Chemical Engineering May 2013 - 59

sor at the maximum capacity, which
is also known as operation at end of
the curve; this corresponds to the far
right-hand side of the normal operation
point). The specific volume ratio
and machine Mach number are closely
related to the test speed. The molecular
weight of the test gas is extremely
important because a significant difference
between the test gas MW
and the actual gas MW can affect the
gas density and the calculated
compressor head.
As a rule of thumb, a deviation
greater than 30-40% could be considered
significant. Any significant difference
in the molecular weights of
the specified gas and the test gas can
result in a test speed that is far from
the rated speed. For the most relevant
results, the test speed should be as
close as possible to the rated speed.
Ideally, a test speed at around 80-95%
of the rated speed is always desirable
(if other limits allow this speed
range). Too often, the above-mentioned
desired speed range is difficult
to achieve for commonly used Type-2
test arrangements (particularly when
a close match to the gas MW cannot
be achieved).
Based on the Type-2 test theory,
the modifications (often called corrections)
to the test results are applied
based on the available gas dynamic
knowledge to estimate the compressor
performance in the specified operating
condition. All correction formulations
are available in the ASME-PTC-10 for
the estimation. In practical terms, this
means that the vendor carries out the
Type-2 test, and then applies the correction
formulation found in ASMEPTC-10
to find expected performance
in operating site conditions.
Balancing the tradeoffs
Some engineers do not trust the Type-2
method, because it allows for so many
deviations from actual conditions. As a
result, the ASME-PTC-10 Type-1 test
is always preferred. However, if the
ASME-PTC-10 Type-1 is not possible
(for example, if the actual process gas
cannot be supplied or used in the manufacturer's
shop), then the only possible
option is to carry out a Type-2 test
using modified test conditions (the
gas molecular weight, the speed, the
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CHEMICAL ENGINEERING WWW.CHE.COM MAY 2013 57
capacity, the power, the pressures, and
others) that are as close as possible to
the specified operating conditions.
The arrangement and details of
Type-2 tests should be fixed in the
compressor bidding stage and before
the machine order placement. Compressor
vendors always prefer the
simplest and cheapest arrangement
for the Type-2 test. Unfortunately, the
ASME-PTC-10 Type 2 code allows the
vendor to use the simplest arrangement.
In other words, the code gives
vendors freedom to use simple setups
and deviations, and vendors usually
opt for the cheapest setup.
Often the Type-2 test gas selections
and testing arrangements are dis
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