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Slab gate valve and wedge gate valves are all designed for use in power, oil and gas industry applications. They are the main and commonly used types of gate valves. They have the similar structure form the appearance, when fully open, they do not have a bore through the gate itself and the gate retracts into the valve body, saves height space that is necessary for slab and expanding gate valves. Today here we will introduce the difference between slab and wedge type gate valve.

 

Slab Gate Valve

Slab gate valves are comprised of a single gate unit which raises and lowers between two seat rings. Due to the fact that the gate slides between the seats, slab gate valves are suitable for the medium with suspended particles. The sealing surface of slab gate valves is virtually self-positioned and is not damaged by the thermal deformation of the body. Even if the valve is closed in a cold state, the hot elongation of the stem does not overload the sealing surface, and slab gate valves without diversion holes do not require high precision in the closing position of the gate. When the valve is fully open, the bore through is smooth and linear, the flow resistance coefficient is minimal, piggable and no pressure loss.

Slab gate valves also have some disadvantages: when the medium pressure is low, the metal sealing surface may not seal completely, instead, when the medium pressure is too high, the highly-frequency opening and closing may make the sealing surface wear too much when there is no medium or lubrication. Another drawback is that a circular gate that moves horizontally on a circular channel controls flow effectively only when it is at 50% of the valve’s closed position.

Slab gate valves applications

Single or double-disc slab gate valves are suitable for oil and gas pipelines with DN50-DN300, class150-900 / PN1.0-16.0 Mpa, operating temperature -29 ~ 121℃. In the case of pipeline with piggable design, use a rising stem gate valve with a diversion hole. The slab gate valve with a diversion hole with a dark rod floating seat is suitable for oil and gas recovery wellhead device. The product oil pipeline and storage equipment shall use a single gate or double gate flat gate valves without diversion holes.

Wedge type gate valves

Wedge gate valves are comprised of a tapered gate that is metal-to-metal sealing. Compared with a slab gate valve, wedge gate valves are not piggable because of the void that is left in the bottom of the valve body when the valve is open. The wedge design increases the auxiliary sealing load, enabling metal sealed wedge valves to seal at both high and low medium pressures. However, wedge gate valves with metal seals are often unable to achieve the inlet seal due to the specific pressure of the inlet seal caused by the wedge action. Wedge gate valves have a certain Angle, generally, 3 degrees or 5 degrees, resulting in accumulated material in the lower groove of the valve, the medium with the particulate matter may damage the sealed seat, make loose closure.

Wedge gate valve application

Wedge gate valves are generally used where has no strict requirements of the size of the valve and harsh occasion. Such as high temperature and high pressure working medium, the requirements to ensure the closure of the long – term sealing conditions. Normally, for the environment with reliable sealed performance, high pressure, high-pressure cut-off (differential pressure) and low pressure by the (small) differential pressure, low noise, have spirit point and evaporation phenomena, the high temperature, low temperature, cryogenic medium, it is recommended to use wedge gate valve such as the electric power industry, oil refining, petrochemical, offshore oil, tap water and wastewater treatment engineering of urban construction, chemical industry, etc.

The ball valve has been widely used in industrial applications due to its small flow resistance, a wide range of pressure and temperature use range, good sealing performance, short opening and closing time, easy installation and other advantages. The ball is an important part that plays a key role in the opening and closing function for the ball valve. In order to enhance the sealing performance and hardness of the ball, it is necessary to pre-treat the surface of the ball. So what do you know about common surface treatments for the ball body?

1. Nickel or chromium plating

Carbon steel body soft seated ball valve has poor corrosion resistance, the surface of the ball can avoid corrosion by electroplating a layer of alloy metal. Electroplating is the process of plating a thin layer of other metals or alloys on the metal surface by using the principle of electrolysis, so as to improve the corrosion resistance, abrasion resistance and surface aesthetics of the metal. When the ball is Austenitic stainless steel and the sealing ring is PEEK, it is suggested that the surface of the ball be plated with Nickel (ENP) or Chromium (HCr) to improve the hardness of the ball and seal. The coating thickness is generally 0.03mm ~ 0.05mm and above if there are special requirements that can be appropriately thickened, through this the plated ball hardness can be up to 600HV ~ 800HV.

2. Cold sprayed tungsten carbide

Cold spraying is a process in which compressed air accelerates metal particles to a critical velocity (supersonic) and physical deformation occurs after the metal particles impact directly on the surface of the substrate. The metal particles are firmly attached to the substrate surface, and the metal particles are not melted during the whole process. The advantage of cold spray is that the ball does not need to be heated, deformation and internal stress will not be generated in the spraying process, the thickness is well controlled, but the surface adhesion is not as good as that of spray welding.

