{"id":7861,"date":"2020-10-06T16:09:00","date_gmt":"2020-10-06T14:09:00","guid":{"rendered":"https:\/\/www.idro-elettrica.it\/esperto-risponde\/lower-and-upper-beams-key-differences\/"},"modified":"2026-07-01T09:46:30","modified_gmt":"2026-07-01T07:46:30","slug":"lower-and-upper-beams-key-differences","status":"publish","type":"esperto-risponde","link":"https:\/\/www.idro-elettrica.it\/en\/esperto-risponde\/lower-and-upper-beams-key-differences\/","title":{"rendered":"Lower and Upper Beams: Key Differences"},"content":{"rendered":"\n<p class=\"wp-block-paragraph\"><em><strong>Sub-suction and Over-suction<\/strong><\/em> These are two inlet conditions that are distinguished by the position of the pump relative to the free surface of the water in the tank. Below is a practical explanation, with references to the relevant points and the main operational implications. <\/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=\"#riferimenti-normativi-citati-nella-risposta\">Regulatory references cited in the response<\/a><\/li><li class=\"\"><a href=\"#come-si-individuano-sottobattente-e-soprabattente-piano-di-riferimento\">How to Identify the Bottom and Top Beams (Reference Plane)<\/a><\/li><li class=\"\"><a href=\"#sottobattente-perche-e-considerata-la-condizione-intrinsecamente-sicura\">Sub-case: Why is it considered an \u201cintrinsically safe\u201d condition?<\/a><\/li><li class=\"\"><a href=\"#soprabattente-fattori-che-ne-sconsigliano-lutilizzo\">Overhead doors: factors that make their use inadvisable<\/a><\/li><li class=\"\"><a href=\"#i-rischi-operativi-tipici-del-soprabattente-aria-ebollizione-prestazioni\">Typical operational risks associated with the upper tank (air, boiling, performance)<\/a><\/li><li class=\"\"><a href=\"#valvola-di-fondo-quando-serve-e-perche-diventa-indispensabile\">Bottom Valve: When It\u2019s Needed and Why It Becomes \u201cIndispensable\u201d<\/a><\/li><li class=\"\"><a href=\"#limiti-e-condizioni-richieste-preferenza-per-il-sottobattente\">Required limits and conditions (preference for the sub-batten)<\/a><\/li><li class=\"\"><a href=\"#conclusione\">Conclusion<\/a><\/li><\/ul><\/nav><\/div>\n\n<h2 id=\"riferimenti-normativi-citati-nella-risposta\" class=\"wp-block-heading\">Regulatory references cited in the response<\/h2>\n\n<p class=\"wp-block-paragraph\">The applicable standard cited is <strong>UNI EN 12845:2020<\/strong>, specifically:<\/p>\n\n<ul class=\"wp-block-list\">\n<li><strong>Section 10.6.1<\/strong> (inlet conditions);<\/li>\n\n\n\n<li><strong>Section 10.6.2<\/strong>, with a focus on:\n<ul class=\"wp-block-list\">\n<li><strong>10.6.2.2 (sub-section)<\/strong>;<\/li>\n\n\n\n<li><strong>10.6.2.3 (overhead panel)<\/strong>.<\/li>\n<\/ul>\n<\/li>\n<\/ul>\n\n<p class=\"wp-block-paragraph\">Reference is also made to <strong>Chapter 8 of UNI\/TR 11438:2016<\/strong>, which addresses the same topic.<\/p>\n\n<h2 id=\"come-si-individuano-sottobattente-e-soprabattente-piano-di-riferimento\" class=\"wp-block-heading\">How to Identify the Bottom and Top Beams (Reference Plane)<\/h2>\n\n<p class=\"wp-block-paragraph\">To distinguish between the two conditions, we start with a \u201cbasic\u201d configuration in which the <strong>reference plane<\/strong> is the <strong>horizontal plane passing through the center of the pump\u2019s inlet<\/strong>.<\/p>\n\n<p class=\"wp-block-paragraph\">From here, the hydraulic conditions <strong>BELOW<\/strong> and <strong>ABOVE the weir<\/strong> can be directly determined by observing the relationship between:<\/p>\n\n<ul class=\"wp-block-list\">\n<li>pump section (inlet\/shaft, depending on the diagram in question);<\/li>\n\n\n\n<li><strong>free water surface<\/strong> level in the reservoir (including the minimum level).