March 2009

www.che.com

Feature Report Part 2 Getting the Most Out of Your

For optimum rupture-disc performance, pay attention to installation, operation and maintenance

Dean Miller Fike Corp.

upture disc devices provide over- Liquid service protection for a variety Liquid-full systems cre- of storage and process vessels ate a number of process- FIGURE 1. This viscous-tee holder design con- Rand equipment. The objective of ing challenges, many of tinually wipes the disc surface, preventing prod- the rupture disc is to maintain a leak- which apply to rupture uct buildup tight seal and be a passive bystander discs. Pressure spikes and until called upon to relieve excess pres- water hammer generated sure. While this is generally the case, by rapid opening or closing there are times when rupture disc of valves somewhere in the process fre- using pressure accumulators to absorb performance can be adversely affected quently do affect the rupture disc. The the unavoidable pressure spikes. through various installation, operation typical rupture disc begins to respond Common indications that you have and maintenance practices. to pressure in excess of the burst pres- a water hammer or another pressure This article reviews some of these sure in less than 1 millisecond. This spike problem include the following: practices, real-life observed conse- means that a short-duration pressure • The rupture disc appears to burst at quences, and corrective or preventative spike that is not detectable by normal a pressure lower than the marked measures that can improve rupture disc process instrumentation can and will burst pressure performance. Part 1 of this feature report affect the rupture disc. Ways to avoid • The rupture disc is only partially (Chemical Engineering, March 2009, pp. pressure spike problems include: avoid- open 42–44) discusses situations where com- ing the use of rupture discs on long, • Ruptures are observed during or binations of rupture discs with relief liquid-filled lines, eliminating fast-clos- immediately after some non-steady valves should be considered. ing-and-opening process valves, and state condition in the process

11-26114205.indd-26114205.indd 1 44/7/09/7/09 1:28:061:28:06 PMPM Feature Report

FIGURE 2. This rupture disc holder has been subjected to heavy corrosion

FIGURE 3. The damage on this forward-acting rupture disc is a result of freezing condensate

Solidifying materials Piping Flange connections This is a combination rupture-disc- The influence of piping and piping- Most rupture discs function in con- selection and piping-design problem. support design on the rupture disc junction with a holder that contains Many process materials are prone to is often overlooked but has been the key interface features that help con- building up and solidifying on internal source of many problems. The most trol bursting characteristics and effect surfaces. This can create a problem common problem is the transfer of a leak-tight seal. The goal of the flange for the typical vessel discharge port excessive piping loads to the rupture connection is to apply the proper that may become partially or com- disc. This can be caused by very tall amount of clamping load for the rup- pletely blocked, resulting in poten- discharge pipes; long, horizontal pipe ture disc and holder to function cor- tially higher than expected rupture- runs; or severely misaligned pipe rectly. This clamping load is affected disc burst and restricted flanges. by the torque applied to the studs, the discharge flow. Common to The most common symptom of a type of lubrication on the studs, the this problem include the following: piping problem is rupture disc leakage type of flange gaskets used, and the due to uneven loading of the metal- general alignment of the pipe flanges. • Use of heat tracing or steam jack- to-metal sealing surfaces. This issue Indications that you have an under eting around the vessel nozzle and can usually be resolved with strategic or unevenly loaded rupture disc in- rupture disc holder placement of piping supports. clude the following: • Use of a “viscous tee” style holder Another piping-related source of (Figure 1), which continually wipes trouble is the failure to vent or ac- • Leakage between the rupture disc the disc surface, preventing product count for drainage of discharge lines. and holder buildup Heavy corrosion of the rupture disc • Rupture disc slippage • Use of a flush-mount-style rupture- holder can result (Figure 2). Also, • Increased variation in burst pres- disc assembly, which eliminates condensate in the discharge lines can sure any in the vessel nozzle freeze, and the expansion during the Indications that you may have an that could accumulate solidified freezing process can collapse a for- over-loaded rupture disc include the materials ward-acting rupture disc (Figure 3). following:

11-26114205.indd-26114205.indd 2 44/7/09/7/09 1:28:081:28:08 PMPM FIGURE 4. Wrinkles on the outer FIGURE 5. This holder shows deformity due edge of a rupture disc indicate to excessive torque. Ideally there should be no slippage from uneven torque gaps between the holder and a straight edge

