Mud Drum Heater Coil Question

[mpeck1982]

I have a water tubed boiler with a superheater that produces 400-psi steam at 650 deg F. There are currently 3 boilers at the plant. One boiler supplies the steam, another boiler is on hot standby, and another boiler is on what I call cold standby. The difference between cold and hot stand by is on hot standby the rapid mix burner is fired once per shift where the boiler on cold standby isn't fired. The hot standby boiler is more of a back up if the boiler producing steam trips offline. A lead/lag/standby operation.

All boilers have a mud drum heating coil. The boiler that is used for cold standby has the inlet boiler feedwater valve closed and the outlet superheater valve closed when in cold standby. The superheater inlet/outlet vents are closed. The mud drum heater coil was designed for 400-psi at 650 deg F. However, there is a low pressure steam line going to the mud drum heating coil at 150-psi at 230 deg F. There is a heat transfer process between the low pressure steam inside the mud drum heating coil and the water in the mud drum and water tubes and superheater tubes. On a perfect day there will be 40 to 70 psi of positive pressure in the tubes.

When the feedwater is closed off and superheater steam outlet is closed I am pulling a vacuum in the tubes with the steam condensing to water. I believe the inlet/outlet superheater vents need to be cracked open partially since there is some small amount of pressure in the boiler. What consequences do I have with those vents being closed? Will these cause possible vacuum conditions in seals or gaskets in valves, drums, etc? possibly damage seals/gaskets?

 

[dougarthur42]

Very good question. It sounds like you have a good designed plant and it sounds like it is being operating properly. It is bottled up and not fired so it doesn't need the SH outlet cracked. If you were firing a flame directly on to waterwall tubes then you would need to have a proper flow through it to disapate the heat. This increased heat would transfer to steam and to superheater, also heated flue gas approximately 1700* would heat the superheater tube. this is when you would need a proper flow through a cracked open SH vent. Because of the temperatures that you are talking about and an indirect heating of water in the muddrum in stead of direct flame heating, you should be fine. even at 40 to 70 psi the saturation temperature of the steam water mix is only about 300 *f and never hotter. I hope this helps
Doug

 

[mpeck1982]

From the heat transfer process of the low pressure steam in the mud drum coil, the resulting temperature of the water in the mud drum and water tubes is less than 212 deg F. I am below the boing point of water. Will vacuum conditions exist with the vents closed?

 

[rmw]

Essentially you are not providing enough heat in your drum heaters to enable the whole boiler to stay warm enough to prevent condensation of the steam in the steam spaces.

You can increase the heat to the mud drum coils which I doubt will work.

Is your boiler manufacturer still in business? Some PV's designed for high pressure aren't necessarily well designed for vacuum and especially deep vacuum.

If you open the vents, then you will bring air into the boiler internals which will accelerate corrosion.

Unless you can absolutely isolate the air/flue gas side of the boiler (think guillotine dampers) you won't be able to prevent the steam inside the boiler tubes, etc. from condensing and even then you might have to add more heat. Not only are you faced with the potential for internal corrosion, but if you are located in a climate where the ambient goes below the dew point often, you are sweating on the outside of all the boiler surfaces shortening their lives too.

rmw

 

[mpeck1982]

The boiler isn't designed for conditions below atmospheric boiling temperature. So an equilibrium has been reached where the boiler is at a lower pressure than the pressure required for the boiling point temperature of water. So then the boiler will be under a vacuum. It will pull the wrong direction on seals, gaskets, and valves... Therefore, if a vent is cracked open, the boiler will be open to atmosphere. The boiler has some small amount of positive pressure from mud drum heating coil so the pressure from the boiler would have a positive force out of the boiler. This vent being cracked open would minimize the boiler creating a vacuum, it would protect the water side by cirulating and maintaining the water chemistry, and on the flue gas side by keeping it dry with the heat from the heating coil.

All in all, this is what the manufacturer suggested. Having a drum vent cracked would minimize air leakage from valves, gaskets, and seals. Since there is some positive pressure in the boiler it would prevent air from coming in the vent...

What do you take on this?

 

[rmw]

For you to prevent air ingress into the steam spaces, your mud drum heaters will have to keep the water above 212F or 100C if you perfer. If it could do that we wouldn't be having this discussion about the boiler going under vacuum.

rmw

 

[Duwe6]

How about putting 30 - 100 psig of nitrogen on the boiler, using a small instrument connection? Pressure stays positive at all times, with no air/oxygen to corrode the tubes.

Aerated feedwater is INCREDIBLY corrosive. Please don't try to Lay-Up a boiler without either inhibiting the feedwater/condensate left in the tubes, or inerting the vapor space in the tubes and steam drum.