<\/li>\n<\/ul>\n\n<h2 id=\"sottobattente-perche-e-considerata-la-condizione-intrinsecamente-sicura\" class=\"wp-block-heading\">Sub-case: Why is it considered an \u201cintrinsically safe\u201d condition?<\/h2>\n\n<p class=\"wp-block-paragraph\">The <strong>submersed<\/strong> condition is described as <strong>inherently safe<\/strong> because the pump is <strong>always<\/strong> located <strong>below the free surface of the water<\/strong>. During inlet, this results in a favorable energy condition due to: <\/p>\n\n<ul class=\"wp-block-list\">\n<li><strong>atmospheric pressure<\/strong>;<\/li>\n\n\n\n<li><strong>severity<\/strong>.<\/li>\n<\/ul>\n\n<p class=\"wp-block-paragraph\">The text also states that <strong>NPSHd<\/strong> values are <em>typically<\/em> <strong>always &gt; 10.33 m.w.c.<\/strong> In practice, the inlet system is more \u201cstable\u201d and less prone to problems typically associated with unfavorable conditions. <\/p>\n\n<h2 id=\"soprabattente-fattori-che-ne-sconsigliano-lutilizzo\" class=\"wp-block-heading\">Overhead doors: factors that make their use inadvisable<\/h2>\n\n<p class=\"wp-block-paragraph\">The &#8221; <strong>soprabattente<\/strong> &#8221; condition is presented as a solution linked to several factors that <strong>strongly discourage its use<\/strong>.<\/p>\n\n<p class=\"wp-block-paragraph\">The starting point is that, in the inlet, the pump relies <strong>solely<\/strong> on <strong>the energy derived from atmospheric pressure<\/strong>. In the text <strong>,<\/strong> this is reflected in <strong>NPSHd values that are typically well below 10.33 m.w.c.<\/strong> <\/p>\n\n<p class=\"wp-block-paragraph\">Furthermore, the design calls for the inlet line to be maintained at a <strong>pressure<\/strong> lower than atmospheric pressure to allow water to flow upward\u2014a condition that makes certain construction and maintenance aspects more sensitive.<\/p>\n\n<h2 id=\"i-rischi-operativi-tipici-del-soprabattente-aria-ebollizione-prestazioni\" class=\"wp-block-heading\">Typical operational risks associated with the upper tank (air, boiling, performance)<\/h2>\n\n<p class=\"wp-block-paragraph\">In the supply line, a vacuum in the pipe means that any connection (such as <strong>a flanged<\/strong> or <strong>threaded connection<\/strong>) can become a potential source of <strong>air ingress<\/strong>, because:<\/p>\n\n<ul class=\"wp-block-list\">\n<li>The pressure inside the pipe is significantly lower than the external pressure.<\/li>\n<\/ul>\n\n<p class=\"wp-block-paragraph\">If the pressure then reaches the <strong>vapor pressure<\/strong>, <strong>water<\/strong> may <strong>boil at room temperature<\/strong>, resulting in the formation of a <strong>two-phase fluid<\/strong>. According to reports, when this fluid enters the pump: <\/p>\n\n<ul class=\"wp-block-list\">\n<li>would prevent us from providing the expected services;<\/li>\n\n\n\n<li>This would result in damage to the pump.<\/li>\n<\/ul>\n\n<p class=\"wp-block-paragraph\">In summary, the overflow increases the likelihood of conditions that could cause the system to deviate from <strong>its design Q\/H<\/strong>.<\/p>\n\n<h2 id=\"valvola-di-fondo-quando-serve-e-perche-diventa-indispensabile\" class=\"wp-block-heading\">Bottom Valve: When It\u2019s Needed and Why It Becomes \u201cIndispensable\u201d<\/h2>\n\n<p class=\"wp-block-paragraph\">The text highlights a practical point: the inlet line can remain full only if it is equipped with a <strong>check valve<\/strong>.