Cooling water system s
26.1 Introduction Cooling water system s usually involve low head pumping plants, and often large flow rates, and aim to minimise the power costs for overall plant efficiency . There often are severe complication s due to the condenser s being located at the highes t point and subjec t there to high temperatures , returning the flow in a loop circuit. There are commo n features in the basic system layout, as shown for example in the Proceedings of the International Instituteon HydraulicTransientsand Cavitation,Sao Paulo, Brazil, July 1982 (in Spanis h and English),coordinate d by Koelle and Chaudhry (1982). The demand for good simulation s of the transient problems has led to a group participation by lAHR for the La Casella circulating water syste m (1980), resulting in a variety of solution s and indicating the difficulties in analysis of this type of system . A case study was also directed to this type of syste m by Safwat et al (1986). It is a matter of debate how well sophisticate d modelling techniques , allowing for dissolve d gases , thermal effects , the probable discharg e level *seen' by the syste m as the pressure s fall in the condense r box and the simulation of the dynamic behaviou r of any anti-vacuum n devices can predict events and ultimate pressures . The analysis of a number of simplified system s is undertaken here merely to test the suitability of discrete cavity modelling for the water column separatio n that invariably occurs .
26.2 Case 1 The example of the cooling water syste m for a nuclear power plant, Safwat et al (1986), is shown diagrammaticall y in Figure 26.1, and the case of a pump trip is examined .
142 Waterhammer:practicalsolutions Condenser 21.3 -^^18.5
Fig. 26.1 Case 1 - Pump Trip - CW plant
The pipeline was 0.591 m in diameter and with aflowrate of 0.385 m^ s"\ the velocity was 1.4 m s~^ and the occurrenc e of separatio n at thefirstmajor 'knee' (5), elevation 20.5 m, is not unexpected . The magnitude of the water hammer was about 97 m compare d with the field measure d result of about 140 m. The result of the analysis is shown in Figure 26.2.
I » i I I I I I I I I I I I » I I I I I t I I I I I I I
11 13 15 17 19 21 23 25 27 29 31 33 m x 15.6
Fig. 26.2 Simulation of pressur e heads - Case 1 120n
-20 H -40 -60-J Fig. 26J Time plot of pressur e heads
1 — I — I — I — I — I — n
Seconds x 30
Coolingwatersystems143 The report by Safwat et al was interestin g in that it was stated that their simulation (more nearly equal to the field test), required a very careful adjustmen t of the values of machine inertia and flowrate to compare with somewha t inaccurate field test measurements . To obtain a result similar to Safwat et al in fact required a slight delay in the closing of the non-return valve at the pump (see Chapter 6 which discusse s this problem) as well as consideratio n of inertia. Figure 26.3 shows the time plot for this case .
263 Case 2 A somewha t more complex syste m is depicted in Figure 26.4, where the condition s downstrea m of the condense r are indicated. The report of the field study of this plant is reported in a number of places , for example, Koelle and Chaudhry (1982), and a detailed study of the effect of bubbly and separate d flow is included with severa l models . 12.8
100 m •!
Fig. 26.4 Case 2 - the Barreiro plant
Again using a simple discrete cavity model the result of the analysis is
1^^>J;69 Second s x 6.7 Fig. 26.5 Case 2 - pressur e heads after power failure
144 Waterhammer:practicalsolutions shown in Figure 26.5 compare d with the field test result. The simulatio n produces somewha t greater pressur e heads , a value of about 33 m maximum compare d with the actual of about 37 m. The analytical solution greatly simplified the syste m which has a significan t complicatio n due to the condition s downstrea m of the condenser , but nevertheles s found values that would be sufficien t for estimate s of the water hammer consequences .
26.4 Case 3 The syste m referred to initially as the La Casella plant is shown diagrammatically in Figure 26.6. The flow rate in this case is much higher at 8.72 m^ s~' and with pipes of the order of 2 m diameter, the velocity is of the order of 2.8 m s"^ This case, like most cooling water systems , is comple x becaus e of the action of control valves , such as the butterfly valve shown in Figure 26.6 and the air inlet valve. The timing of these in the simulatio n has a marked effect on the water hammer produced . The very simple cavity model used here, although allowing for both these valve actions , clearly shows a worse result than actually observe d in tests as seen in Figure 26.7. 61.2 d = 2m
Air inlet valve 56.7
Delivery circuit 400 m
Return circuit 234 m
Fig. 26.6 The La Casella plant
n — I — I — I — I — I — I — I — I — I — I — I — I — I — I — r
169 253 337 421 505 589 673 Seconds x 22
Fig. 26.7 Comparison of experiment and analysis (pump)