High Pressure Steam Pipework in Power Plant 2

[Smith55]

Hi all,

Looking for some tips on best practices in HP steam pipework design in thermal power plant (design pressure up to 200 bar).

I appreciate that there are a number of variables here, but my first question is, at heat exchanger / steam drum nozzles and at valves, would it be the usual practice in power plant design to direct weld these joints, where flanges are not required for regular maintenance?

Second question, where direct welded joints are used at heat exchangers and at valves, would it be the normal practice to re-hydrotest these items of equipment as part of the assembled system at site, or would these joints typically be considered as "golden joints".

Any guidance or references to literature for further reading would be appreciated. Thanks.

 

[davefitz]

In the USA nearly all states require the use of the ASME section I design code for power boilers. Virtually all joints up to and including the first root valve are welded joints, flanged or threaded joints might occur downstream of the first root valve, particularly on steam drum instrumentation. It is possible to have a flanged joint, espescially between different code jurisdictions, such as between a section I and section VIII component. I am familiar with a case where the dissimilar metal weld was failing , and was replaced with a "compact flange" , but that flange was roped off from operating personell just in case. ( Alstom once thru HRSG at the outlet of the once thru HP coolers).

The HP main steam transfer pipe in modern power boilers or steam generators may be sized to limit the piping frictional pressure drop to about 1% of the upstream pressure.

Certain valves cannot be repaired without either a flanged connection ( Gate valve) or cutting it out of the piping system, which extends the outage time as the new weld will need X-ray . Possibly to address that issue, ASME section I requires use of a valve that can be repaired in line as the root valve in some cases, such as a drain valve.

"...when logic, and proportion, have fallen, sloppy dead..." Grace Slick

 

[MJCronin]

Smith55 ....

Q1: "Would it be the usual practice in power plant design to direct weld these joints, where flanges are not required for regular maintenance?"

A1: Yes, it is standard engineering practice for HP steam systems in power plants to MINIMIZE the number of EXPENSIVE flanged piping joints. Capital investment and future maintenance costs are less with BW joints

REFERENCE: Steam piping systems chapter as contained in the PIPING HANDBOOK

Q2: "Where direct welded joints are used at heat exchangers and at valves, would it be the normal practice to re-hydrotest these items of equipment as part of the assembled system at site, or would these joints typically be considered as "golden joints"

A2: Your choice ... YOU are the Power plant Piping Engineer ... The advantage of an integrated EPC contractor can be found here ... where the EPC can integrate the piping hydro with the equipment hydrotest, there can be savings ... BE CAREFUL ... The Code required hydrotest pressure for ASME VIII specified equipment CAN BE LESS than that selected for power plant piping

Additionally, the concept of "GOLDEN JOINTS" does not exist for powerplant piping ... It is NOT part of ASME B31.1, which should be your Code of Record

All of the above are my opinions only ...



MJCronin
Sr. Process Engineer

 

[mk3223]

Depending on the Client requirement, the flanged valve manifold may be used specifically to facilitate the piping isolation and testing purpose on the HP steam piping. The golden joint may be used in lieu of the piping hydrotest but needs to follow the Client Spec and NDE inspection.

 

[Smith55]

Hi all, thanks for the feedback, very much appreciated and all makes sense.

 

[davefitz]

As MJCRONIN advised, B31.1 power piping is the standard to be used on interconnected piping , except where it defers to section I . Flanged connections on large diameter power piping is not recommended- in particular the US's preferred use of legacy B16.5 flanges imply that their extraordinary weight, thickness, and thermal stresses would be imposed on the piping, and pipeline reactions during startup and shutdown ( espescially if any section of pipe hits a bumper or obstruction) could impose excessive stresses on a B16.5 flange. I cannot recall any such use of a B16.5 flanged connection on the HP main steam piping, but my memory is not as accurate as I would prefer.

Another issue that flanged connections on high pressure high temperature steam piping implies the bolting might utilize high strength bolts that are sensitive to stress corrosion cracking, therefore if such bolts are used then the assembly lubricant cannot contain any substance that forms corrosive acid when exposed to steam.

2 examples of unexpected high pipeline stresses include:
a) a 900 MWe supercritical steam generator HP main steam line creeped 12" beyond its design limit and therafter always hit bumpers, the excessive creep was due to forgetting to remove the temporary support pins after a steam turbine outage.

b)a 418 MWe combined cycle plant in Texas experienced extreme differntial settlement of the clay soil between the HRSG and the steam turbine building, such that the transfer pipes between the HRSG and steam turbine would hit bumpers.

"...when logic, and proportion, have fallen, sloppy dead..." Grace Slick