Valve World Americas had the opportunity to speak with Dustin Thompson, Engineering Supervisor at Franklin Valve LP, about expanding gate valves, and dual expanding plug valves, their intricacies, benefits and how they differ from other types of valves.
By Sara Mathov & Stephanie Matas
What is an Expanding Gate Valve?
An expanding gate valve has an internal closure mechanism consisting of multiple pieces that expand in line with the bore of the valve, creating a mechanically energized compression seal. “An expanding gate uses thrust generated by a threaded stem and drive, which pushes the gate mechanism downward, where it then expands horizontally so the seats compress in a horizontal motion,” said Thompson. “This means there is no vertical sliding or abrasion between the seating surfaces.”
An expanding gate valve would be beneficial for applications where isolation is extremely critical. “Examples include metering applications, tank farms, fueling systems and, multi product manifolds,” said Thompson. “If I have diesel on one side, and jet fuel on another, and a valve has any allowable leak rate, it will contaminate the jet fuel with diesel.” In this case, an expanding gate valve with positive double isolation-and-bleed (DIB) isolation, is useful to maintain critical isolation. Expanding plug valves can also be used in similar applications as expanding gate valves, “or whenever you need complete isolation for critical equipment or between two different fluids.”
What is a Dual Expanding Plug Valve?
A dual expanding plug valve has two motions to its operating cycle: a quarter turn and linear motion. “Similar to the gate valve, the internals consist of a taper plug, and two independent slip seals,” said Thompson. “Cycling from open to closed, the valve will quarter turn followed by the linear motion, which expands the slips. As that plug traverses downward, it horizontally expands the slips into the seal.”
Comparing the two valves, the best option would depend on the application, which is a better face-to-face match, and size requirements. “In smaller sizes, an expanding plug valve is more cost effective than the gate valve, but in a larger size, the gate valve may be expensive due to weight and manufacturing technique,” said Thompson. “The face-to-face dimension of a large gate valve can be much smaller than that of a large plug valve, but they are both double block-and-bleed DBB and DIB capable.” With a DBB valve, there are usually two unidirectional self-relieving seats, in contrast, a DIB valve utilizes one or two bidirectional seats.
What Sets Them Apart?
One of the biggest differences between an expanding gate valve and a non-expanding valve is that an expanding gate valve has reduced or even zero friction on seats while cycling. A non-expanding gate valve, such as a slab gate or a wedge gate, has friction on the sealing surfaces. “That friction is going to cause wear over time and limits seal material options,” said Thompson.
Another important differentiating feature is that mechanically expanding valves use thrust generated by the drive mechanism to create the seal as opposed to other valves, which commonly require line pressure or seat springs to energize the seal. “If a seal is line pressure dependent and pressure drops below a certain threshold, the seal could deenergize, and you will have leakage.” He explains that this could lead to contamination if the line pressure drops too low. “Whereas, with an expanding gate valve, because the seat is energized from internal thrust generated by the mechanism, you will not leak due to low pressure situations.”
Expanding valves are usually soft seated. While many gate and quarter turn valves commonly employ PTFE or metal seals. For expanding valves, an FKM or FFKM elastomer seal can be used, with different variants depending on chemical compatibility and operating temperature.
Weighing the Benefits
One of the benefits of expanding gate valves is that they can be soft seated versus metal seated. “What comes with soft elastomer seats is a zero-leakage seal,” said Thompson. “Under API 6D, the acceptance criteria for soft seated valves, is zero leakage during hydrostatic testing. So soft seated valves may perform better than some metal seated gate valves which have allowable leakage rates.”
Compared with more traditional quarter turn valves, a dual expanding plug valve has the benefit of having zero friction between sealing surfaces during cycling. “A ball valve quarter turns and the seats are always touching, so you are getting friction on those sealing surfaces,” said Thompson. “That friction wears on the sealing elements. In an expanding plug valve, the slips are retracted from the wall prior to completing the quarter turn, so you have no friction.” Less friction means less wear, and maintenance over time. “In a traditional quarter turn plug valve, the internals of the valve must be packed with grease which creates the seal. Expanding plug valves do not require any sealant injection for the seats to seal.”
Another benefit of the dual expanding plug valve over a lubricated plug is that it is easier to service if there is a leak due to damage on the plug surface. Normally, plug and body sealing surfaces are machined and lapped to create a fitted pair, making interchangeability of lubricated plugs extremely difficult if not impossible without pulling the valve out of line. With an expanding plug valve, the slips can be removed and easily replaced in line with minimal downtime. “The repairability of an expanding plug valve and expanding gate valves is much better than a non-expanding option,” said Thompson. “The ease of inline maintenance is a highlight for both types of expanding valves.”
One of the potential challenges with expanding gate valves is cost. “They are almost always going to be more expensive than a non-expanding gate valve upfront,” said Thompson. “But it fills a different need, providing a higher integrity seal than a wedge gate for example.” All valves have applications they are suited for, so although expanding gate valves are more expensive, they fill a different niche in the industry. “It really depends on if in the valve is in the right spot with the right specs. If you need it, you could not use anything else.” The ease of maintenance for these valves does help address the running cost of these valves overtime compared to their non-expanding competition.
Another possible challenge is the size and weight of an expanding gate valve. “It may be larger than a non-expanding version, because there are more internal components that have to fit,” said Thompson. Also as expanding gate valves are thrust seated, which gives them their DIB capability, actuation could be more expensive.
Expanding gate valves are relatively new, and different designs for manufacturing are upcoming. “Franklin Valve just released its new compact expanding gate valve, which is called a symmetrical expanding gate valve (SEG),” said Thompson. “This includes some new technologies for the market.” Normally, there is a preferred flow direction because the mechanism is not symmetrical. With the new technology, sealing can be truly bi-directional.
The dual expanding plug valve design has been on the market for nearly 50 years and is well vetted as a design with a history of performing well. “Some manufacturers moving forward will update their packing for these valves to comply with new fugitive emissions requirements,” said Thompson. “There are also innovative coatings being developed, so rather than using standard coatings, customers will have the option to try newer, lower friction coatings to reduce operating requirements.”
About the Expert
Dustin Thompson is the Engineering Supervisor at Franklin Valve LP, Houston, TX. He Graduated Summa Cum Laude with a Bachelors in Mechanical Engineering Technology from the University of Houston, and has worked in valve design and manufacturing for Franklin for the past 6 years. His experience within those years includes dedicated product design, development of quality management processes, hands on research and development and project management.