Steam Generator Instrumentation 2

[moltenmetal]

We have a small non-fired (electric) high pressure (2000 psig operating) continuous process steam generator in our scope on a particular project. The vessel will be ASME VIII-1 and the steam piping will be to B31.3. Both are carbon steels or low alloys.

The steam generator feed will be boiler feed quality and deoxygenated water. We're not sure what, if any, corrosion inhibitor chemicals will be added. The steam is directly used in their process, so inhibitors will be chosen with this in mind.

I've done a bit of searching and know there are stipulations in ASME I and in B31.1 which prohibit the use of stainless steels in these services, presumably over fears of SCC. I'm wondering just how much these rules are extended in practice to instrumentation which serves as part of the pressure boundary, i.e. control valves, level instruments, instrument impulse lines etc. The challenge is that at this scale, many of the preferred instruments are not readily available in carbon steel.

Is the prohibition absolute in practice, and applied to all pressure-retaining components including instruments and valves? Or can stainless steel components be used with periodic inspection and replacement? This plant will not operate continuously and there will be plenty of downtime to permit this, and plenty of process-related reasons to do the PM already.

 

[davefitz]

I know of HRSG economizers that have been installed as section VIII devices ,made of (low carbon < 4% C) stainless steel. The lack of a steam/water interface and the use of an air cooled condenser are 2 factors that may give more confidence to a user of SS in a boiler economizer.

However, if the cycle uses a water cooled condenser, or if the component has a steam-water interface, you would be accepting a large risk of SCC by using a non-stabilized SS. Higher temp components ( such as the furnace waterwall) might not be able to use the low carbon SS, and the stabilized alloys may be too expensive. It would only take one condenser tube leak and you're stuck with a lot of recyclable metal.

 

[moltenmetal]

Thanks davefitz. The stainless instruments would only come into contact with the liquid phase in an upset condition or during start-up when the unit is being heated up, i.e. not at full pressure.

The steam generator is non-fired (electric) and quite small compared to what you're musing about in your post. The components in question are some flow control instruments (orifice flowmeters and control valves) which are downstream of an electric superheater which the electric steam generator supplies- steam there will always be above saturation temperature and liquid contact is very unlikely. The other component of concern is a GWR level transmitter device which will be inserted into a carbon steel bridle. The transmitter is flanged at an elevation well above the max operating liquid level, but the flange on the unit is 316/L.

Everything else on the unit will be carbon steel. At this scale, carbon steel instruments are hard to come by with the features we need for the service.

 

[EdStainless]

Prohibit SS? That is what the tubing in superheaters is made of.

All of the sample and instrumentation lines in a power plant are 304/315 SS today. It is easy to get very high pressure ratings in 1/4" tubing even with "L" grades.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[moltenmetal]

Thanks EdStainless. It appears that the prohibition in the boiler/piping codes is for portions of the system in contact with the liquid contents of the boiler only, due to SCC concerns as I suspected. A quick search of B31.1 didn't turn up anything except a warning that stainless steels are susceptible to SCC. I should be safe with 316/L for the components I'm concerned about, which are either not in contact with the boiler water or are after the superheater and hence will never see water.

Below the waterline we'll stay away from austenitics completely. Even a trace of chloride in the feedwater is going to accumulate, and we can't count on the client to blow the thing down on a frequent enough basis to be safe.

I've never been in a power plant, so I'm a bit surprised to hear that 316/L much less 304/L tubing is being used as widely for instrumentation lines in these plants as you say. Choosing the right thing here is a bit of a tricky situation: carbon steel gives you virtually no corrosion allowance in tubing simply because the wall thicknesses are so low, whereas with the stainless steels you have the SCC risk. Even though the hoop stresses are low (because the tubing wall thicknesses necessary to ensure a seal with OD compression fittings are already rated for very high pressures), there is plenty of residual stress in these assemblies to worry about. Even a 1/4" tube line broken off at the ferrules due to SCC could still do some serious harm with 2000 psig steam.

 

[GenB]

moltenmetals, you mention that it will be a Sect VII-1 boiler, still has to be built per Sect I/B31.1 in full compliance. some instruments and controls can be low alloy but some may not per ASME I. Be aware that some Sect VIII-1 Mfrs will build it and keep quite for the Sect I requirements.
good luck with your project.
GeneralBlr

 

[moltenmetal]

GenB: we'll be buliding the unit ourselves. It will be built to section VIII-1 with piping to B31.3, and the client/owner are on board with that decision. That said, we don't just want to throw out the learning embodied in section I entirely.

 

[GenB]

Forgive me ... When it comes to Code requirements the customer has no choice. Sect I requirements shall be met . And BEP shall be B.31.1

 

[moltenmetal]

GenB: I've reviewed both section I and section VIII in relation to the scope, and am confident that this unit can be designed and fabricated in accordance with Section VIII as an unfired boiler, with piping in accordance with B31.3. There are special provisions in Section VIII (UG-125 and a few others) that must be followed for an unfired boiler, but we're aware of these sections and we are following them.

 

[steris]

MoltenMetals - If I understand your situation correctly the SS parts will only be water wetted under transient conditions. PG-5.5 specifically states that "austenitic alloy steel is permitted for boiler pressure parts that are steam touched in normal operation." Additionally PG-12.1 allows for SS to be used on level indicators. I hope this helps!

 

[GenB]

I would like to see an explanation/Interpretation on this as
you are building an Electric boiler where Sect I PEB has Jurisdiction and it is not an "Unfired" PV

 

[steris]

GenB - Building an electric boiler to Section VIII is explicitly allowed in Section I PEB-3

 

[GenB]

of course, please read on...
The boiler pressure vessel you can build as as you stated on PEB
not the BEP nor the controls it is strictly Sect I...
EB's are power Boilers

 

[metengr]

For BPV I

Prohibit SS? That is what the tubing in superheaters is made of.

These are steam touched service by design, not water wetted conditions.

All of the sample and instrumentation lines in a power plant are 304/315 SS today. It is easy to get very high pressure ratings in 1/4" tubing even with "L" grades.

Yes, and so PG-9.1.2 allows use of austenitic stainless steels for in connector piping and the pressure chamber for fluid level controls.

Sample and instrument lines are considered as misc piping under PG-58.3.7 of Section I. Depending on the point of double isolation, the line falls under ASME B31.1, not Section I.

The main issue for Section I code committee members is that for water touched service, austenitic stainless steels can have susceptibility to stress corrosion cracking in service. At BPV I meetings, this issue has been discussed on occasion and I can tell you this will not be revised. Conservatism regarding use of materials is most important.

If water chemistry is held to strict requirements and material is in the solution annealed condition, most likely you will not have SCC in water-wetted service. However, one cannot guarantee it when it is in a Code book.

 

[steris]

GenB - You are correct about that. I misunderstood your original statement. Boiler trim, external piping and controls must be per Section I but the vessel itself may be to Section VIII. Luckily Section I makes exceptions to allow SS for parts that are steam wetted under normal operating conditions.