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During a pump swap or start-up after maintenance, what precautions are taken to make sure you don’t shut the heater down on low flow if the pump cavitates? Does anyone have some type of bypass that will not keep fuel gas in the heater but still trip the emergency steam if the pump does cavitate and the flow drops off for a moment? What concerns would you have with not tripping the fuel gas and would this be mitigated by the concern of putting cold feed into the online drum?
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For loss of heater feed, it is best to trip the heater fuel gas and inject emergency steam into heater feed coils to prevent coils from coking. After returning heater feed service, try to coke for an additional 4 hours before switching out of the drum to mitigate the effect of uncoked resid in the drum.
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If you can start your charge pumps on a restricted discharge, allow suction to establish itself then transition over by restricting flow to the standby while opening flow to the mainline pump the transition will be seamless. We had the same issue of loss of flow to the heaters which has since all but been elliminated. We utilize Flowserve DSTHF pumps which can tolerate a short run at or near deadhead to allow for a seamless tranition. The root cause for our flow interuptions were due to the violent foaming tendency of our gas oil flushant when it came into contact with the hot bitumen from the diluent recovery plant. In standby the flushant was not shut off so it eventually displaced all the bitumen in the trapped space within the pump. When started this gas oil slug foamed resulting in loss of flow.
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