{"id":56806,"date":"2026-09-11T08:27:30","date_gmt":"2026-09-11T08:27:30","guid":{"rendered":"https:\/\/zobai.com\/?p=56806"},"modified":"2026-09-24T11:42:15","modified_gmt":"2026-09-24T11:42:15","slug":"thermal-expansion-relief-valve-for-blocked-in-liquid-lines","status":"publish","type":"post","link":"https:\/\/zobai.com\/it\/blog\/thermal-expansion-relief-valve-for-blocked-in-liquid-lines\/","title":{"rendered":"Valvola di sfioro per dilatazione termica per linee liquide isolate"},"content":{"rendered":"<style> .zobai-trv-guide { max-width: 100%; overflow-wrap: anywhere; } .zobai-trv-guide .z-table-wrap { width: 100%; overflow-x: auto; -webkit-overflow-scrolling: touch; margin: 1.25rem 0; } .zobai-trv-guide .z-table-wrap table { width: 100%; min-width: 680px; border-collapse: collapse; } .zobai-trv-guide .z-table-wrap th, .zobai-trv-guide .z-table-wrap td { vertical-align: top; padding: 0.75rem; } .zobai-trv-guide blockquote { margin: 1.25rem 0; padding-left: 1rem; } .zobai-trv-guide .z-decision-chain { margin: 1.25rem 0; font-weight: 600; } .zobai-trv-guide .z-cta { margin-top: 2rem; padding: 1.25rem; } @media (max-width: 767px) { .zobai-trv-guide .z-table-wrap table { min-width: 620px; } .zobai-trv-guide .z-table-wrap th, .zobai-trv-guide .z-table-wrap td { padding: 0.625rem; } } <\/style>\n<article class=\"zobai-trv-guide\">\n<p> Una linea di liquido isolata pu\u00f2 diventare un problema di sovrapressione quando il liquido \u00e8 intrappolato all'interno di un confine di pressione definito e la sua temperatura pu\u00f2 aumentare senza spazio sufficiente o un altro percorso adeguato per accomodare l'espansione. La prima domanda ingegneristica quindi non \u00e8 <strong>\u201cQuale valvola di sicurezza per dilatazione termica dovrei acquistare?\u201d<\/strong> \u00c8 <strong>\u201cQuesta sezione crea effettivamente uno scenario di sfioro per dilatazione termica credibile?\u201d<\/strong> <\/p>\n<p> Una valvola chiusa di per s\u00e9 non stabilisce lo scenario di sfioro completo. La dimensione del tubo non determina la capacit\u00e0 di scarico richiesta. Il servizio normale con liquido non prova che il fluido rimanga monofase attraverso il dispositivo di sfioro. Anche una valvola correttamente dimensionata deve comunque essere riesaminata con la sua linea di scarico e la contropressione prevista. <\/p>\n<p class=\"z-decision-chain\"> Definire il confine intrappolato \u2192 confermare lo scenario di dilatazione termica \u2192 stabilire il carico di sfioro richiesto \u2192 impostare la base di pressione \u2192 verificare lo stato del fluido in scarico \u2192 riesaminare il sistema di uscita \u2192 valutare la valvola candidata e le evidenze per la RFQ. <\/p>\n<p> La guida HSE del Regno Unito sulle tubazioni di processo riconosce che il liquido pu\u00f2 essere intrappolato tra valvole chiuse e che le variazioni di temperatura possono quindi causare dilatazione termica e perdita di contenimento. Inquadra inoltre lo sfioro di pressione in base alle effettive condizioni di esercizio e isolamento piuttosto che alla sola presenza di una valvola. <a href=\"https:\/\/www.hse.gov.uk\/comah\/sragtech\/techmeaspipework.htm\" target=\"_blank\" rel=\"noopener noreferrer nofollow\">Guida HSE sulle tubazioni<\/a>. <\/p>\n<p> Per un contesto pi\u00f9 ampio sulla sovrapressione delle pipeline, vedere <a href=\"https:\/\/zobai.com\/it\/applications\/pipelines\/\">la guida applicativa ZOBAI per lo sfioro nelle pipeline<\/a>. Quella pagina copre diversi scenari di sovrapressione nelle pipeline; questo articolo resta focalizzato sulla dilatazione termica del liquido intrappolato. <\/p>\n<section id=\"blocked-in-liquid-thermal-expansion-case\">\n<h2>Quando una linea di liquido isolata diventa un caso di sovrapressione per dilatazione termica?<\/h2>\n<p>\n  Un caso di dilatazione termica inizia con un <strong>effettivo confine in pressione con liquido intrappolato<\/strong>,<br \/>\n  non semplicemente con bassa portata o una singola valvola chiusa.\n<\/p>\n<ol>\n<li>\n    <strong>Definire il confine di isolamento.<\/strong><br \/>\n    Individuare le valvole, le interfacce delle apparecchiature o gli altri punti di isolamento che possono trattenere il liquido.\n  <\/li>\n<li>\n    <strong>Determinare cosa \u00e8 intrappolato.<\/strong><br \/>\n    Stabilire se la sezione pu\u00f2 diventare piena di liquido o comunque sufficientemente confinata<br \/>\n    perch\u00e9 la dilatazione termica sia rilevante.\n  <\/li>\n<li>\n    <strong>Individuare un aumento di temperatura credibile.