{"id":56922,"date":"2026-09-23T23:43:19","date_gmt":"2026-09-23T23:43:19","guid":{"rendered":"https:\/\/zobai.com\/?p=56922"},"modified":"2026-10-27T04:23:20","modified_gmt":"2026-10-27T04:23:20","slug":"thermal-relief-valve-vs-pressure-safety-valve-in-lng-lpg-piping","status":"publish","type":"post","link":"https:\/\/zobai.com\/ar\/blog\/thermal-relief-valve-vs-pressure-safety-valve-in-lng-lpg-piping\/","title":{"rendered":"\u0635\u0645\u0627\u0645 \u0627\u0644\u062a\u0646\u0641\u064a\u0633 \u0627\u0644\u062d\u0631\u0627\u0631\u064a \u0645\u0642\u0627\u0628\u0644 \u0635\u0645\u0627\u0645 \u0623\u0645\u0627\u0646 \u0627\u0644\u0636\u063a\u0637 \u0641\u064a \u0623\u0646\u0627\u0628\u064a\u0628 LNG\/LPG"},"content":{"rendered":"<style>\n.zobai-article { max-width: 100%; }\n.zobai-article .table-wrap { width: 100%; overflow-x: auto; -webkit-overflow-scrolling: touch; }\n.zobai-article table { width: 100%; border-collapse: collapse; }\n.zobai-article th, .zobai-article td { vertical-align: top; }\n.zobai-article img, .zobai-article svg { max-width: 100%; height: auto; }\n.zobai-article a { overflow-wrap: anywhere; }\n.zobai-article .engineering-cta { margin-top: 2rem; }\n.zobai-article .engineering-example { padding: 1rem 1.1rem; border-left: 3px solid currentColor; background: rgba(0,0,0,.035); }\n.zobai-article .evidence-note { font-size: .96em; }\n.zobai-article .article-faq h3 { margin-top: 1.4rem; }\n<\/style>\n<article class=\"zobai-article\">\n<p>\n  In LNG and LPG piping, a <strong>thermal relief valve and a pressure safety valve are not two opposite protection principles<\/strong>.<br \/>\n  A thermal-expansion relief valve is itself a pressure-relief device. The useful difference is the <strong>\u0648\u0627\u062c\u0628 \u0627\u0644\u062a\u0646\u0641\u064a\u0633<\/strong>: what is causing pressure to rise, what section is being protected, what fluid state reaches the device, and what load the device must relieve.\n<\/p>\n<p>\n  <a href=\"https:\/\/legacy.cganet.com\/Publication\/Details.aspx?id=PS-28\" target=\"_blank\" rel=\"noopener noreferrer nofollow\">CGA PS-28<\/a><br \/>\n  describes a thermal expansion relief valve as a pressure-relief device used on piping systems to prevent overpressure caused by thermal expansion of a limited trapped process-fluid volume, including cases involving cryogenic liquids or cold gases trapped between valves or other devices.<br \/>\n  That definition is narrow and useful: it identifies a specific overpressure mechanism rather than a universal valve class.\n<\/p>\n<p>\n  A reliable engineering sequence is therefore: <strong>define the protected section \u2192 identify the credible pressure-rise mechanism \u2192 establish the required relieving duty \u2192 verify the valve and the installed relief path.<\/strong><br \/>\n  Set pressure, overpressure, accumulation, blowdown, relieving capacity and back pressure are related, but they are not interchangeable terms. Their allowable relationships depend on the governing code, protected equipment and valve configuration, so this article does not apply a universal percentage or threshold.\n<\/p>\n<p>\n  A blocked-in liquid warming inside an isolated pipe section can create a thermal-expansion duty. Another system may instead have a blocked-flow, pump-related, vapor-generation, fire or other credible overpressure case. Those cases should not be collapsed into a single \u201cTRV versus PSV\u201d product choice.<br \/>\n  <a href=\"https:\/\/www.api.org\/products-and-services\/standards\/important-standards-announcements\/standard521\" target=\"_blank\" rel=\"noopener noreferrer nofollow\">API 521<\/a><br \/>\n  addresses pressure-relieving and depressuring systems for facilities including LNG terminals, while API&#8217;s pressure-relieving-system framework separates cause identification, required relieving rate and downstream disposal-system review.\n<\/p>\n<p>\n  The valve name is therefore a starting point\u2014not the engineering specification.\n<\/p>\n<section>\n<h2>Thermal Relief vs Pressure Safety Relief: What Is the Real Difference?<\/h2>\n<p>\n  The real difference is <strong>duty first, hardware second<\/strong>.\n<\/p>\n<p>\n  A thermal-expansion relief duty exists because a trapped fluid changes condition and creates pressure as it warms. The protective device still responds to pressure.<br \/>\n  \u201cThermal\u201d describes the cause of the overpressure; it does not turn the device into a temperature-operated valve. CGA PS-28 makes that distinction especially clear by classifying the thermal expansion relief valve as a pressure-relief device.\n<\/p>\n<p>\n  \u201cPSV,\u201d \u201cPRV,\u201d \u201csafety valve\u201d and \u201crelief valve\u201d also should not be treated as universally interchangeable product classes.<br \/>\n  <a href=\"https:\/\/www.iso.org\/standard\/50826.html\" target=\"_blank\" rel=\"noopener noreferrer nofollow\">ISO 4126-1<\/a>, for example, specifies general product requirements for safety valves and explicitly states that it is a product standard rather than an application standard.<br \/>\n  <a href=\"https:\/\/www.asme.org\/codes-standards\/find-codes-standards\/bpvc-section-xiii-rules-for-overpressure-protection\" target=\"_blank\" rel=\"noopener noreferrer nofollow\">ASME BPVC \u0627\u0644\u0642\u0633\u0645 XIII<\/a><br \/>\n  separately provides overpressure-protection rules covering pressure-relief devices, capacity certification, installation and related requirements. The governing project standard, datasheet and valve documentation therefore matter more than the abbreviation alone.