the VTP convergence to a \u201cminimum level\u201d requirement above the inlet.<\/p>\n\n<h2 id=\"da-cosa-dipende-la-sommergenza-vtp\" class=\"wp-block-heading\">What Causes VTP Sommergenza?<\/h2>\n\n<p class=\"wp-block-paragraph\">The VTP convergence <strong>is not a fixed value<\/strong>: <em>it varies<\/em> depending on:<\/p>\n\n<ul class=\"wp-block-list\">\n<li><strong>pump size<\/strong>;<\/li>\n\n\n\n<li><strong>rotational speed<\/strong>.<\/li>\n<\/ul>\n\n<p class=\"wp-block-paragraph\">This point is essential because it avoids a common mistake in technical documentation: treating submergence as a \u201cstandard\u201d depth applicable to every situation. In the text, however, it is presented as a value that must be correlated with the machine\u2019s characteristics and operating conditions. <\/p>\n\n<h2 id=\"indicazione-pratica-sommergenza-vtp-e-diametro-d\" class=\"wp-block-heading\">Practical Note: VTP Overlap and Diameter D<\/h2>\n\n<p class=\"wp-block-paragraph\">As a general guideline, VTP Convergence (S) is linked to a simple rule:<\/p>\n\n<ul class=\"wp-block-list\">\n<li><strong>S<\/strong> is approximately <strong>two and a half times the diameter (D)<\/strong> of <strong>the hydraulic component<\/strong>.<\/li>\n<\/ul>\n\n<p class=\"wp-block-paragraph\">This guideline is useful as a general reference, but the text does not treat it as an \u201cautomatic\u201d value that must always be applied: in fact, it is immediately clarified that the submergence must still be <strong>commensurate with the conditions required<\/strong> of the pump (see the connection to NPSH in the following paragraph).<\/p>\n\n<h2 id=\"collegamento-con-l-npsh-richiesto-coerenza-tecnica-del-parametro\" class=\"wp-block-heading\">Link to the required NPSH (technical consistency of the parameter)<\/h2>\n\n<p class=\"wp-block-paragraph\">In addition to the guideline regarding diameter, the text emphasizes a requirement for design consistency:<\/p>\n\n<ul class=\"wp-block-list\">\n<li>The VTP head <strong>must be commensurate with the NPSH requirements of the pump<\/strong>.<\/li>\n<\/ul>\n\n<p class=\"wp-block-paragraph\">This statement should be interpreted in a very practical sense: even if an indicative value (such as the D ratio) is used, the final choice of submergence depth must be <strong>compatible<\/strong> with the pump\u2019s requirements for operation under the expected conditions.<\/p>\n\n<p class=\"wp-block-paragraph\">In practice, the VTP Convergence serves as a bridge between:<\/p>\n\n<ul class=\"wp-block-list\">\n<li>the <strong>physical phenomenon<\/strong> we want to avoid (vortices and air),<\/li>\n\n\n\n<li>and the <strong>conditions required for the pump<\/strong> (required NPSH).<\/li>\n<\/ul>\n\n<h2 id=\"impatto-sul-volume-utile-della-vasca-uni-en-12845-9-3-5\" class=\"wp-block-heading\">Impact on the usable volume of the tank (UNI EN 12845, 9.3.5)<\/h2>\n\n<p class=\"wp-block-paragraph\">From a design perspective, the text identifies a key criterion: to properly design the water inlet for VTPs, a <strong>minimum hydraulic head equal to the submergence depth S<\/strong> must <strong>be maintained above the inlet<\/strong>.<\/p>\n\n<p class=\"wp-block-paragraph\">This has a direct and significant impact on technical documentation as well:<\/p>\n\n<ul class=\"wp-block-list\">\n<li><strong>Below<\/strong> the level corresponding to submergence <strong>S<\/strong>, the <strong>volume stored in the tank cannot be used<\/strong> for the purpose of calculating the <strong>usable volume<\/strong>.<\/li>\n<\/ul>\n\n<p class=\"wp-block-paragraph\">Therefore, the VTP clearance is not merely a \u201cmechanical\u201d or structural dimension: it affects how the actually usable volume is calculated.<\/p>\n\n<h2 id=\"come-usare-la-sommergenza-vtp-in-progettazione-senza-forzature\" class=\"wp-block-heading\">How to Use VTP Overlap in Design (Without Forcing It)<\/h2>\n\n<p class=\"wp-block-paragraph\">To remain consistent with the above, VTP convergence should be treated as a requirement by:<\/p>\n\n<ol class=\"wp-block-list\">\n<li><strong>specify<\/strong> for submersible pumps (minimum submersion depth);<\/li>\n\n\n\n<li><strong>set<\/strong> to prevent vortices on the free edge and air entrainment;<\/li>\n\n\n\n<li><strong>evaluate<\/strong> based on the pump size and rotational speed;<\/li>\n\n\n\n<li><strong>verify<\/strong> that it is compatible with the required NPSH;<\/li>\n\n\n\n<li><strong>Apply<\/strong> as a minimum layer over the inlet;<\/li>\n\n\n\n<li><strong>Include this<\/strong> in the calculation of the usable volume, excluding anything below that level.<\/li>\n<\/ol>\n\n<p class=\"wp-block-paragraph\">If you&#8217;d like, I can help you turn these points into an internal verification checklist (specifications\/report), but I&#8217;ll need you to confirm the missing information listed at the bottom.<\/p>\n\n<h2 id=\"conclusione\" class=\"wp-block-heading\">Conclusion<\/h2>\n\n<p class=\"wp-block-paragraph\">The <em>VTP Submergence<\/em> is the <strong>minimum submergence depth<\/strong> required to prevent vortices at the free surface and air ingress into the inlet; it depends on the pump\u2019s characteristics and must, in any case, be consistent with<strong>the required NPSH<\/strong>. Furthermore, the entire volume below the submergence level <strong>cannot<\/strong> be counted as <strong>usable<\/strong> tank <strong>volume<\/strong>. <\/p>\n","protected":false},"excerpt":{"rendered":"<p>VTP submersion depth refers to the minimum depth to which the pump must be submerged to prevent vortices at the free surface of the tank and air entrainment. Let\u2019s take a look at the requirements of UNI EN 12845, what factors determine the submersion depth, and how it affects the usable storage volume. <\/p>\n","protected":false},"featured_media":7898,"parent":0,"template":"","meta":{"_acf_changed":false,"_angie_page":false,"footnotes":"","_members_access_role":[],"_members_access_error":""},"tags":[628,630,550,607,632,549,633,629,631,567],"class_list":["post-7894","esperto-risponde","type-esperto-risponde","status-publish","has-post-thumbnail","hentry","tag-convergence","tag-inlet","tag-npsh","tag-sprinkler-systems","tag-storage-tank","tag-uni-en-12845","tag-useful-volume","tag-vertical-turbine-pumps","tag-vortices","tag-vtp"],"acf":[],"_links":{"self":[{"href":"https:\/\/www.idro-elettrica.it\/en\/wp-json\/wp\/v2\/esperto-risponde\/7894","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\/7894\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.idro-elettrica.it\/en\/wp-json\/wp\/v2\/media\/7898"}],"wp:attachment":[{"href":"https:\/\/www.idro-elettrica.it\/en\/wp-json\/wp\/v2\/media?parent=7894"}],"wp:term":[{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.idro-elettrica.it\/en\/wp-json\/wp\/v2\/tags?post=7894"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}