15 replies to this topic

[Lowflo]

I've just become aware of the section in API 520 Pt1 (3.3.3.1.3) that allows higher built-up backpressure (higher than 10%) for cases where the overpressure is > 10%. It says that the built-up backpressure can be >10% provided that it doesn't exceed the allowable overpressure.

Technically, this makes sense but I'd never realized that it was explicitly allowed according to API standards. Primarily, i think this has application in justifying existing installations that exceed the 10% limit, but are sized for a fire scenario. It would also apply to process upset cases in which the PSV is set below the MAWP.

I don't recall ever hearing or reading any discussion on this topic. Is this widely practiced? It hasn't been practiced in my company but I think it should be. Do any of you have any comments on this? Thanks.

Here's the whole text of this section:

3.3.3.1.3 In a conventional pressure relief valve application, built-up back pressure should not exceed 10% of the set pressure at 10% allowable overpressure. A higher maximum allowable built-up back pressure may be used for allowable overpressures greater than 10% provided the built-up back pressure does not exceed the allowable overpressure. When the superimposed back pressure is constant, the spring load may be reduced to compensate for the superimposed back pressure. In this case, it is recommended that the built-up back pressure should not exceed the allowable overpressure. When the downstream piping is designed within the above back pressure criteria, no back pressure capacity correction(Kb = 1.0) is required in the valve sizing equations, for gases at critical flow or for liquids. When the back pressure is expected to exceed these specified limits, a balanced or pilotoperated pressure relief valve should be specified.

[fallah]

I've just become aware of the section in API 520 Pt1 (3.3.3.1.3) that allows higher built-up backpressure (higher than 10%) for cases where the overpressure is > 10%. It says that the built-up backpressure can be >10% provided that it doesn't exceed the allowable overpressure.

Technically, this makes sense but I'd never realized that it was explicitly allowed according to API standards. Primarily, i think this has application in justifying existing installations that exceed the 10% limit, but are sized for a fire scenario. It would also apply to process upset cases in which the PSV is set below the MAWP.

I don't recall ever hearing or reading any discussion on this topic. Is this widely practiced? It hasn't been practiced in my company but I think it should be. Do any of you have any comments on this? Thanks.

For nonfire case:

Allowable Overpressure=1.1*MAWP-Set Pressure

As per above equation,if set pressure would be lower than MAWP,then allowable overpressure could be higher than 10% of set pressure,and therefore maximum build-up back pressure even though is lower than allowable overpressure,could be higher than 10% of set pressure.

All these limitations are to be considered for preventing effect of high build-up back pressure on PSV relif capacity.

Hope above helps out.

Edited by fallah, 13 August 2009 - 04:33 AM.

[Lowflo]

API 520 allows this to be practiced, but I don't see or hear about it being done. What is your experience? Please comment. Thanks.

[fallah]

API 520 allows this to be practiced, but I don't see or hear about it being done. What is your experience? Please comment. Thanks.

Normally set pressure is close to MAWP and 10% overpressure would be considered,but i heard about higher allowable overpressure due to considerable difference between set pressure and MAWP.
As a general statement,there is no logical reason having considerable difference between set pressure and MAWP,one of its effects is increasing size of PSV for specified relief load.

Now,say about your problem in this regard.

[Lowflo]

Please comment if you have any knowledge of the extent to which this is being used in industry. For relief designs that have more than 10% overpressure, is it common to allow more than 10% tailpipe loss? Please comment on how widely this is being practiced?

Thanks.

[fallah]

Please comment if you have any knowledge of the extent to which this is being used in industry. For relief designs that have more than 10% overpressure, is it common to allow more than 10% tailpipe loss? Please comment on how widely this is being practiced?

Thanks.

As far as i know the extent is very limited.

If allowable overpressure is more than 10%,there is no problem to allow more than 10% tailpipe loss.

This is no widely being practiced.

[JoeWong]

Lowflo,
I will try to point out some facts, see it help.

Per my limited project experiences, i still have not seen any pressure vessel code exceeded 10% overpressure from MAWP (as defined in ASME), except the so called "fire case".

