Optimizing drum level measurement
How can steam drum level be controlled to maximize both stability and responsiveness? The best method is to feed forward (FF) the steam flow to the level PID loop and cascade the output of that loop to the setpoint of a boiler feedwater (BFW) and flow loop. Three element (3E) steam drum level control uses level and two flow measurements. See three diagrams.
How can steam drum level be controlled to maximize both stability and responsiveness? The best method is to feed forward (FF) the steam flow to the level proportional-integral-derivative (PID) loop and cascade the output of that loop to the setpoint of a boiler feedwater (BFW) flow loop.
This arrangement is called "Three Element Drum Level Control" (3E) because the algorithm uses level, BFW flow, and steam flow.
"Single Element" (1E) control should be used when either flow is too low or has bad quality. 1E is a simple PID loop with the level as PV, the output going to the valve, and no FF. For more on this topic from Innovative Controls, read: "Advanced PID loop tuning" for more on PID loop definitions, tuning advice, and generic programming methods.
Level measurement and selection
See additional information from Innovative Controls, "Density compensation for steam drum level measurement."
Typically, drums have two level transmitters, often one at each end. Normally these should be averaged, and that average sent to the level PIDs as a process variable. However, if one has bad quality, is being blown down (see below), or is otherwise not trustworthy, the other should be used alone as process variable (PV) for the level loops. Systems with three level transmitters should use the median rather than the average value, or the good one if only one is good.
Level alarms and trips
Typically, drums also have a set of level switches for alarm purposes. The use of those switches for trip purposes varies depending on the existence, number, and type of burner management system (BMS), size, types of fuel, arrangement of blowdown valves and bypass push button (PB), etc. For the combustion control system (CCS), two-out-of-three (2oo3) voting is standard. This means if two out of the group of one set of switches and two transmitters indicate a HI-HI or LO-LO condition, the system should trip. If just one indicates a trip condition, a warning should be raised, but no trip should occur. However, BMSs typically trip on the switches alone.
1E and 3E PID blocks
It is theoretically possible to use the same PID block for both 1E and 3E level control, but you would need to use different tuning settings for the different modes. More importantly, bumpless transfer between 1E and 3E will be difficult.
A better approach is to have three PID blocks:
- LIC-1E: Single element control - level indicating controller (LIC).
- LIC-3E: Three element master - level indicating controller.
- FIC: Three element slave - flow indicating controller (FIC).
FIC gets its setpoint from LIC-3E. To achieve bumpless transfer, when in 1E mode, the 3E loops should track as follows:
- FIC's OP tracks LIC-1E's OP. (OP is output.)
- FIC's SP tracks its PV (in 1E and also if FIC is in manual). (SP is setpoint.)
- LIC-3E's OP tracks FIC's SP (in 1E and also anytime FIC is in manual or local).
- LIC-3E's SP tracks LIC-1E's SP.
And when in 3E mode, the 1E loops should track as follows:
- LIC-1E's SP tracks LIC-3E's SP.
- LIC-1E's OP tracks FIC-3E's OP.
Normally I configure the system so the 3E loops are always in auto and FIC is always in remote (cascade) so that operators can take manual control of the BFW valve in 1E only. This is optional. The OP of FIC and LIC-1E will always be the same, so either can be sent to the BFW valve.
Having multiple loops controlling the same PV via the same valve presents a challenge when designing the human machine interface (HMI) to display the loop faceplates. Approaches include having separate pick points for each of the three loops always visible, or having a drum level pick open the faceplate for LIC-1E when in 1E or LIC-3E when in 3E.
Feed forward to 3E flow slave
The standard 3E level control model includes no FF to the FIC. However, I sometimes find it useful to feed the LIC-3E's OP forward to the FIC (along with sending it to the FIC's SP) with a small FF gain. This allows me to minimize the FIC's proportional action (low gain / high % band) so as to not excessively react to noise on the flow meter, while still being very responsive to flow demand changes from LIC-3E.
Determining whether or not to use 1E or 3E
The operator should have a switch to force the system to 1E. The system should also immediately drop to 1E if:
- The steam flow quality is bad.
- The BFW flow quality is bad.
- The steam flow is below the minimum 3E flow.
- The BFW flow is below the minimum 3E flow.
If none of those conditions indicate the need to swap to 1E, a timer should count when the steam and BFW are both above the maximum 1E flow (which should be set higher than the minimum 3E flow). Switch to 3E when that timer completes (after 5-30 seconds). A typical way to configure the system is to set the minimum 3E flow to around 20% of maximum continuous rating (MCR) steam flow and the maximum 1E flow to around 30%.
2E level control
If the BFW flow is measured using an orifice plate, it is not accurate enough below 20% to control and 1E should be used. However, many modern flowmeters have a better turn-down ratio, good enough so that the cascade loop can be used all the time.
I sometimes set the minimum 3E BFW flow to a little below zero so it always stays in 3E as long as the BFW flow quality is good, and then set a low clamp on the steam flow to the FF input so that there is no FF action for very low (or zero or bad quality) steam flows. This effectively makes it 2E control (just level and BFW flow) in that low steam flow regime.
Experiment with 1E, 2E, and 3E control in low flow conditions to see which performs the best.
Transfer of level setpoint from manual and auto
Whether in 1E or 3E, you must consider what happens to the level SP on a transition of the level PID from manual to auto. Options include:
- Always leave the SP where the operator sets it. The advantage is not having to put it back, but the disadvantage is a significant bump to the BFW valve when switching from manual to auto when the level is not close to the SP.
- Have the SP track the PV when in manual, and leave it when it goes back to auto. This avoids the bump, but the operator must put the SP back to the normal level, and that may cause a bump.
- Combine the advantages of both those schemes with a ramping SP. Have a target SP which stays where the operator sets it, and a ramping SP which tracks the PV when in manual, then slowly ramps to the target after switching back to auto. This also enables operators to make large SP changes while in auto without having to repeatedly make small step changes. It also may simplify SP tracking between LIC-1E and LIC-3E since you can have a single target SP station, and always send the resulting ramping SP to both level PID loops.