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External leakage would be valve cap gaskets, flange gaskets, cylinder outboard and inboard head gaskets, and compressor rod pressure packing leaks. These leaks have no effect on the temperature of the compression process and can be monitored by using ultrasonic listening devices, gas sniffers and flow meters in the case of the packing leaks. External leaks are a safety issue and affect the environment, but do not directly affect the mechanical integrity of the compressor.
Internal leakages reduce the capacity and efficiency of the compressor and can lead to conditions that may cause catastrophic mechanical failure due to excessive temperatures and reciprocating component loading and lubrication. Even if we could physically measure the gas flow into and out of the cylinder, we could not accurately foretell the dangers that exist to the major mechanical components. This is where compressor analyzers (and analysts), both portable and on-line, show their value. These tools allow us to measure internal pressures degree by degree and to visually see and calculate the leakage effects. We can measure the horsepower being consumed and measure the reciprocating component loading and pin reversal requirements. With built in equation of state calculations, we can compare theoretical performance, both pressure and temperature, to measured values. Through trending, we can see how a condition is changing and sometimes see when and how it started and predict where it is headed.
Before the next series of articles on different internal leakages, I would like to bring your attention to the flow balance calculation that some people base their entire condition assessment on. Flow balance is suction flow divided by discharge flow. In a healthy single acting cylinder with 2.0 mmscfd measured inlet flow and 2.0 mmscfd measured outlet flow, the flow balance would be 1.00. If the same cylinder had only leaking rings or valves and the measured inlet flow dropped to 1.5 mmscfd, the outlet flow would do the same resulting in the same flow balance calculation of 1.00.
Our flow balance calculation is going to be made on the calculated suction and discharge flows using internal cylinder conditions. This requires us to measure the internal cylinder pressures at the beginning and end of the compression process, measure the volumetric efficiency at suction and discharge conditions (what percentage of the cylinder displaced volume was used for both the suction and discharge process), what temperatures existed at the beginning and end of the process and what gas properties changed during the process.
In our healthy cylinder, the calculated suction and discharge flows will be the same, resulting in a flow balance of 1.00. Without going into the details at this point, an unhealthy cylinder with a discharge valve leak will result in a flow balance below 1.00. A suction valve or ring leak will result in a flow balance greater than 1.00. The task now is to determine what value above or below 1.00 will trigger a maintenance response.
Consider these observations:
- Calculated flows in a healthy cylinder can be very accurate.
- Calculated flows in an unhealthy cylinder are always too high.
- The lowest calculated flow (suction or discharge) is the most accurate, but still too high.
- In a multistage unit with no other recycle paths between stages, the cylinder with the lowest calculated flow is usually the healthiest.
If you feel it necessary to adjust cylinder phasing offsets to improve flow balance, all cylinders need to be improved at the same time.
We will discuss these characteristics more in our future articles on leakage. In the meantime, Happy monitoring.
Regards,
Warren
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