10 replies to this topic

[panagiotis]

Could you please explain me what is the certified capacity of a PSV? And if it is diffirent than the installed capaciry?
The certified or installed capacities are given by the vendor?

[latexman]

Certified capacity of a PSV is based on flow tests by the PSV manufacturer per jurisdictional authority.  In US, it's ASME.  The outcome of the flow tests is each PSV has a certified flow coefficient for gas/vapor (typically ~ 0.9) and a certified flow coefficient for liquid  (typically ~ 0.7).

 

What is your definition of "installed capacity"?  I don't think I've ever seen it for a PSV.  For a RD or PVSV, yes.

 

For a PSV, there is a worst case sizing flow requirement.  One usually picks the smallest PSV that will do the sizing flow. or more, usually more.  Then, one designs for, or checks, inlet pressure drop and outlet pressure drop.  If inlet pressure drop and outlet pressure drop meet that PSV's criteria, the PSV is okay and the only capacity I recall seeing is the certified capacity of the PSV.  This is because of the "huddling chamber" in a PSV.  This gives it the ability to "pop" open and stay open until the pressure gets to about 93% of set pressure for gas/vapor and about 70% of set pressure for liquids.  Sorry, I've never seen a "installed capacity".  How is it defined?

 

A PSV is either wide open (at it's capacity) or closed.  With that in mind, "installed capacity" would have to be about equal to "certified capacity" or there would be inlet and/or outlet pressure drop problems.

[fallah]

Could you please explain me what is the certified capacity of a PSV? And if it is diffirent than the installed capaciry?
The certified or installed capacities are given by the vendor?

 

Hi,

 

Certified capacity of the PSV is to be stamped on the relevant name plate by the vendor. Installed capacity (or actual capacity) of the PSV is the capacity has been measured in the relevant standard test which should be multiplied by a factor less than one (for gases depend on the vendor but around 0.9) to obtain the certified capacity. Then the installed or actual capacity is always higher than the certified capacity.

[shvet1]

 

GLOSSARY

...

Calculated Relieving Capacity:

The flow rate for a given fluid in a pressure relief system which uses a rupture disk device as the sole relief device at a designated

...

Certified Capacity:

The flow rate through a pressure relief valve calculated for a designated temperature, set pressure, and 10% overpressure (see UG-131©(2) for exception), using a flow area and coefficient of discharge determined in certification tests or otherwise specified in ASME BPV Code. Can be expressed as flow rate of either steam, water (if certified on water), air, and (optionally) other fluids.

...

Coefficient of Discharge:

The ratio of the measured relieving capacity to the theoretical relieving capacity. 

...

Minimum Required Relief Capacity:

Relieving capacity required to prevent the pressure in the protected vessel from exceeding the maximum allowable working pressure by more than allowed by the ASME BPV Code or other applicable codes or practices.

Nameplate Capacity:

See Certified Capacity.

...

Relieving Capacity:

The flow rate through a pressure relieving system calculated for a designated temperature, set pressure and overpressure, using a flow area and coefficient of discharge (or resistance factors for rupture disk device systems) determined in certification tests or otherwise specified in the ASME BPV Code. For pressure relief devices covered under the ASME BPV Code but normally not capacity-certified, and for low pressure relief devices not covered by the ASME BPV Code, the relieving capacity is based on the manufacturer’s calibration data.

...

Theoretical Relieving Capacity:

The computed capacity expressed in gravimetric or volumetric units of a theoretically perfect nozzle having a minimum crosssectional flow area equal to the actual discharge area of a pressure relief valve or relief area of a non-reclosing pressure relief device. The flow path in a perfect nozzle is conventionally taken as isentropic.

Taken from https://a.co/d/1EzkHAU

[panagiotis]

Hello everyone,

 

Thank you for your time to reply me.

 

@latexman,

 

If I understood correct the vendor estimates the certified capacity to have the ability to "pop" open and stay open until the pressure gets to about 93% of set pressure for gas/vapor and about 70% of set pressure for liquids. These values 93% and 70% have any basis? Where the pressure should be 93% of set pressure for gas/vapor?

 

When an engineer sizes a PSV, normally he/she calculated the inlet / outlet pipe sizes based on the famous criteria 3% and 10%. For this the required capacity is used. The required capacity is the relieving capacity, which is calculated differently for each overpressure scenario. And then, when the vendor provides the certified capacity, the inlet/outlet pipe sizes can be checked (if meets the criteria) for the certified capacity. Am I right?

 

@Svhet,

The glossary seems very confusing to me.

[latexman]

If I understood correct the vendor estimates the certified capacity to have the ability to "pop" open and stay open until the pressure gets to about 93% of set pressure for gas/vapor and about 70% of set pressure for liquids. These values 93% and 70% have any basis? Where the pressure should be 93% of set pressure for gas/vapor?

 

When an engineer sizes a PSV, normally he/she calculated the inlet / outlet pipe sizes based on the famous criteria 3% and 10%. For this the required capacity is used. The required capacity is the relieving capacity, which is calculated differently for each overpressure scenario. And then, when the vendor provides the certified capacity, the inlet/outlet pipe sizes can be checked (if meets the criteria) for the certified capacity. Am I right?

 

The 93% (gas/vapor) and 70% (liquid) of set pressure is the approximate reseating pressure of a PSV, when pressure is reduced from a "wide open" flowing status.  The numbers I gave are industry experience numbers.  I do not know if they can be derived theoretically.

 

You are not right.

 

API 520 Part 2 says:

7.3.7.3 Flow Rates for Hydraulic Calculations

A good design practice is to use the PRV rated capacity (e.g., at 10 % overpressure) for inlet pressure drop calculations, since doing so does not constrain future operations and knowledge of the valve’s modulating behavior is not required.

 

PRV rated capacity = the mnfr's certified capacity.  PRV rated capacity is not the required capacity or worst case scenario sizing flow.

 

Definitions are critical during relief sizing/design.  

[panagiotis]

Hi Latexman,

Thanks it is clear now.

[fallah]

 

PRV rated capacity = the mnfr's certified capacity.  PRV rated capacity is not the required capacity or worst case scenario sizing flow.

 

Definitions are critical during relief sizing/design.  

 

 

But a main point for a PSV capacity is that: the "Certified Capacity" should always be higher than (at least equal to) the "Required Capacity"...

[panagiotis]

Hi Fallah,

 

Thanks! Good point. The first information that we have to size the inlet and outlet piping of the PSV is the required capacity. And then from the vendor we can get the certified capacity, which should be used for the sizing of the PSV.

[fallah]

Hi Fallah,

 

Thanks! Good point. The first information that we have to size the inlet and outlet piping of the PSV is the required capacity. And then from the vendor we can get the certified capacity, which should be used for the sizing of the PSV.

 

Hi,

 

When the vendor reports the certified capacity of a PSV it has already been sized by him/her based on the data sheet we provided, and we should check the vendor data sheet to be sure the certified capacity isn't less than required capacity had been reported in our initial data sheet.

[panagiotis]

Hi Fallah,

It is crystal clear now! Thanks!