{"id":7964,"date":"2020-11-18T16:21:00","date_gmt":"2020-11-18T15:21:00","guid":{"rendered":"https:\/\/www.idro-elettrica.it\/esperto-risponde\/useful-volume-of-firefighting-water-reserve-lets-do-some-calculations\/"},"modified":"2026-07-01T09:46:29","modified_gmt":"2026-07-01T07:46:29","slug":"useful-volume-of-firefighting-water-reserve-lets-do-some-calculations","status":"publish","type":"esperto-risponde","link":"https:\/\/www.idro-elettrica.it\/en\/esperto-risponde\/useful-volume-of-firefighting-water-reserve-lets-do-some-calculations\/","title":{"rendered":"Useful Volume of Firefighting Water Reserve: Let&#8217;s Do Some Calculations"},"content":{"rendered":"\n<p class=\"wp-block-paragraph\"><em>Useful Volume of Fire Protection Water Reserves<\/em>: When discussing water storage for fire protection purposes, \u201chow much it holds\u201d does not always equate to \u201chow much is actually usable.\u201d In this answer, I clarify the difference and the technical reason why part of the volume should not be included in the calculation.* <\/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=\"#differenza-tra-volume-geometrico-e-volume-utile\">Difference Between Geometric Volume and Useful Volume<\/a><\/li><li class=\"\"><a href=\"#il-riferimento-per-il-calcolo-della-parte-non-utilizzabile\">The basis for calculating the unusable portion<\/a><\/li><li class=\"\"><a href=\"#perche-esiste-una-parte-non-utilizzabile-le-2-funzioni\">Because there is a \u201cnon-usable\u201d part: the two functions<\/a><\/li><li class=\"\"><a href=\"#testa-idraulica-minima-a-e-distanza-minima-dal-fondo-b\">Minimum hydraulic head (A) and minimum distance from the bottom (B)  <\/a><\/li><li class=\"\"><a href=\"#esempio-pratico-dn-250-e-calcolo-del-volume-non-conteggiabile\">Practical example: DN 250 and calculation of the non-billable volume<\/a><\/li><li class=\"\"><a href=\"#piastra-antivortice-cosa-cambia-nella-quota-a\">Anti-vortex plate: What changes in Category A<\/a><\/li><li class=\"\"><a href=\"#conclusione-come-usare-correttamente-volume-geometrico-e-volume-utile\">Conclusion: How to Correctly Use \u201cGeometric Volume\u201d and \u201cUseful Volume\u201d<\/a><\/li><\/ul><\/nav><\/div>\n\n<h2 id=\"differenza-tra-volume-geometrico-e-volume-utile\" class=\"wp-block-heading\">Difference Between Geometric Volume and Useful Volume<\/h2>\n\n<p class=\"wp-block-paragraph\">The <strong>geometric volume<\/strong> of the tank is the \u201ctotal\u201d volume of the container, which can be calculated using <strong>basic geometric formulas<\/strong> based on the tank\u2019s shape and dimensions.<\/p>\n\n<p class=\"wp-block-paragraph\">The <strong>usable volume<\/strong>, on the other hand, is the volume that can actually be counted toward the fire-fighting water supply: in practice, it is the geometric volume <strong>minus<\/strong> the portion of the tank that <strong>cannot be considered available<\/strong> for achieving the required operating autonomy.<\/p>\n\n<p class=\"wp-block-paragraph\">This distinction is crucial because, in practice, the inlet and hydraulic conditions do not allow for the full utilization of all the water contained\u2014&#8221;down to the last drop.