Hope all of you are doing well. Today's topic is focusing on the general practice sometimes followed in industry related to the set point of Pressure Relief Devices of process equipment. Many times during PHA, I have come across a general philosophy of keeping the set point of the reactor's pressure relieving devices equivalent to its design pressure. This might be adequate for a non-reactive system. But the question is, is it adequate for a reactive system?
Here, the reactive system is one where there are hazards due to a chemical reaction, including the possibility of decomposition or polymerization or some side reaction, etc. We know based on normal kinetics that the rate of reaction is directly proportional to the temperature which in turn is proportional to pressure. In simple terms, it is said that the rate of a chemical reaction doubles with every 10 deg C rise in the reaction temperature. Hence it can be said that:
Higher set pressure leads to a correspondingly higher "set" temperature (i.e. the relieving temperature). This, in turn, leads to a higher rate of reaction which results in higher self-heating or higher temperature rise rate (dT/dt i.e. deg C/min) and a higher pressure generation rate (dP/dt i.e. bar/min). This is very important because for a chemically reactive system the required pressure relief area depends directly on the self-heating and pressure rise rates at the relief conditions.
Referring to the below composite graph for the methanol and acetic anhydride reaction (from an adiabatic calorimetry test), we see that at a given set pressure the corresponding temperature value can be obtained from the graph of pressure versus temperature. For example if the set pressure is 20 psi (about 35 psia) then the set temperature is about 99 deg C. Based on that set temperature, the temperature rise rate can be obtained from the plot of self-heat rate (dT/dt), giving a value of about 20 deg C/min in this illustration. The data show that the temperature rise rate increases exponentially with increasing temperature, and the system pressure rise rate must follow. Notice that if the set pressure is higher, say 35 psig (about 50 psia) then the set temperature is about 110 deg C and the corresponding self-heat rate is about 34 deg C/min. The implication is that at the higher set pressure the reaction rate is higher and the required pressure relief (vent) area is therefore larger.
Recall we have discussed T2 laboratories in my previous blog post, https://staub-ex.blogspot.com/2018/05/relief-device-sizing-and-worst-case.html, where the process was to batch load three different reactants, heat them to 99 deg C, start the agitator and continue heating to the process temperature. There was a provision of cooling water for reactor mass cooling and the reactor was provided with a 4" rupture disc with a set point of 400 psig. But on the day of the incident, cooling system failure resulted in runaway of the desired reaction, which further led to a second undesired exothermic reaction. The set-point of Rupture Disc was too high which led to an even higher temperature before the opening of the rupture disc. Higher temperature resulted in an increased rate of reaction and the 4" rupture disc was not sufficient for the required relieving rate causing the explosion. With lab trials, it was proved that the same 4" rupture disc with a set point of 75 psig would have been sufficient to relieve and prevent the reactor explosion.
This should make it clear that if a pressure relieving device is set at a lower pressure for a chemically reactive system, the size of the pressure relieving device will be smaller as compared to the size of a relieving device set at high pressure for the same system.
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1. What should be the set pressure? Less than equipment design pressure then how much less and what is its basis?
ReplyDelete2. Is it applicable to reactor only or to other systems as well?
3. If we reduce the set pressure of PSV then, the frequency of PSV popping up will increase and if we keep on reducing the set pressure the frequency of popping up the PSV will increase as compared to when PSV set pressure is equal to design pressure of the equipment. We can't judge the frequency of PSV popping. It will be higher for sure, as a result PSV seat might get damaged.
Although the relief rate will be lesser in case of lower set pressure but the reliability of the PSV will be questionable. That means safety.
4. As you mentioned, when reaction proceeds the temperature will increase drastically so we need to take care of design temperature for the equipment also.
First of all I would like to highlight, the article given here is not valid for non reactive systems as they do not need DIERS to be followed compulsorily. For these non reactive systems, such as in oil and gas, one can straight away work on the principle of external thermal load and set point as per API or other standards. However, for reactive chemistry systems:
Delete1. Set point should depend on DIERS and adiabatic calorimetery results. If in a reactive system, one can simply provide whatever size Relief device is calculated from DIERS, then set point can be equivalent to design or MAWP or somewhat higher as well. However, I have seen relief devices being sized as 8” or even more for a small 6 kL vessel due to a possible runaway reaction, so in such a scenario set point will depend more on what we can provide and then do reverse calculations to define set point. Also set point might need considerations for system tempering (i.e. temperature at set pressure equivalent to boiling, such that inventory starts boiling as soon as the relief device opens, to prevent further temperature rise till the boiling material is present in system)
2. This will be applicable to any process equipment where there is a possibility of a reaction. If inside a storage tank, decomposition or polymerization can happen, then same philosophy is valid for that tank as well.
3. For a reactive system, a rupture disc should be preferred. However if PSV is provided then the set point can be evaluated to as high as our PSV relief rate can handle. Same is valid for the rupture disc as well.
4. For reactive systems, DIERS divide runaway chemistry into Vapor, Gassy or Hybrid system. For vapor and hybrid, usually as soon as the relief device opens, the system can get tempered (if set temperature at set pressure matches the boiling point of a material in the reactor). And moreover it is made in the design of process equipment that might handle a runaway chemistry and adequate number of layers of protection should be provided including design of equipment (inherent safety).
I hope these explanations make some sense. Let me know, if I can explain something further.
nice post..
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