Negating A Relief Scenario 1

[barnesed]

Existing plant steam user (i.e. equipment) rated at 125 psig has a steam supply pressure reducing valve set at 90 psig and a relief valve set at 125 psig, sized for steam reducing valve failure. The existing boiler supplying the steam has a relief valve also set at 125 psig. It appears that the reducing valve failure scenario can be negated since the boiler relief valve set at 125 psig also protects the plant steam user rated at 125 psig. (Other valid relief scenarios still apply.) Can I get confirmation that this is a valid assumption and that the reducing valve failure scenario can be negated?

 

[zdas04]

My read on the code is that you cannot take credit for a relieving device when there is a control valve, a check valve, or any control device between the relieving device and the vessel to be protected. The only way that one PSV can protect two vessels is if the only thing between them is piping.

David

 

[barnesed]

Thanks for your reply zdas04!

The existing equipment would still have a relief valve regardless, but since the plant steam source is not capable of overpressurizing the existing equipment by virtue of the boiler relief valve, why can't the scenario of plant steam supply overpressure be negated?

I would understand if the boiler relief valve was the only relief valve servicing the equipment, but as stated, the existing equipment would still have a dedicated relief valve regardless (just not sized for steam reducing station failure).

 

[don1980]

If the steam boiler has a PSV set at 125 psig and your protected system has a 125 psig MAWP, then I see no reason to size your PSV for steam control valve failure. Instead, focus on other potential causes for overpressure. Assuming the boiler PSV is properly sized, the boiler can't cause overpressure of your equipment, even if the control valve goes full open.

Code mandates that you protect each pressure vessel from overpressure, but it's up to the user (you) to determine the sizing basis (causes of overpressure)for each PSV. Code is not prescriptive in defining that for you. So, it's up to you to determine whether or not control valve failure is a cause of overpressure. If the maximum boiler pressure was limied by basic process control instrumentation, then you'd need to consider that this could fail and cause your system to be overpressured. But, since your steam boiler is limited by a PSV, it's not necessary to assume that that PSV will malfunction and cause the downstream system to overpressure.

 

[zdas04]

I agree with don1980. I would have no problem signing off on a credible scenario list that claimed "control valve failure" was not credible because of the 125 psig relief on the supply vessel.

I was thinking (wrongly now that I re-read your original post) that you were using the PSV on the supply vessel as justification for removing the PSV on the other vessel which wouldn't have been OK. Eliminating the credible scenario because of the first vessel's PSV set point sounds reasonable to me. If that were your only credible scenario, I'd question your analysis (steam vessels can always end up liquid full, so you really can't ever get rid of thermal relief), but that doesn't seem to be the case either.

David

 

[Duwe6]

Is "Fire Case" a reasonable scenario? If not, I too would sign off on not needing the PRV.

 

[don1980]

Duwe6, careful with that statement. Just because there are no credible causes of overpressure, that's NOT an automatic exemption from needing a relief device per ASME and most other codes.

Per ASME, you can omit a relief device, but only under the special conditions explained in UG-140. Otherwise, a relief device is always needed, regardless of whether or not there are any identified credible scenarios.

And that's good advice. When there are no credible scenarios, just install a small PSV. The coolant side of exchangers is the only common exception to that (refer to API 521 5.14.1). A small PSV satisfies code, and it provides a world of insurance aginst the unexpected.

Remember, nothing can be made fool-proof, because fools can be VERY ingenious.

 

[pleckner]

Although the final conclusion of the above is correct if analyzing a system for a pressure vessel, I can easliy interpret the original post as one protecting piping and not a pressure vessel. In this case, ASME B31.3 is probably the Code that needs to be reviewed and there are considerations here that are different.