Engineering Control Valves To Handle Black Powder Duty
Overview
In 2005, Saudi Aramco was facing significant challenges associated with black powder contamination of sales gas transmission pipelines. Some control valves were failing after just 44 days in service due to the rapid erosion of internal components. This resulted in excessive downtime and a fundamental lack of process control. In a bid to turn the situation around, Saudi Aramco collaborated with Severn to devise a new specification for pressure control valves (PCVs) deployed in contaminated applications.
Severn’s advanced valve services division was commissioned to develop a highly engineered control valve retrofit solution to meet this specification. The resultant valves were required to handle the rigorous demands of black powder duty under a five-year operational warranty.
Two PCVs were initially selected for retrofit. The combination of the pressure drops they needed to handle and the black powder contamination put them at the extreme end of ‘severe service’.
Following the retrofit, the valves delivered immediate performance improvements and remained in continuous service without incident beyond the five-year warranty period. When the valves were made available for an inspection after five and a half years, only mild internal erosion was observed and the valves were still functioning within the required performance and safety parameters. This paper explores the scale of the black powder challenge, the retrofit solution devised by Severn and outcomes achieved. The full report from the routine inspection of the PCVs is also available on request.
The challenge
Saudi Aramco was experiencing rapid control valve failure at some metering and pressure reduction stations due to black powder contamination. This microscopic contaminant ravages internal components when it travels at velocity within the process medium. It can collect in gas pipelines, leading to flow loss and premature failure of equipment. In some cases, PCVs were failing after just 44 days in service.
What’s more, the erosion of internal components was causing a serious safety concern due to the risk of fugitive gas emissions via the valve body. In 2005 Severn was appointed to conduct a detailed technical review of high profile valve failures associated with black powder at the Berri Junction Pressure Reducing Station in Jubail.
WO 20″ ASTM 600# PCVS FROM DIFFERENT OEMS WERE PRESENTED FOR STUDY
| OEM VALVE 1 | OEM VALVE 2 | |
|---|---|---|
| VALVE TECHNOLOGY | A multi stage pressure reduction trim had been used, with the conventional ‘under the plug’ flow direction. | A multi-labyrinth disc stack trim design was intended to control the velocity of the gas and with it the black powder to minimise its erosive effect. |
| PERFORMANCE | The valve was removed from service due to increasing noise and vibration and an inability to control the process. | The valve lasted 44 days, then failed to provide the level of control required at the metering station and was removed from service. |
| INSPECTION REPORT | The valve trim was severely eroded, passageways had been blocked and the ‘inside to out’ flow had resulted in severe erosion of the valve body walls. | One section of the stack had been eroded completely, creating a direct path for the gas with no velocity control. The unused areas of the stack (above the eroded section) were blocked with black powder and their ability to pass flow if needed was questionable. However, the ‘outside to in’ flow direction had prevented body wall erosion. |
The inspection concluded that primary failure causes were related to four major factors
1 Trim material
2 Trim & pressure envelope flow passage geometry
3 Fluid velocities developed with the pressure let-down process
4 Trim flow passage blockage / seizure
These findings were used to develop a control valve specification addressing the four factors to mitigate risk of failure. Severn was commissioned to retrofit the two OEM valves that had been presented for inspection, in line with the new specification.
Key aspects of the design
- Use of hard materials such as Tungsten Carbide in a combination suited to application characteristics.
- When component size or cost parameters prevented the use of solid Tungsten Carbide, a thick Tungsten Carbide cladding was used. This provides far superior erosion resistance to sprayed Tungsten Carbide coatings typically used for anti-galling in ball valve applications.
- Mechanical containment of the hard, relatively brittle components.
- Control of velocity levels and flow geometry through the trim and the valve body at magnitudes capable of sustaining capacity while reducing the erosion rate. Channelling high velocity media through a tortuous path results in better management of its erosive energy and is only achievable with a globe style valve design.
- New low failure rate manufacturing processes for the large Tungsten Carbide parts.
The trim materials were a variation of Severn’s Level 3 erosion resistant trim. Tungsten Carbide components made up the throttling and erosion risk areas whilst other materials of proven durability combined to ensure the five[1]year warranted operational life was met and exceeded. Valve design and material selection were underpinned by knowledge of actual valve performance, which was fed into the process to create a continuous improvement cycle
Both donor valves were successfully retrofitted at an approved third party workshop by local Severn technicians, then tested and commissioned. Severn P-Series 200in2 actuators were also retrofitted to each valve complete with side mounted hand wheel and instrumentation to provide lock-on air failure.
SOLUTION / BENEFIT MATRIX
| SOLUTION | BENEFIT |
|---|---|
| Control fluid velocity | Reduced particle velocity yields high reduction in erosion |
| Hard trim materials | Erosion resisting materials extend component life |
| Significantly increased flow path size, simple geometry | Reduces tendency to block More easily ‘blown through’ |
| Controlled plug-to-cage running clearances | Minimises axial clearance flow erosion |
| Maximised balance seal section in durable material | Prevents extrusion Reduces wear Reduces vibration |
The outcome
In service, the retrofitted PCVs immediately met the required performance criteria. This smoother operation continued for the long term, indicating that internal components were not suffering any adverse effects of erosion, proving the control valves could handle black powder duty. After five-and-a-half years of uninterrupted service, Saudi Aramco needed to install higher capacity valves and the retrofitted valves were made available for inspection. The following observations were made of the three-cage assembly
Conclusions
Black powder is a highly erosive contaminant that can significantly compromise plant safety, integrity and productivity. Ten years ago, Saudi Aramco’s most experienced and talented engineers were tasked with finding a solution to mitigate the problem cost[1]effectively. Severn’s highly-engineered, customised control valve technologies enabled them to achieve this goal.
Previously, OEM control valves were failing in as little as 44 days. Likewise, alternative valve types, such as ball valves, would struggle to handle the extreme pressure let-down requirements, and would be unlikely to last longer than six months.
They deliver on three counts
1 Minimal downtime
2 Maximum productivity
3 Better long term cost profile
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