Detailed Explanation of Main Compressor Components – Worth Learning!
I. Cylinder
The cylinder is the component of a reciprocating compressor for gas compression. It bears gas pressure, friction between the mirror surface, piston rings and support rings, as well as heat generated during gas compression. Featuring a complex structure with air passages, water passages and valve chambers, the cylinder must possess sufficient strength, wear-resistant hardness, appropriate surface roughness and excellent heat transfer performance.
A cylinder head is fitted on the upper part and a cylinder base on the lower part, forming the working volume of the cylinder. Compressor cylinders are made of cast iron. After years of operation, varying degrees of wear or scratches will occur, especially on high-pressure stage cylinders. This will accelerate wear of piston rings and support rings, lead to gas leakage across piston rings, reduce efficiency and increase power consumption.
Requirements for Cylinders and Repair Measures after Wear
(1) The surface roughness of the cylinder mirror shall be Ra 0.8 or higher. No sand holes, porosity, damaged patches, axial or circumferential scratches are permitted. Minor scratches can be polished smooth with oil stones.
(2) There are roughly three types of cylinder wear:
Eccentric wear: After wear, partial cylinder radius deviates unevenly from the original cylinder axis.
Bell-mouthed or concave inner bore.
Uniform enlargement of cylinder bore, mostly with non-uniform wear.
All above conditions shorten the service life and impair sealing performance of piston rings, triggering gas leakage, high exhaust temperature, rapid damage (even fracture) of piston rings. In severe cases, exhaust pressure and pressure ratio of the preceding stage rise accompanied by temperature increase, and even safety valve alarms.Three general solutions for cylinder wear:
If the wear loss is large and the cylinder wall is thin, the cylinder cannot be repaired and must be replaced.
For eccentric wear or concave bore, boring and honing of the cylinder bore can be performed, followed by refitting new pistons and piston rings.
If structurally permissible, a cylinder liner can be inserted. However, this method carries certain risks.
Risks of Installing a Cylinder LinerFirst, the original bore must be enlarged to fit the liner. Enlargement may expose casting defects such as sand holes, air holes and porosity, resulting in water leakage and scrapping of the cylinder. Second, after liner insertion, the shoulder for mounting gas valves becomes thin, prone to insufficient strength or air leakage. Therefore, liner insertion shall be adopted cautiously; replacing with a new cylinder is the most reliable option.(4) The contact surface between the gas chamber and gas valve shall be flat without scars that cause gas leakage.
(5) All gaskets shall be properly installed to avoid air leakage, water leakage or cross-leakage between gas and water. Adequate sealing gaskets for the bottom of gas valves shall be prepared for timely replacement.
(6) Sludge inside cylinder water passages shall be thoroughly cleaned during each major overhaul. Sludge severely obstructs cooling water circulation and reduces cooling efficiency.
Common Cylinder Faults
Abnormal noise, cylinder cracking and cylinder head rupture.
Impact noise and abnormal sound inside the cylinder can be detected by a stethoscope against the outer wall, accompanied by perceptible vibration by hand. Root causes include:
(1) Insufficient clearance at dead centers (especially top dead center) during piston reciprocation, causing collision between piston and cylinder head, often smashing the cylinder head.
(2) Impact and displacement caused by foreign objects inside the cylinder, such as small tools, fragments of valve plates, broken spring pieces or fractured piston rings.
(3) Loosened piston rod nut striking the cylinder head.
(4) Cracks on the water passage wall of the cylinder allowing water ingress, which easily causes liquid hammer. Shut down the unit promptly and take corresponding measures.
(5) Cracks developed from hidden dangers: for example, residual water inside the cylinder freezes and cracks the cylinder during winter shutdown; collisions during transportation, handling or installation induce internal cracks that gradually expand during startup. Collision prevention shall be implemented in all operations.
(6) Cooler leakage allows water to enter the next-stage cylinder along with gas, easily triggering liquid hammer. Coolers shall undergo a hydraulic pressure test before delivery; the test pressure shall be 1.5 times the design pressure.II. Piston
Piston
Reciprocating motion of the piston inside the cylinder completes the cycle of gas suction, compression, exhaust and expansion.
