Become a compressed air sleuth and identify alternative approaches to resolve compressor problems

Examine two case studies with a centrifugal compressor and an integrally geared compressor.

By Amin Almasi, Rotating Equipment Consultant

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It’s critical to look properly at all potential causes for a compressor problem in a plant. It’s amazing how much we don’t know about what we don’t know. It’s interesting how much operators don’t know about compressors they’ve operated for many years. It’s also amazing how much compressor engineers don’t know about the operation of the rest of the plant where they’ve worked for years. Therefore, it’s critical to get input from other engineers and technicians who notice the problem, who are affected by it, and who are actually related to it.

It’s often useful to collect input from other engineers one at a time. Otherwise, for example, in a formal meeting, engineers or operators tend to be inhibited about offering their impressions of the real causes of problems.

It’s generally a good practice to identify alternatives for approaches to resolve a compressor problem. In other words, brainstorm for solutions to a problem. Brainstorming is collecting as many ideas as possible. And the next step is screening them to find the best idea. Other important aspects of the compressor problem-solving process is continual observation and feedback.

Case study 1

This case study is about the inter-section temperature measurement for a centrifugal compressor with a sidestream to measure temperature inside the compressor before and after the sidestream. The operator team claimed that inter-section temperature measurements weren’t provided and they were unable to determine the actual compressor performance and monitor the compressor operation. They also highlighted they were unable to detect emerging problems or properly adjust the anti-surge system because of the lack of the inter-section temperature measurement. This is a request that a machinery engineer may face for any compressor with a sidestream.

A quick assessment of any conventional-type centrifugal compressor with a sidestream can show that installing temperature measurement sensors inside the compressor is a very risky task that requires lots of effort. It’s an extremely difficult task to provide direct temperature sensing for the process gas inside a compressor in predefined locations of a complex mixing section. The compressor can be seriously damaged or even destroyed in the modification process at site.

compressor problem solving
Figure 1. This schematic shows a conventional-type centrifugal compressor with a sidestream.

Investigations showed that temperatures inside the compressor, before and after sidestream mixing, aren’t necessary to estimate power and performance of a centrifugal compressor with a sidestream. The following equation can be used to calculate the power and efficiency of a centrifugal compressor with a sidestream (Figure 1).

PCOM = m3 h3 - m1 h1 - m2 h2  (Equation 1)
= (m1+m2) h3 - m1 h1 - m2 h2
Where:
PCOM represents compressor power (with a sidestream)
m represents mass flow
h represents enthalpy

The isentropic efficiency could be calculated by comparing the ideal power and the actual power.

To better explain the presented method, the compressor performance can be viewed as an overall performance — a compressor overall efficiency — which includes the stages (impellers) from the inlet to the sidestream (Section 1), the sidestream mixing section, and the stages (impellers) from the sidestream to the outlet (Section 2). For a compressor with a sidestream, there are some losses at the sidestream mixing section. The overall efficiency of a centrifugal compressor with a sidestream is lower than one for a comparable centrifugal compressor without a sidestream.

Based on experiences, unfortunately, some compressor manufacturers calculate the efficiency of compressors with sidestreams, neglecting the sidestream mixing section losses. They just simulate impellers and sections without any losses in sidestream mixing sections. Alternatively some other vendors may consider these losses but with inaccurate methods, which result in losses lower than actual ones. This also helps vendors to claim better efficiency and performance, which may be good for their sale and advertising. The compressors with sidestreams are usually used in special processes such as ammonia, syngas, or propane, where the ASME PTC-10 type-1 performance test cannot be implemented in the vendor shop. The ASME PTC-10 type-2 test has many shortfalls for compressors with sidestreams, and this test can’t identify the above-mentioned efficiency gap. The unrealistic efficiencies claimed by vendors never tested before the commissioning of the plant. This efficiency gap between the actual performance and the vendor-claimed efficiency, due to sidestream mixing section losses, has been identified for many compressors. A machinery engineer should always expect this efficiency gap for any compressor with sidestreams.

The compressor theoretical efficiency depends on many details such as the impeller specific-speed and impeller design, as well as details of sections, but this is usually limited to approximately 70–79% for a compressor with traditional 2D impellers. Many compressors with sidestreams, using conventional 2D impellers, have claimed efficiencies in the range of 72-77%. However, the actual efficiency for these machines could be 65-71%, and for some machines even below 65%. The reason is the losses in mixing sections. These losses are relatively high if mixing sections were not designed properly or in cases that there are operational deviations. When operating process conditions in inlet or sidestream don’t fully match with design conditions, much higher losses, compared to losses at design conditions, can be expected. For example, pressure deviations at a sidestream — sidestream pressure lower or higher than the rated pressure — can result in great losses and operational problems.

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