Piping design in Rankine Cycle

[agiraias]

Hi everybody. I´m project engineer from Spain. I´m workig in a rankine cycle project and we are in the state of the project related to the pipes design. We have some doubts:
If we reduce the mass flow, what happens with the pressure? (I think it drops, but I don´t know why). our intention is to mantain the pressure constant in the cycle, but sometimes regulation of mass flow is needed. So, if the mass flow is reduced, how can we mantain the pressure?
The last question: Why does the pressure fall when the mass flow is reduced?

 

[BigInch]

Hola agiraias, Vivo en Malaga.

First of all you have to realize that you are trying to discuss two concepts at the same time, "Pressure" and "Pressure Drop".

Pressure Drop = If you try to move a lot of mass around in small diameter pipes you must overcome lots of friction, therefore pressure drop will be high. If you increase the pipe diameter, the same or even more mass will flow with less friction, hence less pressure will be lost between the inlet of that pipe to the outlet of that pipe and the [bold]pressure drop [/bold](P in - P out) will be lower.

Pressure, or more likely, maximum pressure, is dependent both on how much mass is charged into the system and how much pressure energy it takes to circulate it.

Now, any given closed system will tend to have a point where the pressure is maximum and another point where the presure is minimum. The difference between those maximum and minimum pressures is the overall pressure drop. This is the overall pressure drop that is dependent on how much mass you want to circulate within the system.

Starting with the lowest possible pressure you can have (why begin with something higher?) you add the overall pressure drop you need to circulate the mass through the system and you get the highest pressure that is needed within the system to do it.

Llamame quando quieres. Mis numeros estan aqui en mis paginas bajo "contacts".

http://virtualpipeline.spaces.msn.com

 

[agiraias]

First of all thanks for your reply bigInch.
Ok, but I think you didn´t understand what I´m looking for, that is; I´m designing a Rankine Cycle. sometimes I have less thermal power available, so we have to pump less mass flow to reach the same thermodynamic state. The problem is that we have to mantain the same pressure in the turbine inlet, so my question is; is true that if we reduce the mass flow, pressure is decreasaed? If this happens, how could we mantain the pressure? Do you know any solution for that?
Thanks Again. I have taken a look to your website. It`s really interesting and I have added to my favourites.

 

[BigInch]

Sure, the minimum pressure constraint is turbine inlet pressure, Pt.

To give an example, let's say that minimum Turbine inlet pressure is Pt = 20 Barg and at low mass flow rates the circulation requires an overall pressure drop of 40 Barg. Assume that it will be split to equal 30 barg drop between pump discharge and turbine inlet and 10 barg drop from turbine outlet to pump inlet.

Now let's say that at high mass flow rates the circulation requires an overall pressure drop of 80 Barg. Assume that that it will be split to equal 60 barg drop between pump discharge and turbine inlet and 20 barg drop from turbine outlet to pump inlet.

That will allow us to set the minimum system pressure at the pump inlet, say 5 barg to maintain good suction conditions.

Considering the high mass flow case, 5 barg at pump inlet + 20 barg to turbine outlet = 25 barg at turbine, + 60 barg from turbine inlet to pump discharge = required pump disharge of 85 barg.

Considering the low mass flow case, 5 barg at pump inlet + 10 barg to turbine outlet = 15 + 30 from turbine inlet to pump discharge = 45 barg at pump discharge.

The pump design conditions, you must have a low mass flowrate capacity at 45 barg and high mass flowrate capacity at 85 barg.

Both cases have turbine inlet pressure higher than the minimum required.

One solution might be to buy a pump that can deliver high mass flowrate at 85 barg and buy a pump discharge control valve. Run with the valve (say) 80% open during the high mass flowrate case and run the low flowrate case with the valve at (say) 50% open. The control valve at 50% open should be designed such that it will increase the overall pressure drop in the system to that required for the low mass flowrate case.

http://virtualpipeline.spaces.msn.com

 

[davefitz]

Generally, the HP main steam piping is desinged for a pressure drop ( boiler to steam tubine inlet valve ) of 2.5-3.5% pressure drop at full load. So, for a 100 bar design pressure, a main steam line pressure drop of 2.5-3.5 bar might be the economic pipe size. The allowable % pressure drop for reheater piping is lower than this value.

If the plant is a constant pressure plant, then the pressure drop will decrease quadratically as the steam flow decrease, so at 50% MCR steam flow you have only 25% of the full load pressure drop.

For a variable pressure plant, the pressure drop % remains constant over the load range, so the 2.5=3.5% pressure drop at full load reamins the same 2.5-3.5% pressure drop at part load, but pro-rated to the lower operating pressure.

Of course, it is also neccesary to limit noise and vibration, so an average steam velocity of 0.3 * soundspeed should not be exceeded.