10 replies to this topic

[ankuragarwal]

Right now I am working on PSV orifice size calculation.
There is a PSV having load = 757573 kg/hr for which caculated orifice area is around 1437.28 cm2.
There are 6 PSVs on line.

As per API 526 maximum orifice size is "T" having orifice area = 167.7 cm2.
As per API I have to install 9 PSV in order to achieve 1437.28 cm2.

So please suggest do I need to increase no of online PSVs from 6 to 9
or
there are PSVs having API orifice designation higher then "T" ( Effective Orifice Area > 167.7cm2) .

I have heard about PSVs having orifice designatin 8 FB1 10 (Orifice Area = 268.128 cm2) and 8 FB2 10 (Orifice Area = 387.096 cm2) .

These are as per API or not. I am not sure.

Please suggest wether i can use these higher orifice designation or not.

[fallah]

ankuragarwal,

You should refer to relevant project spec about using PSV for equipment protection and also ask client standpoint regarding the matter.

Fallah

[paulhorth]

ankuragarwal,

You have a very large relief flowrate there. To dispose of this gas will require a 48 inch flare header, or possibly two x 30 inch, a large flare drum, and a high flare stack.
Have you considered ways to reduce the relief load? It may be that there is nothing you can do about it, but this could be a case where fitting a HIPPs system to shut down the source of high pressure gas can save a lot of cost in the flare system.

Paul

[ankuragarwal]

Thanks for your replies:

Actually in current situation I am working on flare system of whole petrochemical complex.
So flare header size is quite higher. More then 48 inches.

Flare load from this PSV cant be lower down. Atthough I have optimized it but cant go down further.

Project Spec says that I have to follow API.
Number of online PSV ( 6 in this case) is provided by licensor. So increasing the no of PSVs is not a good option for me.
Please suggest how can I manage this much of load in 6 online PSVs, as per API.

Is there any PSV having effective orifice area more then 167.7 cm2 as per API?

I have heard about PSVs having orifice designatin 8 FB1 10 (Orifice Area = 268.128 cm2) and 8 FB2 10 (Orifice Area = 387.096 cm2) .

Are these orifice as per API?

Wether I can go for it of not?

[fallah]

ankuragarwal,

IMO, you should follow the issue in two parallel ways in order to find a proper solution:

1) Ask licensor to clarify how did he size and select 6 PSV's based on the relief load in relevant "Flare Load Summary".
2) Send the PSV data sheet to some well known vendors and ask them to submit their proposals.

Actually, when you size a PSV using the API orifice area and API coefficient, you're simply doing a preliminary sizing.
When a vendor sizes a PSV for you, they are doing a final sizing calculation. In fact, they're using their actual orifice area (normally larger than that of API) and their certified flow coefficient.

Therefore, after getting licensor clarification leave the sizing and selection of the PSV to vendors and come back here let's us to know the result.

Fallah

[aju_1807]

I agreed with Fallah's comment.

Please give your PDS to vendor and ask vendor to provide feedback. Normally the difference between API and ASME orifice is roughly 16% (see attachment). Moreover, each vendor uses safety factor of 10% in calculation so basically certified flow rate which relief valve actually can pass is generally higher and that may bring down the no of valves required.

Take caution when you are dealing in low pressure application as T type orifice (with conventioanal type RV) is not available commercially with set pressure higher than 8-9 barg (as per API-526 limitation). In this case you may have to go for pilot operated RV which has bit higehr set pressure limitation then conventaional/BB type relief vlaves.


If you can provide more detail about your system we may help you further. As far as non-API valves are concerned (which you mentioned in your post), it shall be used based if project philosophy allow to do so.

regards,

AJ

Attached Files

•  API vs ASME Orifice Sizes for PSVs.pdf   54.11KB   154 downloads

[fallah]

Take caution when you are dealing in low pressure application as T type orifice (with conventioanal type RV) is not available commercially with set pressure higher than 8-9 barg (as per API-526 limitation). In this case you may have to go for pilot operated RV which has bit higehr set pressure limitation then conventaional/BB type relief vlaves.

aju_1807,

The "Outlet pressure limit" included in tables of API 526 is the limitation value (in temperature of 100F) the PSV may subject to it and basically extracted from the pressure-temperature rating of the flanged valves as per ASME B16.34,

Contrary to your statement as above, it don't relate to set pressure and relates to the PSV back pressure and shows the maximum back pressure value (for T orifice maximum 100 psig equal to around 7-8 barg) which a PSV with specified material of construction can tolerate.

Fallah

Edited by fallah, 27 March 2012 - 03:23 AM.

[aju_1807]

Fallah,

Please refer attachment for more clarification. As per API-526 table based on the relief temperature and size of valve there is a limitation on the set pressure as well (apart from bp limitation). See attachment for example. The back pressure limitation is also there but we need to also see the set pressure limit .

Hope this clarifies.

rgds,

AJ

Attached Files

•  API-526 Extract.pdf   317.05KB   100 downloads

[fallah]

Fallah,

Please refer attachment for more clarification. As per API-526 table based on the relief temperature and size of valve there is a limitation on the set pressure as well (apart from bp limitation). See attachment for example. The back pressure limitation is also there but we need to also see the set pressure limit .

Hope this clarifies.

rgds,

AJ

AJ,

You were discussing T orifice and did attach Q orifice specification...

Anyway, it is obvious that as per API 526 there is limitation on set pressure for each PSV orifice type titled as "maximum inlet flange (set) pressure limit", but you mentioned:

...T type orifice (with conventioanal type RV) is not available commercially with set pressure higher than 8-9 barg (as per API-526 limitation)...

While if you look at the api 526 for T orifice, you can see for this type the maximum set pressure could be as high as around 20 barg (300 psig) and the 8 barg is maximum outlet pressure (back pressure) for T orifice...

Fallah

[aju_1807]

Fallah,

If you see that table carefully with different body material then you will able to find the limitation of 8 bar. Anyway, lets not enter into that buissness the point which I wanted to highlight and which now you also agree upon is that set pressure limitation plays crucial role for PSV selection.

Moreover, please note that sometime body material selection is not in our hand and is driven by various process conditions so its not always necessary that we can buy CS body material which will allow higher set pressure limitation.

My empahsis was on set pressure limitation and not on exact value.

THnks,
Aj

[fallah]

Aj,

OK, agreed, inlet/outlet pressure limits depend on the body/spring materials of the PSV...

Fallah