&#8221;<\/p>\n\n<h2 id=\"il-riferimento-per-il-calcolo-della-parte-non-utilizzabile\" class=\"wp-block-heading\">The basis for calculating the unusable portion<\/h2>\n\n<p class=\"wp-block-paragraph\">To determine the portion (and therefore the volume) that should not be included in the calculation, refer to <strong>UNI EN 12845:2020<\/strong>, <strong>Chapter 9.3, \u201cStorage Tanks<\/strong>,\u201d specifically:<\/p>\n\n<ul class=\"wp-block-list\">\n<li><strong>Section 9.3.5<\/strong><\/li>\n\n\n\n<li><strong>Figure 4<\/strong><\/li>\n\n\n\n<li><strong>Table 12<\/strong><\/li>\n<\/ul>\n\n<p class=\"wp-block-paragraph\">The objective is to estimate the portion of the tank that, for functional reasons and to protect the equipment, <strong>is excluded<\/strong> from the usable volume.<\/p>\n\n<h2 id=\"perche-esiste-una-parte-non-utilizzabile-le-2-funzioni\" class=\"wp-block-heading\">Because there is a \u201cnon-usable\u201d part: the two functions<\/h2>\n\n<p class=\"wp-block-paragraph\">The <strong>unusable<\/strong> portion of the geometric volume serves a dual purpose.<\/p>\n\n<ol class=\"wp-block-list\">\n<li><strong>Pump protection<\/strong><br\/> This is used to prevent the pumps from sucking in <strong>solid bodies or substances<\/strong> that could damage them. For this reason, a <strong>minimum distance (B) from the bottom of the tank<\/strong> is indicated. <\/li>\n\n\n\n<li><strong>Prevention of Vortices and Air Ingestion<\/strong><br\/>This is designed to prevent the <strong>formation of vortices<\/strong> on the free surface of the tank, which would result <strong>in air being drawn<\/strong> into the inlet pipe (or, in the case of VTPs, directly into the pump). This phenomenon is related to the vortex-like movements that form on the surface. <\/li>\n<\/ol>\n\n<p class=\"wp-block-paragraph\">In practice, the goal is to ensure stable inlet flow, without any issues that could compromise operation.<\/p>\n\n<h2 id=\"testa-idraulica-minima-a-e-distanza-minima-dal-fondo-b\" class=\"wp-block-heading\">Minimum hydraulic head (A) and minimum distance from the bottom (B)  <\/h2>\n\n<p class=\"wp-block-paragraph\">To control the risk of air ingress, a <strong>minimum head (A)<\/strong> is maintained above the pump&#8217;s inlet pipe.<\/p>\n\n<p class=\"wp-block-paragraph\">The text points out that vortices can be caused by:<\/p>\n\n<ul class=\"wp-block-list\">\n<li>insufficient <strong>overlap of the head<\/strong> of the tube and\/or<\/li>\n\n\n\n<li>excessive flow <strong>velocity<\/strong> in the pipe.<\/li>\n<\/ul>\n\n<p class=\"wp-block-paragraph\">Assuming that the speed limits specified by the rules have been observed, insufficient hydraulic head remains a possible cause of air ingress. Hence the importance of complying with dimension <strong>A<\/strong>. <\/p>\n\n<p class=\"wp-block-paragraph\">As indicated, <strong>distances A and B depend on the diameter of the inlet pipe<\/strong>, as shown in the reference table.<\/p>\n\n<h2 id=\"esempio-pratico-dn-250-e-calcolo-del-volume-non-conteggiabile\" class=\"wp-block-heading\">Practical example: DN 250 and calculation of the non-billable volume<\/h2>\n\n<p class=\"wp-block-paragraph\">For an <strong>inlet pipe diameter of 250 mm<\/strong>, the example states:<\/p>\n\n<ul class=\"wp-block-list\">\n<li><strong>A = 0.75 m<\/strong><\/li>\n\n\n\n<li><strong>B = 0.20 m<\/strong><\/li>\n\n\n\n<li><strong>A + B = 0.95 m<\/strong><\/li>\n<\/ul>\n\n<p class=\"wp-block-paragraph\">Hence, the calculation method is as follows:<\/p>\n\n<p class=\"wp-block-paragraph\"><strong>USEFUL VOLUME = GEOMETRIC VOLUME \u2212 BASE AREA \u00d7 0.95<\/strong><\/p>\n\n<p class=\"wp-block-paragraph\">Operational interpretation: The volume between the bottom of the tank and a level of <strong>0.95 m<\/strong> <strong>cannot be used for firefighting purposes<\/strong>, in order to ensure the two functions described above (pump protection and prevention of vortices\/air).