\n<\/p>\n<p>\n  In practical review, keep four pressure terms separate. <strong>\u0636\u063a\u0637 \u0627\u0644\u0636\u0628\u0637<\/strong> is the valve setting used as the opening reference under specified test or service conditions. <strong>\u0627\u0644\u0636\u063a\u0637 \u0627\u0644\u0632\u0627\u0626\u062f<\/strong> is the pressure rise above set pressure during relieving. <strong>\u0627\u0644\u062a\u0631\u0627\u0643\u0645<\/strong> concerns the pressure rise of the protected system above its governing allowable basis during an overpressure event. <strong>\u0641\u0631\u0642 \u0627\u0644\u0636\u063a\u0637 \u0628\u064a\u0646 \u0627\u0644\u0641\u062a\u062d \u0648\u0625\u0639\u0627\u062f\u0629 \u0627\u0644\u0625\u063a\u0644\u0627\u0642 (Blowdown)<\/strong> is the pressure difference between opening and reseating behavior. API 520 and ASME overpressure-protection documentation treat these as distinct design or performance parameters; the exact limits are code- and application-dependent.\n<\/p>\n<div class=\"table-wrap\" role=\"region\" aria-label=\"Comparison of thermal-expansion relief duty and other pressure-relief duty\" tabindex=\"0\">\n<table>\n<thead>\n<tr>\n<th scope=\"col\">\u0646\u0642\u0637\u0629 \u0627\u0644\u0645\u0642\u0627\u0631\u0646\u0629<\/th>\n<th scope=\"col\">Thermal-expansion relief duty<\/th>\n<th scope=\"col\">Other pressure-relief duty<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>Primary question<\/td>\n<td>Can a trapped fluid volume build pressure as its condition changes?<\/td>\n<td>What other credible mechanism can overpressure the protected system?<\/td>\n<\/tr>\n<tr>\n<td>First thing to define<\/td>\n<td>Isolated protected section and trapped inventory<\/td>\n<td>Protected system and overpressure scenario<\/td>\n<\/tr>\n<tr>\n<td>Does the valve name establish capacity?<\/td>\n<td>\u0644\u0627<\/td>\n<td>\u0644\u0627<\/td>\n<\/tr>\n<tr>\n<td>Does pipe size establish required capacity?<\/td>\n<td>\u0644\u0627<\/td>\n<td>\u0644\u0627<\/td>\n<\/tr>\n<tr>\n<td>Does correct set pressure prove adequate protection?<\/td>\n<td>No; the device must also have adequate relieving capacity and be connected to the correct protected volume.<\/td>\n<td>No; capacity, relieving state and installation conditions still have to be verified.<\/td>\n<\/tr>\n<tr>\n<td>What ultimately controls selection?<\/td>\n<td>Required duty plus actual service and installed conditions<\/td>\n<td>Required duty plus actual service and installed conditions<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n<p>\n  For a broader terminology comparison, see the<br \/>\n  <a href=\"https:\/\/zobai.com\/ar\/blog\/prv-vs-psv-vs-safety-valve-vs-relief-valve\/\">PRV vs PSV vs safety valve vs relief valve guide<\/a>.<br \/>\n  This article only uses the terminology needed to make the LNG\/LPG piping decision.\n<\/p>\n<\/section>\n<section>\n<h2>When Does LNG or LPG Piping Become a Thermal-Relief Case?<\/h2>\n<p>\n  Start with the <strong>pressure boundary<\/strong>, not the valve catalogue. A thermal-relief case becomes a credible candidate when a finite fluid volume can become isolated and warming, thermal expansion or an applicable cryogenic state change can increase pressure while that section remains blocked in.\n<\/p>\n<ol>\n<li><strong>Define the protected section.<\/strong> Identify the valves, equipment or other boundaries that isolate it.<\/li>\n<li><strong>Confirm whether fluid can actually become trapped.<\/strong> An LNG or LPG line does not become a thermal-relief case simply because the medium is liquefied gas.<\/li>\n<li><strong>Establish the trapped fluid and state.<\/strong> Liquid, cold gas and a potentially changing phase should not be assumed interchangeable.<\/li>\n<li><strong>Identify the pressure-rise mechanism.<\/strong> Determine whether warming or thermal expansion is actually the credible cause.<\/li>\n<li><strong>Only then establish the thermal-relief duty and required capacity.<\/strong><\/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-relief-vs-pressure-safety-img-01.webp\" alt=\"\u0645\u062e\u0637\u0637 \u0645\u0628\u0633\u0637 \u0644\u0623\u0646\u0627\u0628\u064a\u0628 LNG \u0623\u0648 LPG \u064a\u0648\u0636\u062d \u0645\u0642\u0637\u0639\u064b\u0627 \u0645\u0639\u0632\u0648\u0644\u064b\u0627 \u064a\u062d\u062a\u0648\u064a \u0639\u0644\u0649 \u0633\u0627\u0626\u0644 \u0645\u062d\u062a\u062c\u0632 \u0648\u062a\u0648\u0635\u064a\u0644 \u062a\u0646\u0641\u064a\u0633 \u0627\u0644\u0636\u063a\u0637 \u0627\u0644\u062e\u0627\u0635 \u0628\u0647.\" title=\"Thermal Relief Valve vs Pressure Safety Valve in LNG\/LPG Piping - IMG-01\" width=\"1200\" height=\"900\" class=\"wp-image-56961\" decoding=\"async\" srcset=\"https:\/\/zobai.com\/wp-content\/uploads\/2026\/09\/thermal-relief-vs-pressure-safety-img-01.webp 1200w, https:\/\/zobai.com\/wp-content\/uploads\/2026\/09\/thermal-relief-vs-pressure-safety-img-01-300x225.webp 300w, https:\/\/zobai.com\/wp-content\/uploads\/2026\/09\/thermal-relief-vs-pressure-safety-img-01-1024x768.webp 1024w, https:\/\/zobai.com\/wp-content\/uploads\/2026\/09\/thermal-relief-vs-pressure-safety-img-01-768x576.webp 768w, https:\/\/zobai.com\/wp-content\/uploads\/2026\/09\/thermal-relief-vs-pressure-safety-img-01-16x12.webp 16w, https:\/\/zobai.com\/wp-content\/uploads\/2026\/09\/thermal-relief-vs-pressure-safety-img-01-400x300.webp 400w\" sizes=\"auto, (max-width: 1200px) 100vw, 1200px\" \/><figcaption>\u0645\u062e\u0637\u0637 \u0645\u0628\u0633\u0637 \u0644\u0623\u0646\u0627\u0628\u064a\u0628 LNG \u0623\u0648 LPG \u064a\u0648\u0636\u062d \u0645\u0642\u0637\u0639\u064b\u0627 \u0645\u0639\u0632\u0648\u0644\u064b\u0627 \u064a\u062d\u062a\u0648\u064a \u0639\u0644\u0649 \u0633\u0627\u0626\u0644 \u0645\u062d\u062a\u062c\u0632 \u0648\u062a\u0648\u0635\u064a\u0644 \u062a\u0646\u0641\u064a\u0633 \u0627\u0644\u0636\u063a\u0637 \u0627\u0644\u062e\u0627\u0635 \u0628\u0647.