10 replies to this topic

[SawsanAli311]

Dear All, 

 

In-line with API 520 and ASME SEC VIII UG-134 item a,multiple relief valve device configuration mandates setting the first PSV at the equipment's MAWP/design pressure while the subsequent multiple device(s) at 105% of the MAWP/design pressure.

 

I have a case of 3 working multiple PSVs and one common standby PSV among them all. These PSVs are installed on a separator vessel having a design pressure of 19 barg. Governing over pressure scenario is two phase blocked outlet. 

 

- Set pressure of 1st PSV=19 barg

- Set Pressure of 2nd & 3rd PSV = 19.95 barg

- Set pressure of the isolated inlet spare PSV(number 4) is 19 barg.

 

Now, in case one of the 2nd or 3rd PSV is taken for replacement, how to ensure that the spare PSV would have its PSV set point also staggrered with  respect to the 1st PSV. 

 

If one selects the spare PSV number 4 to have a set pressure of 19.95 barg  instead of 19 barg and PSV number 1 is taken for maintenance, then staggering would be defeated since all available (2,3,4) PSVs would pop at the same higher set pressure of 19.95 barg and allowable accumulation of 16% as per ASME would not be achieved but reduced to around 10%. 

 

Appreciate your views on the following:

- Optimum setting of the spare PSV. 

- Whether planned scheduled PSV replacement shall consider the above setting sequence. What if one of the PSVs urgently required replacement on an emergency basis and hence it can't be planned.

- The safest design is to have one spare PSV for each installed working PSV, however this will lead to having 6 PSVs in total instead of 4 optimized PSVs in addition to structural challenges and complexities. 

 

Thank You,

 

Regards, 

[fallah]

Hi,

 

In general, if you are going to consider one spare for multiple PSV's you have to consider same rated capacity for such multiple PSV's as well. Hence, the spare PSV should be set at the pressure same as the first operating PSV because if it being set at higher value, in the case of using spare PSV relieving can be started at a value higher than MAWP and this may not be acceptable in code limitations standpoints...

[Pilesar]

I believe the first sentence of the originator's initial post is incorrect. There is no mandate that the set pressure of the first PSV be at the MAWP. The setpoint of your primary relief device should be set NO HIGHER THAN the MAWP of the vessel. If your normal operating pressure allows, consider setting the spare PSV at 18 barg. That way you are assured of keeping the staggered setting.

[SawsanAli311]

Thanks Fallah and Pilesar, 

 

It is absolutely incorrect to set the PSV at a value higher than MAWP, for this purpose setting the spare PSV to be equal to the setting of the primary PSV. Yet, if it is set at 18 barg then I believe the size of this spare PSV would be non-identical to the remaining PSVs and accordingly rated capacity would be different.If this spare PSV becomes the primary one then with 18 barg, the area will have to be increased to limit the code allowable accumulation to 16% otherwise accumulation would be higher than 16% which is not acceptable by code.

 

Thanks

 

Regards, 

[Bobby Strain]

Accumulation has to do with the vessel, not the PSV. The maximum relieving pressure has nothing to do with the PSV set pressure.

 

Bobby

[SawsanAli311]

Accumulation has to do with the vessel, not the PSV. The maximum relieving pressure has nothing to do with the PSV set pressure.

 

Bobby

Thank you Mr.Bobby, I agree with you,I may have confused overpressure with accumulation.

 

Can you please advise whether the staggered arrangement common spare PSV should be set as the 1st main PSV. For 2nd & 3rd PSV, as per ASME  code, they shall not be set higher than 5% of MAWP do you think with such close setting, the set pressure tolerances defined in UG-134 as +/- 3% would be affected?

[Bobby Strain]

Pilesar's advice has merit. And, what you choose will depend on the jurisdiction responsible for enforcing pressure vessel code adherence. And your insurance company requirements. Is this a new facility? Or are you just buying a spare valve as a hedge against downtime? Either option will provide protection for the vessel.

