Of course, it is possible that there is water in the instrument air. Most dryers are designed for a dew point of -20°F. There's a remote chance the valves could not fight the spring because the ambient temperature fell below this point, the dryer failed or oil was allowed into the instrument air.
There are some process workarounds that may help. Some of the heat in the furnace can be reduced by changing the shutdown plan to include a cooldown using the combustion air blower. But this won't completely eliminate the risk of blowing the relief valve if the feed valve is closed. Perhaps it would be better in this case to change the actuator in the feed valve to a fail-open orientation. This, too, should be decided by a new HAZOP.
Dirk Willard, senior process engineer
Middough Consultants, Holland, Ohio
As part of our refinery expansion, we must confirm the capacity of a schedule-80 crude-oil-desalter wastewater line. It's a 2,500-ft pipeline consisting of two heat exchangers and a mix of 3-in. and 4-in. pipe emptying into a slop-oil decanter operating at 5 psig. We're hoping that capacity can be increased by 25%. We collected some data using existing pressure gauges. An initial hydraulic study with software using equations from Crane Technical Paper No. 410 indicated there is an "extra" 65 psi when 125 gal/min of brine flows at 160 psig. However, another study done using different Crane-based software gave about-50-psig extra pressure. How could two programs produce such different results?
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