Hi all-
I have 2 questions on a PSV application; please hlep me in answering the questions.

Thanks a lot.

Regards
srin

Application: PSV on steam drum

Background:
2 PSVs are provided on steam drum (from waste heat boiler) and the steam drum is constructed as per ASME Section VIII. Design pressure of the steam drum is 39.3 bar g and the maximum steam generation rate is 21000 kg/hr @ operating pressure (35.5 bar g). I need to calculate relief load (blocked exit scenario) for these PSVs and also I have to ensure that the PSV inlet nozzle size is adequate (the PSV inlet nozzle size has been fixed by process licensor as 2" each).

Set pressure of PSV 1 is 39.3 bar g
Set pressure of PSV 2 is 40.4 bar g (1.03*39.3)

Relief load basis:
I am planning to share the total steam generation rate between the two PSVs equally; so, PSV relief load would be 21000/2 = 10500 kg/hr each.

Query:
1. Is it acceptable to use 6% accumulation for PSV1 & 3% accumulation for PSV2? By doing this, I am able to get the size 2J3 which matches with the 2" PSV inlet nozzle; also, pressure drop across the PSV inlet piping is below 3% of PSV set pressure. ASME Section I says that total accumulation should not exceed 6% of MAWP and with the above proposal, the code requirement is also not violated.

2. For calculating PSV orifice size & rated flowrate, I am using 10500 kg/hr as required relief rate; I have been told by my senior that 10500 kg/hr is the mass flowrate at normal operating pressure (35.5 bar g) and so I should not use it; instead I may need to use the mass flowrate at relieving pressure (41.7 bar g). If that is the case, how do I need to calculate the mass flowrate at relieving pressure? My thinking is as per mass conservation theory and so the steam massflow rategenerated will be the same irrespective of pressure (whether 35.5 or 41.7 bar g). Please clarify my queries.

Srin,

For question 1, I'd have to think about that a bit, so I can't answer right now.

As to question 2, I think your senior is correct. For blocked case, you state that the max steam generation rate is 21,000 kg/hr at 35.5 barg. Using steam tables, look up heat of vaporization of saturated steam at 35.5 barg. Hfg x 21,000 kg/hr should give you the heat duty on your waste heat boiler (Q).

Assuming Q is not changing in the blocked outlet case, look up Hfg at relieving pressure. Divide Q by Hfg at relieving pressure to get the vapor generation rate. I did it quickly, and it is not 21,000 lb/hr, but slightly more as the latent heat drops as the pressure increases.

That is the way I would look at this problem.

Hi Merrill-
Thanks for your response. My doubt is this-

The steam generation rate is based on maximum boiler feed water inflow to steam drum which is 21,000 kg/hr. My understanding is we can't create mass (mass conservation theory); then, how can it be possible to increase the mass flowrate from 21,000 to 21595 kg/hr (as per the calculation method suggested). Please enlighten me. Also, pardon me for my ignorance.

I can quote an example similar to this where the relief load varies substantially

Background:
Ammonia vaporiser is provided with safety valve and relief load has to be calculated for blocked exit case. The normal inflow into the vaporiser is 3900 kg/hr and the normal heat load given in the vaporiser is 1470 kW; In case of blocked exit, the relief load can be given as follows:

Case 1:
Relief load is 3900 kg/hr of ammonia at relieving pressure & temperature

Case 2:
Relief load = Normal heat load/Latent heat of ammonia at relieving conditions => 1470/1028*3600= 5147 kg/hr

I would choose 3900 kg/hr since this is the maximum inflow into the system. Please share your views.


Thanks once again.

Regards
srin

I would say that your logic or thinking is correct. When we size for "blocked outlet" conditions, we take maximum possible flow irrespective of the relief pressure. The effect of relief pressure would change the relief temperature. The temperature will rise. The steam will become superheated than what it is during normal operation.

However, please note that if you are doing calculations for "fire" case, relief pressure does matter to find the flow rate.



Well, for understanding, I would suggest to refer Fig.4 of API RP521, Pg.30. Do not go directly on accumulation %. Do it step by step. Decide set pressure of the valves when there's multiple installation. There should not be more than 5% difference in set pressure with respect to the valve with lower value of set pressure. You are following this criteria. Now, consider same % of accumulation for both these valves. There is no problem. Remember, this % accumulation would be with respect to set pressure of each valve. So, absolute value would be different for both the cases due to difference in set pressures. This value should not be higher than 16% for multiple valve cases. The logic is like below.
Multiple installations
PSV A : Set Pr = 100 ==> Max. Overpressure = 100 + 10% = 100 x 1.05 = 105
PSV B : Set pr = 105 ==> Max. Overpressure = 105 + 10% = 105 x 1.05 = 115.5 ~ 116.

This is how the value of 116% has been set.

If i am not mistaken, there is a design code for boiler where the overpressrue is limited to 3% accumlation. So the figure in API STD 521 which specifically for vessel design to ASME pressure vessel code may not be used here. Correct me if i am wrong...

Nirav-
Thanks for sharing your views. My feedback are as follows:

The PSV set pressures are 39.3 bar g & 40.4 bar g; as you pointed out the set pressures follow 5% criterion. The design pressure of steam drum is 39.3 bar g.

Since the steam drum is of ASME Section VIII construction, safety valve requirement has to be followed as per ASME Section I; section I says total accumulation should not exceed 6% which means the maximum relieving pressure should not exceed 41.7 bar g (39.3*1.06).

The problem here is, the PSV inlet nozzle is fixed which is 2" and I have to relieve the total steam generated using these 2 X 2" nozzles.

Option 1:
PSV 1 - 3% Accumulation (Relieving pressure: 40.5 bar g); set pressure 39.3 bar g
PSV 2- 3% Accumulation (Relieving pressure: 41.7 bar g); set pressure 40.4 bar g

With the above option and with the required relief flow rate, 2" inlet nozzle is inadequate.

Option 2:
PSV 1 - 6% Accumulation (Relieving pressure: 41.7 bar g); set pressure 39.3 bar g
PSV 2- 3% Accumulation (Relieving pressure: 41.7 bar g); set pressure 40.4 bar g

With the above option 2" inlet nozzle is adequate and so I would like to check whether Option 2 can be used or not for this application.


Joe Wong-
I have verified ASME Section I and it says total accumulation should not exceed 6% (not 3%). But, one of my friends also said 3% accumulation and I am not sure where this 3% is quoted. Please let me know if 6% is not correct.

Regards
srin

Srin,

I believe your option 2 in previous post is a valid approach, if the 6% allowable accumulation is correct. I also see 6% in the code, but like you have heard 3%.

As to the relieving rate that I commented on before....I think that I may be mis-interpreting your application. I think that if there is no liquid water to evaporate in this system then you and nirav are correct. Mass conservation applies and the steam gets a higher degree of superheat than normal operation due to the increased pressure/temperature. My thought, however, was that if the steam drum you are looking to protect is connectedl to the source of water being boiled (or the mud drum), then the blocked case and higher temperature will result in more water being boiled and thus higher steam generation rate. Does this make sense to you?

It should be noted that I have worked almost entirely in chemical plants and have very little understanding of boiler construction/operation methods. So it could certainly be a case of my line of thought being off base, but I would probably calculate relief load in this manner.