Test Codes

The basis for code testing is the ASME Power Test Code, PTC 10 [ 1J or ASME PTC 9 [2] as applicable. Several specific points made in the code were intended as guiding principles, yet are often misunderstood. The following facts must be considered.

• The codes establish the rules for a test, including the definitions of code or non-code.

• The codes are not a textbook on testing.

• The codes have to assume that the properties of the gases involved are known. It recognizes that this is not always the case, but must place the burden of knowledge of gas on the persons performing the tests.

• The codes establish a basis on which to agree or disagree. Ultimately the final test procedure and method must be agreed on by the purchaser and vendor.

The PTC 9 establishes the deviation limits from the contract conditions that the test may use and still meet the code (see Table 10-2).

The PTC 10 defines three classes of testing. The code attempts to categorize testing and thereby establish an inherent degree of accuracy. These categories are based on methods of test and methods of analysis.

Table 10-2

Maximum Allowable Variation in Operating Conditions

Table 10-2

Maximum Allowable Variation in Operating Conditions

Deviation

Fluctuations

of test

from average

from value

during any

specified

test run

Variable

(plus or minus)

(plus or minus)

(a) Inlet pressure

(b) Pressure ratio1

(e) Discharge pressure1

(d) Inlet temperature

(e) Inlet temperature deviation for any stage

(0 Speed

(g) Cooling water inlet temperature

(h) Cooling water flow rate

(i) Metering temperature

(j) Primary element differential pressure

(k) Voltage (1) Frequency (m) Power factor (n) Belt slip

2% of abs pressure

(a) Inlet pressure

(b) Pressure ratio1

(e) Discharge pressure1

(d) Inlet temperature

(e) Inlet temperature deviation for any stage

(0 Speed

(g) Cooling water inlet temperature

(h) Cooling water flow rate

(i) Metering temperature

(j) Primary element differential pressure

(k) Voltage (1) Frequency (m) Power factor (n) Belt slip

2% of abs pressure

1 Discharge pressure shall be adjusted to maintain the pressure ratio within the limits stated. Source: [2], Reprinted by permission of the American Society of Mechanical Engineers,

Class I includes all tests made on the specified gas (whether treated as perfect or real) at the speed, inlet pressure, inlet temperature, and cooling (if applicable) conditions for which the compressor is designed and is intended to operate, that is, an air machine or a gas-loop test on the specified gas within the limit set by Table 10-3.

Classes II and III include all tests in which the specified gas and/or the specified operating conditions cannot be met. Class II and Class III basically differ only in method of analysis of data and computation of results. The Class II test may use perfect gas laws in the calculation, while Class III must use the more complex "real gas" equations. An example of a Class II test might be a suction throttled air compressor. An example of a Class III test might be a C02 loop test of a hydrocarbon compressor. Table 10-4 shows code allowable departure from specified design parameters for Class II and Class III tests.

Table 10-3

Allowable Departure From Specified Operating Conditions for Class I Test

Table 10-3

Allowable Departure From Specified Operating Conditions for Class I Test

Variable

Symbol

Unit

Departure (%)(!)

(a) Inlet pressure

Pi

psia

5 (2)

(b) Inlet temperature

T,

R

8 {2)

(c) Specific gravity of gas

G

ratio

2 (2)

(d) Speed

N

rpm

2

(e) Capacity

qi

cfm

4 O)

(f) Cooling temperature difference

°F

5 (4)

(g) Cooling water flow rate

gpm

3 —

(1) Departures are based on the specified value where pressures and temperatures are absolute,

(2) The combined effect of items (a), (b) and (c) shall not produce more than 8 per cent departure in inlet gas density.

(3) See Par. 3.13 of PTC 10 for limitations on range of capacity.

(4) Difference is defined as inlet gas temperature minus inlet cooling water temperature. Source, fl J Reprinted by permission of the American Society of Mechanical Engineers.

(1) Departures are based on the specified value where pressures and temperatures are absolute,

(2) The combined effect of items (a), (b) and (c) shall not produce more than 8 per cent departure in inlet gas density.

(3) See Par. 3.13 of PTC 10 for limitations on range of capacity.

(4) Difference is defined as inlet gas temperature minus inlet cooling water temperature. Source, fl J Reprinted by permission of the American Society of Mechanical Engineers.

Table 10-4

Allowable Departure From Specified Design Parameters for Class II and Class III Tests

Range of Test Values Limits—% of Design Value

Variable

Symbol

Min

Max

q/qd

95

105

q/N

96

104

Mm

50

105

95

105

Volume ratio Capacity-speed ratio Machine Mach number

Machine Reynolds number where the design value is

Below 200,000 centrifugal Above 200,000 centrifugal Below 100,000 axial compressor Above 100,000 axial compressor Mechanical losses shall not exceed 10% of the total shaft power input at test conditions.

* Minimum allowable test Machine Reynolds number is 180,000

** Minimum allowable test Machine Reynolds number is 90,000 Source: [1] Reprinted by permission of the American Society of Mechanical Engineers.

105 200 105 200

It becomes apparent, the farther the test gas parameter deviates from chose of the contract gas, the more difficult the correlation. This leads to test error.

In a multistage compressor, these errors cumulate and play a significant role in mismatching the inlet conditions at every successive stage. If a multistage compressor has side steams, testing becomes quite complex because each section may require a different test speed. A compromise must be made if sections are to be tested at the same time. This compromise is sometimes not feasible, and the sections must be tested separately.

An equation to determine the equivalent speed that would simulate the test conditions for testing the compressor on a gas different from the design process (contract) gas is given as:

N = rotative speed, rpm T, = inlet absolute temperature, °R Z = average compressibility n = polytropic exponent rp = pressure ratio s = contract gas constants (process gas) t - test gas constants

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