How to select the right gate valve

1) Wedge design

The wedge is the sealing part of a gate valve and is therefore crucial. Consider the following:

Wedge nut:

The wedge nut connects the wedge to the stem. There are two basic wedge nut designs:

• A loose wedge nut design where the brass nut slides in a slot in the wedge core
• And a fixed wedge nut design where the nut is expanded in the wedge core. With a fixed wedge nut design the number of movable parts is reduced, thus:

• Minimal vibration and thus no abrasion of the rubber
• Dezincification resistant brass with lubricating ability
• Sealed with rubber at the top and bottom
• No corrosion, malfunction or water hammer

Wedge guides and shoes:

The wedge is exposed to friction and stress forces when the valve is opened and closed during operation of the pipeline. Guides in the wedge fitting to corresponding grooves in the body help stabilizing the wedge position during operation and ensure that the stem does not bend downstream due to the flow velocity.

Double bonding process:

The wedge core is immersed in two different baths providing:

• a primer to prevent corrosion
• bonding between rubber and ductile iron core

When peeling off rubber from the core, it is still covered by rubber, and ensures ultimate adhesion and prevents creeping corrosion

2) Rubber quality

The rubber quality is critical for the durability as well as for the valve function. The rubber must be able to withstand continuous impact from impurities and chemicals without being damaged and it must be able to absorb small impurities in the seat to close tight. Consider the following: 

• Compression set: The compression set means the rubber’s ability to regain its original shape after having been compressed. The EN 681-1 standard states the minimum requirements for the compression set value, but the better the compression set, the better is the rubber’s ability to regain its shape and close 100% tight year after year. 
• Formation of biofilm: Organic substances migrate from the rubber compound and act as nutrients for microorganisms, which will then start forming biofilm causing contamination of the drinking water. Select valves with a wedge rubber that ensures minimum formation of biofilm. 
• Resistance to water treatment chemicals: Chlorine and other chemicals are commonly used to clean new pipelines or disinfect old ones. Ozone and chlorine may also be added in low concentrations to make the water drinkable. The rubber compound must not degrade or crack as a result of chemical treatment of the drinking water, as it would cause corrosion of the wedge core. 
• Drinking water approval: All rubber components in contact with the drinking water should carry a drinking water approval. If no local approvals are required, the rubber in direct contact with the drinking water should hold one of the major approvals like DVGW/KTW, KIWA or NF. 

3) Stem design: 

 Stem shall be made of stainless steel with rolled threads increase the strength and bring about low operating torques

4) Tight construction: 

 There are two important design issues:

• Stem sealing: The sealing placed in the bonnet around the stem retaining the pressure inside the valve/pipeline. Stem sealings should always be designed to be maintenance-free and should last the service life of the valve or at least fulfil the service life demands according to EN 1074-2. The main seal retaining the inside pressure should preferably be designed as a hydraulic seal giving tighter seal with increased internal pressure. Backup seals should be placed around the stem. To protect the sealings against contamination from outside, a sealing should be placed around the stem on the top. 
• Bonnet/body sealing: Tightness between the bonnet and the body can be obtained by using a gasket embedded in a recess in the valve. This design ensures that the gasket will remain correctly positioned and not be blown out as a result of pressure surges. To protect the bonnet bolts against corrosion the bonnet gasket should encircle the bolts, and the bolts should be embedded in the valve in such a way that no threads are exposed to the surroundings. 

5) Corrosion protection

The internal and external corrosion protection is critical for the service life of the valve. A uniform and even epoxy coating in compliance with DIN 30677-2/ DIN 3476 part 1, EN 14901 and GSK* requirements is recommended and involves the following: 

Blast cleaning:

According to ISO 12944-4.

Layer thickness:

Min. 250 μm on all areas. 

Pore-free coating:

The coating must be completely free of penetrating pores to avoid subsequent corrosion of the casting underneath. How to make it? - A 3V holiday detector with a brush electrode is used to electrically reveal and locate any pores in the coating.

Impact resistance:

The impact resistance test is carried out at room temperature right after the coating process by means of a stainless steel cylinder dropped on the coating surface through a one meter long tube corresponding to an impact energy of 5 Nm. After each impact the component is electrically tested, and no electrical breakthrough shall occur.

Cross linkage:

One drop of methyl isobutyl ketone are put on a horizontal epoxy resin coated surface of the test piece at room temperature. After 30 seconds the test area is wiped with a clean white cloth. It is checked that the test surface has not become neither matt nor smeared, and that the cloth remains clean. The test is carried out 24 hours after the coating process.

Adhesion: 

The adhesion of the powder coating is tested on one side of a test plate four times a year for each coating plant according to GSK guidelines using the punch separation method according to DIN 24624. The coating thickness over a dispersed area of the test item shall be within the range 250 μm to 400 μm. The test pieces are immersed for seven days in deionised water at 90oC, and then dried in an oven for 3 hours. A conditioning phase of 3 to 5 days in normal atmosphere is then allowed to elapse. No blisters may arise during the period immersed in the water bath. The surface of the test piece is degreased and then roughened with abrasive paper. The roughened surface is cleaned from dust with oil-free compressed air and recleaned. The adhesion is tested with a minimum pulling force of >12 N/mm2.

Cathodic disbonding: 

Cathodic disbonding tests are carried out on one of each type of component at least twice a year. No bubbles in the coating may develop during the test for cathodic disbonding. For this test, the coating thickness over a dispersed area of the test item shall be within the range 250 μm to 400μm..  

6) General performance

When operating a gate valve either by handwheel or by means of an electric actuator it is important to pay attention to the operating and closing torque.  

Operating torques:

The torque needed to operate the valve from the open position to the closed position, should be between 5 Nm and 30 Nm depending on the valve size.

Closing torques:

The torque needed to close the valve to a drop tight position. This torque should for handwheel operated valves be balanced against the handwheel diameter in such a way that it does not present the operator with a rim-force in excess of 30-40 kg. When operating the valve with an electric actuator or manual gearbox the torque should be within the limits of a standard range actuator. It is important to notice that the actuators normally have a torque range that is quite wide, and often it is the ISO flange connection between valve and actuator that determines the actuator choice. As a main rule valves with ISO flange connection should have max. closing torques as stated below:

• ISO flange F-10, maximum 120 Nm
• ISO flange F-14, maximum 500 Nm 
• ISO flange F-16, maximum 1000 Nm 

• Full bore: To enable the use of pipe cleaning devices the inside diameter of the valves should correspond to the nominal size of the valve.