The global natural gas market continues to develop as additional liquefied natural gas (LNG) facilities come online across the world. These LNG facilities have many challenges including equipment such as valves which come in contact directly with the cryogenic process.
Considerations not only include fit for the application, but also how to safely and effectively perform any ongoing maintenance.
This article was first published in the July/August 2017 issue of Fluid Handling International Magazine, Issue 4, Volume 5.
…specifying valves with a lower amount of flange connections is critical and welding valve bodies to the pipe appears to be an obvious solution.
Taking this approach requires a way to service these valves. First, before we discuss designs to service the valve, let’s share how Alberto, Marco and Sergio describe triple-offset valves.
They define a triple offset valve (TOV):
…a valve that provides closing with no rubbing across the 90° rotation (differently from double offset valves) in view of a single, instantaneous point of contact between sealing elements only when closed position is reached. TOVs can handle bidirectional flow and are tight in both sealing directions up to extreme pressures due to torque seating and non-rubbing rotation achieved with three “offsets”.
Here are the three offset locations:
- The shaft is placed behind the plane of the sealing surface;
- The shaft is placed to one side of the pipe/valve centerline;
- The seat and seal cone centerlines are inclined in respect to the pipe/valve centerline.
This design does:
…ensure longer lifecycles, require lower maintenance, and provide an improved leakage performance compared to concentric types.
Critical in cryogenic applications is the seal ring, which:
…represents the key flexible element necessary to perform sealing against a seat…
For Vanessa triple offset valves, this seal ring:
…is always a one-piece solid ring in Nitronic 50 [hyperlink added], a material highly resistant to corrosion able to preserve important mechanical features at low temperatures.
There are two primary designs in how to service these buttweld valves—side-entry and top-entry. Side-entry valves have access to the valve body from a location on one side of the trim. There are several disadvantages including:
…the number of bolts exposed to the flow is doubled compared with integral seat TOVs and there is no effective prevention against bolt loosening. Overall, this design makes trim components prone to deterioration; therefore, regular maintenance becomes necessary.
From a safety standpoint:
…the trim cannot be extracted and maintenance must be performed with direct exposure of service personnel to safety risks.
On the other hand, a top-entry TOV:
…allows for all possible extraordinary repairs via complete removal of internal components. It is safe to claim that on a clean LNG service maintenance requirements are unnecessary thanks to the 100% metal seating and only extraordinary repairs may become necessary to fix tightness and operability-related issues.
…
The trim can be completely removed from the body and every component can be repaired or replaced.
They conclude:
Whenever inline access is required, the arguments in support of a top entry design are predominant… if a high-quality torque seat, metal-to-metal non-rubbing TOV is being selected, the top entry access will be used for extraordinary repairs (and not for ordinary maintenance) on clean cryogenic services.
Read the article for more specifics about side-entry and top-entry advantages and disadvantages and more about the suitability of the materials of construction for this demanding application.
You can also connect and interact with other valve experts in the Valves group in the Emerson Exchange 365 community.
