DESIGNING BOOLEAN CONDITION TURBINE TRIPS
by Kim A. Lovejoy, Lovejoy Controls Corporation Copyright 1996, All rights reserved. Reproduction by permission only.
Most Turbine Supervisory Instrument (TSI) systems currently in service are liabilities to reliable operation. Whereas modern digital logic redundancy has improved almost all areas of equipment control and monitoring, TSIs have lingered in the single-point failure dark ages. Although spurious trip events occur regularly with enormous lost generation costs, few instrument engineers are aware of much more reliable designs. This paper introduces the concept of replacing conventional TSI trip logic with Boolean condition turbine trips in a modern digital TSI system.
Vibration and thrust TSIs were originally analog comparitors dedicated to sensor inputs, usually provided in pairs for alarm and trip level relay drives. In the late 1970s and well into the 1980s the traditional TSI system manufacturers began to slowly "digitize" their TSIs by replacing the analog comparitors with A/D converters and microprocessors. Unfortunately, most of the so-called modernizations were really just digital substitutions, and single point trip logic was routinely maintained.
A single point trip in a TSI system exists when a single sensor output may, if its output signal varies enough, induce a turbine trip which will shut down the unit. The use of such trips is not a good idea, since many events can induce a spurious, or false trip, including:
* Loose sensor or mounting bracket.
* Kicked sensor.
* Sensor amplifier failure.
* Detector A/D failure.
* Detector filter failure.
When any of the false trip events occur the results are the same, -a forced outage of the turbine.
The Boolean Condition Approach
A much more secure design is available in using Boolean Conditions as trip criteria rather than single point comparisons. A typical Boolean Condition logic chart is shown in Figure 1, where up to five (5) independent sensor value tests are performed to determine if a Boolean Condition Trip exists. Each individual Test consists of a comparison of one of many sensors to a programmable setpoint (either Less Than or Greater than). The overall Boolean Trip Condition then exists if the Boolean expression of (TEST 1 OR TEST 2) AND TEST 3 AND TEST 4 AND TEST 5 is true, allowing secure multi-point event confirmations. The Boolean logic prevents any one sensor output (or any one test comparison hardware) from initiating a trip due to the spurious causes.
In application, however, we must go to considerably more detail than Figure 1 affords to successfully implement Boolean Conditions in modern distributed digital instrument systems. The design method to accomplish this utilizes library blocks of code which are linked to form a TSI "system".
The primary block element is the individual test shown in the Figure 2 flowchart. Each of the comparison tests for each Boolean Condition has one (1) logic input or entry point and four (4) logic outputs. Referring to Figure 2, the first test determination is whether the test itself is active or bypassed. Since not all Boolean Conditions will require the full five tests, provision must be made to bypass those not to be used. If the test is bypassed, the block is exited via the "B" (Bypass) output.
The second test determination must determine whether or not the data used in the comparison is "stale". Stale data must be detected in distributed systems to avoid continued use of old data upon an interruption of communications between the data acquisition system and the Boolean Condition test processor. A time-based interrupt is generated which re-writes data memory to a (stale) code value if not reset upon normal communications. If the test detects the stale code for the data, the block is exited via the "S" (Stale) output.
The third test determination performs the actual numerical comparison between the data value and the setpoint, according to pre-programmed greater-than or less-than logic. If the test passes, the "P" (Pass) output is taken, if failed the "F" (Fail) output is used.
The five individual tests are then assembled with external logic to form a Boolean Comparison as shown in Figure 3. Additional logic required to activate the trip includes an overall enabled condition and the repetitive confirmation of a confidence counter. Only when all the condition criteria have been met is the condition active and the trip processed.
APPLICATION of BOOLEAN TRIP CONDITIONS
Boolean Trip Conditions (BTCs) may be tailored to suit particular applications both to prevent single point spurious trips and to accomodate operating strategies. The following are examples of each.
1. A generation turbine TSI system has x-y proximity probes. The unit is important to the load grid, and requires manual operator confirmation of trips while loaded. During startup, however, automatic trip protection is desired to prevent damaging critical speed operation or seal destruction due to a bowed rotor. Solution: Apply a BTC to each bearing. Use Test 1 as the bearing y-probe greater-than the desired protection limit, bypass Test 2, use Test 3 as the bearing x-probe greater-than the desired protection limit, use Test 4 as the turbine speed less-than five RPM below the synchronous speed, and bypass Test 5. Assign a confidence counter of five (5) and enable the condition. The result will be an enabled BTC which activates only when the unit reaches load speed and requires both the x and y probe at the programmed vibration levels for a confirmed period.
2. A mechanical turbine thrust wear trip is needed which uses opposing direction probes. Solution: Apply a BTC with one probe in a wear limit comparison as Test 1, Test 2 bypassed, the other probe in a wear limit comparison as Test 3, and both Test 4 and Test 5 bypassed.
3. A journal bearing temperature trip is required which occurs at temperatures in excess of 210 F. Solution: Wire redundant thermoucouples. Apply a BTC with the temperature comparison to the first thermocouple as Test 1, Test 2 bypassed, the other thermocouple temperature comparison as Test 3 and both Test 4 and test 5 bypassed.
Many other configurations are possible, including digital logic inputs as test conditions. These are useful to enable or disable trips based on operational status of multiple units. For example, tripping one condensate booster pump for high vibration may not upset operation unless it is the only one running. Tying the operational status of the pumps together can qualify trips.
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