Since the mid-1980s, a fair number of compressor users have taken another look at piston rod wear. With sophisticated coatings finding widespread acceptance in aerospace and gas turbine applications, it was only a matter of time before a number of coating types replaced the more conventional surface hardening employed previously. In particular, the high-velocity-oxygen-fuel (HVOF) coating process has achieved commercial acceptance in the past two decades.
THERMAL SPRAY SYSTEMS INVOLVED IN THE PROCESS
Both HVOF and its predecessor, the detonation or D-Gun system, are thermal deposition systems. The two have many strengths in common. Both operate at high deposition rates and lay down highly adherent coatings of 0.002-in. to 0.020-in. thickness per layer.
The operating principle behind thermal sprays is to produce a high enough temperature to melt virtually anything, from metals to ceramics, then blast the melted metal out of the chamber of an application gun and onto the metal component to be coated. The faster the melted material leaves the chamber, the less time it spends being contaminated by the ambient oxygen atmosphere it needs to traverse on its way to the target surface and the harder its impact. High-impact speeds increase coating density, hardness, uniformity, cohesion, and adherence. Leading HVOF systems and their providers are described on the Internet. (We saved ourselves the task of mentioning them here after lengthy permissions correspondence with the marketing and legal staffs of companies that coat piston rods.)
FIRST STEP: INVESTIGATING THE VENDORS’ REFERENCE LISTS
Vendors and providers have reference lists, and consider them proprietary for reasons that (sometimes) make sense, and at other times are rather unreasonable. Explore the various sources and and buy only from quality providers.
Chances are that you will come across a modified and generally advanced HVOF system version called “HVLF,” which denotes a “High Velocity Liquid Fuel” system. It uses liquid kerosene fuel which is more economical and safer than pressurized explosive gases. When liquid kerosene hits the hot ignition chamber, it vaporizes and raises the chamber pressure to about 120 psi. This relatively high pressure boosts particle velocities to perhaps 4,000 psi. Powder feeds are injected into the gun beneath the flame front and the lower pressure “pulls” these feeds into the flame.COATING
COMPOSITIONS SHOULD MATCH THE APPLICATION
A number of coating materials are available for piston rods. These include tungsten carbides of different properties. Depending on anticipated compressor piston rod service condition, an experienced coating company will recommend the proper tungsten carbide and bonding matrix best suited for a particular application.
For example, say a representative coating combining 83% tungsten carbide with 17% cobalt was applied with an average thickness of 0.037 inches. Its bond strength exceeded 8,000 psi and its apparent porosity was below 0.25%. At an average coating hardness of 67 RC, application in the wear-prone packing area of compressor piston rods will, no doubt, prove beneficial.
DOCUMENTING YOUR REPAIR PROCEDURE
You should expect a knowledgeable repair facility to offer their repair procedures for review and comparison. An above-average supplier will normally proceed with an appropriate step-by-step sequence. In general terms, this type of sequence will probably include the following actions:
1. Checking the rod for straightness, amount of wear, thread damage, piston fit size, and hardness of packing/wiper section, and the like.
2. Documenting the “as received” condition on a sketch,
3. Gaining packing and wiper ring sections plus at least ½ in. (~13 mm) at each end of area undersize, so as to remove damage and wear. The edge of the undercut should be configured with a radius to preclude the formation of stress risers.
4. Documenting the before-coating surface hardness.
5. If the rod has been hardened or nitrided, grinding must penetrate through this layer until the original hardness substrate is reached. Hardness testing must be performed and documented.
6. Performing magnetic-particle inspection of the rod to verify absence of cracks throughout.
7. Demagnitizing the rod to a residual level not to exceed 2 Gauss.
8. Heat-soaking the piston rod in an oven at 400 F for four hours, then allowing it to slow-cool so as to remove residual gases before.
9. Masking and protectively taping all surfaces, except those to be grit blasted.
10. Blasting with aluminum oxide grit to provide a surface finish of 200 RMS to 350 RMS before commencing coating operations.
11. Coating the packing areas with a HVLF (3,400 ft/sec to 4,000 ft/sec particle velocity) tungsten carbide overlay in all packing areas. Using an infrared (IR) thermography gun aimed directly at the point of impact, verifying that the rod temperature does not exceed 350 F during this operation.
12. Applying coating so as to achieve a diameter 0.010-in. to 0.015-in. greater than the specified finished (post-grinding) diameter.
13. Attaching a coupon of like material substrate to be sprayed with the rod, and submitting this coupon to owner/purchaser for follow-up evaluation.
14. Finish-grinding with a diamond wheel, then diamond honing and super-finishing to the agreed-upon RMS value. (Common oversight: A finish that’s better than 8 RMS may not be desired since oil will not stick to it!)
15. Polishing all relief grooves and fillet radii.
16. Checking dye penetrant for indications of blistering, spalling, flaking, cracking, or pitting.
17. Seal-coating the area with a proven PTFE (“Teflon”) or suitable, perhaps proprietary, epoxy sealant.
18. Identifying the rod by die-stamping its end face and providing a report including:
⇒ dimensional data recorded on a sketch
⇒ magnaflux results
⇒ “as received” and “as corrected/finished” dimensional sketch
⇒ coating lengths and thicknesses shown on sketch
⇒ type of coating and lot number identifying material and properties
⇒ final RMS finish, as documented with profilometer tape.
ON A RELATED NOTE
It should be noted that several competent non-OEM coating companies in the United States are also producing entire new piston rods for OEM suppliers. The base material for these rods is typically AISI 4140, although AISI 4340 is used for sweet-gas service and rods larger than four inches in diameter. To avoid stress-corrosion failure, the rod manufacturer normally pays close attention to the hardness value of base materials in sour-gas services.TRR
Editor’s Note: Click Here To Download A Complete List Of Heinz Bloch’s 22 Books
ABOUT THE AUTHOR
Heinz Bloch’s long professional career included assignments as Exxon Chemical’s Regional Machinery Specialist for the United States. A recognized subject-matter-expert on plant equipment and failure avoidance, he is the author of numerous books and articles, and continues to present at technical conferences around the world. Bloch holds B.S. and M.S. degrees in Mechanical Engineering and is an ASME Life Fellow. These days, he’s based near Houston, TX. Email him at heinzpbloch@gmail.com.
Tags: reliability, availability, maintenance, RAM, reciprocating compressors, asset management
