Basic training operator Oil & gas Advisafe Risk Management B.V.
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Content
1.
Casinghead-housing and spools ... 5
1.1.
Summary of surface well completion ... 5
1.2.
Casing head housing and spools ... 7
1.3.
The Cameron CA casing hanger ... 9
1.4.
The Cameron BRX type 2 casing hanger ... 11
1.5.
X-bushing with P-seal ... 12
2.
Tubingheads and metal seals ... 14
2.1.
Summary of tubing heads and metal seal ... 14
3.
Tubingheads and metal seal ... 16
3.1.
LDO tubing head ... 16
3.2.
SRT tubing head ... 17
3.3.
Metal seal... 20
3.4.
Boll-weevil tubing head ... 21
4.
Christmas-tree ... 23
4.1.
Summary of the Christmas-tree ... 23
4.2.
The solid block Christmas tree ... 24
4.3.
The components of the composite Xmas tree ... 26
4.4.
The Xmas tree for the production cross for gas lift oil wells ... 28
4.5.
The Xmas tree setup on clusters ... 29
5.
Gate valves ... 31
5.1.
Summary of Gate valves ... 31
5.2.
The principle structure of a gate valve ... 31
5.3.
Requirements set for valves ... 32
5.4.
The method of sealing gate valves ... 33
5.5.
Sealing compound method ... 33
5.6.
The McEvoy gate valve, model C ... 35
1.
Casinghead-housing and spools
1.1.
Summary of surface well co
m
pletion
The functions of the surface well completion are:
• hanging the successive casings and sealing them off from one another and the surrounding area;
• providing the option of shutting down well production;
• providing the option of making observations and having controlled access to the well for various activities.
The completion consists of the following components:
• Casing head housing
• Casing head spools
• Tubing head
Overview surface well completion 5 1/8” API 5000 type ‘F’ solid-block tree 6” double ‘P’ seal Check port Metal seal SRT tubing head 10” x 6” API 5000 control head Tubing hanger nippel 7 5/8” double ‘P’ seal Plastic packing ports
Casinghead spool 16” API 3000 x 10” API 5000 Casinghead type ‘WF’ Check port Casinghead housing 16” API 3000 x Casinghead type ‘WF’ 16” casing 10 3/4" ” casing 7 5/8” casing 5 1/2” casing
Plastic packing ports
Intermediate flange
Check port
CA slip and seal assembly 10” x 7 5/8”
x-busing 16” x 10 ¾” o.d. casing
CA slip and seal assembly 16” x 10 ¾”
1.2.
Casing head housing and spools
The casing head housing is screwed (and sometimes welded) on top of the conductor and serves to:
• anchor the blowout preventer to the first casing stack during drilling;
• support the above-ground well completion;
• support the next, smaller casing stack via the casing head spool and the casing hangers;
• seal the annular space between the two largest casing stacks;ensure access to the annulus
Casing head housing and casing head spools
The casing head housing is installed on the casing (see figure). There is a flange connection on top, on which the casing head spool will be mounted at a later stage. The housing has two 3” drain holes opposite each other, with
flanges threaded on the inside. The annulus shut-off valves are attached to this in order to enable mud circulation.
On the inside, the casing head housing is a bore that is partially straight and partially tapered.
Tubing head spool
Test channel 10” casing outlet 16”x10” casing head spool BRX type 2 Casing hanger x-bushing 16” casing outlet Injection channel Tubing 5 1/2" 16” casing outlet Tubing 5 1/ 2“ Annulus 5 1/2 ” x 7 3/4” Casing 7 3/4” Annulus 5 1/2 ” x 10 3/4” Casing 10 3/4" Annulus 10 3/4” x 16” Conductor 16”
The first casing head spool is connected to the casing head housing by means of a flange fitted with a metallic packing (ring joint). The number of casing head spools depends on the number of casing stacks with which the well is equipped. In the case of the figure above, only two casing stacks (the 10¾” and 7¾” casings) are taken in, in addition to the 16” conductor, and only one casing head spool is used. The spool connects the various flanges with reduced diameters, which correspond with various consecutive casing stacks.
The spool consists of a housing with a conical notch for the casing hanger at the top. When the
Cameron CA casing hanger is used, there is space for the X-bushing with P-seal in the lower flange. A filling and test port for the P-seal are also installed in the lower flange. When the Cameron BRX casing hanger is used there are two filling ports and two test ports in the lower flange. Two ports with thread and flanges in the side wall of the housing provide access to the annular space.
