Quality Requirements and Control of API Flat Gate Valves

As one of the important pieces of equipment in API Spec 6 A "Specification for Wellhead Equipment and Christmas Tree Equipment", the flat gate valve is mainly used in working conditions with high working pressure [14-140 m PA (2,000-20,000 psi)] and harsh environments, such as oil production or gas production. Once problems occur, they will bring great potential safety hazards and economic losses. Therefore, strict quality control is necessary to ensure the safety of the gate valve. Generally speaking, the quality of flat gate valves is mainly measured from several aspects such as pressure bearing performance, sealing performance, operation performance and service life. To ensure the performance of the above aspects, it is necessary to find out the factors affecting these performances and strictly control them, so as to effectively improve the quality of gate valves.
 
Analysis and control of influencing factors of pressure-bearing performance
 
Pressure bearing capacity is the most important index to ensure the quality of flat gate valves. The pressure-bearing parts (valve body and valve cover) of the flat gate valve should have sufficient strength to ensure normal operation under the rated pressure. A large number of tests have proved that the main factors affecting the strength of pressure parts are design method, mechanical properties and chemical composition of materials, etc. Choosing a reasonable calculation method can effectively ensure that the strength of pressure-bearing parts can meet the use requirements. The design and calculation methods of pressure-bearing strength mainly include API 6X method, finite element analysis method and experimental stress analysis method, etc. Selecting one method and verifying with the other method can theoretically ensure the strength of pressure-bearing parts. It is also one of the effective methods to select the materials of pressure-bearing parts reasonably according to the working conditions. The mechanical properties (including tensile strength, yield strength, elongation, area shrinkage, low-temperature impact energy, etc.) and chemical composition of pressure-bearing materials are clearly defined and controlled in the A P ISpec 6A standard, so they will not be repeated here.
 
Analysis and control of influencing factors of sealing performance
 
Sealing performance is also an important index to ensure the quality of flat gate valves. The flat gate valve has a good sealing ability, so it must be ensured that three important parts cannot leak: between the valve plate and the valve seat; Between the valve seat and the valve body; Between the valve stem and bonnet. The main factors affecting the sealing of the above three parts include the coating performance, surface roughness, flatness and parallelism of the sealing surfaces of valve plates and valve seats; Valve body opening size and shape and position tolerance; Fit clearance of valve body, valve plate and valve seat; And the surface roughness, fitting clearance, etc. of the sealing mating surface between the valve stem and the valve cover and the rod.
 
The factor influencing the sealing performance of the valve plate and valve seat and the sealing between quality control valve plate and valve seat is metal-to-metal sealing, that is, the valve seat and valve plate are closely attached to achieve sealing. Therefore, the main factors affecting the sealing between them are the coating performance, surface roughness, flatness and parallelism of the joint surface between the valve plate and the valve seat. The coating properties of the valve plate and valve seat sealing surface mainly include the thickness, hardness, porosity and bonding strength between coating and substrate, etc. There are two processes: spray welding and supersonic spraying.
 
The spray welding process is to spray weld nickel-based alloy on the sealing surface of the valve plate and valve seat, with the thickness of the spray welding layer greater than 0.6mm, and Ni60 on the sealing surface of the valve plate, with the hardness generally HRC 55-60; The valve seat is spray-welded with Ni45, and the hardness is generally HRC45-50. The welding process will reduce the hardness of the substrate, resulting in large deformation of the valve plate and valve seat under high pressure, which will affect the sealing effect. It is mainly used in working conditions where the rated working pressure is less than or equal to 35M Pa(5,000psi).
 
