6 replies to this topic

[mykid]

Hi all,

I can't get my head around this, it may be a stupid question to some of you, so i sincerely hope you can clear my doubt.. thanks in advance

I know external pressure of 14.7psi refers to full vacuum, 0kPaA.

If a vessel is designed for external pressure of 40kPa, is the design pressure equals to 40kPaA or 61kPaA?

If the PVSV has a set pressure of 20kPa vacuum, is it refers to 20kPaA or 81kPaA? how do we normally define the PVSV set pressure? Using kPaA or -kPag?

[shan]

A PVSV has two settings vacuum setting and overpressure setting. Both vacuum setting and overpressure setting are in reference of atmospheric pressure (gage pressure). If a PVSV vacuum setting is 20 kPa, this means when the vessel internal pressure drops 20 kPa less than atmospheric pressure (81 kPaA) the PSVS will allow air soak in. If a PSVS overpressure setting is 40 kPa, this means when vessel internal pressure reaches 40 kPa higher than atmospheric pressure (141 kPaA) the PVSV will allow inside vapor release out.

[tarafdar]

Hi Mykid,
You are confused with gauge pr. and vacuum pr.
1.14.7 psi equals 104.7 kPa (not 0 kPaA)
2.If the design pr. is 40 kPa (nothing mentioned about gauge pr. or absolute pr),it can not be absolute pr.Because design pr. then will be below atmospheric pr.
So it must be 40kPaG meaning 144.7(40+104.7) kPaA ,not 40kPaA or 61kPaA.
3.PVSV set pr. 20 kPa vacuum means set pr. is 20 kPa below atmospheric pr.So the set pr. is 84.7(104.7-20) kPaA ,not 20 kPaA or 81 kPaA)
I don't know wheather this will help you to clear your doubt.

[mykid]

What about vessel external pressure? When we said the vessel is subjected to 0.4bar external pressure, is the vessel designed for 0.6bara internal pressure?


Shan,
Thank you for the excellent explanation.
i understood about the vacuum setting of PSV now, just wonder how do you show in the P&IDS?
If the overpressure setting is 4bar and vacuum setting is 0.2bar
Do you show pressure 4/ vacuum 0.2 bar on P&IDs and operator will understand that that 0.2 refers to 0.2 below atm? or better to show it as 4 barg / -0.2barg?

Tarafdar,
Why is 14.7psi = 104.7kPa? it should equal to atm 101.3kPa.

[shan]

So far as I know, there is no API or ASME or ANSI standard that specifies PVSV label formats on P&ID. I think it is important that your PVSV labels are understandable to other engineers and designers and consistent with those on the other P&ID's of the same set.

[tarafdar]

Mykid:

Yes, you are correct. Thanks

[fallah]

Do you show pressure 4/ vacuum 0.2 bar on P&IDs and operator will understand that that 0.2 refers to 0.2 below atm? or better to show it as 4 barg / -0.2barg?

mykid,

It is better to show it as 4 barg / -0.2barg.

Fallah