Valves are installed on the side flanges of the casing head housing and casing head spools. These valves provide access to the annular spaces to measure the pressure or, if necessary, to vent the pressures accumulated.
The tubing head spool is installed on the top flange of the top casing head spool, in which the tubing is hung.
1.3.
The Cameron CA casing hanger
After a casing stack is inserted at depth, the string with slips is hung from the rotary table. The casing hanger is installed around the pipe. The casing, with the casing hanger around it, is lifted with the hoisting facility and passes through the hole in the rotary table and the BOP stack to the desirable depth, after which the casing hanger is secured.
The hanging system consists of two slips (components that enclose a flexible packing component) and is called slip and seal assembly.
Cameron CA casing hanger with ‘slip and seal assembly’
16” casinghead housing Slip A Slip C Seal Slip D Slip B 10 3/4" casing
The whole packet is pressed together and kept together by long threaded bolts. By hanging it the serrated slips are pushed against the outside of the casing. Slip D drags against the conical inside wall of the casing head housing and ensures the hanging process. The flexible packing component is pressed against the wall laterally and forms a seal between the two successive casing stacks.
An advantage of the CA hanger system is that the hanger drops to the desirable level around the casing and catches at the place with the serrated slips.
The disadvantages are:
• The casing hanger is not installed until the casing head housing or spool is attached. In practice the hanger is placed through the BOP with the risk that the hanger will get stuck or drag to a halt in the BOP.
• The 'slip and seal assembly' often leaks, while the additional seal formed by the X-bushing with single P-seal is also inadequate.
To eliminate these disadvantages, a Cameron modified BPX hanger was developed, which, working in collaboration with Cameron, resulted in the BRX type 2 casing hanger.
1.4.
The Cameron BRX type 2 casing hanger
This hanger is likewise used in a casing head housing as well as in a casing head spool.
Cameron BRX type 2 casing hanger
The conical casing hanger is screwed to the last casing pipe, which must be cut off at a length accurately specified in advance and threaded. All this is lowered into the well from a running tool which is screwed into the top of the casing hanger, till the conical section drags against the conical support in the casing head spool. Dropping is a lot easier through the BOP than with the slip and seal assembly.
The BRX hanger and the conical support of the casing head housing/spool both have a smooth finish. The seal is obtained by two canvas seal rings in the BRX hanger. The BRX type 2 hanger is tapped with a left-hand thread into which the running tool can be screwed. After shut off, the running tool is unscrewed. At a later stage, the polished surface right above the left-hand thread can be sealed with a double P-seal.
Injection port
P-seal
Casing head spool
Test port
Ring joing
16’ casing head housing
Ring joint
BRX hanger
The advantages of the hanger are:
• improved control of casing hanging through the BOP.
• better sealing systems.
1.5.
X-bushing with P-seal
The purpose of the X-bushing with P-seal is to provide additional sealing in the casing hanging (for CA casing hangers, see figure on the left).
Casing head spool
The X-bushing with P-seal is enclosed in a chamber bored at the bottom of the casing head spool. The seal by the P-seal can be servo-assisted via an injection port in the flange and inspected via a test port. The X-bushing is a metal, ring-shaped component that is slid over the cut-off casing end and is locked into the chamber bored into the casing head spool above.
Tubinghead spool Test channel Casing hanger Casinghead spool X-bushing Casing hanger Casinghead housing Injection channel
The sealed-off space between the X-bushing ring joint and the casing hanger is pressure tested via a ½” test port and checked for leakage through the P-seal.
Only one P-seal is used for the X-bushing, which was found to be a shortcoming.
Nowadays a different type of casing hanging is used, with a double P-seal as a seal above it. If one leaks, a good seal can still be obtained with the other P-seal.
The seal between the housing of the casing head spool and the outside circumference of the X-bushing is obtained by two l-shaped seal rings in the outside circumference of the X-bushing.
The seal between the casing and the X-bushing on the inside is obtained by a canvas ring in the inside circumference of the X-bushing. This packing ring can be activated via an injection port with plastic packing material, whereby the P-seal comes in contact with the casing.
