When specifying or designing valves, reference should be made to the information contained in the Operating Envelope for that particular well.
All valves purchased by must at least meet the standards set in API 6A PSL 1, 2, 3 or 4 and in several areas it must exceed those standards Cases where the standards are exceeded are specialised exceptional cases. The preferred route is to purchase to API 6A latest edition at the PSL-3, PR-2 level.
End connections
For safety reasons valves or pipe fittings with screwed connections are not recommended. The valve flanges on wellheads should be to API standards although the recommended standard on line pipe or processes is ANSI. All wellhead valves should have flush internal joints to prevent erosion and the build up of sand.
Body and bonnet
Preference should be given to forged bodies and bonnets with the minimum number of penetrations. The bonnet should be bolted to the body with standard bolts or studs.
To ensure a good seal, an understanding of seal technology should be applied so that seals are not used to transfer loads, align components etc. Ideally, the internal cavity of the valve should be round with the body to bonnet seal a metal to metal one. Although metal to metal seals are more difficult to install, these are currently preferred as, once fitted correctly, they have a longer lifespan.
In some low pressure, low temperature, clean service process applications it may be cost effective to have elastomeric seals.
Gate
The true floating single slab, double seat arrangement has preference. This helps to prevent build up of debris between the gate parts and also prevents pressure locking of the valve when fluid is retained between the gate slabs (in split gate assemblies) with no pressure in the body. Ideally the gate should have a minimum number of components with no chance of a component becoming detached from the gate and flowing down the process line.
When open, the gate should provide a smooth bore within the valve to prevent erosion and an internal build up of sand.
Care should be taken to ensure that pressure locking of the valve cannot occur under normal operating circumstances. This is particularly important when the process is equipped with an automatic shutdown and bleed down emergency system. The valve should be examined to ensure it can withstand throttling under normal open and closing conditions and that it is not possible for hydrates to build up inside a valve in wet gas service.
Hardfacing
Although hardened faces on the gates is preferable, the type of hardfacing will depend upon the environment to which the valve is subject, i.e. sand production or wirecutting service.
Seat
The valve should have metal to metal seats (gate to seat and seat to body). The valve should seal and be operable under full pressure and flow conditions. More importantly, it should seal effectively in both directions in the low pressure, low differential case. It is not necessary for the valve to have a block and bleed capability (a single valve is considered "one" block in the double block and bleed situation.
The best approach is to have valves that only need 'standard' tool box tools for assembly and disassembly.
Stem
All Wellhead and Christmas tree valves should have non-rising stems, with a (preferably selective) backseating capability. The preference is to have a lockable back seat feature.
Stem packing
When used in a high pressure corrosive environment, the stem packing should be metal to metal. With a non-rising stem the seal is only subjected to linear movement and not a linear and rotational movement. In some process applications elastomeric seals could be suitable provided that the seal is exposed only to fluid pressure and temperature and is restricted to one degree of freedom.
Special consideration must be given to extraordinary service such as steam injection.