<\/strong><br \/>\n    Considerare le condizioni di esercizio, di arresto, di standby o ambientali pertinenti che possono riscaldare<br \/>\n    il liquido intrappolato.\n  <\/li>\n<li>\n    <strong>Confrontare lo scenario con il confine in pressione protetto.<\/strong><br \/>\n    Determinare se la condizione risultante pu\u00f2 mettere in crisi la base di pressione applicabile per<br \/>\n    le tubazioni o l'apparecchiatura.\n  <\/li>\n<\/ol>\n<figure class=\"wp-block-image size-full zobai-figure\" data-image-slot=\"IMG-01\"><img loading=\"lazy\" src=\"https:\/\/zobai.com\/wp-content\/uploads\/2026\/09\/thermal-expansion-relief-valve-img-01.webp\" alt=\"Schema di una sezione di tubazione riempita di liquido, intrappolata tra punti di isolamento ed esposta a un possibile aumento di temperatura.\" title=\"Thermal Expansion Relief Valve for Blocked-In Liquid Lines - IMG-01\" width=\"1200\" height=\"900\" class=\"wp-image-56863\" decoding=\"async\" srcset=\"https:\/\/zobai.com\/wp-content\/uploads\/2026\/09\/thermal-expansion-relief-valve-img-01.webp 1200w, https:\/\/zobai.com\/wp-content\/uploads\/2026\/09\/thermal-expansion-relief-valve-img-01-300x225.webp 300w, https:\/\/zobai.com\/wp-content\/uploads\/2026\/09\/thermal-expansion-relief-valve-img-01-1024x768.webp 1024w, https:\/\/zobai.com\/wp-content\/uploads\/2026\/09\/thermal-expansion-relief-valve-img-01-768x576.webp 768w, https:\/\/zobai.com\/wp-content\/uploads\/2026\/09\/thermal-expansion-relief-valve-img-01-16x12.webp 16w, https:\/\/zobai.com\/wp-content\/uploads\/2026\/09\/thermal-expansion-relief-valve-img-01-400x300.webp 400w\" sizes=\"auto, (max-width: 1200px) 100vw, 1200px\" \/><figcaption>Schema di una sezione di tubazione riempita di liquido, intrappolata tra punti di isolamento ed esposta a un possibile aumento di temperatura.<\/figcaption><\/figure>\n<p>\n  I liquidi sono molto meno comprimibili dei gas. Quando un volume isolato \u00e8 sostanzialmente<br \/>\n  riempito di liquido, il volume aggiuntivo di liquido prodotto dal riscaldamento pu\u00f2 quindi tradursi in<br \/>\n  un brusco aumento di pressione, a meno che il sistema non disponga di un modo adeguato per assorbirlo o scaricarlo.\n<\/p>\n<p>\n  L'apporto termico dipende dall'installazione. A seconda del sistema, le fonti rilevanti possono includere<br \/>\n  il riscaldamento ambiente o solare, il riscaldamento elettrico, un'area circostante riscaldata o apparecchiature<br \/>\n  di processo adiacenti. La presenza di una di queste fonti non la rende automaticamente il caso<br \/>\n  determinante; la condizione credibile deve essere stabilita per l'installazione effettiva.\n<\/p>\n<blockquote>\n<p>\n    <strong>Bloccato non significa semplicemente \u201cassenza di flusso\u201d.\u201d<\/strong><br \/>\n    Significa che il sistema pu\u00f2 creare un confine di pressione del liquido vincolato in condizioni<br \/>\n    in cui l'espansione termica pu\u00f2 mettere alla prova tale confine.\n  <\/p>\n<\/blockquote>\n<p>\n  Questa \u00e8 la condizione da stabilire prima di dimensionare o selezionare un dispositivo di sfioro.\n<\/p>\n<h3>Scenario ingegneristico illustrativo: una linea di riserva tracciata termicamente<\/h3>\n<p>\n  Si consideri un tratto di trasferimento completamente pieno di liquido, isolato a entrambe le estremit\u00e0 mentre il suo tracciamento termico rimane<br \/>\n  alimentato. Il caso di riscaldamento credibile non \u00e8 stabilito dalla sola etichetta del tracciamento:<br \/>\n  l'ingegnere deve confermare la configurazione delle valvole, il volume intrappolato, i controlli del tracciamento, le propriet\u00e0 del fluido<br \/>\n  e la classe di pressione di ogni componente nel confine isolato. Se si propone un percorso di espansione<br \/>\n  disponibile in modo indipendente, la sua disponibilit\u00e0 in questa stessa configurazione deve essere dimostrata.<br \/>\n  Questo \u00e8 un caso di screening illustrativo, non un calcolo n\u00e9 una pressione di taratura raccomandata.\n<\/p>\n<\/section>\n<section id=\"dedicated-thermal-relief-valve-decision\">\n<h2>Ogni sezione di liquido isolata necessita di una valvola di sfioro termico dedicata?<\/h2>\n<p><strong>Non automaticamente.<\/strong><\/p>\n<p>\n  L'identificazione di uno scenario credibile di espansione termica stabilisce che il rischio di sovrapressione<br \/>\n  richiede una risposta ingegneristica adeguata. Non dimostra, di per s\u00e9, che<br \/>\n  una specifica configurazione con valvola di sfioro termico dedicata sia l'unica soluzione accettabile.