<\/p>\n\n<p class=\"wp-block-paragraph\">If this component does not function properly (for example, due to <strong>a leak<\/strong>), the pump and the inlet line may be drained down to the free surface level. The resulting consequence is clear: <strong>the pump will be unable to deliver the design flow rate (Q\/H)<\/strong>. <\/p>\n\n<p class=\"wp-block-paragraph\">For this reason, it is stated that in the case of the <strong>SOPRABATTENTE<\/strong>, the bottom valve is:<\/p>\n\n<ul class=\"wp-block-list\">\n<li><strong>not only necessary<\/strong>, but also <strong>essential<\/strong>.<\/li>\n<\/ul>\n\n<h2 id=\"limiti-e-condizioni-richieste-preferenza-per-il-sottobattente\" class=\"wp-block-heading\">Required limits and conditions (preference for the sub-batten)<\/h2>\n\n<p class=\"wp-block-paragraph\">The response states that the standard recommends using <strong>the &#8220;under-sash&#8221; configuration<\/strong>, subject to the following limitations:<\/p>\n\n<ul class=\"wp-block-list\">\n<li><strong>At least two-thirds of the tank&#8217;s capacity<\/strong> must be <strong>above the pump&#8217;s axis<\/strong>.<\/li>\n\n\n\n<li>The <strong>minimum level<\/strong> cannot be more than <strong>2 meters<\/strong> below the pump shaft.<\/li>\n\n\n\n<li>If the minimum level is below the pump shaft, a <strong>foot valve<\/strong> must be installed.<\/li>\n<\/ul>\n\n<p class=\"wp-block-paragraph\">Finally, it is noted that, in the case of <strong>the SOPRABATTENTE<\/strong>, the <strong>maximum distance between the pump axis and the free surface<\/strong> is <strong>3.2 m<\/strong>.<\/p>\n\n<h2 id=\"conclusione\" class=\"wp-block-heading\">Conclusion<\/h2>\n\n<p class=\"wp-block-paragraph\">&#8220;<em>Under-inlet&#8221; and &#8220;over-inlet<\/em> &#8221; are not just definitions: they describe two inlet conditions with concrete consequences for reliability and performance. In the text, the <strong>under-pressure condition<\/strong> is presented as the preferred and safer solution, while the <strong>over-pressure condition<\/strong> is associated with issues related to negative pressure, air ingress, possible two-phase flow, and the need for a foot valve. <\/p>\n","protected":false},"excerpt":{"rendered":"<p>Lower and upper flanges: What do they really mean in the inlet conditions of a fire pump? In this answer, I summarize the definitions, operational differences, and limitations specified in UNI EN 12845:2020, highlighting why the sub-flange solution is preferable and which risks increase with the over-flange configuration. <\/p>\n","protected":false},"featured_media":7866,"parent":0,"template":"","meta":{"_acf_changed":false,"_angie_page":false,"footnotes":"","_members_access_role":[],"_members_access_error":""},"tags":[558,566,556,552,554,564,560,550,562,549],"class_list":["post-7861","esperto-risponde","type-esperto-risponde","status-publish","has-post-thumbnail","hentry","tag-bottom-valve","tag-design-q-h","tag-fire-protection-system-maintenance","tag-fire-pumps","tag-fire-safety-regulations","tag-inlet-conditions","tag-lower-sash","tag-npsh","tag-overhead","tag-uni-en-12845"],"acf":[],"_links":{"self":[{"href":"https:\/\/www.idro-elettrica.it\/en\/wp-json\/wp\/v2\/esperto-risponde\/7861","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\/7861\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.idro-elettrica.it\/en\/wp-json\/wp\/v2\/media\/7866"}],"wp:attachment":[{"href":"https:\/\/www.idro-elettrica.it\/en\/wp-json\/wp\/v2\/media?parent=7861"}],"wp:term":[{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.idro-elettrica.it\/en\/wp-json\/wp\/v2\/tags?post=7861"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}