The supersonic spraying process is mainly to spray tungsten carbide on the surface, with a coating thickness greater than 0.15mm and hardness greater than 900HV. The coating hardness and bonding strength of the supersonic spraying process are far superior to those of other processes, and the coating has high density and low porosity. It is mainly used in the rated working pressure of 69MPa(10,0 0 0psi), and it is more suitable for gas production wellhead. And the required roughness, flatness and parallelism can be achieved only after the spray welding layer or spray coating of the valve plate and valve seat is finished, through rough grinding, semi-fine grinding, fine grinding, and then grinding and polishing. When the gate valve is used for oil production, the roughness of the valve plate and valve seat is generally controlled at Ra0.2μm, and the parallelism and flatness are controlled at 0.02mm; When the gate valve is used for gas production, the roughness of the valve plate and valve seat should be controlled at Ra0.05μm, and the parallelism and flatness should be controlled below 0.01mm
 
Influencing factors and quality control of valve body opening size tolerance and shape tolerance
 
The opening size of the valve body refers to the distance from the outer end faces of the two valve seats installed inside the valve body, which mainly affects the clearance between the valve plate and the valve seat after assembly. According to experience, its tolerance is generally controlled at 0.1 ~ 0.2 mm, which can be guaranteed by the ordinary processing equipment. The shape and position tolerance of valve opening mainly refers to the roughness, flatness, symmetry with the middle flange and verticality of the opening end face. Figure 1 shows the schematic diagram of the PR2 valve body opening structure.
 
The shape and position tolerance of valve opening mainly refers to the roughness, flatness, symmetry with the middle flange and verticality of the opening end face. Roughness and flatness mainly ensure the sealing performance of the sealing ring between the valve body and the valve seat; symmetry of the two end-to-end flanges mainly ensures the central position of the valve plate and the valve seat inside the valve body after assembly; verticality mainly ensures that the valve plate keeps a parallel state with the valve stem after assembly. Because of the irregular shape of the valve body and large end distance, it is difficult to control the quality of form and position tolerance. When the valve body is processed by a special machine tool, one-time clamping can ensure the valve body's opening size and shape and position tolerance. If an ordinary lathe is used for processing, the following methods can be used for control to ensure the shape and position tolerance of valve body opening.
 
(1) Finish the flanges at both ends, and the excircles of the flanges at both ends shall be used as the benchmark for subsequent processing. The coaxiality of the excircles of the flanges at both ends and the dimensional tolerance between the two end faces and the center of the flange shall be strictly controlled.
 
(2) Adopt the method of tooling. Based on the two end face flanges, finish machining the end face of the middle flange, the inner hole and the sealing groove of the valve cover to ensure the distance between the upper-end face and the main through the hole.
 
(3) Based on the two end faces, finish machining the main bore, and finish machining the inner bore of the valve seat with a special cutter to ensure dimensional tolerance and roughness.
 
Influencing factors and quality control of fit clearance of valve body, valve plate and valve seat
 
Taking the PR 2 gate valve (as shown in Figure 2) which is sealed by the end face and the back of the valve as an example, it is necessary to strictly control the clearance between the three. If the fit clearance is too large, it will lead to the sealing failure of the gate valve, and external leakage, and if the opening clearance is too small, it will lead to the slow internal leakage of the gate valve, which will lead to too long pressure stabilizing time and affect the pressure testing efficiency. To control the fit clearance, it is necessary to control the opening size tolerance of the valve body, the thickness size tolerance of the valve plate and the valve seat. And the dimensional tolerance of the valve plate and valve seat is easy to control, generally within 0.05mm; Generally, the size tolerance of valve opening is controlled within 0.1mm. Therefore, according to many years' of experience, the PR1 gate valves can properly select the fitting clearance according to the spring elasticity of the valve wave seat and the convenience of assembly. For the PR2 gate valve, in order to ensure that the gate valve meets the sealing requirements at the low pressure and high pressure (PSL3G) required by the standard, the fitting clearance should be controlled at 0.2 ~ 0.4 mm.
 
Influencing factors and quality control of valve stem seal
 
The main factors affecting the valve stem seal are the surface roughness of the valve stem sealing surface, the surface roughness of the inner hole of the bonnet and the selection of the stem seal type. Due to the regular shape of the valve stem and bonnet, the roughness of the joint with the stem seal can be achieved by turning or grinding. When the stem seal is made of rubber, the surface roughness can reach Ra1.6μm, and when the stem seal is made of polytetrafluoroethylene, the surface roughness should reach Ra0.8μm or more.
 
