Closed circuit - Effect of a pump on the suction and discharge pressures

[Florian R]

Hello everyone, first question here!

I searched through the forum and I didn't find a similar question.

We are doing the engineering of the (closed) hot water circuit of a boiler. To give you a bit more details, my water circuit is expected to have a static pressure of about 4 bar, which corresponds to the pressure inside the hot water tanks to which the boiler will be connected. The maximum operating pressure of my boiler is 6 bar, and we would place the pump at the inlet of the boiler. We calculated the pressure losses inside my water circuit, to and back from the hot water tanks, and we estimate them to about 2 bar (including a safety margin) at nominal flow rate (45 m3/h).

Here comes my question: will my discharge pressure be equal to the static pressure (4 bars) + the delta_P of my pump (2 bars) ? In this case I might then experience some issues with my pressure relief valve which is set at 6 bars.

Or will the pressure decrease at suction? Or will the two happen at the same time?

I tried to illustrate this under:

Another question if I can, for which I am more confident in my answer: when my pump is not running, I have to take both the static and the hydrostatic pressure into account when calculating the pressure inside my boiler. Right? Which means that as I have 4 bar inside my hot water tanks, I cannot put my boiler more than 20 m (2 bars) lower than the tanks? This will also be a source of problem in my installation.

Best regards,
Florian

 

[LittleInch]

Well you need to give us the full design and description of the system and location, with some sort of lengths of pipe of

the pump, the boiler, the tanks and most importantly the connection point of the device that regulates the pressure.

Without knowing that it could be any of the head graphs you show.

However you also need to think about how this system starts stops. Even if you middle one is valid when flowing, right at the start the initial pressure will equal the first graph. I think the only way you'll get close to the middle graph is to place your pump right at the inlet into the tanks and assuming your flow and return lines are more or less equal head loss the right hand graph would occur by placing the pump on the outlet of the boiler.

Your last point is correct, You can't place the boiler more than 20m below the tanks if indeed the tanks are the point where the pressure is maintained. In that case you would also need to place the pump on the discharge of the boiler so that the pressure could never exceed your 6 bar.


A Relief valve SET at 6 bar has a lower margin on it normally of 10%, so in fact your realistic max operating pressure is closer to 5.4 bar....

You seem to working far too close to the limits for my liking and really need to consider whether you need 4 bar or not at your tanks.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.