Don't Scoff At the Expert

Readers recommend significant upgrades to a relief system.

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Connecting one or more relief devices to a common discharge though ubiquitous is often wrong; different corrosion requirements and handling a refinery-wide fire make this a bad idea. Common headers are okay if the manifold is large or the PSVs are pilot or bellows, which allow greater backpressure — but only if a complete scenario study is conducted. Another issue is reliability: pilot PSVs are complicated. Remove the check valves; consider isolation rupture disks (PSEs) for corrosion isolation.

While API 520 Section 6 allows an inlet automatic valve, bleed valves must be installed between the inlet and relief device to test for leaks, the same as for a PSE/PSV combination. A pressure gauge or switch is required here; use a switch for inaccessible applications.

The combination of a PSE and a smaller PSV addresses two different scenarios. Clearly the PSE is for a less frequent event, probably a fire, and should have the higher setting; the relief valve is probably for a blocked valve scenario. The PSV should be set to a lower pressure, as allowed by ASME code but not greater than 80% of the disk burst pressure; check with the disk vendor. A blocked valve could be from a feed pump or liquid expansion from a reboiler, not vapor, so an isolation pot is required for the PSV discharge.

Now, consider the globe valve below the PSE. According to API RP 520, Part II, Section 6.3.1, "valves shall be full bore." ASME Section VIII requires that the opening through all pipe and fittings between a pressure vessel and its pressure relief valve shall have the area of the pressure-relief-device inlet. I doubt if a 4-in. globe valve meets this requirement.

Last, consider the storage tank in the left-hand corner. Flame arrestors are not required if they will impede flow. API-2000 Section 4.4.1.2 states, "A flame arrestor is not considered necessary in conjunction with a PV valve venting to the atmosphere because flame speeds are less than vapor velocities across the seats of PV valves." Remove the flame arrestor immediately. There are two other relief issues that require attention: 1) only a floating head tank does not require an emergency vent (fire); and 2) fume control, e.g., an activated carbon bed, is usually required to catch benzene and other carcinogens.
Dirk Willard, process engineer
Fluor Global Services, Inver Grove Heights, Minn.


JUNE'S PUZZLER
Our liquid ring vacuum pump repeatedly tripped during high pressure, high flow conditions at startup. When it finally started, the pump ran noisily. On inspection, we noticed it was installed backwards. We reversed the direction of the pump but got only low vacuum and the motor became very hot — exceeding the temperature ratings on the motor insulation. We're operating at 300 torr with water. Why did the pump operate, though noisily, with the flow reversed? What is causing it to run so poorly now? Is the pump shot?

Send us your comments, suggestions or solutions for this question by May 11, 2012. We'll include as many of them as possible in the June 2012 issue and all on ChemicalProcessing.com. Send visuals — a sketch is fine. E-mail us at ProcessPuzzler@putman.net or mail to Process Puzzler, Chemical Processing, 555 W. Pierce Road, Suite 301, Itasca, IL 60143. Fax: (630) 467-1120. Please include your name, title, location and company affiliation in the response.

And, of course, if you have a process problem you'd like to pose to our readers, send it along and we'll be pleased to consider it for publication.

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