Calculating flow capacity of an Air Receiver PSV? 2

[ipocoyo]

Hi everyone, I currently have an air receiver with a design pressure of 500kPa. It is connected to the same line as 3 air compressors which each has a flow rate of 18.54 m3/min. How would I go about calculating the required relief capacity of a PSV on the air receiver?

Thanks in advance!

 

[Latexman]

A good starting point is API 520 Part I and II and API 521.

Good luck,
Latexman

To a ChE, the glass is always full - 1/2 air and 1/2 water.

 

[RVAmeche]

Keep in mind that you'll need the compressor curves not just use the design flow rate. Once you've read thru API 521 you determine the credible overpressure scenarios (such as blocked outlet) and determine what required relief flow rate associated with those scenarios. Then using API 520 and the required relief rate, you can size the relief valve (ie, determine it's orifice) and select a valve from your preferred manufacturer.

 

[MJCronin]

I believe that the relief device will be sized as per ASME BPV Code Section VIII, Div. 1, paragraph UG-125.

Remember, the purpose here is to protect the vessel shell from rupture, not to accomplish any process goals.

The API rules for relief devices on chemical process systems do not, IMHO, apply ...

Here is a good reference from the National Board.

https://www.nationalboard.org/index.aspx?pageID=134&ID=284

MJCronin
Sr. Process Engineer

 

[RVAmeche]

The device and piping must meet ASME BPV Section VIII. However it doesn't really provide guidance on how to analyze a system or size the device; it provides testing/certification/set pressure/overpressure requirements for those devices. I've always heard of it explained as ASME tells you what you need to do and API (and other standards) provide guidance on how to meet those requirements.

For example, that National Board page provides no guidance on sizing the device itself.

 

[MJCronin]

Orifice sizing for ASME Air Receivers should be in accordance with the rules outlined in Section V of this document:

http://www.iceweb.com.au/PressRelief/TechManual.pdf

In the same way that the ASME Boiler Safety Valves should be sized in accordance with the ASME Boiler rules ...... even if the boiler is located at a chemical process facility

The relief device orifice on an ASME Coded Air Receiver located at a chemical plant should also be sized by accepted ASME Rules.

No API reference is necessary

MJCronin
Sr. Process Engineer

 

[RVAmeche]

Note that section 5.0 of that document says that all sizing formulas come from API 520 Part 1. ASME contains no sizing equations where you determine the required orifice area based off the required relief rate. The only equations found in ASME BPV related to relief rate/capacity are the capacity conversion equations in Appendix 11. Everything else in ASME BPV is requirements for the valve itself, ensuring the vessel does not experience excessive overpressure, and stipulations about acceptable piping arrangements.

 

[chicopee]

Safety valves sizing is based on having a set pressure at the MAWP of the receiver and the free air delivery of the compressors. That's the standard by insurance companies and bi fired and unfired pressure vessel codes of several States in the North East of the USA.

 

[georgeverghese]

Required relief capacity would depend on whether each of these compressors has a reliable high press trip switch or transmitter set at <500kpag. If none have a PSHH,then required relief capacity would be sum of all 3 compressors. If all are fitted with trip switches, then would presume using flow from 2 compressors would pass muster; this assumes that 2 out of these 3 switches fails to trigger during a high press incident.

 

[ipocoyo]

Thanks for all the help. I managed to calculate the effective discharge area required for my air receiver. Just another question, do we usually set the PSV at the design pressure or the test pressure of the air receiver? Is there a standard stating anything about this?

Thanks again.

 

[Latexman]

A PSV is set at design pressure/MAWP or lower. Don't forget to check that inlet and outlet pressure drop does not exceed standards.

Good luck,
Latexman

To a ChE, the glass is always full - 1/2 air and 1/2 water.

 

[RVAmeche]

To expand on Latexman's response, both ASME BPV Section VIII and API 520 Part 1 discuss the set pressure and allowable overpressures.

 

[Latexman]

API 520 Part I and II discuss the inlet pressure drop and outlet pressure drop/backpressure.

Good luck,
Latexman

To a ChE, the glass is always full - 1/2 air and 1/2 water.

 

[georgeverghese]

Assumption implied in my previous post is that each compressor is wired up to a dedicated local control panel, which contains this high pressure trip logic. Hence there would be no possibility of a common failure mode from a trip logics perspective.