I am an operations engineer with the Hawaiian Electric Company.  We have been plagued by continuous probles with our boiler feed pump recirculation valves.  I was wondering if anyone could give information as to what type of valves you use at your plants.

Things to consider are:
Unlike a mainland utility our grid is in isolation.  This requires even our base load units to cycle.  What I am saying is that even our units that stay online 24 hours a day, come down to a low power at night due to low system load.  
This meens
1-our BFP recirc valves cycle almost every day.
2-Unlike most, more modern units our BFP recirc discharges straight to the condesor, NOT to a deairator.
3-Our feedpump discharge pressure when the valve opens is around 3000psig and the condensor is around 30" of vacuum.   
4-OUr feedwater temp at this point is between 250-280F

So, we have this extremely large d/p and flashing occuring at the valve.  OUr valves last lest then a few months.  

We tried duabla(spelling?), fisher, mogas and other valves.  Currintly we have had mild success with valtek and are considering cci drag valves.  Depending on which unit the valves are either 2 or 3".

Any comments as to what works best for you guys would be very much appreciated.

It sounds as if your BFW pumps remain online even through the low load night hours.  I could see where the feeedwater recirculation valves last only a few months.  You could ask the valve manufacturers, but I think a short term flow of the recirculation valves (minutes not hours) to protect the pumps would be a more realistic service for the valves.
If your BFW pumps could be set up for staged operation, then the recirculation valves would not be subjected to as many hours of operation.  For example, normal operation of 2 or 3 BFW pumps would be reduced to one pump for the low load night hours.  This would save energy as well as the recirculation valve wear.  The idled pumps could be kept warm with a small side flow from the discharge header back to idled pump(s) through a pressure reduction orifice assembly.  The orifice assembly would have a number of designed plate orifices to minimize flashing and noise while the 3000 psi is letdown in steps to the BFW supply pressure.  
This type of orifice assembly could be designed by the BFW pump vendor, and should be designed for flow in either direction for differing selection of operating / standby pumps.
The recirculation valve discharges might be fitted with similar pressure reduction orifice assemblies to reduce cavitation in the valves.  The orifices would be larger for the flow, and there would not be any energy savings of any idled pumps.  The potential energy savings might even justify variable speed drives on the pumps to match pump output to load demands.

There are several modes of failure for a BFW leak off valve. They can fail due to seat leakage casuing micro cavitation whilst in the shut position. The solution here a valve that has drop tight seat leakage such as some of the metal seated ball valves (VTI) with a downstream inline diffuser (Mitech) to dissipate the pressure. However, in your case it sounds as if the damage occurs during operation. I assume that you are operating these valves in a modulating mode rather than on-off. A disk stack valve would be the solution if it were not for the flashing that is taking place. The best solution to that is to put a restriction orifice in the line as close as possible to the condensor. To ensure that the flashing always takes place across the orifice the flow rate must be constant - thus the valve should be operated in on-off mode bringing the solution back to the metal seated ball valve plus inline diffuser.

There is some merit to what msessions says. But I don't agree with all of his analysis.  I've never been happy with on-off control of a BFP Bypass.  My experience is that a BFP requires a LARGE percentage of bypass (~40%), so controlling the bypass on-off is really shocking to the system. Imagine that your load eases back to 40%, then BAM ! the recirc valve slams open and the pump is at 80%.  It's just as bad when the load creeps up and you get to 45% load and the BFP goes from 85% to 45% essentially instantly.

You're paying for a lot of BTUs that you are flushing into the ocean, so discharging the BFP into a higher point of the system and reclaiming that energy should be a future goal.  

But working with what you have, A lot of wear is caused by high velocity downstream of the valve. WHen the BFW flashes, it expands and accelerates a LOT.    Putting the valve on a large nozzle directly on the side of the hotwell will allow expansion of the 2-phase discharge stream without high velocity.  Just be sure the large nozzle is connected to a generously-sized sparger inside the hotwell to avoid erosion of the tube bundle, sloshing of the hotwell level, and other undesirable effects.

Work with your valve vendor and make sure that there is not cavitation in the intermediate stages of the trim.  Flashing is not nearly as damaging as cavitation, and cavitation happens when the flow flashes and then the pressure recovers to back above the saturation pressure, so it is likely to occur in the stages of your trim just previous to the discharge of the trim stack.  You mentioned you had Valtek.  They have designs to tailor the drop per stage to avoid or minimize this effect.  The idea is to take all the pressure drop (Enegy dissipation) you can in the high-pressure stages where cavitation is less likely, then throttle the flow more gently as it approaches saturation pressure.  

THe valve body and all the piping downstream of the valve should be Chrome-moly to resist erosion from flashing.  Carbon steel is more susceptible to flashing damage.  

If your boiler feedwater is exceptionally clean you might consider the CCI valve.  The inlet orifices to the trim stack are tiny, and the CCI valve has been likened to the most expensive strainer you will ever see.  If it strains out a weld-bead it will stop between the plug and the trim stack: you could be in for a REALLY expensive repair bill.  Interstage cavitation can happen in that design, too, and that tends to straighten out a lot of the corners in the maze the flow is supposed to follow.

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