<\/figcaption><\/figure>\n<p>\n  Within its specific scope for LNG road-vehicle fuelling stations,<br \/>\n  <a href=\"https:\/\/www.iso.org\/standard\/85908.html\" target=\"_blank\" rel=\"noopener noreferrer nofollow\">ISO 16924:2026<\/a><br \/>\n  defines a thermal relief valve around protection against excess pressure caused by cryogenic liquid vaporization or by warming of cold gas trapped in an isolated pipeline section or other small component.<br \/>\n  That definition is useful for understanding the LNG thermal-relief mechanism, but the standard&#8217;s fuelling-station scope should not be expanded into a universal rule for every LNG terminal or process plant.\n<\/p>\n<p>\n  A typical isolated section between two shutoff boundaries is easy to visualize, but it is not the only possible configuration. What matters is whether the proposed relief device has an unobstructed pressure connection to the actual trapped volume throughout the operating and isolation states that create the case.\n<\/p>\n<p class=\"engineering-example\">\n  <strong>\u0633\u064a\u0646\u0627\u0631\u064a\u0648 \u0647\u0646\u062f\u0633\u064a \u062a\u0648\u0636\u064a\u062d\u064a:<\/strong> a transfer line is isolated at both ends after LNG or LPG movement stops. A small trapped liquid inventory remains in the line and subsequently warms from ambient heat leak. No pump has to be running for the pressure to rise. The engineering response is not to choose a \u201cthermal valve\u201d by connection size; it is to define the trapped volume, establish the thermal-relief load, verify the pressure basis, and confirm that the selected device protects that exact section.\n<\/p>\n<p>\n  Identifying a thermal-expansion case also does <strong>\u064a\u062d\u062f\u062f<\/strong> complete the valve selection. Set-pressure basis, required relieving capacity, phase, temperature, inlet and outlet conditions, back pressure and discharge destination remain separate verification items. The fact that thermal-relief loads are often smaller than major process-upset loads does not justify assuming that any small valve will be adequate.\n<\/p>\n<\/section>\n<section>\n<h2>When Is a Thermal-Relief Valve Not Enough?<\/h2>\n<p>\n  A thermal-only assumption is no longer sufficient when another credible pressure-rise mechanism exists\u2014or when the actual relieving condition has not yet been established.\n<\/p>\n<p>\n  API 521 addresses pressure-relieving and depressuring systems rather than treating pressure relief as a catalogue-selection exercise. Depending on the protected system, further review may be required when there is another process pressure source, a blocked outlet or continuing inflow, a pump-related overpressure condition, vapor generation or another significant phase change, external heat exposure where applicable, an undefined protected boundary, an unknown required relieving rate, or an uncertain liquid, vapor or two-phase relieving state.\n<\/p>\n<p>\n  The point is not to build a long list of every possible relief scenario. It is to recognize when the <strong>thermal-expansion model no longer describes the complete problem<\/strong>.\n<\/p>\n<p>\n  Two-phase uncertainty is particularly important.<br \/>\n  <a href=\"https:\/\/www.iso.org\/standard\/76057.html\" target=\"_blank\" rel=\"noopener noreferrer nofollow\">ISO 4126-10:2024<\/a><br \/>\n  provides a dedicated sizing framework for gas\/liquid two-phase flow in pressurized systems including piping. An unresolved LNG\/LPG relieving condition therefore should not be silently forced into a single-phase assumption.\n<\/p>\n<div class=\"table-wrap\" role=\"region\" aria-label=\"Conditions that require broader relief review\" tabindex=\"0\">\n<table>\n<thead>\n<tr>\n<th scope=\"col\">If this is unresolved<\/th>\n<th scope=\"col\">\u0644\u0645\u0627\u0630\u0627 \u0647\u0648 \u0645\u0647\u0645<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>Protected section<\/td>\n<td>You do not yet know what the valve actually protects.<\/td>\n<\/tr>\n<tr>\n<td>Credible relief cause<\/td>\n<td>The required duty cannot be defined reliably.<\/td>\n<\/tr>\n<tr>\n<td>\u0633\u0639\u0629 \u0627\u0644\u062a\u0646\u0641\u064a\u0633 \u0627\u0644\u0645\u0637\u0644\u0648\u0628\u0629<\/td>\n<td>Connection size cannot substitute for sizing.<\/td>\n<\/tr>\n<tr>\n<td>\u0637\u0648\u0631 \u0627\u0644\u062a\u0646\u0641\u064a\u0633<\/td>\n<td>Liquid, vapor and two-phase assumptions are not interchangeable.<\/td>\n<\/tr>\n<tr>\n<td>Inlet piping condition<\/td>\n<td>Pressure loss or restrictions between the protected system and the device can affect opening stability and available inlet pressure.<\/td>\n<\/tr>\n<tr>\n<td>Downstream conditions<\/td>\n<td>Outlet resistance and built-up back pressure can change installed valve behavior.