\n<\/p>\n<div class=\"z-table-wrap\"\n     role=\"region\"\n     aria-label=\"Difference between identifying the thermal expansion scenario and selecting a relief valve\"\n     tabindex=\"0\"><\/p>\n<table>\n<thead>\n<tr>\n<th scope=\"col\">Domanda tecnica<\/th>\n<th scope=\"col\">Che cosa \u00e8 stato effettivamente stabilito?<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>Il liquido pu\u00f2 rimanere intrappolato e riscaldarsi?<\/td>\n<td>Esiste un potenziale scenario di espansione termica.<\/td>\n<\/tr>\n<tr>\n<td>Tale condizione pu\u00f2 mettere in crisi il limite di pressione protetto?<\/td>\n<td>La protezione contro la sovrapressione deve essere valutata.<\/td>\n<\/tr>\n<tr>\n<td>La soluzione deve necessariamente essere una valvola di sfioro termico dedicata?<\/td>\n<td>Dipende dalla strategia di protezione accettata e dalla base di progetto.<\/td>\n<\/tr>\n<tr>\n<td>Una valvola specifica \u00e8 adatta?<\/td>\n<td>Ci\u00f2 richiede un servizio definito e una documentazione specifica del prodotto.<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n<p>\n  A seconda del sistema e della base ingegneristica applicabile, la protezione pu\u00f2 prevedere un<br \/>\n  dispositivo di sfioro o un'altra soluzione ingegneristica che impedisca lo sviluppo di una pressione<br \/>\n  inaccettabile. Una soluzione accettata per un servizio o una giurisdizione non deve<br \/>\n  essere generalizzata a ogni linea di processo.\n<\/p>\n<p>\n  Un'alternativa conta solo se rimane efficace nello stesso stato di isolamento che genera<br \/>\n  the trapped-liquid case. For example, an expansion connection routed through a valve that<br \/>\n  operators close for maintenance cannot be credited without a defensible operating and<br \/>\n  design basis. Record both the proposed protection and the specific lineup in which it works.\n<\/p>\n<p>\n  API Standard 521 is a system-level pressure-relief and depressuring reference for petroleum,<br \/>\n  petrochemical, LNG and related facilities. Its scope supports evaluating the relief scenario<br \/>\n  and system before treating a product name as the design decision.<br \/>\n  <a href=\"https:\/\/www.api.org\/products-and-services\/standards\/important-standards-announcements\/standard521\" target=\"_blank\" rel=\"noopener noreferrer nofollow\">API Standard 521 information<\/a>.\n<\/p>\n<p>\n  The terminology can also mislead procurement. <strong>Valvola di sfioro idrostatica<\/strong><br \/>\n  is used in some industries for devices protecting piping where liquid can be trapped<br \/>\n  between shut-off valves. That practical terminology overlap does not make<br \/>\n  \u201chydrostatic relief valve\u201d and \u201cthermal relief valve\u201d universally interchangeable across<br \/>\n  every fluid, code or industry.\n<\/p>\n<p class=\"z-decision-chain\">\n  Scenario confirmed \u2192 protection strategy defined \u2192 applicable project\/code basis checked<br \/>\n  \u2192 device selected and verified.\n<\/p>\n<p>\n  For broader standards context, see<br \/>\n  <a href=\"https:\/\/zobai.com\/it\/standards\/api-521-pressure-relief-systems\/\"><br \/>\n    ZOBAI\u2019s API 521 pressure-relief systems guide<br \/>\n  <\/a>.\n<\/p>\n<\/section>\n<section id=\"thermal-relief-rate-valve-capacity\">\n<h2>What Determines the Required Thermal Relief Rate and Valve Capacity?<\/h2>\n<p>\n  A thermal expansion relief valve should be selected from the<br \/>\n  <strong>fabbisogno di scarico richiesto<\/strong>, not from pipe diameter or a habitual<br \/>\n  \u201csmall thermal valve\u201d size.\n<\/p>\n<p>\n  For a liquid-filled thermal-expansion case, the relief requirement depends on the thermal<br \/>\n  condition and on fluid properties that determine how the liquid responds as temperature<br \/>\n  changes. Once the required relieving flow is established, actual valve sizing also depends<br \/>\n  on the applicable relieving pressure, temperature, outlet\/back-pressure condition and<br \/>\n  fluid state.\n<\/p>\n<p class=\"z-decision-chain\">\n  Credible thermal condition \u2192 applicable fluid properties \u2192 required relieving rate<br \/>\n  \u2192 relieving pressure and outlet conditions \u2192 applicable sizing method<br \/>\n  \u2192 required flow area\/capacity \u2192 candidate valve capacity verification.\n<\/p>\n<figure class=\"wp-block-image size-full zobai-figure\" data-image-slot=\"IMG-02\"><img loading=\"lazy\" src=\"https:\/\/zobai.com\/wp-content\/uploads\/2026\/09\/thermal-expansion-relief-valve-img-02.webp\" alt=\"Flow diagram showing the process and fluid inputs used to establish thermal relief duty before checking valve capacity.