It’s important to include checking reciprocating compressor performance in your maintenance routine. Here are a few methods to keep in mind.
Reciprocating compressors – also known as piston compressors – are used extensively in several industrial applications. This compressor often features one or more pistons that a crankshaft drives. The crankshaft also drives the connecting rod as well as the piston rod.
How Reciprocating Compressors Work
When the piston travels back and forth within the cylinder, it increases the gas pressure. This gas pressure increment helps in its compression. After compression, the gas is discharged into high-pressure receiving tanks.
There are numerous types of reciprocating compressors, though the more popular ones include:
- Single-cylinder reciprocating compressor
- Double-cylinder reciprocating compressor
- Multi-cylinder reciprocating compressor
- Multi-stage design reciprocating compressor
- Diaphragm reciprocating compressor
The single-cylinder compressor features a suction and discharge area as well as compression.
The double-cylinder reciprocating compressor has dual suction and discharge areas. These help users achieve higher gas pressures than single-cylinder reciprocating compressors.
The multi-cylinder compressor features as many as six cylinders which help boost gas pressure beyond the action of its double-cylinder counterpart.
Diaphragm reciprocating compressors are somewhat different from traditional compressors. Gas compression within this type of compressor is achieved via the movements of a flexible membrane. The crankshaft and a connecting rod facilitate the membrane’s movements.
Checking the Performance of a Reciprocating Compressor
The performance of a reciprocating compressor is usually determined by operating speed. Every cylinder of the compressor discharges the same volume – except for small or insignificant variations that atmospheric changes cause – at the same discharge pressure every time the cylinder completes a discharge stroke.
Changes to the rotation speed of the reciprocating compressor results in significant changes in the discharge volume.
A few variables also affect the overall performance of a reciprocating compressor. This includes the inlet-discharge valves that control flow – into and out of – every cylinder. Though reciprocating compressors make use of several valve designs, it is highly essential as it helps the valves function reliably.
If the valves are somewhat damaged and fail to function effectively at the proper time or don’t seal correctly, the overall performance of the compressor will be considerably reduced.
Spot-checking the overall performance of a reciprocating compressor is essential and must never be taken for granted. It is especially important after rebuilding the compressor, when something feels off or different, or as part of commissioning.
The following are the things you can do when spot-checking your reciprocating compressor:
- Compare discharge temperatures
You can also compare discharge temperatures for cylinders on the same stage. Bear in mind that single-acting cylinders usually have slightly larger discharge temperatures.
Comparing discharge temperatures helps pinpoint which cylinder is faulty. But it only applies perfectly to a minimum of 2+ cylinders on stage.
- Compare measured flow
Consider comparing measured flow with the one that OEM sizing software has predicted. This is usually a walk in the park, especially if it is done immediately after creating a model.
Creating a model is also easy to do; however, it often requires accurate clearances or load step numbers.
Comparing measured flow helps you spot the throughput in order to see if it is below capacity. But it doesn’t help pinpoint the cause behind this problem. It will also require a highly dedicated or customized meter as well as a software model or loading curves in order to achieve significant success.
- Compare inter-stage pressures
You should compare inter-stage pressures using software model predictions. This helps pinpoint the exact stage that is faulty.
However, this method only applies to multi-stage compressors and also requires a software model for a successful evaluation.
- Calculate the overall capacity of every stage separately
You should consider calculating the overall capacity of every stage individually. This spot-checking method is based primarily on observed suction temperatures and pressures in order to pinpoint discrepancies.
No model is required for this spot-checking method. However, in order to obtain the best results, you should consider setting up an Excel calculation that will require gas properties.
If any significant difference pops up, you should take the lower value as the more accurate result. This means that the other stage is defective and requires urgent attention for repairs.
- Checking for hot valves
You should check for hot valves by comparing the temperatures of the equivalent valve cap.
This is capable of identifying which of the valves is faulty or leaking. But if the valve leak is substantial or big, the valve temperatures may not be elevated.
- Perform a highly detailed analysis or examination of the pressure-volume curves
You may also carry out extensive analyses of pressure-volume curves, including ultrasonic patterns.
But this is not a simple method of checking the performance of a reciprocating compressor. Not only does it require incredibly detailed analysis, but it is also time-consuming to execute and requires highly specialized equipment in order to attain success.
Reciprocating Parts That Impact Performance
Reciprocating compressors are built up of several components. Here are a few that significantly impact the overall performance of the equipment:
- Cylinders
Cylinders are part of reciprocating compressors and usually accommodate the suction and discharge valve plates.
Large, low-pressure cylinders are forged from cast iron, while small, high-pressure cylinders are constructed from steel. In a few cases, ductile iron is employed. But in many cases, steel is the go-to material used in creating cylinders in reciprocating compressors utilized in the hydrocarbon industry.
- Piston
According to experts, the piston is considered to be the ‘heart’ of a reciprocating compressor.
Pistons used in reciprocating compressors are made from aluminum alloy, aluminum, and other low-weight materials to minimize shaking and rod load. The piston helps translate energy from the crankcase to the gas within the cylinder.
Pistons usually come with sliding piston rings, self-lubricating seals that help prevent leakage. These seals help boost efficiencies via the reduction of frictional force.
- Crankshaft
Reciprocating compressors above 200 horsepower make use of crankshafts formed from steel.
Other essential parts of reciprocating compressors that impact performance include:
- Piston rod
- Rod packing
- Distance piece
- Connecting rod
- Bearing
- Crosshead, etc.
Conclusion
Reciprocating compressors are vital machines used extensively across a wide variety of industries. Checking the performances of reciprocating compressors from time to time is highly essential. This helps ensure that the equipment is always at its peak and early signs of wear and tear are quickly discovered and taken care of.
Moreover, spot-checking reciprocating compressors ensure a high level of performance. This means you need not be afraid of unexpected and costly repairs or severe downtime as a result of faulty equipment.