There are three (3) predominant valve failure mechanisms, as follow:
- Internal passing when valve is expected in close position.
- External leakage to environment via gland, body split or drain-vent connections.
- Valve not operable, locked in certain positions.
Due to numerous populations of valves and its operational complexity in oil and gas plants, individual assessments are critical yet not viable. Consequently, maintenance strategy for valves run to failure.
Only valves with critical function or special handling application can be justified for periodical inspection and preventive maintenance.
As a valve specialist, the process to predetermine valve failure and its root cause is through observation and every information available.
Below are examples of valve failures, but not limited to:
- Scratch; noticeable by one or more hairline sizes marking over the sealing area.
- Scuffing or Scoring; relatively bigger indentation compared to scratch. Scuffing direction is parallel against the direction of component movement.
- Galling or Seizing; relatively wider area and located on the circumferential area of sealing contact. This failure usually indicates whether stem-gland packing or round obturator-seat(s) affected by excessive friction while attempted to be operated. Or whether female versus male thread type connection were not sufficiently lubricated or fully centered.
- Installation error; all valves should be delivered along with its IOM (Installation and Operation Manual). However, this is not always the case, some valves delivered without IOM or IOM available but not being followed by installation team. Failure below can be prevented by installing the rubber liner lug type butterfly valve in partial close position. Which causes the seat being bulged by the disc, thus when flanges are tightened, it will keep the bulge in place and act as an obstruction whenever the valve is about to be operated.
- Miss-handling valve operation that exceeds its design limit in terms of one or more of following aspect: more than 110% Cv, high kinetic energy endured by its component, design pressure threshold, or design temperature threshold.
- General corrosion is the most common type of corrosion caused by a (electro)chemical reaction that results in the deterioration of the entire exposed surface of a metal. This can be prevented by coating method or cathodic protection. As for the valve internal surface, the thickness of the wall lining already takes into account for corrosion thus the thinning rate takes a relatively long period.
- Localized corrosion; usually targets certain area of the metal component exposed to a service. There are three types:
- Pitting: initiated from small hole(s) or cavity, usually forms in the metal surface, which results a de-passivation of a small area. This area becomes anodic and the remaining metal becomes cathodic, simultaneously producing a localized galvanic reaction. The corrosion appears as moon-surface like craters in small diameters. If it’s located on the sealing area it will cause leaking and if left untreated, it may result deeper crack that will jeopardize the integrity of a particular component.
- Crevice corrosion: Like pitting, crevice corrosion occurs at a specific location. This type of corrosion is often associated with a stagnant micro-environment, like those found under gaskets, washers and clamps. Acidic conditions, or a depletion of oxygen in a crevice can lead to crevice corrosion.
- Filiform corrosion: A special form of crevice corrosion in which the aggressive chemistry build-up occurs under a protective film that has been breached.
- Galvanic Corrosion; occurs when two different metals are located together in a corrosive electrolyte. One metal act as anode and the other as cathode. The anode, or sacrificial metal, corrodes and deteriorates faster than it would alone, while the cathode deteriorates more slowly than it would otherwise.
- Environmental Cracking; corrosion process that result from a combination of external conditions affecting the metal (e.g. chemical, temperature and stress-related conditions, etc). Normally segregated in several categories: Stress Corrosion Cracking (SCC), Corrosion fatigue, Hydrogen-induced cracking and Liquid metal embrittlement
- Flow Assisted Corrosion (FAC); results when a protective layer of oxide on a metal surface is dissolved or removed by the medium, exposing the underlying metal to further corrode and deteriorate. Depending on the cause, it can be differentiated into following:
- Erosion-assisted corrosion;
- Impingement; particle impact against metal surface
- Cavitation; kinetic energy resulted by this phenomenon further damage the surface
- De-alloying; is a selective corrosion of a specific element in an alloy metal which results a porous copper.
- Intergranular corrosion; is a chemical or electrochemical attack on the grain boundaries of a metal. This often occurs due to impurities in metal, which are present in higher contents near grain boundaries.
- Fretting corrosion; due to repeated wearing on rough surface between relatively two rotational moving components.
- High-temperature corrosion; some medium might contain vanadium or sulphates can which under high temperature can form a compound with a low melting point. These compounds are very corrosive towards alloy metals, including stainless steel. This corrosion can also be caused by high-temperature oxidation, sulphidation and carbonization
7. Deposit of scale;
Valve in general have sub-volumes within which are exposed to medium. However, in this area the medium is not necessarily flowing, hence creates a dead-end effect which traps fouling and scaling. This area usually named cavity / pocket / pit / groove. Once the scaling is built-up, hardens or coagulates, it tends to block certain parts’ movement. If scaling built-up located inside the valve’s cavity, it can block the valve movement to reach fully close or open state, thus causing damage on the sealing area.
When excessive force applied towards the plate or axial rod, it experiences plastic deformation, in radial direction. This is caused by hand-wheel over torque by cheater bar or a pre-existing stuck valve being operated with force without prior prudent assessment.
- Adhesive Wear – material loss due to sliding motion between metal surfaces
- Abrasive Wear – material loss due to cutting action of hard particles
- Cavitation Wear – bubbles form in liquids due to sudden pressure change and then collapse producing mechanical shock on the metal surface
- Corrosive wear – material loss due to chemical attack
- Erosive wear – form of abrasive wear due to particles suspended in a fluid
- Surface fatigue wear – material loss from repeated sliding or rolling motion
Instead of being radially deformed, some spindle/stem are tangentially deformed. Often due to limited room in the radial direction and occurs for ductile material. Some O-rings may also be twisted during assembling due to excessive clearance of the groove
Improper installation of certain parts against counter parts with respect to perpendicular angle. Normally caused by human error during installation or wear lead to volume loss over-time.
Relatively softer metal in certain area being deeply truncated.
14. Poor workmanship;
During design or manufacturing, mistake over valve material selection or not adhering to formal applied standards.
15. Lack of caring during preservation or warehouse handling