The boiler at my plant was installed without a low pressure option.  I am in the process of justifying it as a Capital Expenditure so I can keep the boiler firing on a regular basis and keep things warm overnight as well as over the weekend.  At present, I cannot do that and I am having condensation and oxidation issues on the fire side of my boiler.  (See my posting)

     A concern I have is, is a surface blow controller just as accurate at 100 psi, which is our production operating pressure, as it is at 10-15 psi, which will be our overnight/weekend pressure?  I have one in service now, but am concerned when we do go to low pressure over the weekend and overnight, if it is going to be accurate. May need to shut it off, but this could cause upsets in my boiler water chemistry.  Anyone with any opinions, ideas or input, please feel free to respond.

     Are there any other concerns or issues I should be aware of when adding a low pressure option to a high pressure system?  Thank you!

Jeff Gardner - Plant Engineer - Healthcare Waste Solutions

Jeff,

The only possible problem with accuracy regarding your blowdown controller might be the size of the orifice in your blowdown line. 

The orifice in your automatic blowdown line is sized to provide the right amount of flow @ 100 psig.  If you only have 15 psig pushing water through the same orifice there may not be enough flow.  This could cause the orifice to foul or you may just have problems getting enough blowdown to keep your TDS in line.

You will probably need to install an extra orifice in parallel with your current one.  Set them up so one orifice is for high-pressure and the other is for low-pressure and you can isolate the one that isn't in use. 

The electronics will still work just fine.  The valve will open when conductivity is too high and shut when it's too low.  The logic of your controller won't care what the pressure is.

You could calculate the size of the new orifice you'll need, but I would have to dig to come up with the formulas for you.  Try checking the docs that came with the controller.  If there is no chart showing the right size, just try one about twice as big as your current one.  If your TDS is fine then stick with that size.  If your conductivity creeps up over the weekend then increase the orifice size.

There are generally 3 different things to look at when you are converting your boiler to a HP/LP switchable system:

1.  Get the boiler controls set up properly to meet state code (see my post here).

2.  Is your piping big enough?  In your case it is because you aren't using the steam for anything in the evenings or weekends.

3.  Anything unusual.  This would include the automatic blowdown issue.  I can't think of anything else for your situation.  Most of the big problems are usually out in the facility somewhere, which doesn't apply to you.

Hope this helps.  Post again if you have any more questions.

Reed

Jeff,

      You may just be able to add a “T” inline after the blowdown controller. I'm assuming that the controller activates a solenoid valve. Another reason for the orifice would be to not have H.P. steam going to drain/ blowdown pot.

The orifice is needed for a number of reasons, but the main reason is to maintain a reasonable flow rate.  Flow needs to go through the controller for sampling purposes and without the orifice the flow would be excessive.  The orifice also maintains a more desireable flow rate while the controller is opening the valve to bring TDS down.  It's better to bring TDS down slowly rather than doing it with a wide open pipe.

I think it is important to set this up so there is a “low-pressure” orifice in parallel with the high-pressure orifice so the high-pressure orifice can be isolated when not needed.  I think you may run into fouling problems with the high-pressure orifice if you leave it on line while the pressure is too low.  The flow will be very low which could obviously allow sediment to build up, etc.

Reed