CASE STUDY | HY-GRADE CHECK VALVES: A LOW COST ASSET FOR CSG

The client has a unique slurry and tailings system which operates at a pH of 0.9 to 1.2. The slurry is not only abrasive, but also corrosive, scale forming and slightly radioactive. Valve failures not only have an effect on plant mechanical reliability but also contribute to safety and environmental challenges. These failures have a negative impact on the total cost of ownership of the valving assets as there are costs associated with maintenance and repairs.

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First Stage of Extraction

Coal seam gas is extracted by pumping ground water into the well, which carries the CSG to the surface. This process is done with pressure through a wellhead and into the first stage of separation.

The CSG and water mixture flows out through the wellhead into a skid arrangement and wellhead manifold. It then enters the first stage of separation through a two-phase gas/water separator. This separator is essentially a gas scrubber and skid formation which removes 99% of the moisture and solid particles from the gas.

The pictorial arrangement is true for the GLNG and APLNG operations – a number of wellheads feeding into one manifold and a two-phase gas/water separator. The client utilises one gas/water separator for each wellhead to ensure maximum total up time if one separator stops producing.

It is predicted that after a number of years, the pressure of the wellhead will drop considerably before production ceases due to the declining reserve of CSG in the well.

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Fig. 1: Typical First Stage Dewatering Skid for CSG-LNG

The Problem

There are thousands of wellheads spread across a large, remote area which becomes expensive to access and maintain over time.

Every wellhead skid has two check valves. Each gas and water flow line from the separator also has a check valve. It is common for these check valves to fail and at that point, it ceases to perform its single function of preventing back flow either to the wellhead or separator. This issue has a negative impact on components upstream and downstream. Dirty media or, as in the case of one operation where the media is hypersaline, corrosive media can cause significant erosion and corrosion issues to the wellhead skid work and gas separator or to the valve itself.

The check valves specified for all three CSG-LNG operations are wafer duo checks. These are the most common type of check valves supplied in the world today. They operate when two half discs, attached to a shaft and stopper through the center of the valve, are opened by upstream flow velocity, and have springs to assist the discs to close when flow velocity drops. Duo check valve failures can be attributed to valve design, poor valve sizing and incorrect choice of materials.

Typical duo check valve bodies are cast in moulds and are therefore made with a standard inlet bore diameter. This design often results in disc chatter, overworking of the springs and shafts as the fluid velocity is often insufficient to fully open the discs of the valves. When the valve is only partially open it is subject to increased wear which leads to the valve not being able to isolate properly against reverse flow and eventually forcing the operator to replace the valve. Wear can also lead to loss of internal pieces of the valve downstream or failure of the shaft retainers and allowing product to leak into the atmosphere.

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The Solution

CGIS has been successful in providing a check valve into the first stage of separation for a CSG-LNG operation, that gives repetitive zero-leakage shut off while significantly outlasting the previously installed duo check valve

HY-GRADE is a US-made check valve using only US or Western European raw forged or wrought materials for all pressure containing components.

The HY-GRADE advantages over the current duo check valves are:

The ability to be sized correctly on an individual valve basis to prevent disc chatter.

In single phase flow or multi-phase applications where density is inconsistent, the inlet of a HY-GRADE check valve can be sized so that it provides enough flow velocity to keep the disc in the fully open position with no chatter. Due to the replacement seat insert, the same valve body can be used as the pressure and flow in the line changes over the course of the well life, the orifice size can be changed in the field for a reduced cost in comparison to a new valve.

Full bore retainer-less body- zero leaks to the atmosphere while giving the highest Cv (see figure. 2).

When comparing the fully machined HY-GRADE valve to a standard duo check valve, the duo check valve has a shaft and seat retainer positioned through the bore of the valve which reduces pipeline Cv and is subject to the abrasive/corrosive nature of the media. The shaft and discs are held in place by nuts, bolts and retainers external from the valve body making them a potential leak path. As can be seen from figure 2, the HY-GRADE design eliminates all those failure modes whilst producing a high Cv.

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Figure 2

Robust, single piece disc and stem design outside the flow path, with the ability to outlast other check valves with disc chatter (see figure. 3).

Sizing the inlet of a check valve is difficult when there is two or three phase flows as the density at any one time going through the valve is inconsistent and variable. If disc chatter is inevitable, then the one piece disc and stem significantly outlasts the duo check arrangement of 5 to 7 pieces.

Tilt seat – no spring design for lower cracking pressures – eliminates a failure point (see figure. 4).

For lower cracking pressures, HY-GRADE can eliminate the need for a spring and hence reduce the number of potential failure modes by producing a tilted seat which allows gravity to force the flap down and find its seated home. The angled seat and seating area on the flap means that even with the tilted disc design, HY-GRADE performs with significantly lower allowable leakage rate for metal seated valves as per API 598.

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Figure 3

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Figure 4

Conclusion

CSG-LNG operations use typical duo check valves to prevent backflow into their wellhead and separator arrangements at the first stage of dewatering.

Failure of conventional duo check valves in this application have a detrimental effect on the process both upstream and downstream. In addition, the cost to maintain these valves in such remote locations is high.

Duo check failures can be attributed to the design of the valve, lack of sizing and poor material choice.

HY-GRADE has a unique design which eliminates the failure modes currently seen when using duo check valves in this application. HY-GRADE’s manufacturing process is such that each individual valve can be correctly sized for the application as well as the ability to utilise special alloys without the need to produce large quantities for cost efficiency. Future wellhead flow reductions can easily be accommodated by simply switching out the seat retainers for ones resized to future flow regimes.

HY-GRADE has successfully proved itself as a low cost asset by reducing wellhead down time and decreasing the cost of wellhead ownership for a CSG-LNG operation.

Written by Pascal Hatzipalousis

Business Development Leader, CGIS

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