<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n<p>\n  <a href=\"https:\/\/www.api.org\/products-and-services\/standards\/important-standards-announcements\/520part-ii\" target=\"_blank\" rel=\"noopener noreferrer nofollow\">API 520 \u0627\u0644\u062c\u0632\u0621 \u0627\u0644\u062b\u0627\u0646\u064a<\/a><br \/>\n  is specifically the installation part of the API 520 pressure-relieving-device standard. That is why inlet and outlet piping cannot be treated as accessories to the valve-selection calculation. The exact allowable inlet-loss and back-pressure limits depend on the governing standard, valve design and service; they should be checked against the project basis and manufacturer documentation rather than copied from a generic percentage.\n<\/p>\n<p class=\"engineering-example\">\n  <strong>\u0633\u064a\u0646\u0627\u0631\u064a\u0648 \u0647\u0646\u062f\u0633\u064a \u062a\u0648\u0636\u064a\u062d\u064a:<\/strong> a spring-loaded relief valve was originally reviewed against one discharge arrangement, but the plant later ties its outlet into a common header. The new header can increase built-up or variable back pressure during relieving. Even if the valve&#8217;s set pressure has not changed, its opening\/reclosing behavior and effective capacity can require re-evaluation. The prevention step is to recheck the outlet hydraulic case whenever the disposal system changes, not just when the valve itself is replaced.\n<\/p>\n<p>\n  For the complete capacity calculation and certified-capacity workflow, use the<br \/>\n  <a href=\"https:\/\/zobai.com\/ar\/blog\/safety-valve-sizing-and-certified-relieving-capacity-guide\/\">\u062f\u0644\u064a\u0644 \u062d\u0633\u0627\u0628 \u0645\u0642\u0627\u0633\u0627\u062a \u0635\u0645\u0627\u0645\u0627\u062a \u0627\u0644\u0623\u0645\u0627\u0646 \u0648\u0633\u0639\u0629 \u0627\u0644\u062a\u0646\u0641\u064a\u0633 \u0627\u0644\u0645\u0639\u062a\u0645\u062f\u0629<\/a><br \/>\n  rather than expanding this page into a sizing manual.\n<\/p>\n<\/section>\n<section>\n<h2>Can One Relief Device Cover Both Thermal Expansion and Other Overpressure Cases?<\/h2>\n<p>\n  <strong>Possibly\u2014but only after the relevant cases are checked independently.<\/strong>\n<\/p>\n<p>\n  Neither \u201cthe main PSV is larger, so it automatically covers thermal expansion\u201d nor \u201cthermal expansion always requires a physically separate valve\u201d is a reliable selection rule.\n<\/p>\n<p>\n  <a href=\"https:\/\/www.api.org\/products-and-services\/standards\/important-standards-announcements\/520parti\" target=\"_blank\" rel=\"noopener noreferrer nofollow\">API 520 \u0627\u0644\u062c\u0632\u0621 \u0627\u0644\u0623\u0648\u0644<\/a><br \/>\n  \u064a\u062a\u0646\u0627\u0648\u0644 \u062d\u0633\u0627\u0628 \u0648\u0627\u062e\u062a\u064a\u0627\u0631 \u0623\u062c\u0647\u0632\u0629 \u062a\u0646\u0641\u064a\u0633 \u0627\u0644\u0636\u063a\u0637\u060c \u0628\u064a\u0646\u0645\u0627<br \/>\n  <a href=\"https:\/\/www.api.org\/products-and-services\/standards\/important-standards-announcements\/520part-ii\" target=\"_blank\" rel=\"noopener noreferrer nofollow\">API 520 \u0627\u0644\u062c\u0632\u0621 \u0627\u0644\u062b\u0627\u0646\u064a<\/a><br \/>\n  addresses installation. The engineering review therefore needs to consider both the required duty and the installed system.\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-relief-vs-pressure-safety-img-02.webp\" alt=\"Engineering screening path for checking whether one relief device can be reviewed for multiple LNG or LPG overpressure cases.\" title=\"Thermal Relief Valve vs Pressure Safety Valve in LNG\/LPG Piping - IMG-02\" width=\"1200\" height=\"900\" class=\"wp-image-56962\" decoding=\"async\" srcset=\"https:\/\/zobai.com\/wp-content\/uploads\/2026\/09\/thermal-relief-vs-pressure-safety-img-02.webp 1200w, https:\/\/zobai.com\/wp-content\/uploads\/2026\/09\/thermal-relief-vs-pressure-safety-img-02-300x225.webp 300w, https:\/\/zobai.com\/wp-content\/uploads\/2026\/09\/thermal-relief-vs-pressure-safety-img-02-1024x768.webp 1024w, https:\/\/zobai.com\/wp-content\/uploads\/2026\/09\/thermal-relief-vs-pressure-safety-img-02-768x576.webp 768w, https:\/\/zobai.com\/wp-content\/uploads\/2026\/09\/thermal-relief-vs-pressure-safety-img-02-16x12.webp 16w, https:\/\/zobai.com\/wp-content\/uploads\/2026\/09\/thermal-relief-vs-pressure-safety-img-02-400x300.webp 400w\" sizes=\"auto, (max-width: 1200px) 100vw, 1200px\" \/><figcaption>Engineering screening path for checking whether one relief device can be reviewed for multiple LNG or LPG overpressure cases.<\/figcaption><\/figure>\n<div class=\"table-wrap\" role=\"region\" aria-label=\"Checks before one relief device is evaluated for multiple duties\" tabindex=\"0\">\n<table>\n<thead>\n<tr>\n<th scope=\"col\">\u0633\u0624\u0627\u0644 \u0627\u0644\u062a\u062d\u0642\u0642<\/th>\n<th scope=\"col\">Why it can change the decision<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>Does the device protect the required boundary for each case?<\/td>\n<td>A valve elsewhere in the system may not protect an isolated section.<\/td>\n<\/tr>\n<tr>\n<td>Have the relevant relief causes been identified?<\/td>\n<td>An omitted case cannot be covered by assumption.<\/td>\n<\/tr>\n<tr>\n<td>Is the required relieving duty established for each applicable case?<\/td>\n<td>Valve label and connection size do not establish load.<\/td>\n<\/tr>\n<tr>\n<td>Does the candidate have appropriate capacity evidence?<\/td>\n<td>Available capacity has to be compared with the required duty.<\/td>\n<\/tr>\n<tr>\n<td>Is the relieving phase known?<\/td>\n<td>A liquid case does not prove suitability for vapor or two-phase relief.<\/td>\n<\/tr>\n<tr>\n<td>Is the set-pressure basis compatible with the protected system?