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 eliminating the risk of corrosion as a result of moving parts damaging the rubber surface of the wedge core. A fixed wedge nut design is therefore recommended.
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. Wedge shoes help ensuring that the rubber on the wedge surface is not worn through as a result of the friction between the wedge and the guiderail in the body. Make sure that the wedge shoes are fixed to the wedge and that the rubber layer underneath is sufficient to prevent corrosion of the wedge core.
Rubber: It is vital for the tightness of the valve that the wedge is fully vulcanized with rubber and that the rubber volume on the sealing area of the wedge is sufficient to absorb impurities in the seat. A strong bonding between the rubber and the wedge core is important to ensure a correct seal even when the rubber is compressed, and to prevent creeping corrosion even if a sharp object penetrates the rubber during closing of the valve.
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) External corrosion protection
The external corrosion protection is critical for the service life of the valve. A uniform and even epoxy coating in compliance with 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 ¦Ìm on all areas.
MIBK test: The curing of the epoxy coating is to be checked in a cross linkage test (MIBK test). One drop of methyl isobutyl ketone is put on a test piece. After 30 seconds the test area is wiped with a clean white cloth. The test surface may not become matt or smeared, and the cloth must remain clean.
Impact resistance: A stainless steel cylinder is dropped on the coated surface through a one meter long tube. After each impact the component is to be electrically tested, and no electrical breakthrough shall occur.
Freedom of pores: A 3kV detector with a brush electrode is used to reveal and locate any pinholes in the coating.
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 fulfill 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. For safety and health reasons a drinking water approved high quality EPDM rubber compound must be used where direct contact to drinking water occurs.
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) 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. It is important to consider that valves having an operating torque less than 5 Nm encourages the operator of the valve to close the valve to fast thus risking water hammer and pressure surges in the pipeline.
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.
* GSK stands for G¨¹tegemeinshaft Schwerer Korrosionsschutz, and is an independent quality association with about 30 members, all leading European valve and fittings manufacturers. GSK outlines requirements for the coating itself and for the control procedures of the finished coating.