You are working in a gas plant and an upset condition occurs.\u00a0 It\u2019s 3 AM and cold.\u00a0 The flare is going off, alarms are sounding, lights are flashing, and your adrenaline is pumping.\u00a0 The operators correct the situation, and the system returns back to normal.\u00a0 However; a relief valve (Figure 1) on a high pressure ANSI 900#sSeparator failed to reseat and is leaking a lot of gas to flare.\u00a0 You\u2019re asked to line-up the stand-by relief valve, and then isolate the leaking valve so that it can be sent in for repairs. Table 1 presents the portion of ANSI class rating.<\/p>\n
T<\/strong>able 1 – Flange Pressure, Temperature Rating<\/strong><\/p>\n Question: Do you close VALVE #1 or VALVE #2 first, or does it matter?<\/strong><\/p>\n Figure 1.\u00a0<\/strong>Relief Valve Block Closing Sequence\u00a0<\/em> Figure 2.\u00a0<\/strong>Valve Closing Sequence Answer<\/em><\/p>\n Can your operators and engineers answer this question correctly?<\/strong>\u00a0 It may be a great topic for your next safety moment\/tailgate safety meeting.<\/p>\n Another source to review is API 14C [1].\u00a0 Spec breaks are highlighted in Figure 3. (API 5000 to ANSI 600#; and ANSI 600# to ANSI 150#).\u00a0 One example is discussed below:<\/p>\n The well flows into the system at 5,000 psi and its pressure is reduced by a fixed choke.\u00a0 The vessel is operated at 1,000 psi and is protected by a relief valve set at 1200 psi.\u00a0 The vessel has a manual drain valve #5 that is closed.\u00a0 The level is maintained by a level control valve and a bypass exists around the separator for maintenance.<\/p>\n Figure 3.<\/strong>\u00a0<\/strong>Specification Breaks Example: What’s Wrong?<\/em><\/p>\n Figure 4.<\/strong>\u00a0<\/strong>Spec Break Error<\/em><\/p>\n In Figure 4 if you close\u00a0red<\/strong>\u00a0highlighted valves 2 and 8 then the\u00a0yellow<\/strong>\u00a0highlighted piping system is isolated from the relief valve and is currently unprotected from overpressure from the well with a shut-in tubing pressure of 5,000 psi if either of the following valves are closed #3, #6, #9, or the check valve 4 is plugged. If any of these valves are closed the high pressure source will equalize across the choke causing a loss of containment in the downstream piping which is blocked from the relief valve.<\/p>\n If Valve #2 and Valve #8 are closed, and the others are open, then the atmospheric tank will be catastrophically destroyed.<\/p>\n In order to find \u201cspec break busts\u201d and design errors, start at the highest pressure source and then open and close downstream valves to check that each piping component is protected by a relief valve.<\/p>\n For example, by performing simulation, start with the shut-in tubing pressure of 5,000 psi.\u00a0 Close the next downstream valve which is the choke.\u00a0 The upstream piping and closed choke must be rated to 5,000 psi. There is no issue since the spec break is the downstream flange\u00a0of\u00a0valve 1.\u00a0 All piping and components upstream of this break are rated to API 5,000.\u00a0 Next open the choke, open valve 1, close valve 2 and valve 3.\u00a0 The piping segment is not protected by a relief valve and is only rated to an ANSI 600 # system.\u00a0 This piping segment will fail and cause a lack of containment.\u00a0 A solution would be to eliminate valve 2 or provide a PSV upstream of valve 2.<\/p>\n To learn more about similar cases and how to minimize operational problems, we suggest attending ourPS4<\/strong>\u00a0(<\/a>Process Safety Engineering)<\/a>,\u00a0G4 (<\/strong>Gas Conditioning and Processing),<\/strong><\/a>\u00a0G5 (<\/strong>Advanced Applications in Gas Processing,<\/a>\u00a0PF3\u00a0<\/a><\/strong>(Concept Selection and Specification of Production Facilities in Field Development Projects)<\/a>\u00a0and\u00a0PF49\u00a0<\/strong>(Troubleshooting Oil & Gas Processing Facilities)<\/a>\u00a0courses.<\/p>\n PetroSkills\u00a0| John M. Campbell <\/em>offers consulting expertise on this subject and many others. For more information about these services, visit our website at\u00a0http:\/\/petroskills.com\/consulting<\/a>, or email us at\u00a0consulting@PetroSkills.com<\/a>.<\/p>\n By: James F Langer, P.E.<\/em><\/p>\n Sign up to receive Tip of the Month email updates!<\/em><\/p>\n![\"\"](\"https:\/\/i0.wp.com\/www.jmcampbell.com\/tip-of-the-month\/wp-content\/uploads\/2018\/05\/table1-e1649190496833.png\")
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\nAnswer:\u00a0<\/strong>As seen in Figure 2, you should close VALVE #2 first.\u00a0 It isolates the high pressure gas from the relief valve.\u00a0 What many people do not know is that the relief valve has a spec break across the valve.\u00a0 The downstream flanges are only rated for 150#.\u00a0 If you mistakenly closed Valve #1 a loss of containment would probably have occurred with\u00a0serious\u00a0injury.\u00a0 In a recent class, approximately 50% of students\u00a0answered this question incorrectly. This design exists all over our hydrocarbon industry.<\/p>\n<\/a><\/p>\n<\/a><\/p>\n<\/a><\/p>\n