 

Bobby

[SawsanAli311]

Hi,

 

In general, if you are going to consider one spare for multiple PSV's you have to consider same rated capacity for such multiple PSV's as well. Hence, the spare PSV should be set at the pressure same as the first operating PSV because if it being set at higher value, in the case of using spare PSV relieving can be started at a value higher than MAWP and this may not be acceptable in code limitations standpoints...

Dear Fallah, 

 

With regards to the subject query.. for the same multiple PSV arrangements (where I have 3 PSVs working & 1 standby).. considering the design selection of this staggered arrangement was driven by having multiple contingencies where the smaller relieving rate scenario would be much less than the governing scenario i.e. would go below 25% of the one single PSV rated capacity. 

 

IF we consider the 3+1 SB PSVs as EXISTING in the plant... and during design we are considering one of the low relieving rate scenarios which doesnt mandate 3 PSVs to be operating i.e. one PSV is only required to be in operation.. then my questions are:

1- Shall I consider the overpressure for this scenario on the EXISTING PSV is 16% or 10% since the PSV EXISTING area has been calculated considering a 16% overpressure at the design pressure (or less). If we have a single PSV mandated to be in operation then considering 16% would deviate from the code the pressure inside the vessel would be allowed to increase beyond the value acceptable by code for a single relieve contingency. Therefore, what overpressure to be considered knowing that I will not need the staggered PSVs. 

2- If in reality we see that the 3 PSVs were not actually required.. and 1 is enough + 1 SB do you think we can tackle the deviation from 10% overpressure requirement would be reducing the PSV set point.. dont you think this will narrow down the operating envelope of the system?

3- Do you think that for the above cases.. designer will have to really consider the proposal given by  Pilesar by setting the 1st PSV less than the design pressure such that even if one PSV is required.. the 10% overpressure would not be defeated because of the installed area. 

 

Appreciate your kind views

[fallah]

Dear Fallah, 

 

 

 

With regards to the subject query.. for the same multiple PSV arrangements (where I have 3 PSVs working & 1 standby).. considering the design selection of this staggered arrangement was driven by having multiple contingencies where the smaller relieving rate scenario would be much less than the governing scenario i.e. would go below 25% of the one single PSV rated capacity. 

 

IF we consider the 3+1 SB PSVs as EXISTING in the plant... and during design we are considering one of the low relieving rate scenarios which doesnt mandate 3 PSVs to be operating i.e. one PSV is only required to be in operation.. then my questions are:

1- Shall I consider the overpressure for this scenario on the EXISTING PSV is 16% or 10% since the PSV EXISTING area has been calculated considering a 16% overpressure at the design pressure (or less). If we have a single PSV mandated to be in operation then considering 16% would deviate from the code the pressure inside the vessel would be allowed to increase beyond the value acceptable by code for a single relieve contingency. Therefore, what overpressure to be considered knowing that I will not need the staggered PSVs. 

2- If in reality we see that the 3 PSVs were not actually required.. and 1 is enough + 1 SB do you think we can tackle the deviation from 10% overpressure requirement would be reducing the PSV set point.. dont you think this will narrow down the operating envelope of the system?

3- Do you think that for the above cases.. designer will have to really consider the proposal given by  Pilesar by setting the 1st PSV less than the design pressure such that even if one PSV is required.. the 10% overpressure would not be defeated because of the installed area. 

 

Hi,

 

1-The 16% over pressure should be applied on the whole multiple PSVs system, hence if the first PSV will be activated alone, relevant over pressure might be 10%, 16% or a value in between and it wouldn't be considered as a deviation to the vessel design code. 

 

2-If there is a doubt for incorrect design of existing multiple PSVs sizing/configuration, you should re-design the PSV and then make a proper decision.

 

3-The 1st PSV can be set at a value lower than the design pressure if won't narrow down the vessel operating range.

