Dec-2009

AGI case study

An integrator saves time and expense with new ball valves and end connections for pressures up to 15 000 psig.

Bill Menz, Swagelok Company
Tony Taylor, AGI Packaged Pump Systems

Article Summary

Integrators charged with building chemical injection skids for medium-pressure applications face some formidable obstacles. Two of the chief obstacles concern critical fluid system components: medium-pressure instrument ball valves and reliable end connections for use with SAF 2507TM tubing. Under certain circumstances, instrument ball valves in the medium-pressure range are known to leak across the seat. After sealing at high pressure, they may have difficulty resealing at low pressure, especially if pressure in the system is being built up slowly. End connections for the valves pose a special challenge when the tubing required is made of SAF 2507, a corrosion-resistant material preferred in many aggressive chloride-containing environments.

Until recently, the market has not provided simple, easy-to-use, mechanical grip-type tube fittings for SAF 2507 tubing at pressures up to 15 000 psig (1034 bar). For such applications, the industry has relied on cone and thread fittings, which can be temperamental and difficult to fit and assemble.

AGI beta tests a new valve
An integrator that has built skids for most of the major oil companies, AGI Packaged Pump Systems, was acutely aware of the challenges concerning medium-pressure instrument ball valves and end connections for SAF 2507 tubing. Therefore, when it accepted a contract to build two medium-pressure skids with SAF 2507 tubing — a chemical injection skid and a methanol injection skid — AGI looked for a company that could provide new solutions.

Swagelok Company had developed, but not yet released, the Swagelok FKB series medium-pressure ball valve, employing a new technology that would enable the valve to seal and reseal reliably across the entire pressure range up to 15 000 psig.

Offering to beta test the valve, AGI built a testing fixture designed to put the valve through a rigorous set of cycles simulating conditions on a methanol injection skid for deep water exploration. The test employed methanol as the fluid medium and involved the following steps:
• A small pneumatic pump slowly built pressure to 15 000 psig on a closed FKB series medium-pressure ball valve
• The valve was opened and the pressure was released to atmosphere
• The valve was closed
• The process was repeated.

AGI planned to cycle the test valve until failure. The test ran for two-and-a-half days, with the valve completing more than 4000 cycles without failure. At that point, AGI was satisfied with the performance and stopped the test. Over the 20-year life of a chemical injection skid, an instrument ball valve is typically cycled 300 times, about once per month.

The challenges concerning medium-pressure ball valves apply to other oil and gas applications as well. In wellhead control panels, workover vessels or workover panels, medium-pressure ball valves are used to deliver hydraulic pressure to the large wellhead safety shut-off valves at the subsea wellhead. Pressures up to 15 000 psig are required because of the depth of the wells. Medium-pressure ball valves are also used on hydraulic power units, where hydraulic pressure for platform utilities is maintained. In all of these applications, leakage is a serious issue. The substances under pressure are often flammable.

Finding the right connection
Integrators are frequently under very tight time constraints when they are building skids for platforms. The project may be behind schedule even before the fabrication begins. Therefore, efficient component assembly is critical to keep a project moving.

Cone and thread fittings are time consuming to assemble. Any imperfection in the cone or the nipple increases the likelihood of a leak. Concerns about quality have led some owner companies to require that the nipples be purchased from the factory rather than made on site. If the tubing length turns out to be short, a new piece must be ordered, resulting in a delay.
Integrators are also concerned about vibration. A fitting must be able to hold up under vibration, not only during operation on the platform but also during transport to the platform.

AGI found a solution in Swagelok Company’s mechanical grip tube fitting rated for medium-pressure connections to SAF 2507 tubing. The fitting contains two ferrules. Both the front and back ferrules grip the tube during installation (see Figure 1). It is this robust grip that allows for working pressures up to 15 000 psig.

SAF 2507 is considerably harder than 300 series stainless steels, with a Rockwell C hardness of up to 32 (HV 318), as compared to AISI 316 and 304 stainless steels, which have a Rockwell B hardness of 90 or less (HV 185).
The stainless steel ferrules on the tube fitting are able to grip even SAF 2507 tubing because they are fully case hardened with the SAT12 service, a patented, low-temperature carburisation process. Surface hardness of the treated ferrules measures 1200 Vickers, which is comparable to tool steel or about three times harder than untreated stainless steel.

This carburisation process preserves the ductility of the stainless steel even as it hardens it. The ductility of the back ferrule enables it to hinge downward onto the tubing, creating a grip on the tubing over a large surface area. This grip helps to protect the fitting from vibratory stress.
The result is a tube fitting that can be assembled easily and tightened with simple tools. No electronic swaging unit is required. The connection can be taken apart and remade several times.

The medium-pressure tube fitting can be checked positively for sufficient pull-up at initial installation with a handheld gauge, a calibrated flat piece of steel that checks for the right distance between the nut and the body of the fitting (see Figure 2).