What steps do we need to take to keep our sour water stripper at peak performance following a shutdown?

First published in PTQ Q1 2021

Carmella Alfano, Technologist, Axens,
Guillaume Dalary, Technical Service Engineer, Axens

Shutdown operations of the sour water stripper unit should maximise the removal of H2S and NH3 from water, especially for long duration shutdowns, to minimise the safety concerns and corrosion issues during maintenance. If the sulphur recovery unit is not available, the sour gas should be routed to flare, then start the shutdown by decreasing the sour water feed to the unit manually and slowly.

If applicable, by-pass the storage tank, stop the stripped water pumps, and open the start-up recirculation line from stripped water outlet to surge drum. Demineralised water can be introduced into the surge drum to maintain the flow. As H2S and NH3 content in the sour water in the closed loop will start to decrease, sour gas production will be reduced accordingly and should be diverted to the sour flare, if not done previously.

Maintain the temperature of the bottom section of the sour water stripper by keeping steam supply. Continue stripping the water. As H2S and NH3 content in the water decreases in the circulating water, the stripper pressure will start to decrease and the flare pressure control valve will continuously close to maintain the stripper overhead pressure. If required, the stripper can be manually pressurised slightly using nitrogen. Continue water circulation until the water no longer contains any H2S and NH3, approximately two hours. Shut the unit down and, depending on scheduled activities, keep the unit in stand-by with a slight pressure of nitrogen (failing that, by opening the flare PV to float on flare header pressure if nitrogen is not available), or drain all equipment using nitrogen for pushing, depressurise and inert the unit, or prepare the unit for safe entry. During the shutdown procedure, care should be taken to not admit air into the system until all hydrocarbons/acid gases are removed from the system and it is thoroughly washed.

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