<\/td>\n<td>Opening pressure cannot be reviewed independently of the pressure boundary.<\/td>\n<\/tr>\n<tr>\n<td>Are operating and relieving temperatures within documented limits?<\/td>\n<td>Suitability at one temperature does not prove suitability at another.<\/td>\n<\/tr>\n<tr>\n<td>Are body, trim, spring, guide, seat and seal arrangements supported for the service?<\/td>\n<td>Component suitability is configuration-specific and can affect corrosion resistance, friction, leakage and stable motion.<\/td>\n<\/tr>\n<tr>\n<td>Are back-pressure conditions supported?<\/td>\n<td>Outlet pressure can affect opening, lift, capacity and reseating behavior depending on valve design.<\/td>\n<\/tr>\n<tr>\n<td>Is the discharge path acceptable?<\/td>\n<td>The valve is only one part of the complete relief path.<\/td>\n<\/tr>\n<tr>\n<td>Are project documentation requirements satisfied?<\/td>\n<td>Technical capability is not the same as project acceptance.<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n<p>\n  What matters here is that <strong>capacity dominance is not the same as complete suitability<\/strong>. A candidate with enough capacity for a larger-flow scenario may still need separate verification for a lower-flow thermal case if that case involves a different protected section, phase, temperature, component limitation, back pressure or discharge route.\n<\/p>\n<p>\n  Capacity evidence also matters more than connection size. For code-covered pressure-relief devices, ASME Section XIII includes capacity and flow-resistance certification in its overpressure-protection scope, and the National Board&#8217;s<br \/>\n  <a href=\"https:\/\/www.nationalboard.org\/Index.aspx?pageID=64\" target=\"_blank\" rel=\"noopener noreferrer nofollow\">NB-18 \u0644\u0627\u0639\u062a\u0645\u0627\u062f \u0623\u062c\u0647\u0632\u0629 \u062a\u0646\u0641\u064a\u0633 \u0627\u0644\u0636\u063a\u0637<\/a><br \/>\n  provides manufacturer\/device certification information and certified relieving-capacity resources. A matching flange does not demonstrate that the candidate can pass the required relief load.\n<\/p>\n<p class=\"engineering-example\">\n  <strong>\u0633\u064a\u0646\u0627\u0631\u064a\u0648 \u0647\u0646\u062f\u0633\u064a \u062a\u0648\u0636\u064a\u062d\u064a:<\/strong> a valve can be adjusted to the intended set pressure yet still be an inadequate protective device if its verified relieving capacity is below the required case. The correction is not to \u201cturn up\u201d the set pressure or choose a larger connection by appearance; it is to compare the scenario&#8217;s required relieving rate with the candidate&#8217;s applicable certified or otherwise project-accepted capacity basis.\n<\/p>\n<p>\n  Spring-loaded and pilot-operated pressure-relief valves also should not be treated as interchangeable upgrades. Back pressure, operating margin, service cleanliness, sensing\/pilot configuration, temperature and maintenance strategy can all influence architecture choice. Baker Hughes technical guidance on pilot-operated safety relief valves notes that severe dirty service can require filtration or isolation features and that clogging of interconnecting tubing can make a pilot-operated design a poor fit in some services.<br \/>\n  <a href=\"https:\/\/dam.bakerhughes.com\/m\/2c919e250c65d509\/original\/Fundamental-Operation-of-Pilot-Operated-Safety-Relief-Valves.pdf\" target=\"_blank\" rel=\"noopener noreferrer nofollow\">Review the pilot-operated SRV technical note<\/a>.\n<\/p>\n<p>\n  Likewise, nothing in this comparison means thermal expansion always requires a physically separate device. Whether one device can serve multiple duties is an engineering conclusion based on the complete system, not a rule that follows from the names \u201cTRV\u201d or \u201cPSV.\u201d This is a candidate-screening principle, not a statement that one valve has already received project, code or regulatory approval.\n<\/p>\n<\/section>\n<section>\n<h2>What Changes Between LNG and LPG Relief Service?<\/h2>\n<p>\n  LNG and LPG can use the same <strong>decision framework<\/strong>, but they should not inherit the same service assumptions.\n<\/p>\n<p>\n  The shared questions are straightforward: What is protected? What causes the pressure rise? What fluid and phase reach the valve? What capacity is required? What temperature applies? What back pressure exists? Where does the relieved fluid go? What component and documentation evidence supports the candidate?\n<\/p>\n<div class=\"table-wrap\" role=\"region\" aria-label=\"LNG and LPG relief service comparison\" tabindex=\"0\">\n<table>\n<thead>\n<tr>\n<th scope=\"col\">Review variable<\/th>\n<th scope=\"col\">\u063a\u0627\u0632 \u0637\u0628\u064a\u0639\u064a \u0645\u0633\u0627\u0644<\/th>\n<th scope=\"col\">\u063a\u0627\u0632 \u0628\u062a\u0631\u0648\u0644 \u0645\u0633\u0627\u0644<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>\u062f\u0631\u062c\u0629 \u0627\u0644\u062d\u0631\u0627\u0631\u0629<\/td>\n<td>Cryogenic exposure can become a primary suitability boundary.<\/td>\n<td>Use actual operating and relieving temperatures; do not inherit LNG assumptions.<\/td>\n<\/tr>\n<tr>\n<td>\u062d\u0627\u0644\u0629 \u0627\u0644\u0645\u0627\u0626\u0639<\/td>\n<td>Establish the actual LNG state at the protected section and during relief.<\/td>\n<td>Establish actual LPG state rather than assuming liquid or vapor from the medium name.<\/td>\n<\/tr>\n<tr>\n<td>Flashing \/ two-phase<\/td>\n<td>Review when process conditions make it credible.