However, you may read the following :

A pressure vessel design pressure may be specified as 100 barg by process engineer. Due to whatsoever reason, mechanical engineer may selected a wall thickness with MAWP of 120 barg.
A PSV provided to protect this vessel, process engineer may probably set it at 100 barg (same as design pressure of 100 barg). Generally process engineer may think maximum allowable overpressure is 10% of set pressure (10 bar), therefore the built-up backpressure will be limited to 10 bar so that the vessel internal pressure will only experience 110 barg maximum.
In fact, the vessel MAWP is 120 barg. Actual maximum overpressure in this case is 20% of set pressure instead 10% as understood by process engineer. Therefore, this system should capable of taking built-up backpressure of 20% of set pressure instead of 10%.

So... Don't look at the percentage...i always keep the following in my mind.

built-up backpressure <= overpressure

This post "Several Impact of Backpressure on Conventional PRV" mentioned similar stuff

Edited by JoeWong, 14 August 2009 - 11:50 PM.

[Lowflo]

From a techical perspective, it makes sense to allow the tailpipe losses (built-up back pressure) to go beyond 10%, as long as it doesn't exceed the overpressure. The inlet and outlet dP rules (3% and 10%) are based on ensuring that the PSV remains stable in the open position. These rules maintain at least 97% of the set pressure as available pressure drop for the PSV itself. As long as the tailpipe losses don't exceed the overpressure (and the inlet losses are < 3%) then you'll maintain 97 % drop across the valve.

For a fire scenario, with a tailpipe loss of 21% for example, the PSV will be just as stable as in a process upset case where the tailpipe losses were only 10%.

Based on the cited text from API 501 Pt 1, the API committee recognizes this and agrees that it is a safe design practice. If you view API 520 as RAGAGEP (Recognized And Generally Accepted Good Engineering Practice), and most do, then it should be acceptable to apply this provision. My point is that I don't see much discussion of this in publications and forums such as this. So, I'm wondering, does that mean that engineers are aware of it but don't recognize it as a safe design practice, or does it mean that engineers are just not generaly aware of this paragraph?

[sheiko]

Lowflo,
I will try to point out some facts, see it help.

Per my limited project experiences, i still have not seen any pressure vessel code exceeded 10% overpressure from MAWP (as defined in ASME), except the so called "fire case".

However, you may read the following :

A pressure vessel design pressure may be specified as 100 barg by process engineer. Due to whatsoever reason, mechanical engineer may selected a wall thickness with MAWP of 120 barg.
A PSV provided to protect this vessel, process engineer may probably set it at 100 barg (same as design pressure of 100 barg). Generally process engineer may think maximum allowable overpressure is 10% of set pressure (10 bar), therefore the built-up backpressure will be limited to 10 bar so that the vessel internal pressure will only experience 110 barg maximum.
In fact, the vessel MAWP is 120 barg. Actual maximum overpressure in this case is 20% of set pressure instead 10% as understood by process engineer. Therefore, this system should capable of taking built-up backpressure of 20% of set pressure instead of 10%.

So... Do look at the percentage...i always keep the following in my mind.

built-up backpressure <= overpressure

This post "Several Impact of Backpressure on Conventional PRV" mentioned similar stuff

Just to fix ideas:
- Accumulation is relative to MAWP.
- Overpressure is relative to set pressure.

I think in Joe's example (fire case), maximum allowable overpressure is rather 44% of set pressure than 20%.
Indeed, max. allowable overpressure = 1.2*MAWP - set pressure = 1.2*120 - 100 = 144 - 100 = 44 barg.

Edited by sheiko, 14 August 2009 - 05:32 PM.

[JoeWong]

Sheiko,
Thanks for highlight.

PSV set pressure = 100 barg
MAWP = 120 barg
With maximum 10% accumulation per ASME, maximum allowable relief pressure is 110% of 120 barg = 132 barg.

Therefore,
If PSV set at 100 barg, maximum overpressure or built-up backpressure is 32 bar or 32% relative to set pressure of 100 barg.
If PSV set at 120 barg, maximum overpressure or built-up backpressure is 12 bar or 10% relative to set pressure of 100 barg.

[fallah]

If PSV set at 120 barg, maximum overpressure or built-up backpressure is 12 bar or 10% relative to set pressure of 100 barg.

Confusing statement.......If PSV set at 120 barg why reletive to set pressure of 100 barg?

