How It Works: Gate Valves


All valves are designed to stop, allow, or throttle the flow of a process fluid. Gate valves—one of the original valve designs—are ideally suited for on-off, primarily liquid, service. A gate valve functions by lifting a rectangular or circular gate out of the path of the fluid. When the valve is fully open, forged steel gate valves are full bore, meaning there is nothing to obstruct the flow because the gate and pipeline diameter have the same opening. This bore diameter also determines the valve size. An advantage of this fullbore design is very low friction loss, which saves energy and reduces total cost of ownership.






Gate valves can have a rising or nonrising stem design. Rising stems are attached directly to the gate and provide a visual indicator of the valve position. Nonrising stems are generally threaded into the upper part of the gate and have a pointer threaded onto the top to indicate position. Nonrising stem designs are ideally suited for applications where vertical space is limited, in well applications, and where scraping or pigging is not required.






Gate valves are designed with a sealing unit to provide a tight seal around the stem. Our patented single loaded-spring (SLS) stem seal design, used in thread and socket gate valve and WKM Pow-R-Seal double expanding gate valves, provides superior leak protection and a self-adjusting seal designed to reduce maintenance.






Bonnets


Gate valves generally have one of four types of bonnets, which provide closure to prevent fluids from leaking out of the valve. Screw-in bonnets are simple, durable sealing units that use pressure to seal. Union bonnets provide easy access to the valve body for applications that may require frequent maintenance or inspection. Bolted bonnets are generally used for larger valves in higher-pressure applications. Pressure seal bonnets are designed for services with pressure in excess of 2,250 psi [15 MPa].






Applications


Because of the diversity of construction materials, trim offerings, and design combinations available with pressure sealing gate valves, they are appropriate for a wide variety of applications. From high-temperature coking units to food and pharmaceutical services, gate valves can be trusted to reliably perform.






The protected seat-face design of double expanding and slab gate valves eliminates degradation of the seat face caused by debris in the process fluid, which makes them ideal for liquid service. When additional protection is needed at points in pipeline applications where operational integrity is vital and the consequences of environmental exposure are higher, such as near waterways and municipalities, double expanding gate valves are a particularly wise choice.






Our smaller 2- to 4-in nonrising stem version of the Pow-R-Seal API 6A expanding gate valve is commonly used in wellhead manifold systems because of its reliable mechanical seal and high pressure capability.






Drilling manifold systems can also be easily designed to use certain gate valves, such as the Cameron DEMCO valve DM series, with space-saving and versatile mounting designs.






In the power industry, NEWCO gate, globe, and check valves and DOUGLAS CHERO forged-steel gate, globe, and check valves are ideal for standard and critical applications, such as steam distribution in power plants. By replacing the body and bonnet flanges with a welded connection, the design of this valve eliminates a leak path, reduces weight, and simplifies the application of exterior insulation. This, in concert with the forged steel body, provides the highest integrity sealing available.






For challenging subsea environments, where pressures are extremely high, temperatures are low, and operation is difficult, subsea manifolds that integrate valves and interface panels are used for critical isolation. The simple design of the Cameron RING-O subsea valve is ideally suited for integration into these systems and can be actuated manually, via ROV, or hydraulically for ease of operation.






The research presented was conducted to quantify the effects of butterfly and forged flange gate valves located upstream water meters with diameters larger than 50 mm. Errors caused by these valves can have an enormous financial impact taking into consideration that a small percentage of variation in the error of a large meter is typically related to a significant volume of water. The uncertainty on the economic impact that a valve installed upstream of a medium size water meter leads to many water utilities to oversize the meter chambers in order to mitigate the potential negative errors. Most manufacturers approve their meters for a specific flow disturbance sensitivity class according to the standard ISO 4064-1:2018. Under this classification, a correct operation of the meters requires a certain length of straight section of pipe upstream the meter. However, this classification of the meters cannot consider all types of flow perturbances. For this study, two types of valves, butterfly and gate, were tested upstream ten brand-new water meters from six different manufacturers constructed in four different metering technologies: single-jet, Woltmann, electromagnetic and ultrasonic. In each meter unit was tested at five flow rates, from minimum to the overload flow rates. The tests were conducted with valves set in different orientations, closing degrees, and upstream distances from the water meters under study. The research shows that the valves used can produce significant deviations in the measuring errors with respect the errors found for undistorted working conditions.






In this article, PIF spoke to leading Scottish valve distributors, BM Engineering Supplies, to explore the similarities and differences between gate valves and knife gate valves. Although these shut-off valves are both used in on/off applications involving viscous fluids, there are some noteworthy design variances to consider when deciding which is best suited your process application. Here, we put the two head to head to see which valve type is better.






How does a gate valve work?


Gate valves open by lifting a round or rectangle-shaped gate out of the path of fluids. The sealing surface found between the gate and the seats is planar. As such, gate valves will often be used when a straight line of flow of fluids is required with minimum restriction. This popular shut-off valve type features a flat fate closure which slides either in and out, or up or down between two parallel plates to open and close the valve. With this in mind, gate valves are often used for shut-off operations as opposed to flow regulation operations as on or off are the only two settings available.






One of the primary advantages afforded by using gate valves is that the pressure which drops can be very low when forged steel globe valves are fully open. While they are useful as on/off valves, they can also equally competent at performing bi-directional action. However, gate valves do require a large force to operate. Larger sized gate valves even require automatic actuators to operate. Gate valves are also not the quickest valve type to open or close. They also not the best suited for space-sensitive applications as they tend to take up more space compared to other valve types. In some cases, gate valves can also be prone to thermal expansion and shrinking when exposed to high-temperature fluctuations. As a result, this may provide applications with unwanted leakage.






How does a knife gate valve work?


Originally designed for the pulp and paper industry, knife gate valves perform exactly at their name implies. Stringy pulp would impinge between the wedge and sand seat of a normal gate valve to prevent flow shut-off. Knife gate valves feature a sharp edge to cut through the pulp and seal, a useful attribute which has seen this shut-off valve type become invaluable to applications which either deal with viscous fluids or ones which have a similar risk of impingement.






Knife gates are cheap, easy to actuate and light. They are advantageous in applications involving viscous fluids such as sludge and slurry because their blades can cut through thick liquids with ease. As such, they are generally specified in larger sizes for the handling of thicker flows in demanding applications. Despite this, knife gates are known for their low-pressure limitations. As such, this makes them a less desirable shut-off valve for applications which require cleanliness or optimal hygienic conditions.






Buy gate and knife gate valves from BM Engineering Supplies


As an established and trusted valve distributor to the Scottish process industry, BM Engineering Supplies stocks a comprehensive range of both thread and socket globe valves and knife gate valves. For gate valves, BM Engineering is the Scottish partner for one of the UK’s market-leading valve wholesaler, Leengate Valves. Their range of gate valves is approved to ISO 9001 standards and other relevant industry governing bodies. While for knife gate valves, BM Engineering partners with Orbinox UK, one of the world’s leading manufacturers of knife gate valves.