What Is the Best Way to Measure Boiler Blow Down Water?

I would go with an automatic blowdown control system.....no

http://www.nrel.gov/docs/fy04osti/33474.pdf

Silly you.

Flow meter; $50.00-$500.00USD No control over much, little or no savings or even losses.

Blowdown control system; $4,000.00-$7,000.00USD. Savings in water useage will pay for system in 5 or less years.

He'll go on the cheap.

Measure what variable?

Wouldn't it be better to measure how much water it takes to make up the level in the boiler as it was before the blowdown?? Having used a blowdown valve in a locomotive boiler and seen many times how it comes out I doubt that it would be possible to measure how much comes out.

It is not just water it is mostly steam with a dribble from the valve outlet i presume it would be possible to collect the steam in a vessel and then measure it after it changes to water

Peugeot man

What do you want to measure? % solids, chemical composition of the blow down water, quantity, quality, flow rate or something else?

A small heat exchanger tee'd off the blow down piping, with it's own discharge, will cool the effluent enough that the steam will condense before it reaches the container if all you want is a sample of the water. The same can be done with a larger heat exchanger if you want to do something else with the steam/water and only water to come out of the discharge outlet.

Good Luck, Old Salt

Why is it necessary to measure the flow?

You can determine cycles of concentration in the boiler brine relative to the feed-water, and have a reasonable handle on losses from your boiler. If you want to know more, you need the boiler feed-water rate, rate of make-up to the boiler (not the same as feed-water rate, or will not be unless this is once through system with no condensate return), also needed is the steam flow rate (corrected back to liquid).

If you insist on metering the boiler blow-down, you have to have a heat recovery system on this to ensure the entire portion flowing to blow-down flow meter is liquid at a known temperature.

Without cooling the sample, the blow-down has to be back-pressured so that it is indeed liquid, in which case the flow probe is subjected to some very high temperatures. No cheap flow meter will handle that, and there are reliability issues.

If you know the other flow rates in the system, and one or two key chemistry parameters, then the calculation is more or less straightforward.

OK, so it's all gone quiet on the Original Poster's keyboard. Never mind.

The most useful parameter for measuring blowdown is the boiler water conductivity, and hence a conductivity instrument that can meet the pressure and temperature parameters of the boiler is the best measurement device. Somewhere in the Operating Manual for the boiler will be a manufacturer's recommendation for the maximum Total Dissolved Solids [TDS] that the boiler can tolerate, together with a correlation to conductivity at the operating temperatures. If the conductivity indicates a TDS that is lower than that figure, then there is no need to blow down, and no point as all such an operation does is waste water and energy. Which costs.

the problem to get a conductivity sensor that can tolerate high temperature and pressure of the blowdown, temperature near 265C and pressure 55 bar ...also is there any indirect way to now nearly how much blow down I am taking from the drum?? like from the steam and BFWflow?? also to control the bottom blow down automatically ,can we make it by DCS control like we take the conductivity value as a PV and set point as we need then open the Blowdown control valve accordingly??

Posts #8 and #9 both missed the fact that there things called sample coolers, pressure regulating valves, and flow controlling valves called VREL valves that precisely control the condition of the sample to lower pressure and temperature well within the temperature compensation range of nearly all OEM conductivity probe such as SWAN, Hach, Signet, and a host of others. As with most sample panels, most instruments, skimping on price can be lead to issues down the road. The cadillac sample panels all temperature cut-off switches that block-in the flow of sample if the high high temperature limit (above high alarm is reached) as a protection to the online instrument.

It also depends on the boiler system, and one at 265 C and 55 bar (corresponds to ~800 psig steam) will usually have a much greater purity boiler feedwater and makeup water than one operating at 16-17 bar (250 psig). In such higher pressures, one typically will have a much higher percentage of condensate return to the boiler circuit, some feedwater heaters, etc., all of which help with operation at much lower TDS. The operator of the 55 bar boiler needs to know the TDS limit, any silica limit (if steam is for use in prime mover such as steam turbines), what the treatment program requires, and all the other information that is normally provided by the OEM, or the contractor who installed the system.

What about the automatic control of the bottom blow down??? any advice?? I need to install such a system in my boiler???

That will depend on the design class of the boiler? "A" frame? "D" frame? No mud drum??

If you are referring to blowing the mud drum during operation, this should not be done automatically, IMHO. The reasoning is that valve controllers may fail to operate when commanded by the PLC, or that they can fail open is something is wrong in the control logic. On the other hand, the manual valves should be present if indeed it is intended to have a "bottom" blow-down periodically by the OEM. There will be an inside blocking valve, and an outside valve. Operators should open the inside valve 100% once they determine that the outside blocking valve (the control valve) is 0% open, and seated. Then operators will open the outside blocking valve (flow directed to flash tank most often), and close it immediately to that the entire interval open is usually not more than 30 seconds. Longer periods of time can and will upset the water circulation through the steam generating tubes, and can result in tube overheat and failure.