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.

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.

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/Instrumentation Ball Valves 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.

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