\" title=\"Thermal Expansion Relief Valve for Blocked-In Liquid Lines - IMG-02\" width=\"1200\" height=\"900\" class=\"wp-image-56864\" decoding=\"async\" srcset=\"https:\/\/zobai.com\/wp-content\/uploads\/2026\/09\/thermal-expansion-relief-valve-img-02.webp 1200w, https:\/\/zobai.com\/wp-content\/uploads\/2026\/09\/thermal-expansion-relief-valve-img-02-300x225.webp 300w, https:\/\/zobai.com\/wp-content\/uploads\/2026\/09\/thermal-expansion-relief-valve-img-02-1024x768.webp 1024w, https:\/\/zobai.com\/wp-content\/uploads\/2026\/09\/thermal-expansion-relief-valve-img-02-768x576.webp 768w, https:\/\/zobai.com\/wp-content\/uploads\/2026\/09\/thermal-expansion-relief-valve-img-02-16x12.webp 16w, https:\/\/zobai.com\/wp-content\/uploads\/2026\/09\/thermal-expansion-relief-valve-img-02-400x300.webp 400w\" sizes=\"auto, (max-width: 1200px) 100vw, 1200px\" \/><figcaption>Flow diagram showing the process and fluid inputs used to establish thermal relief duty before checking valve capacity.<\/figcaption><\/figure>\n<p>\n  Two shortcuts are worth rejecting. <strong>La dimensione della connessione non \u00e8 capacit\u00e0<\/strong>:<br \/>\n  a valve may fit the line mechanically while still lacking the capacity required for the<br \/>\n  relief case. And <strong>\u201cthermal relief flow is often relatively small\u201d is not a sizing<br \/>\n  metodo<\/strong>: even a modest duty has to be established before a candidate valve can<br \/>\n  be accepted.\n<\/p>\n<p>\n  API 520 Part I is specifically concerned with pressure-relieving-device sizing and<br \/>\n  selection within its scope.<br \/>\n  <a href=\"https:\/\/www.api.org\/products-and-services\/standards\/important-standards-announcements\/520parti\" target=\"_blank\" rel=\"noopener noreferrer nofollow\">API 520 Part I information<\/a>.\n<\/p>\n<p>\n  For the detailed general sizing workflow rather than repeating it here, see<br \/>\n  <a href=\"https:\/\/zobai.com\/it\/blog\/safety-valve-sizing-and-certified-relieving-capacity-guide\/\"><br \/>\n    ZOBAI\u2019s safety valve sizing and certified relieving capacity guide<br \/>\n  <\/a>.\n<\/p>\n<p>The practical inputs for this thermal-expansion case include:<\/p>\n<ul>\n<li>the protected section and relevant trapped volume;<\/li>\n<li>the credible thermal condition or heat input;<\/li>\n<li>fluid identity and the physical properties required by the applicable calculation;<\/li>\n<li>pressione e temperatura di scarico;<\/li>\n<li>the required relieving rate;<\/li>\n<li>expected back pressure; and<\/li>\n<li>the fluid-state assumption used by the sizing method.<\/li>\n<\/ul>\n<p>\n  Only after the required duty has been established should the selected valve\u2019s documented<br \/>\n  capacity be compared against it.\n<\/p>\n<p><strong>Thermal relief is duty-sized, not simply small-sized.<\/strong><\/p>\n<h3>Illustrative engineering scenario: equal pipe size, different relief duty<\/h3>\n<p>\n  Two isolated spools may have the same nominal diameter while one is short and shaded and<br \/>\n  the other is longer and exposed to a credible heat source. Diameter alone cannot establish<br \/>\n  either relieving rate. The duty assessment needs each trapped volume, heat-transfer case,<br \/>\n  liquid expansion properties and applicable pressure limit; only then can a candidate&#8217;s<br \/>\n  documented capacity be compared. This example is illustrative and supplies no sizing result.\n<\/p>\n<\/section>\n<section id=\"thermal-relief-set-pressure-basis\">\n<h2>How Should the Set-Pressure Basis Be Established?<\/h2>\n<p>\n  Set pressure should be derived from the <strong>pressure boundary being protected<\/strong>,<br \/>\n  not copied from normal operating pressure or selected from a generic percentage rule.\n<\/p>\n<p>\n  A Baker Hughes 2478 Series ordering example lists operating pressure, set pressure,<br \/>\n  operating\/relieving\/design temperatures, allowable overpressure, back pressure and required<br \/>\n  capacity as separate inputs. That is a useful specification illustration, not a generic<br \/>\n  set-pressure rule or a claim that this particular product suits every thermal-relief duty.<br \/>\n  <a href=\"https:\/\/dam.bakerhughes.com\/m\/3b234d808f4b5dbd\/original\/BHCN-2478-TS-20201D-1020-English-pdf.pdf\" target=\"_blank\" rel=\"noopener noreferrer nofollow\"><br \/>\n    Baker Hughes 2478 Series technical specifications<br \/>\n  <\/a>.