<\/td>\n<td>Review when process conditions make it credible.<\/td>\n<\/tr>\n<tr>\n<td>\u0627\u0644\u0645\u0648\u0627\u062f<\/td>\n<td>Verify the configured valve for the required low-temperature duty.<\/td>\n<td>Verify compatibility and actual temperature requirements.<\/td>\n<\/tr>\n<tr>\n<td>\u0627\u0644\u0645\u0642\u0639\u062f \/ \u0627\u0644\u0625\u062d\u0643\u0627\u0645<\/td>\n<td>Low-temperature performance may become a model-level constraint.<\/td>\n<td>Fluid, temperature and leakage requirements still control.<\/td>\n<\/tr>\n<tr>\n<td>\u0627\u0644\u0636\u063a\u0637 \u0627\u0644\u062e\u0644\u0641\u064a<\/td>\n<td>Check the installed outlet system and valve architecture.<\/td>\n<td>Check the installed outlet system and valve architecture.<\/td>\n<\/tr>\n<tr>\n<td>\u0648\u062c\u0647\u0629 \u0627\u0644\u062a\u0641\u0631\u064a\u063a<\/td>\n<td>Define it for the actual project.<\/td>\n<td>Define it for the actual project.<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n<p>\n  Cryogenic pressure-relief equipment deserves model-level verification rather than a generic statement such as \u201cstainless steel is suitable for LNG.\u201d<br \/>\n  <a href=\"https:\/\/valves.bakerhughes.com\/news\/pressure-relief-valves-cryogenic-applications\" target=\"_blank\" rel=\"noopener noreferrer nofollow\">Baker Hughes technical guidance on cryogenic pressure-relief valves<\/a><br \/>\n  identifies construction materials, trim design, anti-galling measures and sealing features as cryogenic design considerations. That evidence supports the need to verify the configured product; it does not establish that any specific ZOBAI model has the same construction or service capability.\n<\/p>\n<p>\n  Material review should therefore cover the full moving and sealing system, not only the pressure-retaining body: nozzle and disc surfaces, guides, spring environment, trim interfaces and seat\/seal materials can all affect corrosion, galling, leakage or stable movement under the actual temperature and medium. For LNG service, differential contraction and low-temperature suitability must be confirmed from the product documentation rather than inferred from a generic alloy name.\n<\/p>\n<p>\n  Seat tightness is a separate purchase and test question from set pressure and certified capacity. API&#8217;s current catalog lists <a href=\"https:\/\/www.api.org\/products-and-services\/standards\/digital-catalog\" target=\"_blank\" rel=\"noopener noreferrer nofollow\">API Standard 527, Seat Tightness of Pressure Relief Valves<\/a>; the standard covers test methods for metal- and soft-seated PRVs. If the project has a leakage requirement, state the required test basis and acceptance criteria in the purchase specification instead of assuming that \u201csoft seat\u201d or \u201cmetal seat\u201d alone predicts field tightness.\n<\/p>\n<p class=\"engineering-example\">\n  <strong>\u0633\u064a\u0646\u0627\u0631\u064a\u0648 \u0647\u0646\u062f\u0633\u064a \u062a\u0648\u0636\u064a\u062d\u064a:<\/strong> a valve can fit the line mechanically and meet the intended set-pressure basis, yet still be a poor LNG candidate if its trim, guide or sealing materials are not supported for the actual low-temperature condition. The prevention step is to review the documented temperature\/material configuration as a complete assembly, not to infer cryogenic suitability from body material alone.\n<\/p>\n<p>\n  For the broader application context, see<br \/>\n  <a href=\"https:\/\/zobai.com\/ar\/applications\/lng-and-lpg\/\">LNG and LPG pressure-relief applications<\/a>.<br \/>\n  For deeper component-level selection, see the<br \/>\n  <a href=\"https:\/\/zobai.com\/ar\/blog\/safety-valve-material-selection-guide\/\">\u062f\u0644\u064a\u0644 \u0627\u062e\u062a\u064a\u0627\u0631 \u0645\u0648\u0627\u062f \u0635\u0645\u0627\u0645\u0627\u062a \u0627\u0644\u0623\u0645\u0627\u0646<\/a>.\n<\/p>\n<p>\n  For engineering review, the same questions can be used for LNG and LPG, but the answers should be verified separately for the actual service.\n<\/p>\n<\/section>\n<section>\n<h2>What Should You Confirm Before Selecting a Valve or Sending an RFQ?<\/h2>\n<p>\n  Do not send only \u201cNeed a thermal relief valve, LNG, DNxx.\u201d That does not define the protection duty.\n<\/p>\n<p>\n  A useful engineering enquiry should describe the system first and let the supplier verify the candidate second.\n<\/p>\n<h3>Define the protected system<\/h3>\n<ul>\n<li>equipment or piping section being protected;<\/li>\n<li>isolation arrangement or system boundaries;<\/li>\n<li>credible overpressure scenario or scenarios;<\/li>\n<li>process reason pressure can rise.<\/li>\n<\/ul>\n<p>\n  If one relief device is expected to cover more than one case, identify every credited protected section and scenario instead of supplying only the \u201clargest\u201d case. A lower-flow thermal case can still have a different isolation boundary or different relieving state.\n<\/p>\n<h3>Provide the process and relief basis<\/h3>\n<ul>\n<li>medium and relevant composition;<\/li>\n<li>normal fluid phase;<\/li>\n<li>\u0627\u0644\u0637\u0648\u0631 \u0627\u0644\u0645\u062a\u0648\u0642\u0639 \u0639\u0646\u062f \u0627\u0644\u062a\u0646\u0641\u064a\u0633\u061b;<\/li>\n<li>\u0636\u063a\u0637 \u0627\u0644\u062a\u0634\u063a\u064a\u0644\u061b;<\/li>\n<li>design or allowable pressure basis;<\/li>\n<li>required set-pressure basis;<\/li>\n<li>\u062f\u0631\u062c\u0629 \u062d\u0631\u0627\u0631\u0629 \u0627\u0644\u062a\u0634\u063a\u064a\u0644\u061b;<\/li>\n<li>minimum relevant service temperature;<\/li>\n<li>expected relieving temperature;<\/li>\n<li>required relieving rate or the approved calculation basis.