If there's no pressure compensation on a steam flow meter, the accuracy really suffers. If the steam pressure drops below what the meter is set for, the meter reads high, if it rises, the meter reads low. Most industrial boilers fire to maintain a set header pressure, rather than drum pressure. It's very common for the header pressure to be 125 PSIG, while the boiler safety valves are set for 150 PSIG. At low steam loads, the pressures are almost equal. As the firing rate increases, the header pressure remains constant, so the boiler pressure has to increase to deliver the increased flow. At high fire, the boiler pressure could easily be something like 145 PSIG. If the steam meter was calibrated for 125, the meter is now reading low.
This really causes problems on boiler trips where the steam flow signal is sent to the feedwater control valve. The steam pressure starts to fall as soon as the burner trips. This means the uncompensated steam meter starts to read high. As the pressure continues to fall, the steam flow is actually dropping off, but the meter (with no pressure compensation) erroneously reads it as going higher and higher. This bogus reading gets sent to the level control valve, which is driving open, because it's getting a signal that the steam load is increasing. The valve is doing what it's supposed to, but the drum level is heading off the chart and out of the glass.