\n<\/p>\n<div class=\"z-table-wrap\"\n     role=\"region\"\n     aria-label=\"Pressure terms used when specifying a thermal expansion relief valve\"\n     tabindex=\"0\"><\/p>\n<table>\n<thead>\n<tr>\n<th scope=\"col\">Termine di pressione<\/th>\n<th scope=\"col\">Role in the thermal-relief decision<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>Pressione operativa<\/td>\n<td>The pressure the system normally experiences.<\/td>\n<\/tr>\n<tr>\n<td>Protected design \/ allowable pressure basis<\/td>\n<td>The applicable pressure boundary for the piping or equipment being protected.<\/td>\n<\/tr>\n<tr>\n<td>Pressione di taratura<\/td>\n<td>The specified pressure associated with the relief device beginning its defined opening action.<\/td>\n<\/tr>\n<tr>\n<td>Pressione di scarico<\/td>\n<td>The pressure condition used for relief sizing or capacity assessment.<\/td>\n<\/tr>\n<tr>\n<td>Contropressione<\/td>\n<td>Pressure acting on or downstream of the valve outlet.<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n<p>\n  Those values are related, but they are not interchangeable. Identify which piping and<br \/>\n  components lie inside the blocked-in boundary, then establish the governing pressure basis<br \/>\n  under the applicable design code, equipment rules and project specification before<br \/>\n  assigning the valve set pressure.\n<\/p>\n<p>\n  Scope discipline matters here. A pressure rule written for a vessel should not automatically<br \/>\n  be transplanted into generic process piping, and a regulation written specifically for LPG,<br \/>\n  ammonia or another service should not be generalized to unrelated fluids or jurisdictions.\n<\/p>\n<p>\n  The protected boundary may contain a lower-rated instrument, flexible connection, filter<br \/>\n  housing or valve body as well as the pipe. The governing allowable-pressure basis therefore<br \/>\n  requires a component-by-component review at the relevant temperature. Normal pump discharge<br \/>\n  pressure is useful operating data, but it is neither a substitute for that review nor a<br \/>\n  universal set-pressure formula.\n<\/p>\n<p>\n  This article therefore does not create a universal rule such as<br \/>\n  <strong>\u201cset the thermal relief valve at a fixed percentage above operating pressure.\u201d<\/strong><br \/>\n  Where a numerical relationship is prescribed, it belongs to the applicable equipment,<br \/>\n  code, project and jurisdictional basis.\n<\/p>\n<p>\n  For an RFQ, set pressure should be treated as the result of a protected-system engineering<br \/>\n  decision, not merely as a field to complete on a vendor datasheet.\n<\/p>\n<\/section>\n<section id=\"relieving-fluid-state\">\n<h2>Why the Relieving Fluid State Must Be Checked, Not Assumed<\/h2>\n<p>\n  A line can be liquid-filled before the relief event without remaining a simple<br \/>\n  single-phase liquid system throughout the complete relieving path.\n<\/p>\n<p>\n  Relief-valve engineering guidance distinguishes straightforward liquid sizing from cases<br \/>\n  in which subcooled liquid can flash as pressure falls through the valve. That makes the<br \/>\n  relieving-state check a real sizing boundary rather than a semantic detail.<br \/>\n  <a href=\"https:\/\/www.leser.com\/en-us\/support-and-tools\/engineering\/\" target=\"_blank\" rel=\"noopener noreferrer nofollow\">LESER engineering guidance<\/a>.\n<\/p>\n<p>\n  As pressure changes through the valve and downstream piping, the relationship between fluid<br \/>\n  pressure and temperature can change. Under some conditions, the liquid remains adequately<br \/>\n  subcooled and a liquid-sizing basis remains appropriate. Under others, flashing or a more<br \/>\n  complex phase condition may need to be considered.\n<\/p>\n<figure class=\"wp-block-image size-full zobai-figure\" data-image-slot=\"IMG-03\"><img loading=\"lazy\" src=\"https:\/\/zobai.com\/wp-content\/uploads\/2026\/09\/thermal-expansion-relief-valve-img-03.webp\" alt=\"Concept diagram comparing an initially liquid condition with possible liquid-only or flashing conditions during relief.