9 replies to this topic

[srenfrolamb]

Normally, when evaluating inlet and outlet piping losses for PSVs I use a hydraulics software to determine losses for the PSV rated capacity.  I am working on a project with limited access to fluid property data.  While I should be able to request property values to size the orifice at the relieving pressure, I do not have actual fluid compositions so it is difficult to account for changing conditions along the pipe to evaluate pressure drops for compressible fluids. 

 

It was suggested that I may be able to use standard air to evaluate piping pressure drops. Is it appropriate to use the rated capacity converted to standard air (instead of actual fluid properties) to evaluate pressure drops for a relief system?  Although I have seen standard air used in pressure drop calculations for conservation vents, I have not seen this method used for relief valves.  Why or why wouldn't this method be appropriate?

 

ASME Section XIII - Section XIII- Rules for Overpressure Protection includes Mandatory Appendix IV - Capacity Conversion, which gives information on converting rated capacity to a different capacity such as standard air, but this seems aimed at determining whether vendor-certified capacity meets relieving requirements of your fluid. My tendency is to think that this does not work because the fluid properties such as density and viscosity would be different and not give the same pressure drops as the equivalent flow of air. In addition, it would not address possible condensing as pressure drops through the piping.  

 

Thoughts?

 

Thanks,

Shara

 

[Bobby Strain]

You should get the required information. Nothing else will suffice. What prompted this evaluation? And are you or your employer hired to perform these tasks? One must consider what liabilities are inherited with this task, too.

 

Bobby

Edited by Bobby Strain, 28 October 2021 - 07:01 PM.

[latexman]

Let’s assume you have an air PSV set at 10 barg. Can you use air properties at standard pressure, 1.01325 bara, to calculate the inlet pressure drop?

No, absolutely not.

You need the actual properties.

[sonu19921103]

Hi 

 

Relief valve are safety critical items and its very much important to size them properly. Please get the necessary data and perform the detailed calculations.

 

Also understand the local rules and regulations they will dictate the documentation required.

 

Best Regards

Sonu Singh

[breizh]

Hi,

my 2 cents , 

You mentioned PSV , how have they been specified  ? If you can get access to their data sheet you should be able to get info about the fluid .

There is no way to perform hydraulic calculation without fluid properties data  .

Good luck

Breizh 

[srenfrolamb]

Thank you all for the input.

 

The client is supplying property data that we request for specific conditions and my company is sizing the relief valves.  For example, they are generating the latent heat and the vapor properties of the relieving fluid from their model at the allowable accumulation pressure and supplying us with the physical properties needed to size the orifice. Although we have an understanding of the process, knowledge of what chemicals are used and credible cases, we do not have access to their model with specific compositions.

 

I agree that I have found no reference to using standard air to calculate pressure drops in the code - only conversions of the rated capacity to different conditions (relieving fluid, air, steam). Given your input, I believe that the client will have to give me more information as to the composition of the relieving fluid for the hydraulics - not just the physical properties needed to size the orifice. I am familiar with applicable codes and sizing relief valves, but am also used to having more access to physical property data to complete calculations. When it was suggested to me that I use the rated flow converted to standard air, similar to conservation vent capacities which are typically based on standard air, my gut feeling was no. However, I really wanted some other thoughts/explanations to validate.

 

Thanks again for your input!

[Bobby Strain]

So, are you getting all you need from your client? If you asked for the correct properties, then you can proceed when they supply them.

 

Bobby

[katmar]

There are two factors which I believe you are missing.

The first is that you are not designing the pipe size - you are selecting the pipe size. There will be relatively large steps in pressure drop from one standard pipe size to the next and you will likely find that the target pressure drop falls somewhere between 2 standard sizes. This means that you do not require extreme accuracy in your calculations.

The second aspect that is one that is too often neglected in technical calculations. This is the concept of the "design envelope" or sensitivity analysis. By this I mean that even if you cannot fix the physical properties exactly, you can often define an envelope of properties within which the actual values will fall. With modern software it is very easy to calculate for 20 or more cases to check if the actual conditions will fall within the envelope. This concept is always used in the financial analysis of projects, but somehow we have come to believe that we can calculate technical factors exactly - but in reality we cannot and we should be making much more use of this sensitivity analysis technique.

 

Even if you could get the exact values that will apply at start-up, they are likely to be have changed a few months down the line, so you should be thinking along these lines of varying conditions anyway.

 

[srenfrolamb]

Thanks for your responses. I have been able to obtain needed compositions from the client to do the sizing and analysis of the inlet/outlet piping.  

[breizh]

Hi,

Great , without proper data it's difficult to come to conclusion.

All the best in your career .

Breizh