Condition analysis for safety valve calculation
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Dear sea friends, when analyzing the performance of safety valves, how are plant-wide utility system failures taken into consideration? In the event of a power outage across the entire plant (especially a shutdown of the return pump) or a stoppage in the circulation of water throughout the plant (especially in the tower top coolers), it may lead to overpressure in the towers; if this is to be taken into account, then the discharge capacity of the towers that are at risk of overpressure must be considered as well. I don’t think it’s necessary to consider this (as factories usually have backup circuits), and there are also backup pumps for the circulating water transfer pumps, which can be used in place of the main ones. How do you consider this issue?
Be targeted; analyze each system specifically. It’s unrealistic to strive for greater security in every single system
However, to some extent, this does need to be taken into consideration, and the amount of leakage is not a simple matter of addition; you need to learn more about this topic
Let’s put it this way: it’s enough to be able to convince yourself. All engineering companies make decisions on a gut feeling basis; it’s just a matter of who makes those decisions.
I disagree; it mainly depends on the severity of the hazards associated with that accident scenario
It can’t be stacked, right? Check API520
If they are combined, it can only be considered a waste; a reasonable value should be determined. After all, this is an investment, and while ensuring safety, the lower the cost, the better
Power outages and water supply interruptions do not occur simultaneously; otherwise, what would be the required flow rate for the flare? Set up a PSHH interlock for tower pressure, which will directly cut off the reboiler’s heat source or the feed (depending on the safety analysis to determine which is the ‘primary cause’); assign a SIL level of 2 or 3. Now analyze again to see how much displacement there is left. . . .
I agree with your point; it is also about considering various possibilities, or rather, taking some measures to prevent the safety valve from activating. However, the specifications also state clearly in the section on operational condition analysis that \"measures such as designing fault-proof automatic control interlock systems, automatic start/stop systems, and other conventional instrumentation systems cannot replace the protective function of safety valves for the equipment.\" In other words, the calculations still need to be carried out as usual (it’s just that the use of such systems is less likely). I think now, regarding power outages and water interruptions, although the standards specify such scenarios, it’s better to handle them flexibly during analysis. Even in the event of a power outage, the resulting values can be calculated separately and the largest one can be chosen. The most typical scenario is when the reflux pump at the top of the tower loses power; in such a case, the reflux is interrupted and the liquid level in the reflux tank rises (assuming all other conditions remain unchanged, the top of the tower and the reflux tank are part of the same pressure system, with safety valves located on the reflux tank). During calculations, two scenarios need to be considered: one where the reflux is interrupted (the amount of gas at the lowest tray in the tower plus the amount of feed gas), and another where the pump in the reflux tank stops working (with the outlet closed), in which case calculations are based on liquid release. If there is an air cooler at the top of the tower, then another scenario must also be taken into account, namely when the air cooler stops functioning (and the amount of gas in the vapor phase at the top of the tower is determined by natural cooling) ； It can be seen that although it is caused by one factor (power outage), the various operating conditions can be considered and calculated separately; the larger of the discharge areas should be adopted in the end. This is because, even though it is a power outage, it is not certain which of these conditions will trigger the safety valve first, so it is reasonable to consider them separately rather than adding their effects together. The conditions for water shutdown are simpler; those that occur frequently involve a water cooler at the top of the tower. But, but, according to the specifications, the calculation is based on the maximum volume of gas under normal operating conditions at the top of the tower. If the amount of gas at the top of the tower is indeed quite high (for example, over 150 tons per hour), there is no other solution. At this point, considering the scale of the torch, one must either increase it according to the actual amount of gas or disregard this operating condition. What are everyone’s thoughts on this? ? In the case of a shutdown of all utility services in the plant, it is also necessary to take into account other possible scenarios: what if the materials and heat supplied are not at normal levels? Or what if there is some form of backup system in place (in that case, we need to consider that backup system, haha). As one of the users upstairs said, find reasons that can \"convince oneself\" or \"convince the designer\"; take certain operating conditions into consideration appropriately. It’s not necessary to consider too many conditions as specified in the standards and calculate overly complex values, as that would be uneconomical. Are there any evaluation methods for HAZOP analysis in this regard? Know how to explain it.
Just a few remarks: it cannot be simply added up; generally, the maximum leakage rate under various possible operating conditions is taken as the basis for selecting a safety valve.
Aspen Plus simulation of safety valves