8. Study of the vacuum pulling/maintaining phase
Regulation strategy used in this process is relay control on the vacuum pump to keep RCP pressure between MIN1 and MAX1 (see Table 19). Consequently no frequency analysis is going to be performed.
Scope of simulation is to provide pressure trend in RCP during filling and the number of time pump is called in operation accounting for the constraint on time interval between pump stop and start imposed by pump supplier. Moreover RCP pressure trend has been derived without constraint on pump rest time to find the minimum interval between stopping/starting of the pump.
Main parameters peculiar to vacuum pulling/maintaining during filling are summarized in Table 19.
PARAMETER UNIT VALUE
T-H related
Gas in RCP free volume [-] AIR
RCP air temperature °C 30 and constant
State of BP valve [-] CLOSED
Vacuum pump compression ratio [-] 6
Filling starting pressure bara 0.2
Filling stopping level %MR of PZR 90
Filling rate %MR/min of PZR 1.5
MR m 11 I&C related Pressure set-point MIN1 bara 0.2 MAX1 bara 0.25 Hysteresis bara 0.02 Pump protection Rest time s 300
Instrumentation
Time constant of pressure transducer s 0.25 Time constant of the ACQ software
filter
s 0
Sampling period s 0.4
Table 19: Vacuum pulling/maintaing – Main parameters The scheme for this process is reported in Figure 62.
Figure 62: Vacuum pulling/maintaining block scheme
8.1.
System performance
Data about the performance of the system are reported in the following paragraph according to the two phases that are encountered during the filling procedure of the RCP, namely
Figure 63: Pulling – RCP pressure vs. time
8.1.2. Vacuum maintaining
Once a pressure of 0.2 bara has been reached after the pulling phase, RCP filling takes place at a filling rate of 1.5% of the MR of the PRZ Wide Range (WR) level transducers per minute.
Vacuum pump is started up when RCP pressure is higher than MAX1 and switched off when pressure is smaller than MIN1. Logic signals are provided by ACQ complex block related to the pressure transducer on the PZR top to the complex block DRV that drives the vacuum pump (see
Figure 19).
Figure 64: Maintaining –RCP pressure vs. time during filling
Figure 64 shows that with the present configuration it is not possible to maintain the RCP pressure in the range from MIN1 to MAX1 at the end of the filling of the RCP when the level is near the top of the pressurizer and the pressure increment is faster due to the reduced volume available as shown by Figure 65.
Figure 65: Maintaining – RCP Pressure & PRZ Level vs. Time
Maximum rest interval 3444 s
Minimum rest interval (300 + 2) s
Maximum pressure reached in RCP 0.284 bara Minimum pressure reached in RCP 0.2 bara
Number of times pump is started 1+11
Time required for RCP filling (90% MR) ~367 min Table 20: Vacuum maintaining simulation relevant results
A pressure peak of 0.28 bara in the RCP is registered with current limits on rest time (cfr. Table 20) and this peak is at the end of the filling phase. Two ways can be undertaken if this peak is not tolerable:
̶ Set the pump rest time to ~214 s. In fact this is the minimum rest time that assures no pressure peaks (see Figure 66). The drawback is that this value is smaller than the value required by the supplier so inspection on the vacuum pump side is required. Anyway there will be only two times in which the 300 s rest time imposed by the supplier is not fulfilled: last two times before reaching the target RCP water level (∆t1=270 s ∆t2=214 s)
Figure 66: Pressure time trend during RCP filling without limitation on the vacuum pump
Maximum rest interval 3444 s
Minimum rest interval 214 s
Maximum pressure reached in RCP 0.25 bara Minimum pressure reached in RCP 0.2 bara
Number of times pump is started 1+11
Time required for RCP filling (90% MR) ~355 min Table 21: Main datas extracted from Figure 70
8.1.3. Optimization of the vacuum maintaining phase
A first note to be done refers to the vacuum pump: in order not to trig on the timer, the pump should be always on running. To do so pressure in RCP volume must be in the range 0.2 – 0.25 bara and never should fall down 0.2 bara.
So a first thing to do is start the filling of the RCP before 0.2 bara are reached. In the simulation the value of 0.21 bara has been chosen.
Phenomena to be accounted for in this phase are gas mass inventory change and free volume change. By considering these phenomena, a variable filling rate has been found that allows keeping pump on during the entire process.
Figure 67: Variable filling rate in different units
Figure 68: Filling phase results with variable filling rate
Maximum pressure reached in RCP 0.248 bara Minimum pressure reached in RCP 0.21 bara
210 mbar reached after 88.73 min
Time required for RCP filling (90% MR) ~135 min Table 22: Main datas from Figure 72
8.1.3.a.Pros&Cons PROs
̶ Vacuum pump is started only 1 time: it is held in continuous running till the end of the process;
CONs
̶ This is a typical example of feed forward control: the set-point generator does not know anything about the actual state of the plant. This may result in trouble operation if process and/or parameter changes from that ones used to calculate the theoretical set-point trend; anyway this will not be a major issue since all pump protection are still implemented and it is possible to place a limitation on the minimum filling rate that can be reached. Obviously this limit will be the reference filling rate that is 15% MR/min;
̶ Greater complexity of the RCV I&C structure: it should be able to generate a linear time varying set-point at least;
̶ Perhaps, certainly, charging pumps can assure neither such volumetric flow rates, nor the required dynamics capability to follow this set point.
References
[3] Areva’s internal documentation
