Venting the boiler to the condensation loop

[JonRiley56]

Hi,

Is there a reason why there our boiler systems are set up to only dump through the safety valve if the pressure rises too high and we dont provide for a way to manually dump the excess steam to the exhaust side of the engine and out through the condensation loop ?

It would seem like it would make more sense to conserve the steam/water.

thoughts ?

[fredrosse]

A "steam dump" system is frequently used for that very purpose, especially where the heat source is not quickly controlled to match the engine's steam consumption. In the power industry, the steam dump systems (also called steam bypass systems) are almost always applied to nuclear reactors (turning their heat output down quickly is difficult) and in combined cycles, where the heat source is the hot exhaust gas from an internal combustion (IC) engine (typically a gas turbine) which is routed thru a boiler (Heat Recovery Steam Generator), and the IC engine will continue to run at full load when the steam engine (typically a steam turbine) is out of service.

For our launches, one must consider the complications of installing a steam dump system that recovers condensate, compared to the water inventory savings that can result. Most of us can anticipate a reduction of steam use, and cut down the fire as required. Even if the fire is too much, most of us can dump steam to atmosphere thru various paths before the safety valve opens. This provides no recovery of the water, but this is a relatively minor event.

[JonRiley56]

Thanks fred,

I didnt know if the amount of expansion that would happen in dumping high pressure steam to the condensation loop without expending any of the energy in the engine would cause too much turbulence in the hotwell etc.

I am going to be running primarily on salt and with the large grate area and square footage of my boiler I am afraid of over firing. I guess I am paranoid about running low on water and overheating the boiler. If I do end up going this direction at some point do I need to have a check valve in the dump line coming from the engine so that the regular exhaust doesnt back up that route and condense ?

jon

[preaton]

In the situation you describe a better option may be to install an injector. A slug of water to the boiler is a common trick to prevent blowing off in a lock or similar.

Cheers

Paul

[fredrosse]

As stated above, pumping some excess of cool feedwater into the boiler is another method of keeping the safety valve from popping, and no water is lost by doing this. An injector or displacement pump will accomplish this.

As far as overfiring the boiler, I think you will easily learn to regulate your fire and generally avoid any serious loss of water inventory due to overfiring. You will lift the safety once in a while, and learn how to manage things.

If you do install a dump line from the boiler to the condenser, I would think a check would not be required. Boiler pressure should always be greater than condenser pressure except perhaps in a cold shutdown condition. If the dump line is shutoff, and some engine exhaust goes into the dump line and condenses, this is not a problem.

[Maltelec]

Don't forget that a safety valve relies on the pressure differential between its input and output. Should you feed the safety output into the condenser for instance you will create back pressure which would alter the safety valve's function.

A steam dump system would compare the pressure to a known point (i.e. atmospheric) regardless of the output pressure. This creates quite a bit of complexity.

Another solution would be to have two safety valves, one feeding into a condenser at a slightly lower pressure, pushing water back into the hotwell, and one at working pressure to blow off as normal. That way you should satisfy any safety requirements while loosing as little water as possible. I wouldn't recommend feeding the dump valve into the engine's condenser as the vacuum would cause it to dump the steam too early.

[fredrosse]

Safety valve rules dictate that the discharge of a boiler's safety valve be directed to atmosphere, never to a potentially closed vessel of any kind. The piping run to atmosphere should be as short and direct as practical. These rules should definately not be broken.

Dumping steam to atmosphere, or to a condenser, is allowed, provided measures are taken to assure that the dump steam cannot overpressurize the vessel to which the dump steam is directed. If the condenser does not have an assured vent area (with no valve) at least the size required to keep the condenser vessel below its design pressure, then relief valves must be added to the condenser to protect it from potential overpressure. Depending on the design pressure of the condenser vessel, this relief valve may have to be considerably larger than the dump valve size.

A small manual valve, dumping to atmosphere, removes all of these complications, and is easily capable of letting off steam to keep boiler pressure from lifting the safety valve.