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Chemical Process Technology

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Monday, August 25, 2008

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A pressure relief valve (PSV) protecting a pressure containing system, it is possibly be tapped from a vessel or piping. The location of the tapping is very much subject to the source of overpressure, flow path of the fluid, fluid quality, etc.

Top of Vessel
A PSV protecting a vessel generally located on the highest location of the vessel. The intention is to minimize potential of liquid get into the inlet line and subsequently passing the PSV. Present of liquid in PSV throat will potentially limit the flow path of a the PSV and lead to overpressure.

Apart, as fluid is entering a vessel during overpressure and PSV lift, fluid may carry some liquid and solid will slow down when it enter the vessel. It potential partial knock out liquid and solid in the vessel and avoid carry into the PSV.

Vapor Outlet Pipe (Highest point)
Sometime PSV is tapped from the vapor outlet piping. One of the advantage is it located on the highest point of the system and minimize the potential of liquid entering PSV which may reduce the vapor flow path. Another advantage is reduce number of nozzle located on the vessel. One of the downside the locating PSV on the vapor outlet is additional service platform and structure required dedicated to these PSVs. It would increases the installation cost.

Side of Vessel
Present of internal i.e. demister in a vessel, the tapping point would possibly be located at the side of the vessel but below the demister. This is to avoid the potential of demister blockage and limiting flow passing demister where it possibly cause overpressure of compartment upstream of demister (lower compartment) while downstream of demister may possibly operate at pressure below set pressure. Apart, it may also cause PSV chattering due to large pressure drop across demister.
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Present of solid particle in fluid and clear flow path in demister would be the determining factors. However, one may have to understand very low possibility to have a complete blockage of demister and demister is still mechanically fitted on the vessel when exposing to large differential pressure. Thus in many occasion, this could be an issue with long debate and it end up one PSV at lower compactment of demister to protect vessel from process upset whilst another PSV located on the vessel top protect vessel upper compartment from fire contingency.

Another issue related to vessel side tapping is it reduce the distance between PSV inlet nozzle and liquid level in the vessel. Dynamic turbulence and vapor movement may result severe entrainment and lead to large amount of liquid enters PSV. Again it reduce the flow path of PSV.

Inlet of Vessel
Separator inlet nozzle may be fitted with inlet device. The may potential cause partial blockage and lead to overpressure of section upstream of feed nozzle. Thus, a PSV may be tapped from the inlet of vessel. While considering PSV tap from inlet pipe, one may understand liquid (and solids) can be easily carried into PSV.

Concluding Remarks
PSV tapping location is significant affect the performance of the PSV. Proper location may sometime determining the effectiveness of an overpressure protection system. Extra care shall be paid into this aspect.

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posted by Webworm, 1:52 PM

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