Plant InSites: Look Beyond the Lore

Don’t rely on recollections about how a unit had performed

By Andrew Sloley, contributing editor

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Plant legends arise from a blend of hearsay, lack of time for proper analysis, and insufficient understanding of process fundamentals. Such mythology often stands in the way of effective scope definition and troubleshooting, as a recent experience at a trans-shipment terminal illustrates.

The terminal is currently releasing hydrocarbons to atmosphere because its flare system is out of service. To get the flare running again, engineers propose to replace components to bring the unit back to its original as-designed condition.
Figure 1
System Layout: Problems in the lines from the tanks to the flare seal drum undermined successful operation.

Three tanks connect to a common flare seal drum (Figure 1). These fixed-roof tanks contain light hydrocarbons. Temperatures significantly rise during the day, causing tank pressure increases. So, some gas is vented. The flare system is intended to burn the venting gases to prevent their atmospheric release.

The drum uses a water seal. This provides a nearly fail-safe method of setting a back pressure on the drum to minimize breathing losses from the day/night cycle while keeping an open route to the flare at all times. Because turning a single valve in the flare vent line could isolate a tank, each tank has a local conservation vent. So far, so good — at least in theory.
Postponed maintenance has rendered the flare useless. Neither the flare tip nor the ignition system is physically capable of working anymore. The operations department argues: “Why bother to fix the flare? It hasn’t worked for years anyway.”

Operations has good reason for this opinion — after all Figure 1 shows three mutually inconsistent factors:
1. The conservation vent on the tank is set to relieve at 4 in. of water column (0.14 psi).
2. The water seal in the flare drum is 10 in. into the seal water (0.36 psi).
3. The piping has a 7.5-ft.-deep low spot in the system that eventually will fill with condensate (roughly 2 psi).

The hydrocarbon vapors from the tanks obviously can’t get to the flare. Even if the piping low spot is empty, the seal drum requires 10 in. of water column to push the water out of the seal legs. Yet the tanks only will hold 4 in. of water column before the conservation vents open.
Now plant mythology enters the situation. Many people on site remember that the flare system “worked” when first installed. But it turns out that simply means they saw flame from the flare. Likewise, the lack of visible flame was the only consistent explanation of “not working.” So, the legend arose that a working flare is one with a visible flame.

However, fundamentals dispel that myth. The pressure balance proves it was impossible for the system ever to have met its objective. Restoring the equipment to original mechanical condition won’t reduce light hydrocarbon emissions. Meeting that goal demands an in-depth repair — removing the seal loop in the piping system and shortening the seal leg depth in the seal drum — to reduce the seal back-pressure on the tanks.

Unfortunately, many plants have areas where the definition of “working” hasn’t anything to do with the actual process objective or fundamentals of how equipment functions.
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