• Leakage between the rupture disc rupture disc. Consult the disc manu- Maintenance frequency and holder facturer to verify the higher load will While rupture disc manufacturers • Increased variation in burst pres- be acceptable. would like to recommend replacement sure of every rupture disc every year, that • Permanent damage to the holder Maintenance activities is not a realistic expectation. As shut- Rupture disc devices require periodic downs for maintenance activities are Torque: All rupture discs have a rec- maintenance just like any other piece stretched further and further apart, ommended torque provided by the of equipment. it is not unusual to expect 3 to 5 years manufacturer. This information may Device removal: The entire device of service from the rupture disc. Rup- be located on the rupture disc tag or (rupture disc and holder) can be re- ture disc life is generally tied to the in the installation instructions pro- moved from the flanged connection history (pressure and tempera- vided. This torque value is generally and inspected. The device may be ture) applied to the disc. Relatively associated with some common thread re-installed if desired as long as the low pressures and static conditions lubricant. Torquing should always be disc hasn’t been removed from the will generally result in very long disc done in a criss-cross pattern in incre- holder. Pre-torque style holders are life, while cyclic conditions approach- ments of no more that 25% of the final frequently used in applications where ing the burst pressure will result in torque value to ensure evenly devel- physical inspection and re-use of the shorter disc life. There is no one, right oped loading (Figure 4). It is gener- rupture disc is desired as the holder answer regarding replacement fre- ally a good practice to re-torque the maintains load on the disc even after quency, but common considerations studs after a period of time and after removal from the pipe flanges. Once include the following: the system has come up to operating the disc has been removed from the to compensate for the holder it is recommended that the • Severity: What is the severity of the normal relaxation of bolt loads. disc be replaced. Due to the metal- service the rupture disc is subjected Lubrication: Thread lubrication can to-metal sealing surfaces, any re-in- to? Consider corrosion, operating drastically affect the resultant load stallation of a rupture disc is likely pressure, cyclic duty and so on. achieved from an applied torque. Dry to seal less effectively. Also, some rup- • History: What is the history of the unlubricated threads are more prone ture disc types may exhibit increased rupture disc location? Has it been to corrosion and galling, resulting in burst pressure variability when re- prone to premature fatigue type an under-loaded rupture disc that installed. failures? might exhibit leakage, slippage, or in- Disc inspection: The value of rup- • Cost: What is the cost of an unex- consistent burst pressures. Threads ture disc inspection is limited to pected outage due to premature lubricated with high performance signs of damage, corrosion, or product burst versus the cost of a replace- lubricants, such as PTFE, moly-dis- buildup. No information about the ment rupture disc during planned ulfide or graphite, may result in an disc integrity or remaining life can be downtime? overload condition. determined through visual or dimen- • Upgrade: Are there other disc ma- Gaskets: Flange gaskets have an ef- sional inspection. terials or newer disc technologies fect on the applied load as well. Soft Holder inspection: Holder inspec- that will allow the maintenance gaskets, such as PTFE, while great at tion should include examination of interval to extend beyond conforming to imperfect surfaces, are the seating surfaces for damage, cor- limitations? What is the payback subject to cold flow. The result is that rosion, or product buildup. A quick on such an upgrade? a short period of time after applying way to check for holder damage due the correct load, the gasket will have to over-torque is to put a straight When a rupture disc bursts unex- further compressed and released most edge along the gasket face and look pectedly, the user should look for the of the load. Some gaskets may have a for signs of significant bending (Fig- obvious pressure or temperature ex- higher required load than that of the ure 5). cursions in the process that may have

11-26114205.indd-26114205.indd 3 44/7/09/7/09 1:28:111:28:11 PMPM initiated the bursting of the disc. If • Discharge to atmosphere, pressure and reduce long-term costs. ■ the cause is not apparent, it is time or header Edited by Dorothy Lozowski to engage the manufacturer to as- • Rupture-disc performance history Author sist in troubleshooting the problem. in this location Dean Miller is the manager When rupture disc problems are ex- • Photos of rupture disc, holder, in- of mechanical engineering with Fike Corp. (704 SW 10th perienced, it is important to gather as stallation St., Blue Springs, MO 64015; Phone: 816–655-4729; Email: much information as possible to ar- • Signs of abnormal operation: for [email protected]). He rive at resolution quickly. The follow- example, pin-hole opening, partial has technical responsibility for a variety of products in- ing items form a good starting point opening, slippage and corrosion cluding rupture discs, explo- sion vents, and mechanical for data collection: products explosion and fire-protection products. He received a B.S. degree in mechanical engi- • Rupture disc lot number Rupture discs generally provide neering from the University of Missouri-Kansas City and an M.B.A. from Keller Graduate School • Date installed highly reliable pressure relief, so in of Management. He has 23 years of experience in the pressure relief industry and serves as a • Date of burst the case of problem installations there member of the ASME subcommittee on Safety • Actual normal and maximum oper- are likely actions that can be taken to Valve Requirements, ASME committee on Boiler & Conformity Assessment, API ating pressure improve rupture disc performance. Task on RP520, STD521, and STD2000, and the U.S. Technical Advisory Group for ISO • Actual normal and maximum oper- Partnering with your rupture disc TC-185. ating temperature supplier to investigate and evaluate Editor’s note • conditions: yes or no these situations will promote oppor- Both disk and disc are commonly used spellings • Cyclic conditions: If yes, describe tunities to improve rupture disc per- of the word in industry. The editors have chosen to use the spelling of the author in each article • Liquid or vapor formance, extend maintenance cycles of this series.

Posted with permission from Chemical Engineering. Copyright © 2009. All rights reserved. #1-26114205 Managed by The YGS Group, 717.399.1900. For more information visit www.theYGSgroup.com/reprints.

11-26114205.indd-26114205.indd 4 44/7/09/7/09 1:28:121:28:12 PMPM