What is the optimal pressure for a closed district heating system?

[paparaco]

Hello,

I need help with defining pressure in closed district heating system. As shown in attached drawing, we have a heating station which supplies hot water (110/70 degrees C) to various households.
The lowest house is 90m below station and the highest one is 20m above. I need to determine final pressure in system and release pressure of safety valve.
I assume final pressure in system would be 11bar (90m+20m) and release pressure of safety valve 12bar (10% above system pressure)?

 

[theodoros.mihos]

If this is real work we must calculate and shock when valves close. Usually used air-caves to make shock smoother. Speed of water is critical for these calculations.

 

[paparaco]

I don't understand air-caves. It is closed system without any air inside.

 

[theodoros.mihos]

air bottle valves that used for shocks absorption.

 

[paparaco]

OK, but are my assumptions for pressure occurring in system correct?

 

[theodoros.mihos]

For unmoved liquid yes they are. But if this is a real application it is dangerous to use this.

 

[paparaco]

It is real application and the pump is running right now. Flow is 71 m3/h, pressure in supply pipe is 8.5bar and in return pipe 4bar.

 

[theodoros.mihos]

Details are not clear but you can use hydrostatic pressure difference plus pressure ref. You must include pump pressure as n%*power = pump pressure * Volume/sec where n is about 60% for electrical motors.
When pump starts, liquid accelerate smoothly. When pump stops, there is a shock F=dp/dt to all pipe system. dt for water is about 1-2 seconds.
For this reason may you use valves to allow one way flow. The shock pressure adds to total pressure may be larger than total hydrostatic pressure. If pipes are on one line, one way valves take the load. On curves needed mechanical stability to take the load.

 

[256bits]

It is real application and the pump is running right now. Flow is 71 m3/h, pressure in supply pipe is 8.5bar and in return pipe 4bar.

This is from guages at the pump??
One would presume that is where you are referencing the pressures. Due to the difference in elevations the pressure is not uniform across the whole network.

If so, with the pump running, with no losses,
Bottom house 9 meters below is receiving another 9 bar at his sevice to the house, 17.5 bar.
Top house is receiving 2 bar less.

Where is the pressure safety valve supposed to be located?

Supply = 8.5 and return =4 bar, there are losses of 4 bar somewhere in the system. Pump is adding 4.5 bar of pressure to the water to compensate.

I assume final pressure in system would be 11bar (90m+20m) and release pressure of safety valve 12bar (10% above system pressure)?
Here you are referencing the pressure at the bottom house with no flow.

When stating pressures, always state the location.

 

[theodoros.mihos]

I think that pressure safety valve can be only (8.5 - 2) + 10% on the top. But if you put it on the bottom may be (4+9) + 100% except you using one way valves. For this reason bottom safety valves is not good idea.

 

[paparaco]

This is from guages at the pump??

Yes at given moment. See below pic of how it is done.

Top house is receiving 2 bar less.

So 6.5 bar

Supply = 8.5 and return =4 bar, there are losses of 4 bar somewhere in the system.

Of course, there is a couple km of pipe network.

Here you are referencing the pressure at the bottom house with no flow.

When stating pressures, always state the location.

What would be the pressure before the pump and after the pump? It has to be less than safety valve release pressure and high enough to circulate water through whole network.

 

[paparaco]

I think that pressure safety valve can be only (8.5 - 2) + 10% on the top. But if you put it on the bottom may be (4+9) + 100% except you using one way valves. For this reason bottom safety valves is not good idea.

Safety valve is inside heating station right behind heat exchanger as shown on pic.