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Feedwater Heater Drain Sizing

Feedwater Heater Drain Sizing

Feedwater Heater Drain Sizing

(OP)
Hello,

I am hoping to get some help on how to properly size feedwater heater drain lines downstream of the control valves.

The feedwater heater drain (condensate) will flash across the control valve, so the line size downstream of the control valve should be larger than the inlet line. I was advised that typically line size downstream of the control valve is 1 or 2 sizes larger than inlet.... however, my hydraulic calculation shows unless I increase the line size dramatically, the flow will exceed critical flow or erosional velocity limits.

I read thread378-144043: Feedwater Heater Drain to Condenser, someone had posted a similar question. However, the responses to that question did not talk about "critical flow" or "erosional velocity" of the two-phase flow. Shouldn't these be considered when sizing the FWH drain lines?

I'd appreciate any help on this subject. Please advise. Thank you.

RE: Feedwater Heater Drain Sizing

You seem to be concerned with Flow Accelerated Corrosion (FAC) in one specific type of system

FWH drain pipe line sizing would be a product of the control valve design and selection process. This is commonly done by a computer program for the control valve that evaluates choked flow

It would not be unusual to find a reducer (increaser ?) to be located immediately downstream of the drain control valve to make the pipe size larger.

You also should be aware that the condensate pH and piping material selection also play a role here. I assume that pH is out of your control... but a lower pH causes more erosion in these systems

For many decades, it has been a powerplant design practice to upgrade piping materials downstream of drain control valves to a more erosion resistant material - such as a low chrome molly material( Say from A53/A106B to A335-P22)

A much more complete general discussion of FAC (by O Jonas)can be found here:

https://www.ccj-online.com/wp-content/uploads/2011...

MJCronin
Sr. Process Engineer

RE: Feedwater Heater Drain Sizing

As you and MJCronin have pointed out, there are several factors that need to be taken into account if you want to size this pipe from first principles. If you are sure you have taken all the factors into account then you should be able to get to a good design. However, the usual way to size a pipe like this would be to rely on the accumulated experience of power plant engineers and published by vendors such as Spirax Sarco and others.

There will of course be situations where the published rules of thumb do not apply so check through the underlying assumptions for the published tables to see if they are applicable to your situation. If not, then you will have to go to a first principles design.

Katmar Software - AioFlo Pipe Hydraulics
http://katmarsoftware.com

"An undefined problem has an infinite number of solutions"

RE: Feedwater Heater Drain Sizing

I suggest that you consult ASME TDP-1 2013, Prevention of Water Damage to Steam Turbines Used for Electric Power Generation: Fossil-Fueled Plants. This reference is a definitive guide for design and operation of drain systems to prevent turbine water induction.

Please note two drains are required from each closed feedwater heater: a "normal" drain and an "alternate" drain. Sizing criteria for these two types of drains are different. The "normal" drain usually cascades to a lower pressure heater. For a "normal" drain, two-phase flow conditions may exist downstream of the drain control valve since the pressure difference between the heater shells is relatively small.

The "alternate" drain is routed directly to the condenser. For an "alternate" drain, drain flow will flash to steam downstream of the drain control valve. For an "alternate" drain, you also need to consider the higher drain flow when you operate with the next lowest pressure heater out of service.

In both types of drains you should locate the drain control valve as close to the receiving vessel (heater shell or condenser)as practical. Besides using A335-P22 material downstream of the control valve, as suggested by MJCronin, you can select a pipe wall thickness one schedule heavier than dictated by pressure and temperature.

Best of luck!

RE: Feedwater Heater Drain Sizing

What was your final decision about this system ?

MJCronin
Sr. Process Engineer

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