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Valve Design

valveWhen specifying or designing valves, reference should be made to the Operating parameters for that particular well. All valves purchased must at least meet the standards set in API 6A PSL 1, 2, 3 or 4 and in several areas 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.

1 Design Features

1.1 End connections

For safety reasons we do not recommend valves or pipe fittings with screwed connections. 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.

1.2 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.

1.3 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.

1.4 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.

1.5 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 as the approach is that a single valve is "one" block in the double block and bleed situation.

Ultimately the approach is to have valves that only need 'standard' tool box tools for assembly and disassembly.

1.6 Stem

All Wellhead and Christmas tree manually operated valves should have non-rising stems, with a (preferably selective) backseating capability. The preference is to have a lockable back seat feature.

1.7 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.

2. Gate valves

All tree valves are normally gate valves of identical design. The actuated valves on a tree differ in their mode of operation from the non-actuated valves in that they are reverse acting. This means that the valve is closed when the gate is in the fully "out" position. All the valves fitted to the tree and "A" annulus should be capable of withstanding the same pressure as the tree.

2.1. Specification guidelines for tree valves

The valve configuration of a Christmas tree should conform to the wellhead safety principle of always providing an ultimate safety barrier, complying with international standards such as ISO, and API, etc..

2.2. Recommended features of tree valves

All tree valves should preferably have forged bodies and flanges with a minimum number of penetrations. The recommended design is the true floating single slab and double seat arrangement, with a (preferably selective) backseating capability. It is recommended that manual valves have non-rising items, where as actuated valves must have a rising item. Ideally the stem packing should be metal-to-metal. However, in some cases elastomeric seals could be allowed.

2.3. Wire cutting valve

Sometimes, in an emergency during wireline operations, it may become necessary to cut the wire. In such a situation it is essential that this can be done quickly and efficiently, and may be achieved by installing a wire cutting valve in the tree. If installed, this valve should be fitted in the upper master gate position. The wireline BOP may have this capability, therefore the installation of a separate cutting valve in the tree is optional.

The difference between wire cutting valves and non-wire cutting valves is in the design of the slab, and the power of the actuator. With a wire cutting valve it should be possible to cut the largest size of braided wire used (7/32"), repeatedly without causing damage to the gates or seats. In some applications a booster, or add-on actuator is used to transform a normal actuator into a wire cutting one.

2.4. Coiled tubing cutting valve

Occasionally during coiled tubing operations it will be necessary to shear the coiled tubing. A coiled tubing quad BOP has this capability, therefore the installation of a separate coiled tubing cutting valve in the tree is not considered necessary.

Tree valve ace has also shown manual gearboxes on valves to be maintenance intensive.

3. Production chokes

3.1. Adjustable chokes

Adjustable chokes are designed to withstand very high pressure drops and vary the fluid/gas flow at the same time. Control of the chokes may be by simple on/off local manual panels, or sophisticated distributed control systems.

The high pressure drops which are normally associated with chokes often cause severe turbulence and eventual erosion in the downstream pipework. A solution is to install hardened pup pieces directly downstream of the choke, or target tees instead of flowline bends.

Chokes must always be fitted in accordance with the manufacturers instructions and never be inverted. Flow reversal may cause premature catastrophic failure of the choke internals.

There are several types of chokes available, the main types being:

  • control valve types, with trims and plugs similar to process control valves, but designed for high pressure drops;
  • variable orifice chokes, normally a needle valve configuration;
  • multiple orifice chokes, normally two rotating discs with one or more holes in each disc.

When considering the selection of chokes, the following factors should be considered:

  • very high pressure drops;
  • erosion of downstream pipework;
  • the effect of high pressure differentials on start up;
  • the degree of required choke control;
  • valve sequencing and shutdown philosophy;
  • maintainability, access for the removal of internals, etc.;
  • vibration levels to be expected as a result of the pressure drop;
  • acoustic levels during the high pressure drop production phases.

There should be a device to ensure pressure is bled off from the choke internals prior to any maintenance operation.

3.2. Positive chokes

Positive chokes are mainly used in remote, non-critical areas, in low pressure applications and where the choke may be some considerable distance from the tree. Once the fixed bean or orifice is fitted, the flow rate from the well cannot be changed. To vary the flow rate a different sized bean must be fitted. This normally entails breaking open the pressure envelope, which should only be done by trained personnel who are aware of the consequences of mis-aligned seals or badly fitted chokes.

As replacing a large bean in a vertically installed choke is a difficult operation, this should be considered during the design phase if it is not to become a serious operating problem. Fixed bean devices should always be installed in accordance with the manufacturer's instructions.

4. Tree Valve Actuation

Valves are opened or closed either by hand or actuators. Actuated valves can be triggered automatically or manually. However, whether the tree has two actuated valves (UMG and FWV) or a single SSV, they must close immediately when triggered.

4.1. Automatic actuator design

  • actuator fluid volume and overall dimensions should be minimised to reduce size and response times;
  • for maintenance and replacement, size and weight should be minimised but should not compromise the pressure integrity of the tree;
  • all actuators must have a fail-safe action;
  • actuators should not interfere with the back seating capability of the valve;
  • in an emergency the tree valves should close in the quickest possible time. The normally accepted time is 10-20 seconds; including the cutting of wire.
  • during well intervention operations normal control of the well is transferred to the equipment operator. In an emergency it must be possible for the operator to shut in the well;
  • control of the well during well intervention is traditionally done by disconnecting the fixed hydraulic line to the actuator and connecting a flexible hydraulic hose. The hydraulic flexible line may be approximately 200 feet long and 1/2" in diameter. When this is done it must be demonstrated by calculation or otherwise, that the actuator closure time is not affected;
  • in a wire cutting application the actuator should have enough force to cut the largest size of braided wire (7/32") in use, independent of well pressure.

4.2. Manual actuation of tree valves

Manual valves should not be hydraulically or pneumatically triggered, or have a gearbox. These devices isolate the operator from the "feel" of the valve during its travel and therefore do not provide direct control. Experience has also shown manual gearboxes on valves to be maintenance intensive.