Pressure is equal to the force divided by the area (P~F/A). That is a very basic concept. I am sure you already familiar with this formula in the very beginning of junior high school. In talking PSV, the force is generated by mass relieving rate. I am sure, all of us know well that relieving rate is depends on cases that to be considered (e.g. fire, blocked discharge, gas blow by etc.)
Let’s focus to Kc, Kd and Kb. I will make other posting for the explanation of the other factors.
Effective Discharge – Kd
Kd is effective discharge coefficient used for the mass flux capacity correction for the real nozzle. The higher the Kd value, the closer the mechanical to an ideal design - ideal nozzle, Kd = 1-. It is very obvious that the PSV has Kd value lower than 1. For instance, API relief valve has Kd = 0.975 and 0.65 for gas and liquid respectively.
We can use API data as preliminary calculation only when we don’t have any reference of Kd value. We usually do this at proposal stage.
Some vendors have a better design PSV than standard API. In other word, they have Kd value higher than 0.975. So, I suggest you to ask your vendor the exact value of Kd, especially at project stage. There is potential that the smaller orifice area is required.
Kc is correction factor when rupture disk to be installed upstream of the PSV.
Rupture disk is required to be installed at upstream of the PSV for system contain solid that may plug the PSV over time. At HAZID/HAZOP, for toxic service, potential leaking of relief valve shall be considered, and then rupture disk at upstream PSV can be used as positive seal for the safeguard. That is why, based on my experience, the combination rupture disk and PSV is very seldom to be applied in the gas processing.
Actually, the Kc value is complex. But I am sure, we seldom use it. So, I am not too interest makes longer explanation. I understand many engineers hate with the long article with something not practical, me too. If you want to know more detail about Kc, please read API 520 part I by yourself.
Back pressure is defined as a pressure existing at PSV’s outlet. It impact to opening pressure, reduction capacity, instability or may combination of all. Kb is required for correction of reducing capacity.
For the low back pressure system that the impact is not significant, conventional type can be used. And for the excessive back pressure service, pilot type PSV is required to overcome it. Then, imagine that low back pressure for the conventional type and no impact back pressure on pilot type due to mechanical design. That’s all, I think, very clear why Kb is required for balance below type only.
Actually, for conventional type, when the condition is non-critical due to superimposed back pressure, Kb is required. But for now, rather than it will make you confuse, forget it since there are not much system likes that.
The understanding of the back pressure is very important for process engineer. At this time, at least, we know why Kb is required. I will make a separate posting for explanation of back pressure for more detail. I have plan it will have been finished before the end of this month. Don’t miss it.
That’s all I can share this week. The required orifice area calculation procedure is covered in API 520 part I. The standard relief valve orifice is also already stated in API 526 and ASME Sec VIII. Please refer to those documents for the detail.