Abnormalities in Testing

Even if the test is conducted in accordance with code requirements, some errors can still occur. A test was carried out with a four-impeller gas booster which gave an unexpected result. As an experiment, the test was repeated with different test gases.

All of the tests were with code conditions except for C02 where ihe Mach number variance was more than plus 5%. The results are shown in Figure 10-4, It was concluded that the R12 test provided the more nearly

30 40 SO 60

FLOW M3/HR

Figure 10-4. Test results from different gases. [9] (Reprinted by permission of The institution of Mechanical Engineers)

correct correlation. Table 10-5 shows a comparison of testing with different gases. It should be noted that R-12 is no longer available as a test gas due to environmental regulations. It is still included in the comparison to show the effects of correlation with different gas characteristics.

The preceding experience did lead to a lack of confidence, and it was concluded that an impeller-by-impeller performance check should be carried out theoretically at test speed with the test gas. This idea was carried out on a cold methane compressor with nine impellers. The results of this were fruitful as can be seen from Figure 10-5, and the expected performance at various test speeds calculated in accordance with the code is shown in Figure 10-6. As a result of this work, two test speeds were

Table 10-5

A Comparison of Testing with Different Gases

Item

Design

1

2

3

Gas

Feed Gas

R12

R22

co2

Mol wt

32.36

120

86.48

44

Rprri

11,000

5350

7010

10987

inlet density

11.8

17.9

4.58

3.59

Outlet density

59.4

47.85

9.42

10.67

Machine Mach no.

0.816

0.792

0.86

0.92

Reynolds no.

3.207

1.5114

1.95

2.71

Note: Reynolds number is x I CT6

Source: [9] Reprinted by permission of the Institution of Mechanical Engineers,

Note: Reynolds number is x I CT6

Source: [9] Reprinted by permission of the Institution of Mechanical Engineers,

Figure 10-5. Effect of deviation difference R22 test gas and cold methane [9]. (Reprinted by permission of The institution of Mechanical Engineers)

adopted and an agreed correction factor applied on test head and efficiency for flow test point.

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