Steam Flow calculations 2

[TJT]

When measuring steam flow from a Steam Drum the question has been raised as to the accuracy of the reading when the temperature / density changes. If the pressure of the steam drum drops, the density changes, so what, if any impact does this have on the orifice plate calculation? ie) for a calculated dp of 80" WC is there a noteable error? And if so, can it be factored out.
Thanks

 

[TBP]

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.

 

[TD2K]

Density changes do affect the accuracy of an orifice meter, whether they are significant depends on the size of the change and the accuracy you need.

Roughly, the change in accuracy in an orifice meter is proportional to 1/(rho)^0.5. So, if the actual density is 95% of the 'design' value, the indicated flow rate through the meter in lb/hr will be higher than it actually is by about 2.5%. Over and above that, the actual meter calculations will be affected but for most cases, that's a more minor effect.

 

[benga]

Changes in pressure and also temperature affect the accuracy of an orifice meter if it is has not been correctly compensated for it. But what is the right formula and what accuracy do does the meter give when it is compensated?
To get answer on this question you could try to use the program FlowCalc32 CE. It can calculate a flow formula with compensation for you. You can also get charts of the output of the compensated flow formula versus flow calculated through ISO 5167 at different levels of temperature and pressure. The program can be found at

http://www.controlengineering.se/flowcalc/flowcalceng.htm