9 replies to this topic

[Sumanth]

Hi,

 

I am a process engineer with one year experience.There is a 3 phase horizontal separator and has got both BDV & PSV. The operating pressure ranges from 30 to 110 kg/cm2 (g). The design pressure is 203.3 kg/cm2 (g). The initial pressure considered for BDV Depressurization (Depressurization tool in Hysys) is the PZHH of the separator, i.e 121 kg/cm2 (g). The PSV is set at 150 kg/cm2(g) (usually PSV set pressure will be the design pressure, but here in order to protect the downstream facilities which is designed at lower pressure, the PSV is set at 150 kg/cm2(g).)

 

As per my understanding, BDV being the primary protection for the separator, when fire occurs, BDV opens and evacuates the inventory and the BDV is kept at open unless it is reset. The pressure drop is achieved across the RO and relieved to flare. So if BDV operates correctly, PSV will not pop up.

 

PSV will act only if the primary protection fails (BDV fails to open).

 

Kindly advice on my above understanding.

 

Whether the initial pressure of 121 kg/cm2(g) (PZHH of the separator) selected for depressurization is correct with respect to PSV set pressure of 150 kg/cm2(g)?

 

 

During fire, is there be a possibility of both BDV & PSV opening simultaneously?

 

 

 

 

[aqassam]

I know that company guidelines specify the preferred methodology for analyzing vessels with PSV and BDVs. 

 

When I've analyzed this before, clients have required that the PSV be sized assuming no BDV exists (since it is the last line of failure in the event that the BDV does fail for any reason). 

 

For BDVs you have to consider two cases: non  emergency and emergency scenario. The non-emergency scenario depressuring is done from normal operating conditions to establish the MDMT for the vessel. The emergency scenario assumes that the pressure vessel is at relieving pressure before the depressurization occurs worst case scenario assumption). This will also result in the greatest peak vapor flow rate through the BDV. 

 

I'd be interested to see how others analyze this situation. 

[fallah]

Sumanth,

 

My answers are in red color as follows:

 

 

PSV will act only if the primary protection fails (BDV fails to open). In fire case, yes...

 

Whether the initial pressure of 121 kg/cm2(g) (PZHH of the separator) selected for depressurization is correct with respect to PSV set pressure of 150 kg/cm2(g)? No problem...

 

During fire, is there be a possibility of both BDV & PSV opening simultaneously? No...

 

 

 

 

[Vmm]

Hi Sumanth,

Why the PSV set pressure is reduced to 150 kg/cm2g, even-though the system design pressure is 203.3 kg/cm2g, ff the downstream is not designed for the upstream design pressure, then downstream system shall be protect with a Gas blow-by PSV, rather than reduce the upstream PSV set pressure (as this system is capable to take higher pressure).

 

I do agree with Mr.Fallah answers on your other quires

 

Thanks

Regards.

VMM

[RoyenG]

Hi Sumanth,

 

For some companies, they use below initial conditions for BDV:

 

- Fire case : initial conditions will be the normal operating conditions.

 

- Adiabatic Case : Initial conditions at the PZHH and normally used for estimating the lowest temperature in the system.

                             If you use API RP 521 as the guideline, PZHH is set at the 97% of the PSV set pressure.  

 

The main purpose of RO downstream of BDV is to set the maximum flowrate from the isolation system to the flare.

 

 

Regards,

Royen

 
 

[fallah]

 

For some companies, they use below initial conditions for BDV:

 

- Fire case : initial conditions will be the normal operating conditions.

 

- Adiabatic Case : Initial conditions at the PZHH and normally used for estimating the lowest temperature in the system.

                             If you use API RP 521 as the guideline, PZHH is set at the 97% of the PSV set pressure.    
 

 

Hi,

 

Design pressure is normally assumed to be as starting point for both cases. If it is much higher than operating pressure (above 15-20%) the maximum operating pressure or PSHH is to be selected.

 

As far as i know there is no guideline in API 521 for PSHH setting at 97% of the PSV set pressure, then if there is let's have the evidence...

[Steady]

Agree with Fallah's Post 6;

 

Remeber these guidelines or sizing criteria may vary on company to company basis. Therefore, you need to follow you company/cleint standards.

 

Design pressure is conservatively assumed to be intital pressure for the sizing case of the BDV. In your case since the design pressure is not expected to reach because the PSV is going to be set at a lower value. You may choose to use PSV set pressure as the intital pressure for the BDV. Again, thats something you need to agree with your Principal engineer.

Edited by Steady, 31 March 2014 - 02:57 AM.

[RoyenG]

Hi Fallah,

 

Guidelines for each contractor/owner/company might be different in defining initial conditions for BDV fire and adiabatic case.

 

Taking PSHH or design pressure as the initial conditions for the fire BDV will result in a higher blowdown rate, which will result in an oversize of a Relief system. Probability of all system in the PSHH conditions at the same time should be low (could be verified through SIL?).

 

API RP 521 does not mention specifically that PSHH = 97% of PSV set pressure, but figure 4 - Pressure Levels (API RP 521_1997) provide a graph that shows blowdown set point is located at 97% of PSV set pressure.

 

Regards,

Royen

Attached Files

•  Figure 4_API RP 521_1997_PSV & BDV.pdf   140.51KB   52 downloads

[Steady]

@RoyenG; the blowdown point you referring to in your attachment is the 'closing pressure of the PSV' which is nearly 2-3% below the set pressure due to simmering of the PSV.

With regards to sizing BDV, i dont see any logical reason of not considering the maximum pressure allowed in the system unless one's design criteria/standard says it otherwise. Remember BDVs are to be initiated when required whether its an emergency or any other reason. Considering the maximum intital pressure of the system will result in bigger RO but at least it is going be to be able to depressurise the system in required/expected time.

[fallah]

 

Guidelines for each contractor/owner/company might be different in defining initial conditions for BDV fire and adiabatic case.

 

Taking PSHH or design pressure as the initial conditions for the fire BDV will result in a higher blowdown rate, which will result in an oversize of a Relief system. Probability of all system in the PSHH conditions at the same time should be low (could be verified through SIL?).

 

API RP 521 does not mention specifically that PSHH = 97% of PSV set pressure, but figure 4 - Pressure Levels (API RP 521_1997) provide a graph that shows blowdown set point is located at 97% of PSV set pressure.

 

 

Royen,

 

The depressirizing facility, especially when to be applied in fire case, is a mitigation facility against a safety issue and should be considered for worst conditions, then taking design pressure as starting point even if will lead to a little bit higher blowdown rate is a logical action. Of course, as i mentioned, if there would be a considerable difference between design pressure and maximum operating pressure the choise would be changed to maximum operating pressure or PSHH.

 

And yes, each company has to follow its prespecified guidelines and should accept the responsibility of any probable risk may happen due to conservatism ignorance...

 

Finally, the graph you attached is a typical graph and related to pressure levels in PSV application to protect a pressure vessel and nothing to do with PSHH setting, then is irrelevant to the discussion...Of course, if the maximum operating pressure would be around 90% of the design pressure, the PSHH would be set at the maximum operating pressure or with a margin at a little bit higher value but well below the PSV blowdown ...