Turbine bypass and condenser overpressure protection

[processeng01]

Dear all,

I would like to get your comments to the following:

ASME B31.1 mandates that downstream of a pressure reducing valve shall be protected either by i)designing the low pressure side same as high pressure side ii)or by installing a pressure relief valve with sufficient capacity.

Now, considering the steam surface condenser applications, we have typically single or dual bypass lines to the condenser in CCPP's. Typically there is no PSV located on bypass piping in most of the plants, so designers apparently rely on safety valve/rupture disk of condenser.

My question is as follows:
What do the condenser standards (such as HEI) say about how to size the PSV+rupture disk if there is a bypass line to the condenser?

My take is that we have to consider the case where bypass valve is stuck open and desuperheater is not working properly. This would correspond to worst case with high energy steam dumping into condenser. Is it too conservative??

Appreciate any information. Sorry for poor English

Regards

 

[athomas236]

Hello,

This is not my field but I would expect the condenser to be fitted with bursting discs to provide over pressure protection.

If the bypass valve is open and the spray water flow stops for any reason, I would expect the bypass inlet isolating valve to close. In addition, if the bypass valve is closed but required to open and spray water is not available then I would expect that the bypass valve would not be allowed to open.

I would expect both these cases to cause the reheater valves to open.

It is not usual practice to base design on two independent faults occuring simultaneously.

 

[sailoday28]

processeng01 (Mechanical)
If a turbine bypass failed open, operators should be reducing other loads (main turbine trip, etc) to the condenser.
What is the duty of the condenser under this senario (with out the desperheater)?
With other sudden loads reduced, the other turbine bypass might open. At any rate, I would then assume the design duty of the condenser is adequate to dissapate the load.

I would be concerned with the impact of no desuperheating on the downstream side of the desuperheater. Such as high temperature impengement on condenser tubes.

 

[JohnBreen]

This may interest you:

http://www.ccivalve.com/pdf/372.pdf

 

[JohnBreen]

Also.........

http://www.copesvulcan.com/Literature/cv-1163_turbinebypass_cope.pdf

 

[processeng01]

Does anybody have HEI standard for steam surface condensers?

What does it say about sizing criteria for PSV+rupture disk on the condenser?

Any special requirements against abnormal operation of turbine bypass stations?

 

[stgrme]

For large combined cycle plants, the rupture disc or discs are installed on the low pressure turbine casing/casings rather than on the condenser. These discs are designed to rupture between 5 psig and 10 psig, and to pass the entire quantity of turbine exhaust steam.

I don't have a copy of the current revision of the HEI standard for Steam Surface Condensers. However, the Ninth Edition indicates that the relief device must have sufficient capacity to pass all of the steam admitted to the condenser except for lines already protected by relief devices set to open at pressures not exceeding 10 psig.

Best of luck!