X-bushing P-seal Non-return valve Injection channel Casinghead spool 1/2 “ test port Non return valve Shut-off cap
16”casinghead housing housing
10 3 /4” casing
X-bushing met P-seal
Pressure plug Bleeder plug
Ring Joint Profile RX
2.
Tubingheads and metal seals
2.1.
Summary of tubing heads and metal seal
The tubing is connected in the bottom of the well to the packer, which seals the casing and the tubing. The connection between the tubing and the annulus must also be sealed at the top, meaning above-ground. The total weight of the tubing string must also be absorbed, which is dealt with by the tubing head spool and the tubing hanger nipple connected to the tubing.
Surface well completion
The tubing head spool is a transition from the
narrowest casing to the Xmas tree or to a temporary adapter. Internally, the tubing head spool is processed in such a way that the tubing hangs in it with tubing hanger nipple and seals it. The method of hanging depends to a large extent on the temperature of the medium flowing through during production. Tubing is hung without
prestressing in cold shallow wells. In deep wells, large differences in length can be created between the cold, enclosed well and the warm producing well. In such wells the tubing is prestressed before
hanging, which means installed in the cold well after prestressing. 5 1/8 “ API 5000 type “F” Solid-block tree 6” double “P” seal Check port Metal seal SRT tubing head 10” x 6” API 5000 conrol head Tubing-hanger nippel 7 5/8” double “P” seal Plastic packing ports Casing spool 16” API 3000 x 10” 5000 Casinghead type “WF” Check port Casinghead housing 16” API 3000 Casinghead type “WF” 16” casing 10 3/4” casing 7 5/8” casing 5 1/2” casing
Plastic packing ports Plastic packing ports
Check port CA slip and seal assembly 10” x 7 5/8” X-bushing 16” x 10 3/4 “ o.d. casing
Plastic packing ports
CA slip and seal assembly 16: x 10 3/4"
As soon as the temperature of the well increases during production, this prestress disappears for the most part due to expansion of the tubing material. However, enough tensile strength will be left in the tubing for it to seal properly and not buckle, which would cause production or wireline problems. The following tubing heads are mostly used:
• LDO tubing head
• SRT tubing head
• Boll-weevil tubing head
LDO and SRT tubing head is used in gas wells, in which the tubing is prestressed before hanging in order to ensure that the tubing will always be stressed when hanging at
temperature differences. A boll-weevil tubing head is used for oil wells and injection, wells in which the tubing hangs free in the centre of the casing.
A plug can be installed in the tubing hanger nipple for all three systems, thereby
protecting the well from fluid or gas flowing out during an exchange in completion above-ground or when a blowout preventer (BOP) is installed. The above-above-ground completion can also be pressure tested using this plug after installation or exchanges.
3.
Tubingheads and metal seal
3.1.
LDO tubing head
This head is installed on the top casing head spool.
LDO head
The tubing is hung from the rams in the tubing head spool. The tubing hanger nipple, which is attached to the tubing, is used for this purpose. A packing is installed in the rams to ensure sealing on the polished surface of this nipple and the LDO head. The tubing is hung by closing the rams around the polished section of the tubing hanger nipple so the front of it rests on the rams.
The rams are opened and closed by screw spindles. One must pay attention that the tubing hanger nipple is centred as accurately as possible. The rams are fitted with rubber
Production cross Tubing –hanger -nippel
plastic or P-seal injection ports Soft metal seal ring (metal seal) Control of test ports
Intermediate flange for metal seal Ring joint
Body of LDO head Body of LDO head
Conductor pin
V-packingset cover with packed spindle (2x) Seal check with bleeder plugs
Bleeder plugs Casinghead spool
plastic or P-seal injection ports with bleeder plugs
seals which ensure that by tightening the spindles and the weight of the tubing a proper seal between the tubing and the casing is obtained.
The tubing hanger nipple is threaded on both sides and has a landing nipple profile on the inside so it can shut off a plug by means of wireline.
3.2.
SRT tubing head
Nowadays so-called tension heads, type Cameron SRT, are installed in new gas wells. Also, the LDO heads used before are now often replaced with SRT heads during a workover. This tubing head is also installed on the top most casing head spool.
Cameron SRT tension head
Production cross
Plastic injection port
Metal seal with controlport
Intermediate flange
Packing securing bolts Packingset
Body Ledge cover
Ledge cover
Stops of centring the tubing hanger
Controlof test port
Ring joint Controlof test port
Just as it is for the LDO head, the tubing is hung on the rams here by means of a polished tubing hanger nipple provided with internal notches so that plugs can be shut off.
