9 replies to this topic

[Nagarajan K]

Dear Sir,

 

I am really confused between rated and Relieving Capacity for safety valves.I tried in search engines.

got more confused.

 

I refereed API 520 and got formula to calculating relieving Capacity,but was confused with rated capacity and difference between them.

 

Kindly guide me.

 

Regards,

 

Nagarajan K

 

 

[Sathya R]

Reliving capacity is based on the scenario selected based process design. Rated capacity is higher and is based on the selected orifice.

[Nagarajan K]

Dear Sathya,

 

so can rated and relieving can be same

[fallah]

Hi,

 

There are two relieving capacities for a PSV as follows:

 

Required relieving capacity: Has been obtained after credible scenarios analysis and governing scenario selection

 

Rated relieving capacity: Should be at least equal to required one and would be obtained by using actual ASME orifice size and actual ASME coefficient of discharge...It's same as PSV stamped capacity and is normally provided by PSV manufacturer...

[Nagarajan K]

Thanks for your replies.

 

For

 

Do we consider Overpressure addition during calculating inlet pressure for PSV.

which in turn will be used to calculate Required relieving capacity,

 

NK

[fallah]

Hi,

 

To calculate required relieving capacity of a PSV, always upstream relieving pressure which is the summation of set pressure, allowable overpressure and atmospheric pressure, is to be considered...

[Rafael Espinoza]

Well, I just want to use this tread because others (more appropriate to the subject) are closed,

 

The issue about relieve capacity, fire x blocked scenarios, chattering:

 

Going straight to the point: the rated capacity are always enough for the required capacity, but it's possible that the real scenario could develop much less relief flow than rated. (Partial fire exposure, for instance) In these cases chattering is possible. How to handle this chattering possibility?

 

Thanks

[fallah]

Rafael,

 

You can use modulating pilot operated PSV to avoid chattering...

[Bobby Strain]

Nobody ever bothers to consider what is troubling you.

 

Bobby

[Brocklesnar]

Just to add to whats all said before,

Rated or nameplate capacity of any relief valve is stamped on the valve and matches the flow of compressible fluid (say steam) at saturated conditions of discharge. To obtain the relieving capacity at particular relieving point of set pressure and temperature, use the correction factors given in API-520 or ASME Section-I for superheat temperature, pressure > 1500 psi, type of PSV arrangement.

 

My experience tells that, for everything else remaining constant, the relieving capacity in superheated conditions is less than rated or nameplate capacity (which of course corresponds to saturated condition)