PSV wall thickness calculation. 1

[BFZHA]

I want to discuss an issue we have faced when dealing with PSV body wall thickness.

The PSV is installed on a process line where the corrosion allowance (CA) is required.

The need for the PSV was urgent, unfortunately the available PSV’s did not match such requirement i.e WTrequired >= WTcalculated + CA. The Manufacturer can provide only PSV with wall thicknesses less than required by -0.5 mm.

During discussion of this issue, the following opinions were given;

1- The first opinion is since corrosion allowance is required and since the wall thickness is calculated according to ASME VIII part 1, the corrosion allowance shall be considered and then the Vendor offer is not accepted.

2- The second opinion has considered that since the fluid is in permanent contact only with the nozzle and the disc (made of SS + TCC) and occasionally with a body of the PSV, then the corrosion allowance can be ignored in the calculation of the wall thickness and then the Vendor offer can be accepted.

3- The third opinion is just a combination of the two first opinions i.e. the corrosion allowance shall be considered in the calculation of the body wall thickness, but since the body is not in permanent contact with the fluid and the difference between required and available wall thicknesses is 0.5 mm only, the Manufacturer offer can be accepted.

Please can you advise?

Best Regards.

 

[KoachCSR]

What was the total corrosion allowance specified? Does the missing 0.5mm constitute the full corrosion allowance or only a portion of it.

Given the urgency of your need, and the already observed fact that the process fluid only makes permanent contact with the nozzle and disk, I see no reason why you can't revise your specification and accept the vendor's offer.

If you do that, I would recommend the following:

1. Revise your P&ID to indicate this valve was originally procured without the required corrosion allowance. (So that the information doesn't "disappear")
2. Consider adding a regular (yearly or bi-yearly, perhaps) inspection to confirm the level of corrosion taking place at the nozzle connected to the process. If the PSV is flanged, this inspection should be easy enough. If not, consider UT.

 

[mk3223]

IMO, to discuss with operation for the current PSV valve acceptance since it was urgent need for the work. It should develop a frequent inspection schedule to detect any corrosion issue during the service, and develop a work plan during the future plant turnaround or when any maintenance opportunity arises.

 

[BFZHA]

Thanks a lot for your answers,
First the corrosion allowance is 6 mm, then the missing 0.5 mm do not constitute a big issue if, as you say, a regular inspection is conducted in order to check the level of corrosion.

In summury, the final decision should take into account, both design conditions (Pressure, Temperature, fluid, material, corrosion allowances, etc...) and operation and maintenance activities by scheduling a regular inspection for corrosion level.

Best Regards.

 

[e43u8]

BEZHA,

You mentioned wall thickness is calculated according to ASME VIII part 1, while the PSV is to be installed on a proces line for which ASME VIII isn't applicable and it might be designed based on ASME B31.3.

Indeed, as far as i know PSV manufactuters wouldn't design these valves same as pressure vessels for which they need have to consider CA in wal thickness calculations. In fact, the PSV body material and thickness are to be selected based on the set pressure and the fluid would pass through them and it's supposed the material of PSV body shouldn't be corroded at the specified fluid(s) can pass through them.

 

[BFZHA]

e43u8,
Thank you for your reply.
I would like to notice that ASME BPVC is referenced in ASME B31.3 and as stated in API 526 for Flanged Steel Pressure relief Valves in design part (chapter 7) " Pressure-relief valves discussed in this standard shall be designed and manufactured in accordance with the
applicable requirements of ASME BPVC, Section VIII for pressure-relief devices.". Also on different occasion Manufacturers send us a calculation sheet where the wall thicknesses of retaining pressure parts where calculation were done and checked with ASME B16.34 and ASME VIII part 1.

For the material, only the disk and nozzle are specified with SS + TCC, other parts (body, bonnet and cap) are specified with CS and the corrosion allowance is considered only for the body since it will be occasionally in contact with fluid when the PSV open. This material specification seems to be appropriate otherwise the cost impact will be significant.

Tanks and Regards.

 

[DSB123]

BFZHA,
You say the fluid never comes into contact with the body of the PSV. I do not beleive this. PSV's do leak from time to time and the fluid would then come into contact with the PSV body.
Are you willing to defend yourself in a court of law if the PSV failed and someone was seriously injured? It is up to you to ensure safety of the installation. As KoachSCR says as a minimum you need to ensure that there are regular inspections/thickness measurements taken of the installation.

 

[BFZHA]

DSB123
I totally agree with you, especially since I am reading these days Trevor Kletz's book entitled "learning from accident" and I have learned how a little negligence can lead to human drama.

The use of the CS and a corrosion allowance of 6 mm for the body takes precisely into account the operating conditions (fluid, pressure and temperature) as stated by material selection diagram. Despite this, I remain convinced that a periodic inspection of the thicknesses remains necessary for the safety of people and facilities.

Thanks and Regards.