Do we need a PSV after RO (Restriction Orifice)? 2

[kiade]

Hi all,
I am using the RO (Restriction Orifice) to let-down the pressure from 900# (ANSI Class) to 300#. The steam is flowing through the pipe and sends to some consumers. I have manual valve downstream of the RO. These are my questions:
1. Do I need any PSV (Pressure Safety Valve) for protection of the downstream piping in case of closing the downstream valve inadvertently? Valve is so far from RO and installed next to the consumers. (Note: We know if steam doesn't flow through the RO, upstream and downstream pressure is going to be the same pressure and all system needs to be in 900# class which I don’t want to be).
2. If we need the PSV how can I calculate the PSV’s relief load?
Please let me know your thought. Thanks guys.

Kia

 

[TD2K]

If you close the manual block valve and part of the 300# system is overpressured, you need a relief valve or prevent the overpressure some other way (remove the valve, lock it open, etc).

Personally, I'd install a PSV. To size it, you calculate the amount of steam that would flow through the orifice at the maximum allowable pressure in the 300# system. Depending on the location of valves and users, you may be able to take credit for the flow to some of the users in reducing the flow but you need to be certain they will always be taking that flow. I reviewed a fuel gas letdown valve's PSV that took credit for the normal flow through the let-down valve to the users. The difference between the full open letdown valve capacity and the normal flow was used to size the PSV. The problem was the 'normal' fuel gas flow was for winter operation. During the summer, the fuel gas consumption was almost zero and the PSV was undersized.

 

[zdas04]

You really have to think hard about taking credit for "normal" take-away. It is certainly credible for your one "always" user to have maintenance and stop taking steam. In that case everything that can flow through the RO will. The first check I would do is see if I had choked flow through the RO at downstream MAWP. If so then calculating the flow rate is pretty simple (sonic velocity times area of RO times density upstream of the RO (to get mass flow rate) divided by density at standard conditions (if you need SCF).

If the downstream pressure does not indicate choked flow then I would use nozzle calcs (see Crane 410) get a flow rate. I always check that flow rate to see if it is valid (i.e. if the velocity is less than 0.6 Mach). If velocity is greater than 0.6 Mach then I always revert to choked flow and oversize the PSV slightly since I don't have closed-form equations for transsonic flow. If the nozzle arithmetic results in a velocity less than 0.6 Mach then I use that flow rate for sizing the PSV.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.

 

[kiade]

Thank you TD2K & zdas04 for your answers.

zdas04, why do you choose the 0.6 Mach as a criteria?
How do you find choked flow condition? (Choked Pressure/Flow Rate) Could you please give me a clear example from that you are refering to?

 

[zdas04]

If you look at a chart of velocity vs. density you will see that the change in density that can be attributed to velocity effects is very slow until about 0.3 Mach. At that point the rate of change begins to increase, but it is still fairly slow. At around 0.6 Mach the line turns much closer to vertical. Some investigators use 0.3 Mach as the cut off for incompressible flow, but I find that I get usable results from treating the area from 0.3 to 0.6 Mach as incompressible. Values above 0.6 Mach are clearly inappropriate for any equation that uses the "incompressible flow (i.e., constant density)" assumption.

As to how to tell if flow is choked, see faq135-1198.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.

 

[kiade]

Thanks zdas04