Various piston structures are available with cylindrical skirts. Most pistons consist of two or three split segments; domestic models adopt integral pistons.
Support rings (guide rings for vertical units) and piston rings are installed on the piston.
Approximate Gas Capacity Calculation
Single-acting piston: Gas compression occurs at the upper end of the piston. Estimated flow: \(Q=0.75\times F\times S\times n\)
Double-acting piston: Gas compression occurs at both upper and lower ends of the piston. Estimated flow:
\(Q=0.75\times[F_{p}+(F_{p}-F_{r})]\times S\times n\)
Where:
Q — Gas flow
\(F_{p}\) — Cross-sectional area corresponding to cylinder bore
\(F_{r}\) — Cross-sectional area of piston rod
S — Stroke
n — Rotational speed
Most compressor cylinders are double-acting, and a small number are single-acting.
Assembly of Piston and Piston Rod
Aluminum pistons are widely used in compressors. The locating surface of the piston rod and piston bore must meet tolerance requirements. During assembly, the two contact surfaces between the nut and piston shall fit tightly, as these surfaces transmit piston force. Aluminum has low compressive strength, so an iron pressure block is fitted at the end to engage with the shoulder of the piston rod.
The fitting area between the pressure block and aluminum piston shall exceed 75%, and the fitting area between the piston rod shoulder and pressure block shall exceed 70%. Otherwise, eccentric stress will be generated on the piston rod thread during nut tightening, leading to piston rod fracture. Anti-loosening structures must be equipped after nut fastening to prevent nut loosening.
Requirements for Piston Installation into Cylinder
The piston must be centrally positioned with circumferential clearance after installation to avoid friction between piston and cylinder wall. Uniformity of circumferential clearance shall be inspected.
Inspection method: Theoretical clearance = (Cylinder bore diameter − Outer diameter of piston) ÷ 2. Measure actual circumferential clearance with a feeler gauge and compare with theoretical clearance to identify deviation causes. Normally, the lower clearance is expected to be larger than the theoretical value, which benefits the service life of support rings. For this reason, some European manufacturers lower the central hole connecting the piston (support ring) and piston rod by approximately 1 mm (varies with piston diameter), lifting the piston inside the cylinder, known as an eccentric piston (disadvantage: complicated piston positioning assembly).
Excessive piston sinking requires troubleshooting: enlarged clearance between crosshead and sliding plate leading to misalignment between crosshead and cylinder; eccentric cylinder wear; misalignment between cylinder and intermediate distance piece; non-parallel connecting surfaces or non-perpendicular centerlines between new cylinder and middle body. Corresponding corrective actions shall be taken.The axial side clearance of the piston inside the cylinder can be measured by the lead wire pressing method and adjusted via the connecting thread between piston rod and crosshead. After mounting the cylinder head, measure the head-side clearance. If the clearance deviates significantly from factory specifications, adjust by adding gaskets.
Function of Piston Dead Center Clearance
Dead center clearance forms clearance volume (including clearance volume of gas valves and pistons). Excessive clearance volume is harmful for compressors; excessive clearance of the first stage reduces exhaust capacity. Nevertheless, clearance must be reserved between cylinder and piston for the following purposes:
(1) Piston rods, connecting rods, crossheads expand and elongate when heated; wear occurs at rotating shaft joints and bores enlarge. The clearance prevents collision between piston and cylinder head and avoids accidents.
(2) Moisture contained in air partially remains inside the cylinder after compression and cooling. The dead center clearance accommodates residual liquid. Liquid incompressibility will cause destructive consequences (liquid hammer / water hammer) without such space.
(3) Gas inside the clearance acts as an air cushion to buffer and prevent collision between piston and cylinder head.
Key Points for Compressor Pistons
(1) The ring grooves of three-piece aluminum pistons are prone to widening under continuous impact of piston rings due to the soft aluminum material, especially the first ring groove on high-pressure stages. Some pistons adopt iron ring grooves with better performance. Iron pistons are not widely adopted mainly due to weight: reciprocating piston mass generates inertia force. Unbalanced inertia force causes unit vibration.