<\/p>\n\n<p class=\"wp-block-paragraph\">This step is the key to moving from \u201climited volume\u201d to \u201cactually measurable volume.\u201d<\/p>\n\n<h2 id=\"piastra-antivortice-cosa-cambia-nella-quota-a\" class=\"wp-block-heading\">Anti-vortex plate: What changes in Category A<\/h2>\n\n<p class=\"wp-block-paragraph\">The same table shows that the use of an <strong>anti-vortex plate<\/strong> (with the specified minimum dimensions) affects dimension A.<\/p>\n\n<p class=\"wp-block-paragraph\">In the example with <strong>a DN of 250 mm<\/strong>, it is specified that the plate must have a minimum size of <strong>1.0 \u00d7 1.0 m<\/strong>. In this case: <\/p>\n\n<ul class=\"wp-block-list\">\n<li>Height <strong>A<\/strong> is no longer a function of the diameter of the inlet pipe;<\/li>\n\n\n\n<li><strong>A becomes a constant equal to 0.1 m<\/strong>.<\/li>\n<\/ul>\n\n<p class=\"wp-block-paragraph\">In practical terms, this means that the anti-vortex plate is a component that directly affects the determination of the height above the Inlet (A) and, therefore, the non-countable volume, according to the specified configuration.<\/p>\n\n<h2 id=\"conclusione-come-usare-correttamente-volume-geometrico-e-volume-utile\" class=\"wp-block-heading\">Conclusion: How to Correctly Use \u201cGeometric Volume\u201d and \u201cUseful Volume\u201d<\/h2>\n\n<p class=\"wp-block-paragraph\">In summary, the <strong>geometric volume<\/strong> describes the total capacity of the tank, while the <strong>usable fire-fighting water volume<\/strong> excludes a non-countable lower portion, determined by the distance from the bottom (B) and the minimum head above the Inlet (A). The DN 250 example shows how A and B result in a total elevation (A+B) that must be subtracted\u2014using the base area\u2014to arrive at the usable volume.   <\/p>\n","protected":false},"excerpt":{"rendered":"<p>Useful volume of fire protection water reserves: how this differs from the tank\u2019s geometric volume, and why not all the water \u201ccontained\u201d in the tank can be counted. Let\u2019s look at how to identify the unusable portion and the effect of the anti-vortex plate, using a practical example. <\/p>\n","protected":false},"featured_media":7966,"parent":0,"template":"","meta":{"_acf_changed":false,"_angie_page":false,"footnotes":"","_members_access_role":[],"_members_access_error":""},"tags":[712,708,669,707,706,709,711,710,549,633],"class_list":["post-7964","esperto-risponde","type-esperto-risponde","status-publish","has-post-thumbnail","hentry","tag-actual-tank-capacity","tag-anti-vortex-plate","tag-fire-protection-water-supply-2","tag-fire-suppression-tank","tag-geometric-volume","tag-inlet-line","tag-inlet-vortices","tag-storage-tanks","tag-uni-en-12845","tag-useful-volume"],"acf":[],"_links":{"self":[{"href":"https:\/\/www.idro-elettrica.it\/en\/wp-json\/wp\/v2\/esperto-risponde\/7964","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\/7964\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.idro-elettrica.it\/en\/wp-json\/wp\/v2\/media\/7966"}],"wp:attachment":[{"href":"https:\/\/www.idro-elettrica.it\/en\/wp-json\/wp\/v2\/media?parent=7964"}],"wp:term":[{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.idro-elettrica.it\/en\/wp-json\/wp\/v2\/tags?post=7964"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}