A. Natural gas engine valve failure

The first picture shows two valves with extreme recession.

The following is by Lloyd Leugner of Maintenance Technology International, Inc. It deals with the main cause of valve recession.

Natural Gas Engine Lubrication and Oil Analysis - A Primer in Predictive Maintenance and Condition Monitoring
Lloyd Leugner, Maintenance Technology International, Inc.

Sulfated Ash
Any discussion of the elemental analysis of natural gas engine oils is not complete without a comment concerning the issue of sulfated ash content. Natural gas engine operation tends to form various deposits such as varnish, sludge and an ash residue which remains after the oil is burned during operation.

The varnish and sludge are controlled by the detergent/dispersant additives, however these detergent/dispersant additives tend to leave a grey, fluffy ash residue after the oil has been burned. This ash residue is made up of metal sulfates from such additives as barium, calcium, phosphorus, zinc, magnesium and boron.

Therefore, lubricant formulators must ensure that these additive concentrations are high enough to help prevent valve recession, but not so high as to cause unwanted and harmful deposits, or cause catalysts to become ineffective.

Valve recession is the premature wearing of the valve seat into the cylinder head. The sulfated ash residue helps to prevent premature valve recession by “cushioning” the valve seat area.
Excessively high concentrations of certain additives, such as zinc or phosphorus, can also be harmful to catalyst equipped natural gas engines, because these additives may deactivate the exhaust catalyst by forming glassy-amorphous deposits, which prevent the exhaust gas from reaching the active surfaces of the catalyst, which in turn makes control of harmful emissions impossible.

In addition, natural gas engine manufacturers also list the levels of sulfated ash and the additive concentrations that are acceptable for use in their particular engines. For specific recommendations concerning ash content and additive levels, the engine operator should contact both the engine manufacturer and the lubricant supplier.

Conclusion: The most likely cause of the premature valve wear is improper maintenance procedures for the engine conditions. 

B. Premature valve component wear caused by various external conditions

The first picture shows abrasive wear on valve stems. Wear of this type is due to improper adjustment and is not a defect in material.

Failures due to carbon build-up and/or corrosives
The next several pictures will show the heavy abrasive carbon build up on the valve, valve spring and rotator and the valve guide. Note the heat discoloration of the guide (blue arrow). These parts failed due to carbon build up due to external conditions and not defects in the product.

The next picture shows the stem of the broken valve. Note the break (green arrow) is just above the valve face (strongest part of a valve). Also note the erosion of the valve stem from carbon cutting (red arrow).

This picture below shows guide and stem of failed valve. The red arrow shows erosion at the parting line where the valve stem is attached to the valve head.

C. Valve and valve guide wear caused by poor maintenance 

The first two pictures show the wear in the lower part of the valve guide closest to the valve head. The second picture shows the heat discoloration on the lower part of the guide was caused from excessive heat. Excessive guide wear most likely occurred from improperly set valve bridges and maintenance issues.

Valve wear caused by misadjusted 
This picture shows the seat wear on one of the valves. Most of the valves had similar wear and a few had severe wear.

Valve seat wear will reduce valve lash and effect when a valve opens and closes which changes timing. It also effects scavenging, valve overlap and both valve and valve seat cooling. Lack of proper scavenging can also cause excessive heat build up in the exhaust ports and damage the valve guides. If there is a variation in wear of the two valves that share a common crosshead it can create side loading of the valves and wear in the valve stems. Crossheads must also be inspected prior to installation for both wear and orientation. 

D. Valve failure caused by bending fatigue 

The following picture shows a complete valve and the stem of the failed valve. The green arrows will point out where the failed valve broke in relationship to the complete valve. The blue arrow will point out where the valve head is welded to the valve stem.

The next picture of the stem of the failed valve shows the welded area of the stem a little clearer.

The next picture is a view of the valve stem break that is a bending fatigue failure.

Final picture of damage to cylinder head at failed cylinder. Note valve missing head at bottom shows remains of stem, this occurred as a secondary failure.

The failed valve had no inclusions or flaws in the break area. The valve did not break at the weld attaching the valve head to the shaft as claimed. The most likely cause for the failure is bending fatigue caused by misalignment.

E. Valve failure caused by impact loading

• The yellow arrow pointing at the valve retainers show they are undamaged.
• The rotators indicated by the blue arrow were inspected and function properly showing no damage.
• The two valves in the foreground show no galling in the stem indicating they did not seize in the valve guide.
• The Green arrow points to the broken valve, note location of break.

In the above picture the green arrow points to the valve head and the area the valve broke. This is in the weld area of the valve, the strongest part of a valve. This type of failure only occurs from impact. The second valve is bent in the same location but did not break. All the possible causes such as valve sticking in guide, broken valve retainers show no signs of failure as seen in the first picture.