(2) Pistons are replaced frequently, sometimes multiple times. New pistons are usually manufactured based on field measurement. Different manufacturers produce pistons with inconsistent axial width and radial depth of piston ring grooves and support ring grooves. Consequently, piston rings and support rings of the same model from different regions may fail to assemble, especially support rings. Three solutions are adopted:
① Optimize piston rings and support rings to adapt to grooves of varying width and depth, though this is not optimal.
② Custom manufacturing for large dimensional differences.
③ Adopt European piston design standards for replacement pistons to achieve gradual universalization (including standardization of piston rods).III. Connecting RodThe connecting rod links with the crosshead and converts rotary motion of the crankshaft into reciprocating motion. It transmits piston force, serving as a heavily loaded component with high failure probability. Connecting rod fracture can damage crossheads, slideways, pistons, oil scrapers and other parts. In serious cases, the crankcase may rupture and the crankshaft deformed.
Connecting Rod Structure
The connecting rod consists of the rod body, big end, big end bearing shell, small end bronze bushing, connecting rod bolts and nuts.
(1) The split big end accommodates bearing shells and is assembled onto the crank pin via connecting rod bolts. Strict requirements apply to coaxiality of the two bolt holes on the big end cap and rod body, bolt positioning surfaces and supporting surfaces for connecting rod bolts.
(2) The rod body must have sufficient tensile, compressive and bending strength, and is machined with an oil passage running from the big end to the small end.
(3) An interference-fit bronze bushing is embedded in the small end with clearance fit to the crosshead pin.
Connecting Rod Bolts
Connecting rod bolts bear severe alternating loads and are the most stressed components in the motion mechanism. Most compressor accidents related to connecting rods originate from bolt failures. Bolts must feature sufficient tensile strength and fatigue resistance. Great attention shall be paid to material selection, manufacturing (including transition fillets and surface roughness) and assembly.
Common Hidden Defects
(1) Poor vertical and tight fitting between bolt head / nut and supporting surfaces on the connecting rod.
(2) Loosened nuts and failed anti-loosening devices.
(3) Insufficient precision at stress-concentration sensitive areas of bolts, such as threads, relief grooves, transition fillets and parallelism of big/small end holes.
(4) Bolt fatigue or fracture under additional impact loads near fatigue limits.
(5) Excessive clearance between bolts and positioning holes in rod body and big end cap.
(6) Defective raw material.
(7) Improper fitting clearance with bearing shells and bronze bushings leading to eccentric wear and seizure.
(8) Blocked connecting rod oil passage causing oil cutoff.
Piston Rod
One end of the piston rod connects to the piston and the other to the crosshead. The piston is fixed on the rod by the shoulder and nut on the rod. Threads at the other end screw into the crosshead. Under alternating tensile and compressive stress during operation, piston rod fracture may trigger accidents involving pistons, cylinders and crossheads.
(1) Requirements for Piston Rod
Possess sufficient strength and stiffness to transmit piston force accurately.
Flawless material and proper heat treatment (normally three heat treatment processes).
Friction surfaces contacting sealing rings and oil scraper rings shall be quenched or nitrided according to material to guarantee friction resistance, with surface roughness Ra 0.4.
High machining precision covering threads, positioning surfaces, pressure-bearing surfaces, friction surfaces and relief grooves.
No bending allowed. Bending causes piston rod runout and imposes extra stress on related components. The runout (vertical and horizontal) during operation shall be controlled at approximately 0.06 mm. Excessive runout shortens service life of sealing elements.
No scratches, dents or impacts on piston rod sealing surfaces and threads.
(2) Piston Rod Faults
Both ends of the piston rod are connected to the piston and crosshead via threads. The thread root is a dangerous section prone to fracture. Threads at the crosshead connection bear larger loads than those at the piston connection, so fracture more easily.
A pressure sleeve or pressure pad is installed between the piston rod shoulder and aluminum piston to reduce specific pressure on the aluminum piston. Uneven surface fitting among the piston, pressure sleeve and piston rod shoulder leads to thread fatigue failure after long-term operation. Uneven wear of support rings and crossheads causes inclined movement of the piston rod and additional stress, resulting in thread fatigue damage. Other causes include loosened connecting nuts, failed locking devices, defective piston rod material, machining technology and precision.
