12 replies to this topic

[shan]

Hi Everyone,

Per API 520/GPSA, if MAWP = 100 psig, PSV setting pressure should be 110 psig. What should be high pressure alarm setting and high pressure trip setting? Are 105 psig and 108 psig respectly correct? I have not seen any industry code that specifies pressure high alarm setting value and pressure high trip setting value.

Regards,

Shan

[paulhorth]

Shan,

The PSV set point must not exceed the design pressure of the equipment , which itself is less than or equal to the MAWP. This is perfectly clear from section 3.5, table 2, of API RP 520 pt 1.
The relieving pressure when the PSV is fully open is allowed to rise to 10% above the MAWP. This is called accumulation.
The set point for the high pressure trip should therefore be at some margin below the PSV set point, allowing for the tolerance in the sensing device, thus below the MAWP.The idea is that the PSHH operates to shut the system and avoid opening the PSV.

To repeat:
PSV set point must never exceed the MAWP. That's what MAWP means - Maximum Allowable!

Paul

[shan]

Thank you Paul for clarifying PSV Set Point =< MAWP. However, I did not get your point on settings of high pressure alarm and high pressure trip. Please specify the specific values not just a vague mention "some margin below".

[paulhorth]

Shan,

For setting the high alarm and high pressure trip, there are no specific values. You are paid to use your professional judgment on this matter, as on many other aspects of design.
Let me give you an example.
Suppose I have a vessel with normal operating pressure 50 barg. Then, I might make the design pressure 55 barg, applying a typical margin of 10% above the normal operating, in accordance with my client's preferred standard and my own good practice. However the choice of design pressure should be for the system not just for an individual item, so I might choose to apply a higher design pressure, say 60 barg, because there is a 60 barg system upstream and I want to avoid a relief case from the upstream source. This is how process design is done. Now, I have a difference of 10 bar between normal pressure and design pressure. So I have to set the alarm and trip in that range. I might decide to split the pressure intervals equally, so that the high alarm is at say 54 barg and the trip is at 57 barg. It could be different. I would need to check that the trip setting was not too close to the relief setpoint.

Notice I have not mentioned the MAWP. Thiis is because the MAWP is not known at the process design stage - it is back-calculated by the vessel fabricator at the time of manufacture. The fabricator will use a standard plate thickness which will be thicker than the calculated thickness for my 60 barg design pressure. So the MAWP will be slightly higher than 60 barg and I could choose to raise the PSV set pressure to match it - but there is no benefit in doing this.

Paul

[shan]

Hi Paul,

Thank you for your effors to help me out. However, I have to say that it not a legitimate statement that I am paid to define high pressure alarm / high pressure set points because there are not any rules or guidelines to follow in the industry. Actually, I think I am paid to follow the proper rules and guidelines but not just arbitrarily pick up 2 numbers between normal operating pressure and PSV setting pressure.

Although nobody grant me the answer yet, I believe the set points should somehow relate to probabilities of something wrong in the operation vs. possible normal pressure fluctuation. For example, if the pressure reaches pressure high alarm value, there is 50% possibility something wrong in the operation. If the pressure reaches pressure high trip value, there is 90% possibility something wrong in the operation. My problem is I am unable to defend myself why 50% and 90% not 55% and 95%.

Regards,

Shan

[fallah]

To repeat:
PSV set point must never exceed the MAWP. That's what MAWP means - Maximum Allowable!

Paul

Paul,

Just to complete your valuable statement:

In multiple valve installation there are exceptions that PSV set point may exceed the MAWP: for additional valve maximum set pressure could be equal to 105% of MAWP, and for supplemental valve (in fire case) it could be equal to 110% of MAWP.

Fallah

Edited by fallah, 31 August 2011 - 11:13 PM.

[paulhorth]

Shan,

There is much more to being a professional engineer than simply following rules and guidelines. Being professional means making full use of your education, training and experience to form judgments about the best design solutions to problems. You will not find rules and guidelines to cover many decisions and choices faced by process engineers.
This forum is an opportunity to benefit from the experience and judgment of others, not simply to learn new rules.

If you prefer to work in a job where rules and guidelines cover everything, I suggest you join the army, or become a government official ("Civil servant" as we call them in the UK).