\" title=\"Thermal Expansion Relief Valve for Blocked-In Liquid Lines - IMG-03\" width=\"1200\" height=\"900\" class=\"wp-image-56865\" decoding=\"async\" srcset=\"https:\/\/zobai.com\/wp-content\/uploads\/2026\/09\/thermal-expansion-relief-valve-img-03.webp 1200w, https:\/\/zobai.com\/wp-content\/uploads\/2026\/09\/thermal-expansion-relief-valve-img-03-300x225.webp 300w, https:\/\/zobai.com\/wp-content\/uploads\/2026\/09\/thermal-expansion-relief-valve-img-03-1024x768.webp 1024w, https:\/\/zobai.com\/wp-content\/uploads\/2026\/09\/thermal-expansion-relief-valve-img-03-768x576.webp 768w, https:\/\/zobai.com\/wp-content\/uploads\/2026\/09\/thermal-expansion-relief-valve-img-03-16x12.webp 16w, https:\/\/zobai.com\/wp-content\/uploads\/2026\/09\/thermal-expansion-relief-valve-img-03-400x300.webp 400w\" sizes=\"auto, (max-width: 1200px) 100vw, 1200px\" \/><figcaption>Concept diagram comparing an initially liquid condition with possible liquid-only or flashing conditions during relief.<\/figcaption><\/figure>\n<blockquote>\n<p>\n    <strong><br \/>\n      At the relevant relieving pressure and temperature\u2014and as pressure falls through the<br \/>\n      valve and outlet system\u2014does the fluid remain in the phase assumed by the sizing method?<br \/>\n    <\/strong>\n  <\/p>\n<\/blockquote>\n<p>\n  If the assumed phase remains valid, the case can continue under the appropriate liquid-sizing<br \/>\n  methodology. If meaningful phase change occurs, the required inputs and sizing treatment can<br \/>\n  change. That does not mean every thermal-expansion case is two-phase; it means the phase<br \/>\n  assumption should be verified rather than inherited from the normal operating description.\n<\/p>\n<p>\n  A procurement description such as <strong>\u201cliquid service, thermal relief\u201d<\/strong><br \/>\n  therefore does not by itself prove the relieving-state assumption required for final sizing.\n<\/p>\n<p>\n  This article treats <strong>initial phase<\/strong> e <strong>relieving phase<\/strong><br \/>\n  as related but separate engineering inputs.\n<\/p>\n<\/section>\n<section id=\"thermal-relief-discharge-back-pressure\">\n<h2>Why Discharge Routing and Back Pressure Are Part of the Relief Decision<\/h2>\n<p>\n  A correctly selected valve is not enough if the installed relief path cannot perform<br \/>\n  under the defined scenario.\n<\/p>\n<p class=\"z-decision-chain\">\n  Protected section \u2192 valve inlet \u2192 relief device \u2192 outlet piping \u2192 disposal or recovery destination.\n<\/p>\n<figure class=\"wp-block-image size-full zobai-figure\" data-image-slot=\"IMG-04\"><img loading=\"lazy\" src=\"https:\/\/zobai.com\/wp-content\/uploads\/2026\/09\/thermal-expansion-relief-valve-img-04.webp\" alt=\"Diagram showing the complete thermal relief path from the protected liquid section through the relief valve to the downstream destination and back-pressure source.\" title=\"Thermal Expansion Relief Valve for Blocked-In Liquid Lines - IMG-04\" width=\"1200\" height=\"900\" class=\"wp-image-56866\" decoding=\"async\" srcset=\"https:\/\/zobai.com\/wp-content\/uploads\/2026\/09\/thermal-expansion-relief-valve-img-04.webp 1200w, https:\/\/zobai.com\/wp-content\/uploads\/2026\/09\/thermal-expansion-relief-valve-img-04-300x225.webp 300w, https:\/\/zobai.com\/wp-content\/uploads\/2026\/09\/thermal-expansion-relief-valve-img-04-1024x768.webp 1024w, https:\/\/zobai.com\/wp-content\/uploads\/2026\/09\/thermal-expansion-relief-valve-img-04-768x576.webp 768w, https:\/\/zobai.com\/wp-content\/uploads\/2026\/09\/thermal-expansion-relief-valve-img-04-16x12.webp 16w, https:\/\/zobai.com\/wp-content\/uploads\/2026\/09\/thermal-expansion-relief-valve-img-04-400x300.webp 400w\" sizes=\"auto, (max-width: 1200px) 100vw, 1200px\" \/><figcaption>Diagram showing the complete thermal relief path from the protected liquid section through the relief valve to the downstream destination and back-pressure source.<\/figcaption><\/figure>\n<p>\n  The review should establish where the relieved liquid will go, whether that route remains<br \/>\n  available during the relief case, and what pressure the downstream system will impose on<br \/>\n  the valve.\n<\/p>\n<p>\n  Back pressure can influence relief-valve opening, closing, stability and capacity, depending<br \/>\n  on the device and system. UK HSE&#8217;s relief-system guidance explicitly identifies these<br \/>\n  effects, so the outlet condition belongs in candidate verification rather than being left<br \/>\n  as a post-purchase piping detail.<br \/>\n  <a href=\"https:\/\/www.hse.gov.uk\/comah\/sragtech\/techmeasventsyst.htm\" target=\"_blank\" rel=\"noopener noreferrer nofollow\"><br \/>\n    HSE relief and vent system guidance<br \/>\n  <\/a>.\n<\/p>\n<p>\n  For a deeper treatment of that interface, see<br \/>\n  <a href=\"https:\/\/zobai.com\/it\/engineering\/back-pressure-and-bellows\/\"><br \/>\n    ZOBAI\u2019s back-pressure and bellows engineering guide<br \/>\n  <\/a>.