The keys to choosing a gate valve
Part 1 of 2: Body & Wedge
- 01 Pre-purchase quality checks
- 02 Checking the gate valve body and coating
- 03 Checking the wedge, wedge guides and wedge nut
- 04 Does a gate valve always need a bypass?
- 05 Summary of Part 1
Compromising on quality when purchasing gate valves commonly leads to early replacement when the gate valve inevitably fails. The solution to decreasing total lifecycle costs, as outlined in our previous article titled “The Hidden Costs of Gate Valves”, is to use high-quality gate valves that last longer and have lower maintenance costs. This two-part guide will help you to recognize what makes a quality product stand out among the many gate valve options on the market. Part 1 covers pre-purchase quality checks, valve body & coating checks, and checks related to the wedge, wedge nut, and wedge guides.
Pre-purchase quality checks
Before making any purchases, you can confirm if a manufacturer’s gate valve meets some basic quality criteria essential to long service lifetimes:
Confirm the gate valve meets or exceeds code and standard requirements by checking:
- Is the gate valve safe for potable water? This includes both certification approval from DVGW, ÖVGW, KIWA, WRAS, or your country’s certifying agency, as well as using safe materials as outlined by the 4MS Initiative.
- Is the gate valve compliant with DIN EN1074-2 endurance testing requirements?
- Does the gate valve have both a GSK certification and a GSK production certification? Both are necessary to ensure product compliance.
Does the manufacturer have a proven track record of longevity? Hawle for example, still has gate valves functioning after over 50 years of underground service?
Checking the gate valve body and coating
Before installing a gate valve, you can perform some inspection checks on the valve body to ensure it will stand up to operating conditions. The initial checks should consist of the following:
From which materials is a gate valve made?
Of the two most commonly used materials, GJS400-15 ductile iron offers double the elongation at break over GJS500-7. The improved ductility of this valve makes it less prone to cracking and improves long-term durability. Gate valves made using GJS400-15 are typically heavier because they use increased wall thickness to compensate for the slightly lower tensile strength.
Steel bolts easily corrode, causing them to “stick” which complicates bonnet removal during maintenance or potentially causing the bolts to break when put under stress. Galvanized bolts are more resilient to corrosion than plain steel, but stainless steel bolts are the most resilient and are the best option for gate valve longevity.
Visual inspections of the gate valve body
Visually inspect the gate valve for surface irregularities from the casting process, such as pores or grooves. The internal and external valve surfaces should be smooth, especially along the length of the wedge guides. Ensure that the surfaces of the bonnet flange are machined by running a finger along them. If they are left as-cast and not machined, the bonnet gasket might be squeezed unevenly, resulting in premature failure and leakage.
Tip: Advanced checks
For those that want to be certain their gate valve body is top quality, the following expert inspection checks can be performed:
Conduct testing on the bonding strength and the impact strength of the valve’s protective coating. Coating quality is highly dependent on the coating material, coating process used, and surface preparation. As a baseline standard, GSK specifies that coatings must have a minimum bonding strength of 12 N/mm² or greater. For best-case long-term performance, however, Hawle recommends aiming for a bond strength of at least 18 N/mm².
Checking the wedge, wedge guides and wedge nut
The gate, also known as the wedge, is the central element of a gate valve, and any damage to this component means a shorter life span for the valve as a whole.
Visual inspections of the wedge
- Inspect the wedge guides in the body to ensure there are no irregularities that could lead to high actuating torques and high abrasive forces.
- Confirm the wedge guiding system uses low-friction, plastic-covered nose pieces with a groove in the body. A common alternative is where rubber is forced to glide on the valve body, making valve actuation difficult and eventually causing leakage when the rubber is damaged.
- Ensure that the wedge is completely rubberized, leaving no exposed ductile iron surfaces on the wedge.
Structure of a wedge nut
- Inspect the attachment of the wedge nut to the wedge to determine if it is a form-fit or a press-fit. Form-fit wedges are generally preferred as they offer improved flexibility and can typically absorb water hammers due to their freedom of play. The downside to this freedom of movement is that it can cause abrasion on the rubber where the nut rests. A press-fit wedge nut on the other hand can transfer the force of water hammers to the spindle and subsequently to the bearing. This has the potential to bend the spindle.
- Ensure that the ratio of the wedge nut thread length to the spindle diameter is 1.2 or higher. The lower this ratio is, the higher the wear on the wedge nut threads as the load is distributed over a smaller area.
- Confirm that when the valve is completely opened, movement is stopped by the wedge nut contacting the spindle bearing. The wedge nut and spindle bearing are both brass, so it is unlikely there will be any damage between them. If however movement is stopped when the wedge nut or wedge contact the valve coating directly, this can result in coating damage and subsequent corrosion of the gate valve.
Tip: Advanced wedge checks
- Confirm any brass materials that could be in contact with potable water are, at a minimum, listed in the 4MS Initiative list of safe materials. Even better is to select lead-free materials, such as Ecobrass®, as a number of initiatives for completely eliminating lead from water supply networks are already underway. If not already applicable in your area, future-proof your water supply network by anticipating these lead-elimination initiatives to come in the near future.
- Conduct endurance testing to assess wedge functionality after 50, 250, and 2500 opening/closing cycles at nominal pressures. Also simultaneously record closing torque measurements to observe the difficulty in closing the valve while in service. An accelerated aging test can also be conducted by, for example, completing 250 cycles of opening/closing the wedge in a corrosive environment, such as saltwater at elevated temperature.
- Assess the sealing behavior of the wedge by spreading chalk over the entire sealing area, then close and open the valve. If narrow lines are left in the chalk, this is an indication that in the long term the valve will either not stop water flow reliably, or will require high actuation torques, which puts additional stress on the surrounding components. Also, check the width of the wedge’s “shoulders” to ensure it seals firmly around the entire sealing surface.
Does a gate valve always need a bypass?
For DN 500 and larger gate valves, confirm the valve does not come with a bypass valve as a stock feature. A mandatory bypass valve indicates that opening and closing the gate valve may be difficult without using the bypass.
High-performance gate valves of these sizes should be actuated easily by the handwheel without a bypass, even at 16 bar pressure.
Summary of Part 1
The first part of this guide covers a number of basic, advanced, and expert inspection checks to determine if a gate valve body and wedge components are of high-quality make and will perform well in service. From materials selection to processing methods, and from standards and testing compliance to intelligent design, selecting the right gate valve can make all the difference to your business but improving service life and decreasing lifecycle costs. Read Part 2 of this guide to learn more about the gate valve’s spindle and seals and to see a brief recap of the whole inspection process.
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