Boiler Flow Direction

Hi Buzneg,

Best to do simple observation. When you next boil the kettle of water for a coffee, you will note that only the steam comes out of the spout. Now if you were to add water to the kettle at the same rate as the steam exits while still keeping it boiling.

Now if you make the spout into a jet and put a small fan in the exiting stream, the fan will rotate and if powerful enough, could do some work. If the jet doesn't allow the steam to exit and allows a build up pressure in the kettle, you will need to pump the new water into the kettle.

But the expanding steam must push out equally in all directions, the turbine works because the expanding steam in the boiler pushes the steam through the pipe, but that same steam should push down on the water. There must be a work function, or a rocket engine type of thing happening here.

The Boiler has a water level float/probe control which switches on/off the boiler feed pump with the level changes. The feed pump is selected as per m3/gpm of the boiler capacity and is always at a higher volume and pressure than the boiler maximum steam generation capacity and operating pressure so, the water is pumped in when the water level in the boiler body drops.As a safety there is always a check valve on the feed line entering the boiler to safeguard against high pressure boiler water & steam reversing back in event the pump feeding pressure drops due to any pump failure.

Backflow bad. Backflow very, very bad.

This why we put non return valves on the outlet steam of boilers. The non return valves work as an isolation valve and a check valve to prevent the "dreaded back flow".

not to get to pedantic but; a check valve, as I recall, should not be considered an isolation valve.

The non return valve is an isolation valve with a built in back flow preventer or "check" valve built into the valve. You can have a globe valve as the other isolation valve and this will allow you to run the boiler by manual if needed.

This why we put non return valves on the outlet steam of boilers. Only if you have more than 1 boiler in operation. If 2 Boilers are in connection with the main steam header than each should have a non return/check valve on the outlet to safeguard each other.

It is not to be considered as an isolation valve. However, one can take emergency work prior to the non return if no isolation valve is installed. For example: if a steam trap has a isolation valve prior to it and a non return on the discharge and you need to replace the trap you can close the isolation valve and remove the trap for - the non return will safeguard.

The boiler inlet water pressure is higher than the steam at boiler outlet. The pressure keeps going down from Economizer inlet to drum to Super heater outlet. This cause water/ steam to flow through the boiler.

From the field:

Water can only get into the boiler from a pump that develops greater pressure then the boiler has on it.

Close to the surface of the water is the steam disengaging area. As the water temperature rises above boiling the steam expands.

If the boiler header stop valves are open the steam can exit the boiler at velocities greater that 4,000 feet per minute.

Yes the pressure is very nearly the same at the bottom of the boiler as it is at the top minus the pressure drop caused by the exciting steam.

If you close the valves the pressure will equalize.

By the way the water and steam temperatures will be very close to the same also.

Mike

The Steam Cycle in the power plants use rankines (or its modified version).In this the water is converted into steam and the heat is exchanged isothermally and isobarically subjecting it to the change of state.

It's an special case, when exchanging heat by an isothermal process is valid. But this is due to the change of state. During it P and T are constant ie in this iso-temperature line and a iso-pressure line are the same thing.

The Boiler Feed Pump brings the water to the boiler at the required pressure an temperature. At this temperature the addition of heat changes its state from liquid to gas (steam). This expansion in volume (no increase in pressure) makes it escape thorough the opening it has - towards turbine where the velocity is converted into rotation through the reaction turbine and then the steam goes back to BFP through condenser (where it is brought back to liquid state)

So that answere your question - water flows in and steam flows out in the same pressure. In the turbine stage the pressure is converted into the motion and then the resultant loss of pressure is built up by the BFP.

"So that answer your question - water flows in and steam flows out in the same pressure"

This statement is not true.

There are no extra points given for overstating a simple answer that has already been answered.

As a matter of practicality the feedwater inlet pressure is about 20% higher than the drum/water/steam pressure.

If you have a boiler up to operating pressure with no demand it will have water and steam at, for all practical purposes, the same temperature. You are correct that there will be an equilibrium.

When steam is needed a valve opens and steam rushes to try to reach equilibrium with the condition downstream, be it a turbine or a process, or whatever. There is now a slight pressure differential between the boiler water and the steam. This differential is sufficient to cause the water on the surface of the boiler to flash and turn to steam. This flashing off will continue as long as there is a demand downstream. Water is replaced by pumping it into the vessel at a higher pressure than the operating pressure of the boiler.

The steam moves to the point of lowest pressure. The point of lowest pressure will be the load device, heat exchanger engine turbine etc. As the steams energy is consumed it condenses and the condensate trap then opens and allows the condensate to flow to a collection point, condensate return tank. The tank stores condensate untill the boilers water level drops and controls start the feed pump. check valve in the pump discharge prevent the pressurization of the condensate side of the system.

The feedwater going in a boiler is always higher pressure than the steam coming out, or at least it is this way on all the boilers and feedwater systems I have ever saw. There are other natural reasons that the water wants to travel in and the steam out but the main reason is higher pressure IMO.