\n<\/p>\n<p>\n  The destination itself is service-dependent. A small expected thermal-relief rate does<br \/>\n  not prove that unrestricted atmospheric discharge is acceptable. Conversely, a closed<br \/>\n  return, drain or recovery system is not automatically suitable if it imposes unacceptable<br \/>\n  back pressure or can be isolated when relief is required.\n<\/p>\n<p>\n  ISO 23251 provides an international pressure-relief-system framework for petroleum,<br \/>\n  petrochemical and natural-gas industries. Its current 2019 edition explicitly supplements<br \/>\n  API 521 6th Edition, so project teams should confirm the adopted standard set rather than<br \/>\n  silently treating different editions as textually equivalent.<br \/>\n  <a href=\"https:\/\/www.iso.org\/standard\/75144.html\" target=\"_blank\" rel=\"noopener noreferrer nofollow\">ISO 23251 information<\/a>.\n<\/p>\n<blockquote>\n<p>\n    <strong><br \/>\n      Where can the relieved fluid go without compromising either safe disposal or the valve\u2019s<br \/>\n      ability to perform its relief function?<br \/>\n    <\/strong>\n  <\/p>\n<\/blockquote>\n<p>\n  Detailed flare design, closed-header hydraulics and valve-architecture thresholds belong<br \/>\n  to separate engineering work. This article only needs to establish that the outlet system<br \/>\n  forms part of the relief decision.\n<\/p>\n<h3>Illustrative engineering scenario: a return header that can be isolated<\/h3>\n<p>\n  Suppose the thermal valve discharges into a closed recovery header. The proposed route<br \/>\n  appears contained, but the header has an isolation valve and can already be pressurized<br \/>\n  during the trapped-line event. Before accepting the arrangement, confirm header availability,<br \/>\n  simultaneous-case pressure, fluid compatibility, disposal capacity and the candidate valve&#8217;s<br \/>\n  documented back-pressure limits. A contained destination is not automatically an available<br \/>\n  or hydraulically acceptable destination. This is illustrative; it does not specify a<br \/>\n  universal back-pressure limit.\n<\/p>\n<\/section>\n<section id=\"thermal-relief-rfq-engineering-review\">\n<h2>What Information Belongs in a Thermal-Relief RFQ or Engineering Review?<\/h2>\n<p>\n  A technically useful RFQ should describe the <strong>protected system and relief duty<\/strong>,<br \/>\n  not simply request a valve by nominal size.\n<\/p>\n<p>\n  Separate the process inputs from the supplier verification:\n<\/p>\n<div class=\"z-table-wrap\"\n     role=\"region\"\n     aria-label=\"Thermal relief RFQ data split between the buyer or engineer and the valve supplier\"\n     tabindex=\"0\"><\/p>\n<table>\n<thead>\n<tr>\n<th scope=\"col\">Buyer \/ engineer should define or assign<\/th>\n<th scope=\"col\">Supplier \/ candidate should confirm<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>Protected piping or equipment boundary<\/td>\n<td>Exact valve configuration offered<\/td>\n<\/tr>\n<tr>\n<td>Fluid and relevant composition<\/td>\n<td>Wetted-material, seat and seal suitability<\/td>\n<\/tr>\n<tr>\n<td>Normal fluid state<\/td>\n<td>Pressure-temperature suitability<\/td>\n<\/tr>\n<tr>\n<td>Pressione e temperatura operative<\/td>\n<td>Available set-pressure capability<\/td>\n<\/tr>\n<tr>\n<td>Governing protected-pressure basis<\/td>\n<td>Documented capacity for the specified duty<\/td>\n<\/tr>\n<tr>\n<td>Credible thermal condition or heat source<\/td>\n<td>Selected flow area or valve size<\/td>\n<\/tr>\n<tr>\n<td>Required relieving rate, or responsibility for determining it<\/td>\n<td>Effect of specified back pressure on the candidate<\/td>\n<\/tr>\n<tr>\n<td>Relieving-phase assumption<\/td>\n<td>Connection and pressure-class suitability<\/td>\n<\/tr>\n<tr>\n<td>Contropressione attesa<\/td>\n<td>Applicable inspection or test documentation<\/td>\n<\/tr>\n<tr>\n<td>Relief destination<\/td>\n<td>Requested conformity documentation, where applicable<\/td>\n<\/tr>\n<tr>\n<td>Applicable project code or specification<\/td>\n<td>Deviations, limitations and unresolved points<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n<p>\n  <strong>Required relieving capacity is not the same as connection size.<\/strong><br \/>\n  One is a process-protection requirement; the other is primarily an installation characteristic.\n<\/p>\n<p>\n  <strong>Required capacity is also not the same as compliance evidence.<\/strong><br \/>\n  Capacity documentation addresses the flow-performance question under the applicable basis.