Edited by fallah, 07 April 2020 - 06:26 AM.

[SawsanAli311]

 

Dear Fallah, 

 

 

 

With regards to the subject query.. for the same multiple PSV arrangements (where I have 3 PSVs working & 1 standby).. considering the design selection of this staggered arrangement was driven by having multiple contingencies where the smaller relieving rate scenario would be much less than the governing scenario i.e. would go below 25% of the one single PSV rated capacity. 

 

IF we consider the 3+1 SB PSVs as EXISTING in the plant... and during design we are considering one of the low relieving rate scenarios which doesnt mandate 3 PSVs to be operating i.e. one PSV is only required to be in operation.. then my questions are:

1- Shall I consider the overpressure for this scenario on the EXISTING PSV is 16% or 10% since the PSV EXISTING area has been calculated considering a 16% overpressure at the design pressure (or less). If we have a single PSV mandated to be in operation then considering 16% would deviate from the code the pressure inside the vessel would be allowed to increase beyond the value acceptable by code for a single relieve contingency. Therefore, what overpressure to be considered knowing that I will not need the staggered PSVs. 

2- If in reality we see that the 3 PSVs were not actually required.. and 1 is enough + 1 SB do you think we can tackle the deviation from 10% overpressure requirement would be reducing the PSV set point.. dont you think this will narrow down the operating envelope of the system?

3- Do you think that for the above cases.. designer will have to really consider the proposal given by  Pilesar by setting the 1st PSV less than the design pressure such that even if one PSV is required.. the 10% overpressure would not be defeated because of the installed area. 

 

Hi,

 

1-The 16% over pressure should be applied on the whole multiple PSVs system, hence if the first PSV will be activated alone, relevant over pressure might be 10%, 16% or a value in between and it wouldn't be considered as a deviation to the vessel design code. 

 

2-If there is a doubt for incorrect design of existing multiple PSVs sizing/configuration, you should re-design the PSV and then make a proper decision.

 

3-The 1st PSV can be set at a value lower than the design pressure if won't narrow down the vessel operating range.

 

Thanks a lot Fallah for your response.

When you are referring to 16% I believe you would infer to the 1st PSV which is set at 100% of the vessel's design pressure (or less) since the subsequent other PSVs (which are set at not greater than 105%) would have a comparatively lower overpressure i.e. around 10% with respect to their PSV set point. All PSVs irrespective would eventually have the same upstream relieving pressure.

 

Additionally, I have another question, if the same above system is subject to fire, do you agree that the first valve would in this case pop sooner as the area was designed for 16% overpressure.  would you encourage stating that the overpressure is still 16% and not 21% which would be considered in initial design since the PSV is already existing and it is designed for 16% accordingly.

[fallah]

Thanks a lot Fallah for your response.

 

 

When you are referring to 16% I believe you would infer to the 1st PSV which is set at 100% of the vessel's design pressure (or less) since the subsequent other PSVs (which are set at not greater than 105%) would have a comparatively lower overpressure i.e. around 10% with respect to their PSV set point. All PSVs irrespective would eventually have the same upstream relieving pressure.

 

Additionally, I have another question, if the same above system is subject to fire, do you agree that the first valve would in this case pop sooner as the area was designed for 16% overpressure.  would you encourage stating that the overpressure is still 16% and not 21% which would be considered in initial design since the PSV is already existing and it is designed for 16% accordingly.

 

 

Hi,

 

As mentioned because the 16% over pressure in multiple PSVs sizing is applied on the whole required orifice area and then the calculated area would be divided to multiple area, the first PSV may experience 16% over pressure or lower but what can be declared without any doubt is: when all PSVs will be activated for governing scenario, the maximum over pressure is 16%.

 

Because the governing case is other than fire, in a fire case again the first PSV may experience 16% over pressure or lower and again the 16% over pressure (not higher) might be experienced when all PSVs are being activated because the governing case with 16% over pressure will cover the fire case as a non governing case...