Hot water level measurement using pressure sensor in closed tanks

[pturmel]

I've seen a pair of DP cells used to get density-corrected level in a sealed vessel. Both DP transmitters would have the low leg at the tank bottom. The main level DP would have its high leg at the top as normal. The second DP would have its high leg at about halfway up, and gives you density for the bottom half of the tank. For main level readings between ~3/4 and full, smoothly incorporate the density correction from the bottom half.

 

[drbitboy]

Hol' on there Baba Looey! Some of this does not make any sense.

the first issue is during the heating process the volume of the water starts to increase and this problem appears on the system HMI.

Google search: Eureka.

For a fixed quantity of water in a tank, thermal expansion of the water will change the level. However, if

1. the top of the tank, containing water vapor (vapour), is open to the atmosphere through the overflow pipe, and
2. the overflow pipe is not blocked,

then thermal expansion will not the pressure reading of a pressure sensor at the bottom of the tank.

The second issue is when we start to pump this heated water from the tank, the pump may still operating while the tank is empty because of the fake volume resulting from the vapour pressure.

Google search NPSH.

If the tank is empty, then it is unlikely that the overflow pipe is blocked, at least is cannot be blocked by water. So any excess vapour pressure almost certainly would have been released to the atmosphere through the overflow pipe by the time the actual level reaches the bottom of the tank. To be clear, with the system OP has described, any "fake volume" interpreted via pressure sensor readings cannot be caused by vapour pressure.

The fundamental problem with the pump seems to be inadequate NPSH. The causes of that could be

• that the water is flashing at the pump suction because the water is at 80°C, or
• there was a mistake in the design choice of the elevation of the pump relative to the bottom of the tank.

We readers of this thread do not know enough about OP's system to state any of this as more than speculation.

Caveats

I am basing these claims on the system OP has described so far, with the following assumptions:

• an open* overflow pipe that will maintain the pressure at the top of the water level at atmospheric pressure;
• a pressure sensor that measures gauge pressure, i.e. pressure relative to atmospheric pressure;
• the density of air and water vapour are negligible.

* If the overflow pipe is somehow blocked (e.g. by a closed valve or a seal loop) or otherwise restricted (e.g. it is a capillary and not really an overflow pipe), then all bets are off.