Download - Process Engineer_ PSV Calculation - Kd,Kc,Kb Factor

7/26/2019 Process Engineer_ PSV Calculation - Kd,Kc,Kb Factor

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Hi friends, how are you today? Hopefully, you are not already bored with my articles. Now, we

will learn the concept of relief valve calculation. The spreadsheets not tell you anything, but

we should know what really it does. After read this fully article, I hope you get a better

understanding of the relief valve calculation concept, and then know well with your

spreadsheet.

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.)

We need some corrections since the fluid flows through a relief valve nozzle orifice rather than an ideal nozzle.

F o r t h e s a m e a r e a , a t c e r t a i n condition, the flow capacity of relief valve orifice must be less compared with the

ideal one Its mean tha t m o r e a r e a i s r e q u i r e d t o h a n d l e t h e s a m e m a s s r e l i e v i n g r a t e - . T h a t s w h y , t h e r e a r e

some c o r r e c t i on factors a r e r equired such as Kc, Kd, Kb, Kv, Kp, Kh, Kn etc.

L e t 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.

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 dont 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

s m a l l e r o r i f i c e a r e a i s r e q u i r e d .

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.

Impact of Back Pressure Kb

Back pressure is defined as a pressure existing at PSVs 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

PSV Calculation - Kd,Kc,Kb factor

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Bu i l t U p a n dSu p e ri mp o sse dBa ck Pre ssu re

Bu i l t U p Ba ckPre ssu reC a l cu l a ti o n

Ba ck Pre ssu reEffe ct

PSV In sta l l a ti o n -Gu i d e

R e l i e v i n gC o n d i ti o n Exce e dD e si g nTe mp e ra tu re

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. Thats all, I think,

very clear why Kb is required for balance below type only.

Note that the back pressure correction factor Kb for conventional and pilot type is not required, and then use

Kb=1. Ask your vendor of Kb value as graphic- for using below PSV type or refer to API 520 part 1 for

preliminary design.

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. Dont miss it.

Thats 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

f o r t h e d e t a i l .

Thank you.

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Replies

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8 comments:

Anonymous Oct 24, 2011, 3:59:00 PM

In sizing PSV for steam relief - Kb can be greater then 1.0?

Reply

Triyanto SR Mar 19, 2012, 9:32:00 PM

I don' t think so.

The Kb is correction factor..(reducing capacity of the PSV due to back pressure)

The greater the back pressure, the lower the PSV capacity.

Krittika Sep 17, 2012, 11:47:00 AM

This is such a great blog.Please do continue the good work as it is a lot of help for Process Engineers

r o u n d t h e w o r l d . . .

Reply

Anna Andronikou Jul 11, 2013, 2:46:00 AM

Great job.Your articles are very helpfull.Thank you

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samsamqqqj Aug 8, 2013, 12:25:00 PM

I found your blog when I was looking for a different sort of information but I was very happy and glad to

r e a d t h r o u g h y o u r b l o g . T h e i n f o r m a t i o n a v a i l a b l e h e r e i s g r e a t

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Anonymous Aug 30, 2013, 10:13:00 PM

Thank you, lots of valuable information

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Anonymous Sep 23, 2013, 12:39:00 PM

pleasure to read the your blog...Technical lanaguage is very sample & easy to understand....bravo....keep

it up....

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Anonymous Oct 16, 2014, 4:19:00 PM

Please correct like this: Note that the back pressure correction factor Kb for balanced bellow and pilot type

is not required, and then use Kb=1. Ask your vendor of Kb value as graphic- for using below conventional

PSV type or refer to API 520 part 1 for preliminary design

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