<\/li>\n<\/ul>\n<p>\n  If the phase is uncertain or two-phase relief is credible, state that uncertainty rather than assuming liquid-only or vapor-only sizing. If the governing code or owner specification limits set pressure, overpressure, accumulation or blowdown, provide that basis rather than asking the supplier to infer it from normal operating pressure.\n<\/p>\n<h3>Define the installed outlet conditions<\/h3>\n<ul>\n<li>\u0648\u062c\u0647\u0629 \u0627\u0644\u062a\u0635\u0631\u064a\u0641\u061b;<\/li>\n<li>downstream system or header;<\/li>\n<li>expected superimposed and built-up back pressure where applicable;<\/li>\n<li>relevant outlet piping restrictions;<\/li>\n<li>flare, vent, process-return or other project-defined destination where applicable.<\/li>\n<\/ul>\n<p>\n  Back pressure is not merely a piping detail. Depending on valve architecture, it can affect opening pressure, lift, capacity and reseating behavior. Changes to a shared disposal header should trigger a valve\/application review even when the protected equipment and set pressure are unchanged.\n<\/p>\n<h3>State the mechanical and material requirements<\/h3>\n<ul>\n<li>\u0645\u062a\u0637\u0644\u0628\u0627\u062a \u062a\u0648\u0635\u064a\u0644\u0627\u062a \u0627\u0644\u0645\u062f\u062e\u0644 \u0648\u0627\u0644\u0645\u062e\u0631\u062c\u061b;<\/li>\n<li>pressure class;<\/li>\n<li>spring-loaded, balanced or pilot-operated architecture if already fixed by the project;<\/li>\n<li>project material requirements for body, trim, nozzle, disc, spring\/guide environment and seat\/seal;<\/li>\n<li>medium compatibility requirements;<\/li>\n<li>leakage or seat-tightness test requirements;<\/li>\n<li>installation constraints, including orientation or accessibility where relevant.<\/li>\n<\/ul>\n<h3>Ask the supplier to confirm the candidate evidence<\/h3>\n<ul>\n<li>\u0627\u0644\u0637\u0631\u0627\u0632 \u0648\u0627\u0644\u062a\u0643\u0648\u064a\u0646 \u0628\u0627\u0644\u062a\u062d\u062f\u064a\u062f\u061b;<\/li>\n<li>pressure and temperature limits;<\/li>\n<li>construction materials;<\/li>\n<li>seat and seal arrangement;<\/li>\n<li>applicable relieving-capacity evidence;<\/li>\n<li>supported fluid \/ phase basis;<\/li>\n<li>back-pressure limitations;<\/li>\n<li>connection details;<\/li>\n<li>set-pressure and blowdown adjustment\/test basis where applicable;<\/li>\n<li>seat-tightness test basis where required;<\/li>\n<li>test, traceability and documentation package;<\/li>\n<li>conformity or certification only where it actually applies.<\/li>\n<\/ul>\n<p>\n  For inspection and repair planning, <a href=\"https:\/\/www.api.org\/products-and-services\/training\/inspection-training\" target=\"_blank\" rel=\"noopener noreferrer nofollow\">API RP 576<\/a> is the API recommended-practice framework for inspection of pressure-relieving devices in the oil and petrochemical industries. If the governing jurisdiction or owner specification requires National Board repair or test accreditation, verify the actual organization and scope in the<br \/>\n  <a href=\"https:\/\/buscenter.nationalboard.org\/MfgRepDir.aspx\" target=\"_blank\" rel=\"noopener noreferrer nofollow\">National Board Manufacturer and Repair Directory<\/a>;<br \/>\n  do not assume a VR or T\/O requirement applies globally.\n<\/p>\n<p>\n  The buyer or process engineer defines the protected system and required duty; the supplier confirms what a specific valve configuration can support.\n<\/p>\n<p>\n  For the broader selection workflow, see the<br \/>\n  <a href=\"https:\/\/zobai.com\/ar\/blog\/safety-valve-selection-guide\/\">\u062f\u0644\u064a\u0644 \u0627\u062e\u062a\u064a\u0627\u0631 \u0635\u0645\u0627\u0645\u0627\u062a \u0627\u0644\u0623\u0645\u0627\u0646<\/a>.\n<\/p>\n<p>\n  <strong>Classify the relief scenario before classifying the valve.<\/strong><br \/>\n  Once the protected boundary, relief cause, capacity, phase, temperature and installed conditions are defined, \u201cthermal relief valve versus PSV\u201d stops being a terminology debate and becomes a traceable engineering review.\n<\/p>\n<aside class=\"engineering-cta\">\n<p><strong>Need a candidate reviewed for an LNG or LPG piping duty?<\/strong><\/p>\n<p>\n    Send the protected piping boundary, medium and phase, operating and relieving conditions, required relieving duty, back pressure, discharge arrangement and project requirements.<br \/>\n    ZOBAI can use those inputs to review a candidate configuration and identify what model-specific evidence still needs to be confirmed.\n  <\/p>\n<p><a href=\"https:\/\/zobai.com\/ar\/ask-an-engineer\/\">\u0627\u0633\u0623\u0644 \u0645\u0647\u0646\u062f\u0633\u064b\u0627<\/a><\/p>\n<\/aside>\n<\/section>\n<section class=\"article-faq\">\n<h2>\u0623\u0633\u0626\u0644\u0629 \u0645\u062a\u0643\u0631\u0631\u0629<\/h2>\n<h3>Is a thermal relief valve the same as a pressure safety valve?<\/h3>\n<p>\n  A thermal-expansion relief valve is a pressure-relief device, but \u201cthermal relief\u201d describes a specific overpressure duty rather than proving a universally separate hardware class. The exact device terminology and configuration depend on the governing standard, service and project specification.\n<\/p>\n<h3>Can one pressure safety valve cover thermal expansion and another relief case?