Valve Magazine - Spring 2012

WEIGHT AND SPACE SAVINGS In addition to operational advantages the TOV offers a significant savings in weight and space, which will have a positive impact in the overall spending on valves. The simple rule is that the larger the size and the higher the rating, the larger the savings a TOV can offer. With reduced weight, especially when using exotic materials, the total cost of ownership can be reduced.

Figure 4. Computational fluid dynamic image of a 3-inch ASME Class 300 valve

controlled torque.

For many valves, bubble-tight is adequate, but this fails to recognize that the valve is leaking. Bubble-tight, in accordance with FCI 70-2 Class V or Class VI or 598 and API 6D metal-seated means “allowable leakage.” For example, FCI 70-2 cl.VI for an 8-inch valve allows 40 bubbles per minute when tested at 3. 5 bar air; 598 leakage is measured in drops per minute (liquid) and bubbles per minute (air), while 6D and FCI 70-2 leakage is measured in milliliters per minute.

ADVANTAGES OF A TOV
UPSTREAM/OFFSHORE

TOVs offer several advantages compared to other valves typically used in most upstream applications. Compared to a soft-seated ball valve, the TOV is inherently fire safe with a non-rubbing rotation. No safety factors are required in actuator sizing and the valve opens against full delta P (no equalizing pressure is required). It requires less maintenance, offers better control capabilities, and ultimately reduces the space, weight and cost.

A soft-seated ball valve is also zero leakage. However, the soft components are limiting in terms of applicability and, more importantly, the life expectancy of the valve. A TOV achieves the same performance with metal-to-metal sealing, virtually no temperature limitation and a much more extended product life.

If necessary, a TOV can open against full-rated differential pressure in all classes up to ASME Class 1500. This is not possible with ball valves where it may be necessary to have a small bypass line to equalize upstream and downstream pressure to open the valve. This additional pipe and valves will not only increase the overall cost of the unit but will add potential points of leakage and failure.

Because of the weight and space savings, always an important consideration offshore, installation of TOVs is easy and simple when compared to ball valves. The valves are easy to operate and are an excellent choice for ESD (emergency shutdown) and HIPPS (high-integrity pressure protection system) applications. Whatever the required closing time is, it can be achieved and maintained for a very long time with a TOV. TOVs are used as standard for trip applications by major turbine manufacturers.

This article is focused on the zero leakage capabilities of TOVs in isolation applications. However, TOVs also can be used for control applications. Furthermore, the triple eccentric design offers advantages over traditional high-performance butterfly valves. One TOV can be used at the same time for both control and isolation, replacing the two valves normally used (butterfly for control and ball for isolation). Conventional ball valves are not used in control applications.

COMMON
MISCONCEPTIONS

While replacement of ball valves in downstream applications is common, there is still reluctance to using TOVs in upstream and offshore applications. A common perception is that when a TOV is fully open there is risk of damaging the seal ring because of the high velocity of the media. However, a computational fluid dynamic image of a 3- inch ASME Class 300 valve shows that the valve has the lowest media velocity near to the seal ring both upstream and downstream (Figure 4).

Another perception is that when the TOV is fully open, there may be a considerable pressure drop because of the presence of the disc and shaft in the middle of the line. A calculation with oil at 90 bar and a velocity of 3 meters per second in a 14-inch ASME Class 600 TOV shows that once the valve is in the fully open position, the pressure drop in this particular case would be only 0.07429 bar.

CONCLUSION Triple offset valves are a serious, robust and reliable alternative to traditional ball valves for many offshore applications in the upstream oil and gas segment. TOVs can provide reductions in weight and space and substantial cost savings with medium and large valves on the topsides of floaters and platforms, as well as in FPSO/floating storage and off-take turrets. VM