Tungsten carbide is characterized by high hardness and good wear resistance, but the melting point is much higher than the general metal material point, about 2870℃, so only cold spraying tungsten carbide (WC) process can be used. The 0.15mm~0.18mm thickness of spraying tungsten carbide can achieve the ideal surface hardness, if there are special requirements can be up to 0.5mm ~ 0.7mm, the thicker the thickness of cold spray, the lower the surface adhesion, not recommended to use a thick cold spray thickness. The hardness of cold sprayed on the surface is generally 1050HV~1450HV (about 70HRC).

3. Spray welding or cold spraying of Nickel-base alloy/Cobalt-based alloy

Ball valves usually use spray welding or cold spraying of nickel-based alloy inclnel600 on the ball. The process of spray welding is basically the same as that of thermal spraying, but the remelting process is added in the process of powder spraying. The commonly used cobalt-based alloy on the ball valve ball is STL20, STL6 and STL1, which is usually used for spray welding. The general thickness of spray welding cobalt-based alloy is 0.5mm ~ 0.7mm, and the actual maximum thickness can be up to 2.5mm~3mm. The hardness after spray welding is generally STL20:50~52HCR; STL6:38 ~ 40 HCR; STL1:48 ~ 50 HCR4，

4. Nitriding treatment

Nitriding treatment refers to a chemical heat treatment process in which nitrogen atoms penetrate into the surface layer of the workpiece at a certain temperature and medium. Nitriding treatment can improve the wear resistance, fatigue resistance, corrosion resistance and high-temperature resistance of metal. The essence of nitriding treatment is to infiltrate nitrogen atoms into the surface layer of the ball. During the friction process between the seat and the ball, the nitride layer is easy to be worn or thinned for the hard seated ball valve, resulting in the ball is easy to be scratched by impurities in the medium, affecting the seal and even making the torque increase.

Valve packing is a kind of dynamic sealing structure which is installed between the valve stem and the packing box of the valve cover to prevent outside leakage. Packing material, reasonable packing box structure and installation methods ensure the valve reliable sealing performance. There are various valve sealing packing materials are available and different packing suitable for different working conditions, including asbestos, graphite, PTFE, etc.

• Flexible graphite packing

Flexible graphite packing is the most widely used material in the valve, which can be pressed molding, has been widely used in the field of petroleum, chemical industry, power generation, chemical fertilizer, medicine, paper, machinery, metallurgy, aerospace and atomic energy and other industries wherewith the nominal pressure ≤32MPa. It has the following excellent performance:

Good flexibility and resilience. The incision packing can be freely bent more than 90° in the axial direction and will leak-free due to the change of temperature/pressure/vibration, safe and reliable; Good temperature resistance. The wide range of -200℃-500℃ usages, even in non-oxidizing medium up to 2000℃ and maintain excellent sealing; Strong corrosion resistance. It has good corrosion resistance to acid, alkali, organic solvent, organic gas and steam. Low friction coefficient, good self – lubrication; Excellent impermeability to gases and liquids; Long service life, can be repeated use.

• PTFE packing

Polytetrafluoroethylene packing has good lubrication, weaving polytetrafluoroethylene packing has excellent corrosion resistance, and can be used for cryogenic medium, but its temperature resistance is poor generally only used in the temperature below 200℃ conditions, while can not be used for melting alkali metal and high temperature of fluorine, hydrogen fluoride medium.

• Vegetable fiber packing

Made of hemp or cotton impregnated oil, wax or other anti-seepage materials, used for low-pressure valves below 100℃ and media like water, ammonia and etc.

• Asbestos packing

Asbestos fiber has better heat resistance, absorption and strength can withstand weak acid, strong alkali. Inked asbestos, rubber asbestos and oil-impregnated asbestos are suitable for valves with the steam temperature of 450℃.

• Rubber filler

Rubber cloth, rubber rod, ring rubber packing for temperature ≤140℃ ammonia, concentrated sulfuric acid and other media.

• Carbon fiber packing

The carbon fiber filler is made of polytetrafluoroethylene emulsion impregnated with carbon fiber is a woven rope. Carbon fiber packing has excellent elasticity, excellent self-moistening and high-temperature resistance. It can work stably in the air temperature range of -120 ~ 350℃, and the pressure resistance is less than 35MPa.

• Metal + rubber packing

It can include metal-wrapped packing, metal laminated packing, metal corrugated packing, lead packing, etc. The metal wrapped packing and metal laminated packing is characterized by high-temperature resistance, erosion resistance, abrasion resistance, high strength, good thermal conductivity, but poor sealing performance must be used with plastic packing, its temperature, pressure, corrosion resistance that depends on metal material.

• Stainless steel wire + flexible graphite woven packing

Generally, v-shaped packing is composed of upper packing, middle packing and lower packing. The upper and middle packing is made of PTFE or nylon, and the lower packing is made of 1Cr13, 1Cr18Ni9 and A3 steel. PTFE can withstand high temperature 232℃, nylon 93℃, general pressure 32MPa, often used in corrosive media.

Generally speaking, the valve packing materials are mainly PTFE and flexible graphite, be noted that the packing box dimension accuracy, roughness, stem surface dimension accuracy also affect packing seal performance.