<\/h3>\n<p>\n  It can be evaluated as a candidate for multiple cases, but capacity alone is not enough. The review must confirm that the device protects the required boundary and that set-pressure basis, required relieving rate, phase, temperature, materials, back pressure and discharge path are acceptable for each credited case.\n<\/p>\n<h3>How does back pressure affect a relief valve in LNG or LPG piping?<\/h3>\n<p>\n  Back pressure can change opening, lift, capacity and reseating behavior depending on valve design. Built-up back pressure comes from outlet-system resistance during discharge; superimposed back pressure already exists at the valve outlet. The acceptable limits and the choice between conventional, balanced or pilot-operated designs must come from the governing standard and the specific valve documentation rather than a universal percentage.\n<\/p>\n<h3>\u0644\u0645\u0627\u0630\u0627 \u062a\u0639\u062a\u0628\u0631 \u0633\u0639\u0629 \u0627\u0644\u062a\u0646\u0641\u064a\u0633 \u0627\u0644\u0645\u0639\u062a\u0645\u062f\u0629 \u0623\u0647\u0645 \u0645\u0646 \u062d\u062c\u0645 \u0627\u0644\u0648\u0635\u0644\u0629\u061f<\/h3>\n<p>\n  Connection size tells you how the valve connects to the piping; it does not prove how much mass or volume the valve can discharge under the required relieving conditions. The required relief load must be compared with applicable capacity evidence for the actual fluid, pressure, temperature and valve configuration.\n<\/p>\n<h3>\u0645\u0627 \u0647\u0648 \u0627\u0644\u0641\u0631\u0642 \u0628\u064a\u0646 \u0636\u063a\u0637 \u0627\u0644\u0636\u0628\u0637\u060c \u0627\u0644\u0636\u063a\u0637 \u0627\u0644\u0632\u0627\u0626\u062f\u060c \u0627\u0644\u062a\u0631\u0627\u0643\u0645\u060c \u0648\u0627\u0646\u062e\u0641\u0627\u0636 \u0627\u0644\u0636\u063a\u0637\u061f<\/h3>\n<p>\n  Set pressure is the valve&#8217;s opening reference under specified conditions. Overpressure is the rise above set pressure during relieving. Accumulation refers to the protected system&#8217;s pressure rise above its governing allowable basis during the event. Blowdown is the pressure reduction between opening and reseating behavior. Do not apply generic limits without the applicable code and project basis.\n<\/p>\n<h3>How often should an LNG or LPG pressure-relief valve be inspected or recertified?<\/h3>\n<p>\n  There is no single interval that is correct for every service. Inspection and test frequency should follow the applicable jurisdiction, code, owner mechanical-integrity program, service severity and device history. API RP 576 and the relevant National Board\/NBIC requirements can be part of that framework where they apply; project-specific requirements should control the published maintenance interval.\n<\/p>\n<\/section>\n<\/article>\n<p><script type=\"application\/ld+json\">{\n    \"@context\": \"https:\\\/\\\/schema.org\",\n    \"@type\": \"BlogPosting\",\n    \"headline\": \"Thermal Relief Valve vs Pressure Safety Valve in LNG\\\/LPG Piping\",\n    \"description\": \"Compare thermal relief and pressure safety valve duties in LNG\\\/LPG piping, including trapped liquid, capacity, phase, back pressure and RFQ checks.\",\n    \"mainEntityOfPage\": {\n        \"@type\": \"WebPage\",\n        \"@id\": \"https:\\\/\\\/zobai.com\\\/blog\\\/thermal-relief-valve-vs-pressure-safety-valve-in-lng-lpg-piping\\\/\"\n    },\n    \"publisher\": {\n        \"@type\": \"Organization\",\n        \"name\": \"ZOBAI\"\n    }\n}<\/script><\/p>","protected":false},"excerpt":{"rendered":"<p>\u0642\u0627\u0631\u0646 \u0628\u064a\u0646 \u0645\u0647\u0627\u0645 \u0635\u0645\u0627\u0645 \u0627\u0644\u062a\u0646\u0641\u064a\u0633 \u0627\u0644\u062d\u0631\u0627\u0631\u064a \u0648\u0635\u0645\u0627\u0645 \u0627\u0644\u0623\u0645\u0627\u0646 \u0644\u0644\u0636\u063a\u0637 \u0641\u064a \u0623\u0646\u0627\u0628\u064a\u0628 LNG\/LPG\u060c \u0628\u0645\u0627 \u0641\u064a \u0630\u0644\u0643 \u0627\u0644\u0633\u0627\u0626\u0644 \u0627\u0644\u0645\u062d\u062a\u062c\u0632\u060c \u0648\u0627\u0644\u0633\u0639\u0629\u060c \u0648\u0627\u0644\u0637\u0648\u0631\u060c \u0648\u0627\u0644\u0636\u063a\u0637 \u0627\u0644\u062e\u0644\u0641\u064a\u060c \u0648\u0641\u062d\u0648\u0635\u0627\u062a \u0637\u0644\u0628 \u0639\u0631\u0648\u0636 \u0627\u0644\u0623\u0633\u0639\u0627\u0631.<\/p>","protected":false},"author":2,"featured_media":56961,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[74],"tags":[],"class_list":["post-56922","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-sizing-performance-engineering"],"_links":{"self":[{"href":"https:\/\/zobai.com\/ar\/wp-json\/wp\/v2\/posts\/56922","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/zobai.com\/ar\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/zobai.com\/ar\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/zobai.com\/ar\/wp-json\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/zobai.com\/ar\/wp-json\/wp\/v2\/comments?post=56922"}],"version-history":[{"count":0,"href":"https:\/\/zobai.com\/ar\/wp-json\/wp\/v2\/posts\/56922\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/zobai.com\/ar\/wp-json\/wp\/v2\/media\/56961"}],"wp:attachment":[{"href":"https:\/\/zobai.com\/ar\/wp-json\/wp\/v2\/media?parent=56922"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/zobai.com\/ar\/wp-json\/wp\/v2\/categories?post=56922"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/zobai.com\/ar\/wp-json\/wp\/v2\/tags?post=56922"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}