3 replies to this topic

This query is regarding the relief case finalisation for offshore platforms.
We were involved in the engineering calculations for relief case identification and sizing of PSV.
Our PSV is mounted on the first stage suction KOD of compressor. MOC for KOD is CS.

After doing preliminary calculation, the settle out pressrure for first stage compressor loop is nearly 35 kg/cm2g. Suction pressure during normal operation is ~6 kg/cm2g. Based on the settle out pressure calculation and the operating pressure, KOD has been given design pressure of 35 kg/cm2g, and thus the PSV set pressure of 35 kg/cm2g is selected.

While doing relief temperature estimation based on wetted and unwetted area method, relieving temperature (calculated based on API 520-521 method) exceeds the design temperature of the KOD for the given design pressrue.

Please acknowledge if the methodology is correct. Please give advice, where I might have made a mistake

[fallah]

I think metodology is correct, but probably you have BDV on KOD for its depressurizing leading to higher toleration in fire case. KOD is designed as per settle out pressure and relieving temp. of fire case is not considered in this regard, even is to be greater.

[djack77494]

It does not seem plausible to me that your relieving temperature would exceed the design temperature of the Knock Out Drum. I believe that the relieving temperature for a KOD fire case would be set at the boiling point of the liquid in the base of the drum. Since this is feeding a compressor, the bulk of the fluid moving through your system is presumably vapor. The associated liquid would not typically have a very high boiling point. To say it exceeds the vessel's design temperature does not seem reasonable to me. Tell me what I'm missing.

[fallah]

It does not seem plausible to me that your relieving temperature would exceed the design temperature of the Knock Out Drum. I believe that the relieving temperature for a KOD fire case would be set at the boiling point of the liquid in the base of the drum. Since this is feeding a compressor, the bulk of the fluid moving through your system is presumably vapor. The associated liquid would not typically have a very high boiling point. To say it exceeds the vessel's design temperature does not seem reasonable to me. Tell me what I'm missing.

Relieving temperature in this case (As per your correct statement being the bulk of moving fluid as vapor) mostly evaluated as unwetted (API 521 5.15.2.2.2) ,and we can see in most fire case of unwetted conditions the relieving temperature could be exceeded to design temperature.In many cases the difference would be considerable.
Contrary to other cases,source of overpressure in fire case (unwetted) is increasing gas/vapor temperature due to heating and therefore gas/vapor temperature at relieving pressure would be higher (no need to say in real world before reaching this condition the vessel may has been failed).