3 replies to this topic

[Light]

For the line sizing of the inlet line to a Pressure Safety Valve, why do we consider only non-recoverable i.e. frictional losses for pressure drop calculation? Why don't we consider the accelerational and static head losses for it? 

 

Also, what is the logic behind the 3% rule for the inlet line sizing to the PSV?

 

What are the criteria for sizing the outlet line from a PSV? The outlet line is generally larger in size than the inlet line. Any reason for this?

 

 

 

Regards.

[samayaraj]

Downstream of safety valve will be one or two size bigger than the inlet size. This is because the expanded gas/vapor need to be flown within allowable velocity to avoid pressure buildup. Downstream line may be connected to a header so, frictional losses will be added to it. Hence, you need to reduce/ maintain the velocity within the limit to avoid back pressure buildup in the downstream side.

[ChemEng01]

Because;

 

The PSV set pressure should be adjusted for static head. 

 

The inlet line sizing is based on the rated flow through the PSV so no need for acceleration head. 

 

3% rule is based on the pressure that the PSV begins to close (blowdown) The 3% rule is assuming a 3% blowdown. If this inlet loss is too high the PSV may flutter or chatter during relief. The reduced capacity of the PSV including the inlet losses may be acceptable, if not  your equipment will overpressure to set pressure + overpressure + inlet line losses. If the PSV inlet line losses are above 3% you can do a force balance assesment outlined in API 521. If your inlet losses + built up backpressure are less than the maximum allowable overpressure + blowdown the PSV should not experience chatter. 

Edited by ChemEng01, 15 October 2015 - 12:43 AM.

[fallah]

For the line sizing of the inlet line to a Pressure Safety Valve, why do we consider only non-recoverable i.e. frictional losses for pressure drop calculation? Why don't we consider the accelerational and static head losses for it? 

 

Also, what is the logic behind the 3% rule for the inlet line sizing to the PSV?

 

What are the criteria for sizing the outlet line from a PSV? The outlet line is generally larger in size than the inlet line. Any reason for this?

 

 

Light,

 

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