Feed-forward control sets water flow by comparing flow difference against level controller output.
Errors caused by the inaccuracies of the flow meters and the withdrawal of perhaps 2.5% of the water as “blow-down” (which is not converted to steam) will prevent the two flow signals from being identical. Any error in the steam-water balance will cause a falling or rising of the steam drum level. Therefore, the level controller will slowly readjust the set point of the flow-difference controller to maintain a steady-state balance.
The system assumes the use of orifice-type flow sensors and does not use square-root extractors, because the oscillation period and dynamic gain of a two-capacity level process varies directly with flow. The gain of the feed-water control loop without square-root extraction seems to compensate correctly for the process gain change.
Figure 3 also shows an external feedback from the flow-difference measurement to reset the level controller. This feature will precondition the level controller during startup or at other times when feed water is controlled manually or otherwise is limited. This external reset guarantees that an increase in steam flow or blow-down flow will increase the feed-water flow immediately, without waiting for the steam drum level to change. This means that the feedback portion of the loop (LIC-108) will need only to trim the ΔFIC-109 set point to correct for flow-meter errors.
Béla Lipták
ANSWER: The question is using mixed units of measure for pressure and flow rate. Kg/cm squared should be given as kpa or psi gage. Tons per hour should be lb/per hr. Then steam tables or material and energy balances can be applied.
Morton W. Reed, PhD, PE, Columbus Water Works
ANSWER: The three-element control has a feed-forward element from the steam flow rate. If you know what causes the fast changes in demand, you may add another input, with possible “lead-lag” element to the steam flow signal from that source.
Stephen Gaertner, PE, Sales Engineer, Clipper Controls
QUESTION: How does a loop-powered, 2-wire field transmitter work? How are 24V DC and 4-20mA transmitted within the same two wires?
S. Janakiraman, Technip Geoproduction S/B, Kuala Lumpur, Malaysia
ANSWER: First, 24 VDC and 4-20 mA are not two different signals: They are the same signal. 24 VDC is provided by the power supply. The two-wire device changes the loop current by varying the voltage drop across itself. Essentially it is controlling its own power consumption. If the process variable (PV) is at 0% range, it adjusts its own current consumption to 4 mA. If the PV is at 100%, it increases its own current consumption to 20 mA. 20 mA is achieved by lowering the voltage drop across itself. If a higher load (resistance) is introduced in the loop, the voltage drop across the transmitter is reduced.
Most smart transmitters can drive a signal 3.6-21 mA, and many also meet the NAMUR standard requiring these current levels to indicate measurement beyond the normal range, as well as fault conditions. Therefore, a smart transmitter must operate on less than 3.6 mA. This is achieved by using low-power microprocessors running at low speed, sleep mode or other power management, putting a limit on how fast the device can perform computations.
Fieldbus devices, on the other hand, are allowed to draw 10 mA or more, and although they need more powerful microprocessors to handle the communications, they are also able to make faster, more sophisticated computations.
Jonas Berge, engineer, Emerson Process Management, Singapore