what is KC and KD ??
Answers
Answer:
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.
Kd is depends on the mechanical design. In other word, every vendor has a specific value of Kd. It is important to be realized, so you are not always input 0.975 in the spreadsheet whereas you use PSV other than API.
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.
Combination with Rupture Disk – Kc
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.