7 replies to this topic

[shubhang]

Dear all,

I am working on adequacy check (non coincidental relief scenarios only) for PSVs in crude unit as a part of capacity revamp study. Previous calculations were conducted before the isometrics were made and hence the designer has not accounted for static head in inlet line for PSV in cases where liquid relief is expected.

I do understand that it is a recommended practice to adjust for static head in set pressure, but doing that in most of the PSVs in the unit is an expensive affair.

From the vast experience that this forum has, can anyone help me as to what are the drawbacks if this adjustment to set pressures is not done.

[fallah]

shubhang,

 

The main drawback is: when PSV would pop up at the set pressure, the pressure in the vessel to be protected is higher than set pressure in the amount of liquid static head and at relieving conditions the relieving pressure would be higher than 1.1*set pressure. Then if the set pressure would be equal to vessel's MAWP, code limitation in overpressure standpoint would has been passed...

[shubhang]

Ok got it.

Thanks fallah

[aroon]

Important note: May not be relevant to this particular post. 

Location of the PSV is very much vital in all the cases. The impact can be reversed of what is mentioned by Fallah if the PSV location is at the bottom of the equipment. I have seen some case of overfilling or liquid blocked outlet for the vessels and some columns, where PSVs are located at the bottom line. In this cases, PSV will pop up prior to the equipment reaching its relieving pressure (1.1/1.16 * set pressure).

 

 

Cheers

Aroon

Edited by aroon, 11 March 2014 - 09:52 PM.

[fallah]

In a closed system before your relief valve has lifted there will be no static head. Static head only exists in open systems!

 

Think about it, how could your pressure at your vessel be higher than the pressure at the relief valve before it has lifted? Its a closed system! The pressure will be the same. (Same logic as pump sizing from an open / closed system)

 

As long as your inlet line losses are less than 3% during relief (open system) including the static head then your ok. No need to change set pressure, just ensure inlet line is adequately sized.

 

Please correct me if im wrong, because I've never added a static head and adjusted a relief valve set pressure before. It doesn't make any sense if inlet line losses are ok.

 

Its just Pascals law  " in a closed system an increase at any point will give equal increase at every point of the liquid"

 

Hi,

 

The static head is the head due to liquid column (in gas systems is negligible due to low density) and can exist in either of open/closed/flowing/nonflowing systems anywhere there would be an elevation difference in a liquid system. The pressure due to static head depends on the elevation difference and liquid density. Then in a system to be protected by a PSV that would relieve liquid out of system, the pressure of such static head, if any, should be taken into account in relevant PSV setting. Pascal law is about fluid power; pressure at any point in a closed body of fluid acts equally in all directions..., and nothing to do with liquid elevation change in such closed body of fluid...

 

The 3% rule about the friction loss in PSV inlet line is another story and should be respected in all systems with or without liquid static head at the inlet of the PSV...

[ChemEng01]

Thanks for the guidance Fallah. Any chance you could refer to a standard which explains about reducing set pressure due to static head so I can get my head around it.

 

Thanks

[fallah]

Thanks for the guidance Fallah. Any chance you could refer to a standard which explains about reducing set pressure due to static head so I can get my head around it.

 

 

Hi,

 

Attached from a valid reference helps you out...

Attached Files

•  Pages from pressure safety design practices for refinery and chemical operations1.pdf   74.24KB   626 downloads

[ChemEng01]

Thanks Fallah