Through conduit double expanding gate valves
Through conduit gate valves are manufactured as per standard API 6D. These valves are preferred for dirty (suspended/abrasive) or fouling service because of the fact that, unlike normal gate valves, their wedge/gate (closure member) is not open to flow stream and so free from jamming or obstruction. These valves are suitable for onshore and offshore application and can be used for isolation purposes.
Article by Yogesh Jangra, Fluor Daniel India Limited
Double expanding gate valves have a gate (wedge) split into two parts, mainly male and female segments. The male and female parts of the wedge are often termed as gate and segment by different manufacturers. Normally the gate and segment have a cam retained design (control arm or lever lock arm). Some manufacturers will also use a spring retained design for the gate and segment. The design of the gate and segment is such that the segment engages with the bonnet and body stop while in open and closed positions, so the gate slides up and down respectively.
Due to the angle between the gate and segment a sliding movement will push the gate and segment towards the seat to achieve sealability. During travelling for open and closed positions the gate slides over the segment due to the design of the angle and cam arrangement (control arm or lever lock arm) which holds the two parts together to maintain a tight seal.
This unique feature of the expanding gate valve (along with body drain and relief valve arrangement) makes it suitable for double block and bleeds (DBB) as well as double isolation and bleed (DIB). Sealing is achieved independently of pressure in the flow line. The flow is bidirectional, however for unidirectional flow the preference is for the segment (driven) to be placed downstream. This will avoid unnecessary load on the locking mechanism.
Full bore is quite achievable in these valves, a feature which makes this valve most suitable for the use of a pig launcher or scraper. Full bore also allows these valves to operate with negligible pressure drop. The valves have drain and relief arrangements (with expanding gate feature), so a gas tight shut off feature is also quite achievable.
Expanding gate valves have a heavier valve body casting when compared to a normal gate valve, so special consideration should be given to its layout. Normally, the body height (from valve center line to bottom of the valve) is too high and it becomes necessary to elevate the line (layout perspective) to accommodate the extra body beneath the valve center line, which is not the case with normal gate valves (API 600). The orientation of expanding gate valves is also important since manufacturers often have reservations about installing them with respect to the direction of the flow line and the stem. API 6D does not specify a preferred installation, nor does it recommend any change in design, but advises the manufacturer’s recommendation to be considered (this is normally the installation and operational manual).
The preferred valve orientation is always horizontal bore (flange to flange) and vertical stem. Valve orientation in vertical bore (flange to flange) with horizontal stem is also employed but valve orientation in horizontal bore (flange to flange) with stemhorizontal (or inclined at angle) is forbidden. In other words, we cannot place the valve in a horizontal flow with a horizontal stem or any inclined direction. Although a guide plate (or skirt) can be arranged, whose function is to guide the gate/segment in an open/closed position, it cannot sustain the weight in a horizontal/inclined direction. A robust guide plate (or skirt) cannot be designed, since it can also interfere with the cam arrangement (lever lock arm/control arm).
The weight of segments can also be enormous (many tonnes) and for higher sizes and ratings this can cause valve stems to bend. A last resort may be modification in the design of valve body altogether, and to consider supporting the gate/segment with an additional arrangement as this will require space between gate/segment and valve body. Such a modification incurs additional cost, schedule and risk implications when running projects, and may not be feasible at all.
The API 600 2015 edition mentions that the design of normal gate valves should ensure correct alignment of gate and stem for all orientations. For sizes DN 650 and above, as a minimum, wedge guides and body guides are hard-faced to allow the valve to take any orientation. But then it also talks about manufacturers providing in their installation and operational manual any limitations as a result of stem and valve orientation. API 6D does not specifically address this issue and dictates for installation and operational manual for valves. Designers should discuss the issue extensively when obtaining technical clarifications from manufacturers, rather than waiting for the installation and operational manual after the purchase order.
About the author
Yogesh Jangra has served in engineering roles for Engineering, Procurement and Construction (EPC) companies for upstream and downstream operations. He has a bachelor’s degree in Mechanical Engineering from CRSCE, Murthal, India. Currently, he is serving as Piping Material Engineer at Fluor Daniel India Limited. He has a total experience of 11+ years in Piping Material Engineering. He is also a RDG member of ASME B 31.3 committee.