Now to the alarm and trip setpoints. Your probabilities would be just a fiction to disguise what has to be judgment. There is no way that you could calculate the probability of a pressure excursion of say 5% above normal - and certainly not accurately enough to distinguish between 50% and 55% probability. You have to consider how your process operates and what the likely "normal" and "abnormal" variations would be. Find someone who has worked on a similar process and ask them.

Fallah,

You are of course right, about multiple PSVs and supplementary PSVs. I wanted to keep my reply short and simple enough for Shan to understand, because he thought that the first PSV could be set at 110% of MAWP, which is completely wrong.

Paul

[fallah]

Although nobody grant me the answer yet, I believe the set points should somehow relate to probabilities of something wrong in the operation vs. possible normal pressure fluctuation. For example, if the pressure reaches pressure high alarm value, there is 50% possibility something wrong in the operation. If the pressure reaches pressure high trip value, there is 90% possibility something wrong in the operation. My problem is I am unable to defend myself why 50% and 90% not 55% and 95%.

shan,

As paul mentioned there is no specified rule or guideline for setting alarm and trip points.

But, we usually set pressure high alarm at a little bit value higher than the maximum operating pressure and for pressure high trip at a little bit value lower than the design pressure.

Hope this help you out.

Fallah

[shan]

Hi Fallah,

The term “a little bit” is hardly acceptable in the engineering world. We engineers define almost every thing with numbers, values and percentages. Now, my problem becomes simpler. Let me make rules on high pressure alarm and high pressure trip because there is no such rule yet.

High Pressure Alarm Setting Rule:
Max Operating P (barg) High P Alarm Setting
0-35 Max Operating P + 1.4 bar
Over 35 Max Operating P + 4%

High Pressure Trip Setting Rule:
Max Operating P (barg) High P Tripping
0-35 Max Operating P + 2.8 bar
Over 35 Max Operating P + 8%


Rules are just something that followed by the most of people.

Regards,

Shan

[fallah]

High Pressure Alarm Setting Rule:
Max Operating P (barg) High P Alarm Setting
0-35 Max Operating P + 1.4 bar
Over 35 Max Operating P + 4%

High Pressure Trip Setting Rule:
Max Operating P (barg) High P Tripping
0-35 Max Operating P + 2.8 bar
Over 35 Max Operating P + 8%

Shan,

As per your above rule please set the high pressure alarm/trip for a vessel with, let say, maximum operating pressure of 2.5 barg and design pressure of 3.5 barg.

Fallah

Edited by fallah, 07 September 2011 - 06:41 AM.

[paulhorth]

REMINDER:

In the first post on this thread, Shan said this:

Per API 520/GPSA, if MAWP = 100 psig, PSV setting pressure should be 110 psig.

This is totally wrong. He then said this:

Actually, I think I am paid to follow the proper rules and guidelines

.....although he had just shown his inability even to understand correctly a simple guideline in API RP 520.

Yet now, Shan thinks he can propose rules to Fallah and the rest of us about alarm and trip settings. Confidence together with ignorance can be an expensive and even dangerous combination.

Paul

[deltaChe]

For ASME pressure vessels, the set pressure of the relieving device shall not
exceed the MAWP of any equipment in a covered system. If the opening pressure
of the selected relief device is affected by operating temperature, such as a rupture
disk, the coincident relieving temperature must also be determined. If multiple relief
devices are used, only one need be set at the MAWP. The others may be set at a
pressure not exceeding 105% of the MAWP. See API 520 Part I, Section 3.5.2 and
Table 1 (7th ed.) for discussions of multiple relief device set pressures.

[Qalander (Chem)]

Somehow I understand that there is some misunderstanding seems to have crept -up,among the contributory colleagues in this post.

I take the opportunity to say that

• the vessels design(s) most usually have incorporation in-place for the venting and reliving API, I.P., A.S.M.E.,E.N.,Shell D.E.P. etc.codes applicable clause(s).
• What has unfortunately became topic of debate is "the residual pressure built-up upstream of any Pressure Safety /Pressure Relief Valve(s) immediate inlet flange" during short releasing or relieving periods .

Hopefully everyone agrees to this?

Edited by Qalander (Chem), 08 September 2011 - 05:17 AM.