<br \/>\n  Material certificates, inspection records or other project-required conformity evidence<br \/>\n  answer different procurement questions.\n<\/p>\n<p>\n  A supplier should not have to guess the relief scenario. If the protected pressure basis,<br \/>\n  thermal condition, required relieving rate, fluid state or expected back pressure remains<br \/>\n  undefined, those gaps should be resolved before a proposed valve is treated as an approved<br \/>\n  candidate.\n<\/p>\n<blockquote>\n<p>\n    <strong><br \/>\n      Can the proposed valve be traced back to a defined thermal-expansion duty, protected<br \/>\n      pressure boundary, relieving state and outlet condition\u2014and does the supplier evidence<br \/>\n      address each of those requirements?<br \/>\n    <\/strong>\n  <\/p>\n<\/blockquote>\n<p>\n  If not, the candidate is not ready for approval merely because a catalogue uses the words<br \/>\n  <strong>sfioro termico<\/strong> o <strong>hydrostatic relief<\/strong>.\n<\/p>\n<aside class=\"z-cta\" aria-label=\"Engineering enquiry\">\n<p><strong>Have a blocked-in liquid relief case to review?<\/strong><\/p>\n<p>\n    Prepare the protected boundary, fluid, operating and relieving conditions, required relief<br \/>\n    duty, set-pressure basis, expected back pressure, discharge destination and applicable<br \/>\n    project requirements before requesting a valve candidate.\n  <\/p>\n<p>\n    <a href=\"https:\/\/zobai.com\/it\/ask-an-engineer\/\"><br \/>\n      Submit the defined duty for engineering and RFQ review<br \/>\n    <\/a>\n  <\/p>\n<\/aside>\n<\/section>\n<section id=\"thermal-expansion-relief-faq\">\n<h2>Domande Frequenti<\/h2>\n<h3>Does closing two valves always require a thermal relief valve?<\/h3>\n<p>No. First establish whether liquid can be trapped, whether credible warming can challenge<br \/>\n  the protected pressure boundary, and whether an adequate protection path remains available<br \/>\n  in that exact isolation state. A dedicated device is one possible engineered response.<\/p>\n<h3>Can I choose the valve by pipe diameter?<\/h3>\n<p>No. Diameter helps define the installation, but the required capacity comes from the<br \/>\n  credible thermal duty, fluid properties, relieving pressure and temperature, phase<br \/>\n  assumption, and outlet condition. Compare that duty with documented candidate capacity.<\/p>\n<h3>Can the valve discharge back into the same system?<\/h3>\n<p>Only if the destination is available during the relief event and its pressure, capacity,<br \/>\n  compatibility and isolation arrangements are acceptable for the selected device and<br \/>\n  project basis. A connection alone does not establish an adequate relief path.<\/p>\n<h3>What if the liquid flashes after passing through the valve?<\/h3>\n<p>Do not continue with a liquid-only sizing assumption without checking the pressure and<br \/>\n  temperature path. Have the relieving state and applicable sizing method reviewed, then<br \/>\n  confirm the valve and downstream system against that result.<\/p>\n<\/section>\n<\/article>","protected":false},"excerpt":{"rendered":"<p>Scopri quando un liquido in blocco crea un caso di sfioro per dilatazione termica, cosa determina la capacit\u00e0 e la pressione di taratura e quali dati verificare prima della selezione della valvola.<\/p>","protected":false},"author":2,"featured_media":56863,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[74],"tags":[],"class_list":["post-56806","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-sizing-performance-engineering"],"_links":{"self":[{"href":"https:\/\/zobai.com\/it\/wp-json\/wp\/v2\/posts\/56806","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/zobai.com\/it\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/zobai.com\/it\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/zobai.com\/it\/wp-json\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/zobai.com\/it\/wp-json\/wp\/v2\/comments?post=56806"}],"version-history":[{"count":0,"href":"https:\/\/zobai.com\/it\/wp-json\/wp\/v2\/posts\/56806\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/zobai.com\/it\/wp-json\/wp\/v2\/media\/56863"}],"wp:attachment":[{"href":"https:\/\/zobai.com\/it\/wp-json\/wp\/v2\/media?parent=56806"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/zobai.com\/it\/wp-json\/wp\/v2\/categories?post=56806"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/zobai.com\/it\/wp-json\/wp\/v2\/tags?post=56806"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}