The cylinder is the component of a reciprocating compressor for gas compression. It bears gas pressure, friction between the mirror surface, piston rings and support rings, as well as heat generated during gas compression. Featuring a complex structure with air passages, water passages and valve chambers, the cylinder must possess sufficient strength, wear-resistant hardness, appropriate surface roughness and excellent heat transfer performance.
A cylinder head is fitted on the upper part and a cylinder base on the lower part, forming the working volume of the cylinder. Compressor cylinders are made of cast iron. After years of operation, varying degrees of wear or scratches will occur, especially on high-pressure stage cylinders. This will accelerate wear of piston rings and support rings, lead to gas leakage across piston rings, reduce efficiency and increase power consumption.
Requirements for Cylinders and Repair Measures after Wear
(1) The surface roughness of the cylinder mirror shall be Ra 0.8 or higher. No sand holes, porosity, damaged patches, axial or circumferential scratches are permitted. Minor scratches can be polished smooth with oil stones.
(2) There are roughly three types of cylinder wear:
Eccentric wear: After wear, partial cylinder radius deviates unevenly from the original cylinder axis.
Bell-mouthed or concave inner bore.
Uniform enlargement of cylinder bore, mostly with non-uniform wear.
All above conditions shorten the service life and impair sealing performance of piston rings, triggering gas leakage, high exhaust temperature, rapid damage (even fracture) of piston rings. In severe cases, exhaust pressure and pressure ratio of the preceding stage rise accompanied by temperature increase, and even safety valve alarms.Three general solutions for cylinder wear:
If the wear loss is large and the cylinder wall is thin, the cylinder cannot be repaired and must be replaced.
For eccentric wear or concave bore, boring and honing of the cylinder bore can be performed, followed by refitting new pistons and piston rings.
If structurally permissible, a cylinder liner can be inserted. However, this method carries certain risks.
Risks of Installing a Cylinder LinerFirst, the original bore must be enlarged to fit the liner. Enlargement may expose casting defects such as sand holes, air holes and porosity, resulting in water leakage and scrapping of the cylinder. Second, after liner insertion, the shoulder for mounting gas valves becomes thin, prone to insufficient strength or air leakage. Therefore, liner insertion shall be adopted cautiously; replacing with a new cylinder is the most reliable option.(4) The contact surface between the gas chamber and gas valve shall be flat without scars that cause gas leakage.
(5) All gaskets shall be properly installed to avoid air leakage, water leakage or cross-leakage between gas and water. Adequate sealing gaskets for the bottom of gas valves shall be prepared for timely replacement.
(6) Sludge inside cylinder water passages shall be thoroughly cleaned during each major overhaul. Sludge severely obstructs cooling water circulation and reduces cooling efficiency.
Common Cylinder Faults
Abnormal noise, cylinder cracking and cylinder head rupture.
Impact noise and abnormal sound inside the cylinder can be detected by a stethoscope against the outer wall, accompanied by perceptible vibration by hand. Root causes include:
(1) Insufficient clearance at dead centers (especially top dead center) during piston reciprocation, causing collision between piston and cylinder head, often smashing the cylinder head.
(2) Impact and displacement caused by foreign objects inside the cylinder, such as small tools, fragments of valve plates, broken spring pieces or fractured piston rings.
(3) Loosened piston rod nut striking the cylinder head.
(4) Cracks on the water passage wall of the cylinder allowing water ingress, which easily causes liquid hammer. Shut down the unit promptly and take corresponding measures.
(5) Cracks developed from hidden dangers: for example, residual water inside the cylinder freezes and cracks the cylinder during winter shutdown; collisions during transportation, handling or installation induce internal cracks that gradually expand during startup. Collision prevention shall be implemented in all operations.
(6) Cooler leakage allows water to enter the next-stage cylinder along with gas, easily triggering liquid hammer. Coolers shall undergo a hydraulic pressure test before delivery; the test pressure shall be 1.5 times the design pressure.II. Piston
Piston
Reciprocating motion of the piston inside the cylinder completes the cycle of gas suction, compression, exhaust and expansion.
