turbine by pass valve: thermal shocks 1

[fvincent]

Some of my clients report problems in 120 bar, 538oC by-pass valves, mainly due to thermal shocks.

Some say that such shocks are intrinsic for high temperature differences between inlet at 538oC and outlet at 18 bar, 485oC (isoenthalpic expansion).


My main concerns are:
- valve does not open when necessary
- maximum opening velocity one can achieve (for certain systems, specially co-generation plants where back pressure steam is the main product, a turbine trip should not evolve to failure in steam dispatch for process
- normal leakage of the valve when closed
- thermal shocks both in the valve itself as in the piping downstream

By the way, I refer to a 500 t/h steam pressure control valve operating as by-pass of a 65 co-generation power plant

Can you tell me more about problems one can face with this kind of equipment?

Thanks

fvincent

 

[davefitz]

This is a well developed technology in europe, and it is being increasingly used in US combined cycle plants. Best primer on the subject is by Sulzer Thermotec valve co. and older papers (1975-1985) by Heinz Termuehlen.

Some valves being sold today as bypass valves are not suitable for the fast thermal shocks imposed on the body and cage, and the result is the plug siezes in the cage or in the body because the plug increases in temp faster than the body. The guideline used in Europe to prevent such seizure ( and limit other valve thermal stresses) is to keep the valve hot, not more than 50C colder than the main steam temp ( ie, the final temp it would reach if it opened). It is also a wise idea to locate the stem in a vertical position. This warming philosophy implies the outlet piping must be low alloy piping.

The max speed needed for the bypass valve actuator depends on the limitations of other equipment. If you are trying to avoid a trip due to drum water level transients on a very large coal fired rankine cycle plant ( ie, 500 MWe or larger) then Sulzer indicates it is neccesary to have a minimum size at least 80% MCR and open speed to 100% open in 0.2 seconds ( fail open, electro hydraulic). However, smaller combined cycle plants with generous drum sizes can avoid a trip due to drum level upsets with a stroke period of about 3 seconds.

Another consideration is the time it takes to overheat the reheater tubes; if they are in a very high temperature radiant heat zone they must recieve cooling steam flow within about 3 seconds to avoid overheat above the austenizing temperature, but in a combined cycle plant which trips the duct burner upon steam turbine trip then they can survive a little longer before overheat.

 

[fvincent]

davefitz,

Thanks for your reply.

My concerns regarding valve open speed are the drum level stability, indeed, and the steam supply for the process, as well. In fact I was considering a maximum of 5s for 0-100% travel.

As to the type of valve/attemperator do you think that integrated reducing/desuperheating valves are more stable than separated reducing valve and desuperheating station?
(for a 500 t/h 120bar/18bar valve)

As to keeping the valve body hot, do you think that the own leakage of the valve can play this role alone?

Regards


fvincent

 

[NGiLuzzu]

FVincent,
just for information, what kind of valves the problems are reported on?

May obturator and/or body bore geometry changes be considered? What are the constraints (e.g.: diameters, face-to-face, etc.)? What about materials' selection?

Thanks and regards, 'NGL

 

[davefitz]

For stability , you need to ensure teh spray nozzle are the wide turndown spring loaded type nozzle. Also, you need to ensure teh measuring thermocouple is located at least 0.3 sec downstream of the spray nozzle to get good temp indication.

To avoid water hammer at low flows, there should be an override to prevent spraying more than a ratio of spray water to inlet steam flow greater than 0.3 : 1.

A nice method of keeping the valve hot without requireing an increase in leakage is to add a 2" line, routed from teh bypass valev inlet to the steam turbine stop valve inlet ( ie, in parrellel with the main steam line). The pressure drop in the main steam line ensures some warmup steam flows thru the 2" warming line.