Power Boiler Continuous Blowdown Control

I am doing a project to automate our continuous blowdowns on our 115,000 lb/hr power boilers. Presently they have a variable orifice
Hancock valve that the operator manually adjusts. I am wondering peoples opinions, on if I should put in a variable orifice automated valve (normal control valve) with PID control, or an open close valve (ball valve) with a restricting orifice and time proportional control.
The control valve would be sized to give me 2% of feedwater flow at 50%, and the flow restrictor for the ball valve would be sized to give me 4% of feedwater flow.
The ball valve with actuator is a cheaper solution, but is it better? The piping is 3/4".
Thanks.
Curtis
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Curtis wrote:

You are aware of the cost difference. The big question is lifetime and operating reliability. The process certainly does not require that the blow down be a continuous process. Before you can really compare the two methods, you have to think about how few on off cycles per hour are really needed. The lower this number can be, the better the reliability for the ball valve. You may well be able to substitute a two threshold operation for a time proportional control.
--
John Popelish

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The control valve in a throttling condition would more prone to plugging with particulate, especially if this is a 600# class boiler with a hot process zeolite softeners or similar equipment.
Are your multiple blowdowns all piped to this point? Do you have multiple valves? 4% blowdown isn't much of a problem if you feedwater pump is already pressure controlled by a recirculation loop.
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If you are considering a ball valve with restriction plates downstream take a look at Valvetechnologies http://www.valv.com/v/pr/xac/index.htm
--

tHe PuNx

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We use a normal control valve and a PID controller witch get it΄s setpoint from a ratiostation connected to the feedwaterflow. Works fine for us.

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Yeah, I thought about that, and then trimming the ratio station with a Conductivity controller, but I think that might be "over-instrumentating" the control for a little 115,000 pound boiler.
By the way, what type of valve do you use?
Thanks
Curtis
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We use a Neles segment ball valve, DN50 (2") PN100. The ratio between feedwaterflow and blowdown are 4% the lab takes a sample once a day just to make sure everythings ok.

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