Various piston structures are available with cylindrical skirts. Most pistons consist of two or three split segments; domestic models adopt integral pistons.
Support rings (guide rings for vertical units) and piston rings are installed on the piston.
Approximate Gas Capacity Calculation
Single-acting piston: Gas compression occurs at the upper end of the piston. Estimated flow: \(Q=0.75\times F\times S\times n\)
Double-acting piston: Gas compression occurs at both upper and lower ends of the piston. Estimated flow:
\(Q=0.75\times[F_{p}+(F_{p}-F_{r})]\times S\times n\)
Where:
Q — Gas flow
\(F_{p}\) — Cross-sectional area corresponding to cylinder bore
\(F_{r}\) — Cross-sectional area of piston rod
S — Stroke
n — Rotational speed
Most compressor cylinders are double-acting, and a small number are single-acting.
Assembly of Piston and Piston Rod
Aluminum pistons are widely used in compressors. The locating surface of the piston rod and piston bore must meet tolerance requirements. During assembly, the two contact surfaces between the nut and piston shall fit tightly, as these surfaces transmit piston force. Aluminum has low compressive strength, so an iron pressure block is fitted at the end to engage with the shoulder of the piston rod.
The fitting area between the pressure block and aluminum piston shall exceed 75%, and the fitting area between the piston rod shoulder and pressure block shall exceed 70%. Otherwise, eccentric stress will be generated on the piston rod thread during nut tightening, leading to piston rod fracture. Anti-loosening structures must be equipped after nut fastening to prevent nut loosening.
Requirements for Piston Installation into Cylinder
The piston must be centrally positioned with circumferential clearance after installation to avoid friction between piston and cylinder wall. Uniformity of circumferential clearance shall be inspected.
Inspection method: Theoretical clearance = (Cylinder bore diameter − Outer diameter of piston) ÷ 2. Measure actual circumferential clearance with a feeler gauge and compare with theoretical clearance to identify deviation causes. Normally, the lower clearance is expected to be larger than the theoretical value, which benefits the service life of support rings. For this reason, some European manufacturers lower the central hole connecting the piston (support ring) and piston rod by approximately 1 mm (varies with piston diameter), lifting the piston inside the cylinder, known as an eccentric piston (disadvantage: complicated piston positioning assembly).
Excessive piston sinking requires troubleshooting: enlarged clearance between crosshead and sliding plate leading to misalignment between crosshead and cylinder; eccentric cylinder wear; misalignment between cylinder and intermediate distance piece; non-parallel connecting surfaces or non-perpendicular centerlines between new cylinder and middle body. Corresponding corrective actions shall be taken.The axial side clearance of the piston inside the cylinder can be measured by the lead wire pressing method and adjusted via the connecting thread between piston rod and crosshead. After mounting the cylinder head, measure the head-side clearance. If the clearance deviates significantly from factory specifications, adjust by adding gaskets.
Function of Piston Dead Center Clearance
Dead center clearance forms clearance volume (including clearance volume of gas valves and pistons). Excessive clearance volume is harmful for compressors; excessive clearance of the first stage reduces exhaust capacity. Nevertheless, clearance must be reserved between cylinder and piston for the following purposes:
(1) Piston rods, connecting rods, crossheads expand and elongate when heated; wear occurs at rotating shaft joints and bores enlarge. The clearance prevents collision between piston and cylinder head and avoids accidents.
(2) Moisture contained in air partially remains inside the cylinder after compression and cooling. The dead center clearance accommodates residual liquid. Liquid incompressibility will cause destructive consequences (liquid hammer / water hammer) without such space.
(3) Gas inside the clearance acts as an air cushion to buffer and prevent collision between piston and cylinder head.
Key Points for Compressor Pistons
(1) The ring grooves of three-piece aluminum pistons are prone to widening under continuous impact of piston rings due to the soft aluminum material, especially the first ring groove on high-pressure stages. Some pistons adopt iron ring grooves with better performance. Iron pistons are not widely adopted mainly due to weight: reciprocating piston mass generates inertia force. Unbalanced inertia force causes unit vibration.
