Superheater relief capacity PG-68.2

[4Pipes]

I apologise in advance for not spotting earlier posts on superheater safety valve (PSV) capacity.

If a SHO PSV had a capacity of 100% MCR, then the steam temperature through the PSV and SH tube temperatures would be the same as for design.
PG-68.2 is interpreted in some places as meaning that the SHO PSV capacity can be typically 15%-25%. Reducing the steam flow to 15% during pressure relief will increase the steam temperature and SH tube temperature. I cannot find any mention of this.

The simple option would seen to go back to first principles and size the capacity of the SHO PSV on either 100% MCR (simple) or determine the actual steam flow rate necessary to keep the tubes and header within their design pressure corresponding to actual temperature experienced at the lower relieving flow. A SH correction for the higher temperature would then be applied to the SHO PSV. I would welcome any comments?

Thanks

 

[davefitz]

I do not know of all asme considerations used to end up with the 15% min requirement, but as applied to a power boiler, the superheat relief valve should not open until after the normal max operating pressure is exceeded- and on most US power boilers, 1 boiler feeding 1 turbine, there is a separate requirement ( B31.1) to automatically runback boiler firing rate so as to lower the main steam pressure to below design pressure.

This means that in most cases prior to 1990 ( ie prior to increase in use in multiple HRSGs feeding one STG) the amount of time the boiler spent in overpressure was normally only a few moments.

Operating at long periods with large steam relief flows to atmosphere is poor operating practice, and also wastes a lot of demin makeup water.

For the case of multiple boilers feeding one STG, it is not explicitly required to have the automatic runback in heat input based on b31.1 code, but its a good idea to implement it anyway.

Some parts of the section I code need to be updated to accomodate the increased use of multiple boilers feeding one STG and the increased use of turbine bypass systems, which were uncommon in the US prior to 1990.

 

[4Pipes]

Thanks Dave,
Operating PSVs for long periods is not intended. How long is long? 30 seconds, 1 minute, 5 minutes? In most practical cases it is not for very long. However for design purposes you would need to know. The codes do not specify a time limit. A safe period of safety valve operation is not listed. (or is it implied). Reducing the steam flow through the superheater to 15% might impose a time limit. How long can a particular superheater operate without reaching an unacceptable temperature. Is 15% high enought to maintain a steady and acceptable metal temperature? What is the design pressure at the new equilibrium temperature?
Andrew

 

[davefitz]

You would need to calculate the time delay prior to initiating a MFT , based on the tube wall thickness and imposed heat flux.

As I recall, for thin wall ( 0.15" thk)reheater tubing located only in the convection pass ( 2000 F gas temp), a "fast valve " event ( ie fast closure/oepning of the IP turbine intercept valve) , flow must be reestablished within (4 sec ) to avoid an MFT- the mft is needed to avoid overheat of T22 tubing above the first critical temp. For thicker walled superheater tubes( but located in a higher heat flux zone) the time delay would need to be calculated on a case by case basis . But for a drum type boiler, the HP bypass would need to open within 0.2 sec ( and be sized for at least 85% MCR steam flow) in order to avoid lifting drum safety valves- refer to Sulzer bypass valves literature.

So of all intents and purposes, unless you have an HP bypass valve, a major interruption of steam flow will require either a fast runback to ignitors only , or an MFT.

 

[4Pipes]

Exactly! Are we implying that the nominal superheater relief capacity of 15-25% might need a little more detailed consideration than is currently interpreted by some code users.

 

[davefitz]

I do not know who the code users are that you are referring to. IN the US , nearly all large central station fossil fired plants ( 1 boiler feeding 1 steam turbine) either automatically runback firing rate if the pressure slowly increases to the lifting of the superheater releif valves, or they execute a MFT following a steam turbine trip which also lifts all boiler rleif valves.

The asme code does not explicitly require the operator to runback firing rate if the steam and metal temperatures exceed design temperature, but the 1986 failure of the hot reheater piping at Mohave station ( 7 dead) following 1 year of operating at 100 F over design reheater steam temperature stands as a warning to prudent opertors not to continue operating above desing steam or metal temperature.