Analysis and control of performance influencing factors
 
The operation performance of the flat gate valve is also an important index to measure the quality of the gate valve. A good flat gate valve is flexible and light in the process of opening and closing, with a small opening and closing torque, and the operator has a good experience. The factors that affect the switching torque of gate valves are mainly related to the friction of each friction pair of gate valves, including the friction between the valve plate and valve seat, the friction between the valve stem and the trapezoidal thread of the valve plate drive nut, and the friction between the valve stem and rod seal. The measures to reduce the friction between the valve plate and the valve seat are mainly to control the surface roughness of the contact surface, which has been stated before. Reducing the friction force of the trapezoidal thread pair connected between the valve stem and the valve plate is mainly to control the roughness of the rotating surface. The trapezoidal thread of the valve stem is an external thread, so the surface roughness is easy to ensure. However, the trapezoidal thread of the valve plate is an internal thread, and its size is small, so it is necessary to select appropriate processing methods, processing parameters and tools of different materials according to the valve plate materials to ensure the roughness of the rotating surface. For gate valves with a rated working pressure of 69MPa(10,0 0 0psi) or above, the roughness of trapezoidal thread needs to reach R a1.6μm or above to effectively reduce the friction force. In addition, the surface of trapezoidal thread can be further reduced by copper plating or Teflon treatment. When the rated working pressure of the gate valve is greater than or equal to 69MPa(10,000psi), it is best to choose PTFE material with a low friction coefficient and self-lubricating performance, and the roughness of the valve stem sealing section should reach Ra0.8μm or more. In addition, the proper selection of bearings can also reduce the rotating torque, with needle bearings being preferred, thrust roller bearings followed by thrust ball bearings.
 
Analysis and control of influencing factors of service life

Service life is an important performance index of gate valves and has great economic significance. The factors affecting the service life of gate valves are as follows: (1) The corrosion of the sealing medium to the parts of gate valves. In API Spec 6A standard, there are clear provisions for the selection of main parts of gate valves with different material grades, but at the same time, the hardness and heat treatment requirements of materials with DD, EE and FF in NACEM0175/ISO 15156 should be taken into account. In an acid environment, the hardness of martensitic stainless steel should not exceed 235HBW, and it should be treated in heat treatment.
 
(2) The corrosion of the gate valve surface caused by the external environment is mainly influenced by the quality of painting, including the treatment of the surface before painting, such as sandblasting, the type of paint, the thickness of paint film and the number of layers.
 
(3) Because the gate valve has been exposed to the harsh environment such as outdoors and humidity for a long time, the connecting thread of the parts of the gate valve exposed to the air also has serious corrosion, so it is also necessary to protect the connecting thread so as to isolate it from the outside air and achieve the anti-corrosion effect. In addition, regular maintenance measures such as injecting grease into the valve cavity and bearing can also improve the service life of the gate valve.
 
Conclusions and suggestions
 
Through the analysis of the influencing factors of API Spec 6A flat gate valve, such as pressure bearing performance, sealing performance, operation performance and service life, the key points affecting the quality of each part of the gate valve are further defined, and the control methods and measures are put forward, which can provide a reference for improving the quality of the gate valve. The gate valve is one of the most important components in wellhead devices and Christmas trees, and it is also the most frequently used and overhauled component. However, the wearing parts of gate valves of each manufacturer do not form a uniform size standard and have no interchangeability, which increases the maintenance time and management cost of gate valves. Therefore, it is suggested to standardize and serialize the wearing parts of gate valves (including stem seal, seat seal, even valve stem, valve plate and valve seat, etc.) to improve the interchangeability of gate valves by different manufacturers and reduce the use cost of users. The quality requirements of spray paint are not controlled by API Spec 6A standards, but the anti-corrosion performance of the external surface of the gate valve is also a guaranteed gate.

The quality of valves is an important part. Although each manufacturer has formulated enterprise standards, the quality is uneven. Therefore, it is suggested to formulate a uniform painting specification, which will help to prolong the service life of gate valves.