All discussion could be summarized as:

-Allowable Overpressure=1.1*MAWP-Set Pressure

-Built-up backpressure <= overpressure

Edited by fallah, 15 August 2009 - 01:02 AM.

[sheiko]

If PSV set at 120 barg, maximum overpressure or built-up backpressure is 12 bar or 10% relative to set pressure of 100 barg.

Confusing statement.......If PSV set at 120 barg why reletive to set pressure of 100 barg?

All discussion could be summarized as:

-Allowable Overpressure=1.1*MAWP-Set Pressure

-Built-up backpressure <= overpressure

Fallah,

I agree except for fire case where max. accumulation is 21% of MAWP as per ASME BPVC Section VIII.

As for Joe's last statement, i think it is simply a typing mistake.

Edited by sheiko, 15 August 2009 - 06:41 AM.

[JoeWong]

Correction...

If PSV set at 120 barg, maximum overpressure or built-up backpressure is 12 bar or 10% relative to set pressure of 120 barg.

[skearse]

While code may 'allow' it, here's the problem you run into: You don't get to pick the overpressure scenario when it happens. Normally, there are several cases that are valid for a particulat vessel, only one of which allows an OP of 21%. The others are all 10%. And honestly, the 10% cases are probably more likely than a unit fire causing an OP. So what happens when the steam valve fails open, or the cooling water system fails, and your tailpipe is sized with 21% backpressure? IMO, and IME, the 10% rule is pretty strictly adhered to, unless you go to a balanced or pilot valve.

[Lowflo]

Thanks to all for your comments. Based on the responses here, and my own experience, this exception to the 10% rule doesn't appear to be widely practiced. Should it be used whenever there's an opportunity? Or, should we disregard this option entirely? Personally, I think the answer to both those questions is, "No". Instead, it should be applied when it makes sense, based on good engineering judgment. If I was reviewing an existing relief design and found that the tailpipe losses were say 15%, then I wouldn't suggest the client spend money to modify the installation if the overpressure was at least 15%.

For new designs, I doubt I'd ever agree to a design that had 20% or more built-up backpressure, even if that design had an overpressure exceeding that value. But, I see little safety risk with the judicious application of this provision. True, we very rarely experience fires that challenge the design of the relief system. If this PSV opens due to another scenario (one limited to 10% overpressure) the real risk is still extremely small. If the inlet line was design properly, in accordance with the 3% rule, then the valve won't chatter when the overpressure exceeds 10%. The inlet dP will actually decrease, thus not affecting the stability of the valve. So, it boils down to the risk of the vessel itself failing when the internal pressure is between 110%-121% MAWP. That risk, too, is extremely small.

The bottom line is that engineers should keep an open mind with respect to this option, and apply it where it makes sense.....cases where it can be used to reduce cost without having any real adverse effect on safety.

[sgoel]

The clause exists in the code. However I have never seen ut being used by any plant designer.

Thanks

Thanks to all for your comments. Based on the responses here, and my own experience, this exception to the 10% rule doesn't appear to be widely practiced. Should it be used whenever there's an opportunity? Or, should we disregard this option entirely? Personally, I think the answer to both those questions is, "No". Instead, it should be applied when it makes sense, based on good engineering judgment. If I was reviewing an existing relief design and found that the tailpipe losses were say 15%, then I wouldn't suggest the client spend money to modify the installation if the overpressure was at least 15%.

For new designs, I doubt I'd ever agree to a design that had 20% or more built-up backpressure, even if that design had an overpressure exceeding that value. But, I see little safety risk with the judicious application of this provision. True, we very rarely experience fires that challenge the design of the relief system. If this PSV opens due to another scenario (one limited to 10% overpressure) the real risk is still extremely small. If the inlet line was design properly, in accordance with the 3% rule, then the valve won't chatter when the overpressure exceeds 10%. The inlet dP will actually decrease, thus not affecting the stability of the valve. So, it boils down to the risk of the vessel itself failing when the internal pressure is between 110%-121% MAWP. That risk, too, is extremely small.

The bottom line is that engineers should keep an open mind with respect to this option, and apply it where it makes sense.....cases where it can be used to reduce cost without having any real adverse effect on safety.