Swedish Cast Iron Steam Vessel from 1966 1

[Even Hestnes]

Hi,
We have a cast iron steam vessel built by KMV from 1966. the vessel was certified to a maximum working pressure of 3.5 bar and was tested to 7 bar. The vessel need to operate frequently at 3.5 bar due to process requirements. my predecessor was instructed by the company setting up the PRV here in scotland that the PRV had to be set at MWP, so the prv is set at 3.5 bar. This obviously causes an conundrum since the pressure should not be close to the PRV for a prolonged period.
So, my questions to the forum are.

1. I seem to recall that we operated with max working pressure being max operating pressure back in the days (I am Norwegian, and our standards/practises are very much the same as the Swedish). and then the PRV was allowed to be set 10% higher than this.
2. does anyone have the swedish steam standard from 1966? I think maybe the 1959 edition was valid in 1966.
3. I want to argue to the insurance company that we can classify the Vessel to have max operating pressure set at 3.5 bar and max working pressure set at 3.5 x 1.1 = 3.85 bar. Has anyone else done similar stunts on these older vessels. And if so, how did you achieve uprating?
4. the fact that the hydrostatic test pressure was 2x MWP kind of give me an argument as well. unless the old standard stipulated such an high safety factor.

To help you understand. this is a steam vessel, 5.5 metre diameter and over 5 metre long, weighing nearly 100 Tonnes and rotating at 55 rpm that is used in the production of paper. So, its a bit of a monster and I don't want to let it loose. However, history tells me that the vessel has been operating at 3.5 bar since new. but the problem has arisen that the PRV now has to be set at 3.5 bar and not any higher.

Thank you

Even

 

[Ericapi]

Hestnes,
I suggest you research that the Scottish jurisdiction regulations concerning steam vessels.

There should be clear guidelines from the jurisdiction re: the settings of the SRV. The regulations for safety relief valves are typically different that pressure relief valves upon pressure vessels. Pay particular attention to the distinction and the differences.

Take care.

Mr. Eric
API 510 & 653

 

[The Obturator]

You need to address certain questions;-

1. What is the design code of the vessel?
2. What is the design pressure of the vessel.
3. How was the vessel protected from overpressure in the past? What was used ? Details ?
4. Clarify your comment that the vessel is rotating at 55 RPM - this sounds bizarre.

Normally, a Pressure-relief Valve is set at the Maximum Allowable Working Pressure (MAWP).

In process vessels the normal operating to PRV set pressure differential is 10 &.

As Ericapi pointed out above, steam vessels can have different requirements, hence you need to determine what applicable code this vessel is.

You also need to speak with the local authority on whether this being an established/old vessel, if earlier rules apply or if UKCE (was PED)regulation applies.

As a long guess, depending on the above, you may need to consider a pilot operated pressure relief valve built for steam to work closer to 3.5 barg, though this would allow some 5% operating to set pressure differential depending on manufacturer/model. These are often used in paper mills.

There are also spring operated safety valves built to ASME I boiler code which work with high op/set differentials.

You will not find a PRV with a true operating to set pressure differential of 2 - 3 % unless you opt for an elaborate (and expensive) Controlled Safety Pressure Relief System CSPRS ala ISO-4126-5.


*** Per ISO-4126, the generic term
'Safety Valve' is used regardless of application or design ***

*** 'Pressure-relief Valve' is the equivalent ASME/API term ***

 

[Even Hestnes]

Thanks for answers.

Yes. PED and pssr is very clear. PSV must be no higher than mwp. However. The definition has somewhat changed over the years on what is designed max working pressure (calculated) and what is operational max working pressure (tested). So that is what I am really trying to find out.
Also. Since the vessel was proof tested to 2xMWP I am hoping I can tweak it since requirements are usually 1.41 or 1.5. mind. I do not have the Swedish code from 1966 so without this I don't have an argument, it may well be that a hydrostatic test to 2xMWP was a requirement back in the days to account for manufacturing uncertainty.

And yes. The vessel rotates at 55 rpm. It is the main MG roll on the paper machine that takes the paper pulp around and dries it into paper towel that is then scraped off continuosly. It is a true spectacle.

 

[The Obturator]

Some additional comments to consider.

Whatever set pressure is decided, it would be very wise, no, recommended, that a sizing calculation is done based on the capacity of the vessel and set pressure. You need to be sure that the PRV is of adequate size for the emergency flowing capacity, or for that matter, not oversized, which will cause other unwanted issues. The PRV of course cannot be undersized. Higher pressure = smaller size. Lower pressure + larger size.

The vessel was most probably hydrostatically tested at x 2 in the past. Hydro testing is completely different to steam testing. You need professional advice on getting this vessel certified and pressure rated for current day use. Generally vessels like these are downrated over time due to internal corrosion and weakness etc.

Regarding the vessel being in rotation, how was/is the existing safety valve installed (MG Roll?)- you have not advised yet how the vessel was/is protected. A photo or 2 of the vessel and valve would be appreciated for better understanding.


*** Per ISO-4126, the generic term
'Safety Valve' is used regardless of application or design ***

*** 'Pressure-relief Valve' is the equivalent ASME/API term ***