Main steam pipes without boiler stop valves

[athomas236]

Gentlemen,

I am involved in the evaluation of a bid by a Chinese supplier for a boiler supplying steam to a 600MW steam turbine. The main steam conditions for the boiler are 240bar and 570C and the steam pipe will be designed to B31.1.

The bidder is proposing not to instal a boiler stop valve (BSV), so I asked if the turbine inlet valves could withstand the boiler hydrostatic test pressure.

The response was that the turbine valves will not withstand the boiler test pressure or even the steam pipe test pressure.

For boiler hydrostatic testing the steam pipe will be cut and a blanking plate installed, After testing the plate will be removed and the pipe rewelded.

The steam pipe will not be hydrostatically tested but will be subjected to 100% radiography.

With 20minutes to respond on this matter, I rejected the proposal and said that either:

1. a BSV (with bi-directional sealing) should be installed anf the turbine valves should be able to withstand the pipe test pressure or,

2. a BSV need not be installed but the turbine valves should be able to withstand the boiler test pressure.

My concern is that this problem has not gone away and not doubt there will be a further reply quoting Chinese practice. If anyone has any advice or comments I would be pleased to hear them.

Best Regards,

athomas236

 

[dougarthur42]

It sound to me that you made the right call. A Stop valve should be installed for safety reasons. If there is more than one boiler on this line then definately so. Also if the turbine trips than steam or water may go the wrong way causing damage to the boiler or turbine. anyway good call on short notice.

 

[rmw]

I am not a gentleman, but I'll weigh in anyway.

I would think it obvious that a boiler for a 600MW seam turbine is not on a header with other boilers. So, isolation from other steam sources to do maintenance isn't an issue.

So that said, what is a BSV for in the first place. Hydro-ing the boiler. How many times are you going to hydro this boiler in its life? Probably more than once.

So it would seem to me that the question would be how many times you anticipated welding the plug in the line to do a hydro and whether the present value of a BSV was more or less than the future cost of cutting, plugging, unplugging rewelding and x-raying the pipe.

How much maintenance will this valve require in its lifetime whether or not it is ever used again?

It is an interesting conundrum. Thanks for throwing out the idea.

rmw

 

[eyec]

another consideration is the cost/chance of damage to the turbine in a trip situation. without the stop valve all kinds of problem events could reek havoc downstream.

then again chinese valves are not known for their reliability, so maybe you sub out the installation of the BSV to someone else.

 

[metengr]

athomas236;
You have made the correct call. In fact, we have a smaller unit that is the only odd ball in our fleet where we have no boiler stop valve from original design. However, the MS piping and turbine stop valves were designed to accommodate pressure testing of the boiler. Why did I say pressure testing? Because hydrostatic testing and pressure testing are basically the same test method (water is used as the pressurized fluid) except hydrostatic testing is either 1.3 or 1.5X MAWP by Code.

You will pressure test this boiler at some point. Now imagine conducting a pressure test after weld repairs of the boiler and subjecting the heavy wall MS pipe downstream of the boiler outlet to cold water. This results in thermal shock and possible thermal fatigue damage.

For the odd ball unit I mentioned above, we perform either air over water or nitrogen over water to avoid filling the entire boiler for a pressure test after repairs. We keep pressure testing water out of the main steam line.

 

[davefitz]

athomas,
I think you made the right call, but I am surprised the turbine stop valve is not suitable for hydrotest. In terms of allowable stress for the valve pressure parts, it seems likely the increase in hydrotest allowable stress at 100 C (compared to the design allowable stress at 570 C)should allow the 1.5 overpressure, so perhaps the limiting valve component is the stem or bonnet mounting bolts or the unusual piping loads during hydrotest.

 

[athomas236]

Gentlemen (and rmw),

Thank you for all your advice. I should find out tomorrow what was agreed during the meetings; project manager still in the air.

Best regards,

athomas236

 

[DIRTYDAVE]

Back in the 1960's it was the passing fad not to use a boiler stop valve on unit systems.
However, I think the many resulting problems changed peoples' minds.

 

[eliebl]

I would suspect that welding in a test plate other than initial test would be out of the question.

If these are P91 materials I would not want to think about it. My preference would be to have the BSV.

 

[athomas236]

eliebi,
The pipes will be P91, thanks for the reminder.

DIRTYDAVE,
Do you have any information about the resulting problems.

Best Regards,
athomas236

 

[rmw]

You are welcome Athomas236. (And, BTW, that is the way I am used to being addressed when with others).

Obviously my post was to try to highlight the horrors of the need to have to plug the line every time you did boiler repairs but my mind didn't even go to the problems of P-91 piping. Good catch, eliebi.

I have had to do the line plugging thing with HP FWH's that lacked adequate isolation (when repairs were made while the unit was on line) but obviously that is a once, hopefully not even twice in a lifetime type of thing with a heater. And, while it was heavy pipe, it wasn't P-91.

rmw

 

[DIRTYDAVE]

Problems:
Plugging the line as others have mentioned & this was long before P-91 came along. A whole lot easier to just close a valve.
Metengr mentioned a thermal concern. It was a long wait for the steam line to cool down so you could do the hydro.
Also you had to install a temporary valve for blowing steam lines.
Oh Oh. I just remembered a job where a contractor welded a steel plate over the end of the steam line & it blew of during the hydro.
Probibly a few more stories i have forgot since 40 years ago.
Add things up & buying a valve would seem a whole lot cheaper.

 

[davefitz]

Don't forget: the P91 ductility at low temps is not great; ensure the hydrotest metal temp is above ( 80 F) espescially the final main steam pipeline components, during the hydrotest.

Also, if the chinese steam turbine stop valve you are wleding the P91 pipe to is made of a lower alloy ( and consequently much thicker) you should be using a F91 transition piece with a very slight transition slope( 18 deg max at OD, 0 deg at matched ID)to the thicker lower alloy valve.

 

[LSThill]

did P92 replace P91

L S THILL

 

[Mr168]

LS: No. P92 is an ASME Code Case material, whose primary difference from P91 material is the addition of Tungsten.

 

[stanweld]

All of the conventional coal fired units that we have constructed in the past 30 years have had only the Turbine Control Valve as the stop valve. If the (Chinese?) CV cannot support hydrotest pressure, one must wonder if you should reject the bid outright. The main steam control valves, for the most part, are manufactured from 1%-1.25% Cr, 1% Mo, .25% V alloy steel castings (same as turbine casing) having high temp strength nearly half way between that of P22 and P91 alloy steels at 1000F to 1060 F.

 

[athomas236]

Gentlemen,

The prospective EPC contractor has changed his mind about his boiler sub-supplier to another Chinese company whose technical description does not mention boiler stop valves.

So its all back to square one.

Best regards,

athomas236