(2) Pistons are replaced frequently, sometimes multiple times. New pistons are usually manufactured based on field measurement. Different manufacturers produce pistons with inconsistent axial width and radial depth of piston ring grooves and support ring grooves. Consequently, piston rings and support rings of the same model from different regions may fail to assemble, especially support rings. Three solutions are adopted:
① Optimize piston rings and support rings to adapt to grooves of varying width and depth, though this is not optimal.
② Custom manufacturing for large dimensional differences.
③ Adopt European piston design standards for replacement pistons to achieve gradual universalization (including standardization of piston rods).III. Connecting RodThe connecting rod links with the crosshead and converts rotary motion of the crankshaft into reciprocating motion. It transmits piston force, serving as a heavily loaded component with high failure probability. Connecting rod fracture can damage crossheads, slideways, pistons, oil scrapers and other parts. In serious cases, the crankcase may rupture and the crankshaft deformed.
Connecting Rod Structure
The connecting rod consists of the rod body, big end, big end bearing shell, small end bronze bushing, connecting rod bolts and nuts.
(1) The split big end accommodates bearing shells and is assembled onto the crank pin via connecting rod bolts. Strict requirements apply to coaxiality of the two bolt holes on the big end cap and rod body, bolt positioning surfaces and supporting surfaces for connecting rod bolts.
(2) The rod body must have sufficient tensile, compressive and bending strength, and is machined with an oil passage running from the big end to the small end.
(3) An interference-fit bronze bushing is embedded in the small end with clearance fit to the crosshead pin.
Connecting Rod Bolts
Connecting rod bolts bear severe alternating loads and are the most stressed components in the motion mechanism. Most compressor accidents related to connecting rods originate from bolt failures. Bolts must feature sufficient tensile strength and fatigue resistance. Great attention shall be paid to material selection, manufacturing (including transition fillets and surface roughness) and assembly.
Common Hidden Defects
(1) Poor vertical and tight fitting between bolt head / nut and supporting surfaces on the connecting rod.
(2) Loosened nuts and failed anti-loosening devices.
(3) Insufficient precision at stress-concentration sensitive areas of bolts, such as threads, relief grooves, transition fillets and parallelism of big/small end holes.
(4) Bolt fatigue or fracture under additional impact loads near fatigue limits.
(5) Excessive clearance between bolts and positioning holes in rod body and big end cap.
(6) Defective raw material.
(7) Improper fitting clearance with bearing shells and bronze bushings leading to eccentric wear and seizure.
(8) Blocked connecting rod oil passage causing oil cutoff.
Piston Rod
One end of the piston rod connects to the piston and the other to the crosshead. The piston is fixed on the rod by the shoulder and nut on the rod. Threads at the other end screw into the crosshead. Under alternating tensile and compressive stress during operation, piston rod fracture may trigger accidents involving pistons, cylinders and crossheads.
(1) Requirements for Piston Rod
Possess sufficient strength and stiffness to transmit piston force accurately.
Flawless material and proper heat treatment (normally three heat treatment processes).
Friction surfaces contacting sealing rings and oil scraper rings shall be quenched or nitrided according to material to guarantee friction resistance, with surface roughness Ra 0.4.
High machining precision covering threads, positioning surfaces, pressure-bearing surfaces, friction surfaces and relief grooves.
No bending allowed. Bending causes piston rod runout and imposes extra stress on related components. The runout (vertical and horizontal) during operation shall be controlled at approximately 0.06 mm. Excessive runout shortens service life of sealing elements.
No scratches, dents or impacts on piston rod sealing surfaces and threads.
(2) Piston Rod Faults
Both ends of the piston rod are connected to the piston and crosshead via threads. The thread root is a dangerous section prone to fracture. Threads at the crosshead connection bear larger loads than those at the piston connection, so fracture more easily.
A pressure sleeve or pressure pad is installed between the piston rod shoulder and aluminum piston to reduce specific pressure on the aluminum piston. Uneven surface fitting among the piston, pressure sleeve and piston rod shoulder leads to thread fatigue failure after long-term operation. Uneven wear of support rings and crossheads causes inclined movement of the piston rod and additional stress, resulting in thread fatigue damage. Other causes include loosened connecting nuts, failed locking devices, defective piston rod material, machining technology and precision.
