How to choose the right gate valve

Definition of a Gate Valve
Gate valves are utilized extensively across various applications and are ideal for both above-ground and underground installations. Choosing an appropriate valve is especially critical for underground installations to avoid expensive replacements. These valves function by being completely open or shut, and they are installed as isolating valves in pipelines, meaning they shouldn't be used for flow regulation. Gate valve operation involves a rotating motion of the stem either clockwise to close (CTC) or clockwise to open (CTO). This rotation moves the gate up or down along the stem's threaded section.

Applications of Gate Valves
Gate valves are preferred in scenarios requiring minimal pressure loss and an unobstructed bore. In their fully open state, gate valves offer a clear path for fluid flow with minimal pressure loss, making them suitable for use with pipe-cleaning pigs. As multi-turn valves, gate valves require several turns to transition between open and closed states, which also helps to minimize water hammer effects.

Gate valves are versatile and can handle numerous fluid types. For example, AVK's gate valves can operate under these conditions:
Potable water, wastewater, and neutral liquids: Temperatures ranging between -20 and +70°C, maximum flow velocity of 5 m/s, and up to 16 bar differential pressure.
Gas: Temperatures between -20 and +60°C, maximum flow velocity of 20 m/s, and up to 16 bar differential pressure.
Refer to AVK's installation and maintenance instructions for water/wastewater and gas applications.

Gate Valves with By-pass
By-pass valves are generally utilized for three primary purposes:
• To balance pipeline differential pressure, reducing valve torque requirements and allowing single-person operation
• Maintaining flow with the main valve shut but by-pass open to prevent stagnation
• Facilitating delayed filling of pipelines

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Parallel vs. Wedge-shaped Gate Valves
Gate valves come in two principal forms: Parallel and wedge-shaped. Parallel gate valves feature a flat gate between two parallel seats, with knife gate valves being a common variant. In contrast, wedge-shaped gate valves employ inclined seats with a similarly inclined gate. Most of AVK's gate valves are solid wedge-shaped, though knife gate valves are available for wastewater, along with parallel slide valves for gas supply.

Metal Seated vs. Resilient Seated Gate Valves
Metal seated gate valves were once prevalent before the advent of resilient seated variants. Metal seats involve a conical wedge design requiring a depression in the valve bottom for tight sealing. Resilient seated gate valves, however, feature smooth valve bottoms allowing free passage for impurities. The resilient rubber surface ensures drop-tight sealing by accommodating impurities within the valve. Depending on project requirements, AVK offers both resilient and metal seated gate valves.

Rising vs. Non-rising Stem Design
Rising stems are attached to the gate, moving up and down simultaneously, giving a visual indication of valve position and enabling stem lubrication. This type is suitable for above-ground installation.
Non-rising stems, conversely, are threaded into the gate and rotate with it, rising or lowering internally. These take up less vertical space and are suitable for both above-ground and underground setups. AVK offers non-rising stem gate valves with factory-mounted position indicators. While most AVK designs feature non-rising stems, rising stem options are also available for water, wastewater, and fire protection applications.

Choosing the Right Gate Valve To avoid expensive replacements, one must select a gate valve with robust construction. Here are five essential selection criteria:

1) Optimal Wedge Design
The wedge, being the sealing part, is crucial. Consider the following:

Different Wedge Nut Designs
The wedge nut connects the wedge to the stem. Two basic designs exist: a loose wedge nut sliding in a slot in the wedge core and a fixed wedge nut expanded within the wedge core. The latter reduces movable parts, thereby mitigating corrosion risks from damaged rubber surfaces. Fixed wedge nut designs are recommended.

Wedge Guides and Shoes
Friction and stress affect the wedge when opening and closing. Guides fitting into body grooves stabilize the wedge and prevent stem bending from flow velocity. Wedge shoes ensure the rubber on the wedge surface remains unworn by friction with the guiderail. Ensure wedge shoes are firmly attached and adequately rubber-coated to prevent wedge core corrosion.

2) Rubber Considerations
For valve tightness, the wedge must be fully vulcanized with rubber, and the rubber volume should absorb impurities in the seat. Strong bonding between rubber and wedge core ensures a proper seal, preventing corrosion even if objects penetrate the rubber. Need more information about the DIN Bellows Seal Gate Valve? Contact us now.

Rubber Quality – Compression Set, Durability, and Approvals
Rubber quality impacts valve durability and function. The rubber must handle continuous impurities and chemical exposure without damage and absorb seat impurities to close tightly. Consider the following:

Compression Set – Ability to Regain Original Shape
The compression set indicates rubber's ability to revert to its original shape after compression. While EN 681-1 standard outlines minimum requirements, better compression sets offer superior long-term tightness.

Preventing Biofilm Formation
Organic substances from rubber compounds nourish microorganisms, creating biofilms that contaminate drinking water. Choose valves with biofilm-resistant wedge rubber.

Resistance to Water Treatment Chemicals
Rubber compounds should withstand disinfection chemicals like chlorine and ozone without cracking, preventing wedge core corrosion.

Drinking Water Approval
Rubber in direct contact with drinking water should have relevant approvals (e.g., DVGW/KTW, KIWA, NF), ensuring its safety and compliance.

3) External Corrosion Protection
External corrosion protection is vital for valve longevity. A uniform epoxy coating compliant with DIN 3476 part 1, EN 14901, and GSK* standards is recommended and involves:

Blast Cleaning
Per ISO 12944-4 standards.

Layer Thickness
Minimum 250 µm uniformly.

MIBK Test
Coating curing checked with a MIBK test (cross-linkage test) using methyl isobutyl ketone.

Impact Resistance
A stainless steel cylinder drop test ensures coating durability.

Porosity Test
A 3kV brush electrode detector identifies pinholes in the coating.

4) Tight Construction

Stem Sealing
Stem sealing in the bonnet maintains internal pressure and should be maintenance-free throughout the valve's lifetime, ideally designed as a hydraulic seal for enhanced tightness under pressure. Backup seals around the stem and top stem seals protect against contamination. Use drinking-water-approved EPDM rubber where direct contact occurs.

Bonnet to Body Sealing
Bonnet/body tightness involves gaskets embedded in recesses to stay properly positioned and resist pressure surges. Corrosion-resistant bonnet gaskets should encircle bonnet bolts, with bolts embedded to avoid thread exposure.

5) General Performance
Pay attention to operating and closing torques for handwheel or electric actuator operations.

Operating Torques
Operational torque from open to closed should range from 5 Nm to 30 Nm depending on valve size to avoid water hammer effects.

Closing Torques
For drop-tight closure, balancing handwheel torque against diameter is crucial to avoid excessive rim force. Electric actuators or manual gearboxes should fall within standard torque limits based on ISO flange connection:
ISO flange F-10, maximum 120 Nm
ISO flange F-14, maximum 500 Nm
ISO flange F-16, maximum 1000 Nm

* GSK (Gütegemeinschaft Schwerer Korrosionsschutz) is an independent European quality association for corrosion protection standards among key valve and fittings manufacturers.

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