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Properly Position Interlock Valves

Oct. 16, 2012
The obvious location may not be the most cost effective

Plants continue to expand use of active safety systems. Interlocks often figure as key components of these systems. For instance, specific events may trigger interlocks to automatically close or open valves. Figure 1 shows a partial view of an overhead system in which a proposed high-level interlock (HLI) on a drum would trip a valve. The question is "where should the valve be?"
Interlocks exist to prevent some consequence. Here, the plant wants to ensure that liquid doesn't fully fill the drum because that would flood the pressure control point. The final consequence of a full drum is liquid running down the purge line to the plant vent gas system. In this case, liquid in the vent system may create safety problems.

The proposed HLI shuts a valve on the line from the overhead condenser to the drum, stopping liquid flow into the drum. The valve closing leads to liquid starting to build up in the condenser. More liquid in the condenser reduces condensation capacity. The decreased capacity causes system pressure to increase. The dual-range controller then shuts the exchanger bypass valve. At this point, the tower system starts to pressure up.

The overhead condenser is heat integrated with another system. So, the reduced cooling duty affects that system, too. Fortunately, the connected system can get heat from another source. It will suffer a small upset, but the consequences are manageable.

An alternative location for the isolation valve is directly on the vapor vent line. When the liquid level gets too near the height of the outlet nozzle, the HLI closes the vapor purge line. Noncondensable gases then start to accumulate in the drum, displacing liquid. That liquid, plus vapor still being sent to the overhead condenser, causes a rise in its liquid level. This reduces the surface area for condensation, decreasing ability to condense the tower overhead. Tower pressure rises.

Figure 1. The proposed high level interlock will work, but so will a lower cost alternative. Both locations lead to very similar direct consequences. So, other factors should drive the decision about where to install the valve. Two major questions are how easy are the valves to access and how much do they cost?The isolation valve on the exchanger outlet would be located very close to the ground. Installation and access would be easy. However, the valve installation might force the exchanger up and increase pressure drop in the overhead system. While not insurmountable, elevation changes required to make the system hydraulics work would increase costs.The isolation valve on the drum outlet would be located above the drum. An access platform already exists to service the pressure control valves. So, installing the isolation valve would involve only a modest expense.The exchanger outlet would require a 6-inch valve. In comparison, the drum outlet would call for a 2-inch valve. In alloy 2205, this translates into a significant cost difference.Overall, the best choice is to put the isolation valve on the vapor outlet line. While both options pose essentially the same process consequences, the vapor line valve offers lower installation costs and the valve itself is cheaper.

ANDREW SLOLEY is a Chemical Processing Contributing Editor. You can e-mail him at [email protected]

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