Batch digester relief valve chattering after a set-pressure increase
Batch digester No. 2 has a MAWP of 1,100 kPa(g) and a design temperature of 175 C. Its conventional spring-loaded relief valve was reset from 965 to 1,035 kPa(g) after a production increase. Since then the valve chatters violently when it lifts, and the upstream 3-inch line has a measured inlet pressure loss of 58 kPa at rated flow. The valve has a 2-inch inlet and 3-inch outlet, with a rupture disk upstream whose burst tolerance is plus or minus 5%. The valve capacity was sized for steam relief during a blocked outlet, not for an exothermic reaction scenario. The mill wants to keep the higher set pressure and add a downstream silencer. The relief header has 12 kPa built-up backpressure. The vessel's next external inspection is in 10 months.
1. Evaluate the proposed set pressure against the vessel MAWP and code accumulation limits; show the calculation.
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At a 1,035 kPa(g) set pressure, 10% accumulation gives 1,138.5 kPa(g), which exceeds the 1,100 kPa(g) MAWP. The maximum allowable set pressure is 1,100 divided by 1.1, or 1,000 kPa(g). The rupture disk tolerance makes this worse: a disk marked 1,000 kPa may burst at 1,050 kPa, leaving only 50 kPa, about 4.8%, before MAWP; the valve may not reach rated capacity. A safer set is about 950 kPa or a disk with a tighter tolerance. The 58 kPa inlet loss is 5.6% of the 1,035 kPa set pressure, above the 3% rule, which is about 31 kPa. The proposed set pressure is therefore not acceptable.
2. Diagnose the chatter and state whether a downstream silencer will help.
Read the worked answer
Chatter is being driven by excessive inlet pressure loss and probably by an oversized valve. The 58 kPa loss is nearly twice the 3% limit, so the pressure under the valve seat drops as the valve opens, the valve closes, and the cycle repeats. A downstream silencer adds backpressure and does not fix inlet loss; it may reduce capacity further. Fix the inlet by increasing the line to 4 inch, shortening equivalent length, removing the rupture disk if the relief scenario permits, or using a pilot-operated valve. Recheck built-up backpressure; 12 kPa is below 10% of a 1,000 kPa set pressure, so it is not the primary cause.
3. What risk-assessment actions would you take before returning the digester to service?
Read the worked answer
Do a management-of-change review and revalidate the relief scenarios, including exothermic reaction and blocked outlet; the existing steam-only sizing is insufficient. Use API 521 and, if reaction can occur, DIERS methods to check for two-phase flow. Confirm the rupture disk and valve combination at the lower set pressure, install an independent high-pressure trip or additional relief, and function-test the valve. Check the inlet line for strainers, debris, and disk fragments. Do not return to service at 1,035 kPa(g).