The advantages over LDO heads are:
• The rams only serve to hang the tubing in a central position, thus not to seal it.
• Because the rams drag to a halt on a stop, we know for a fact that the tubing hanger is in the centre after the rams have been screwed in completely.
• A separate packing bushing (sandwich seal) ensures sealing. If there are any leaks, plastic can still be injected to thereby rectify the leaks.
• A workover mast is no longer required should the packing need to be changed, because the tubing hanger nipple continues to hang on the rams.
Detail of SC-SSV hydraulic control line
though the wall of the SRT tubing hanger nipple
Access to the annulus
The tubing head housing has two openings on the side through which a connection with the annulus can be established. A set of double valves are installed in either opening. Via one set of valves, which are sealed in the open position, the kill pump line is connected from the kill pump manifold to the annulus so that the well can be killed at any time.
A non-return valve is installed in the kill pump to prevent the pipe and the manifold from being subject to the full well pressure in case of a leak between the tubing and the annulus. The servo-condensate mixture that serves to combat the corrosion of the tubing by CO is also pumped into the annulus via these valves. The annulus is filled entirely with servo-condensate and can be injected into the tubing via the injection valve in the bottom most SPM. This is accomplished by pumping the servo-condensate into the annulus by means of a membrane pump. The valves on the additional side openings to the annulus are closed. An inspection port is sometimes installed here.
SC SSV control line
Nowadays the ¼” control line to the SC SSV usually runs via the metal seal through the wall of the tubing hanger nipple and thereafter to the safety valve via the annulus. The purpose of the control line is to operate the SC SSV hydraulically.
Plugged
Metal seal
Connection for hydraulic inspection
Control line to ball valve Tubing hanger nippel
3.3.
Metal seal
The metal seal for the hanger nipple is the most important seal between the tubing and the annulus. The seal consists of a soft metal ring, which is butted up against the tubing hanger nipple between the Christmas tree and the flange beneath it.
Seal around the tubing hanger nipple
An inspection or test port of the space between the tubing hanger nipple and the soft metal ring provides indications on possible leakage through the inside. Leakage passing the soft metal ring on the outside must be detected by listening for air passing or using a gas detector. This test port is also used as a passage for the hydraulic control pressure for the underground safety valve (SC SSV).
However, not one leak has been detected during 10 years of experience with metal seals. Leakage along the plastic seals (P-seals) in the Christmas tree can be rectified by injecting more plastic. This seal only serves to prevent corrosive fluid from the tubing from corroding the metal seal.
Intermediate flange, specially installed for the soft metal ring seal.
Flange of composite production cross. Tubing-hanger-nippel Control of testport
Plastic injection port P-seals Metal seal Ring joint Packing securing bolts Intermediate flange specially installed for the soft metal ring seal.
3.4.
Boll-weevil tubing head
The boll-weevil hanging method is used for oil wells since the requirements imposed on the sealing are not as strict as those for gas wells, as the pressures are lower. A polished hanger nipple is also used here, with notches for installation of plugs by the Wireline department.
Boll-weevil hanging
The short hanger nipple is tube-threaded on both sides: on one hand to establish the connection between the hanger nipple and the tubing, and on the other hand so that the boll-weevil hanger assembly can be put into position in the boll-weevil spool through the BOP by means of auxiliary tubing screwed into the top.
The weevil hanging is a combination of a weevil hanger assembly and a boll-weevil spool, which is installed on the top most casing head spool. The boll-boll-weevil hanging consists of a spherical cone fitted with a packing ring. The purpose of the pressure bolts is to compress the packing component in order to obtain a good seal between the tubing and the casing head. A plug can be screwed into the hanger nipple to allow for pressure testing and for temporary protection when the above-ground well completion is exchanged. Sometimes a Cameron type H two-way check valve (a non-return valve that works in two directions) is installed so that the well can be temporarily shut down when the above-ground well completion or adaptor are exchanged for a BOP, or vice versa. This valve has
Hanger nipple Packing ring
Spherical cone with packing component Packed pressure bolt
Guide pin
Wireline plug rabbet Lip
Chamber
Boll weevil
Polished for sesaling the wireline plug
two seats and seals both above and below the valve (function of an inside BOP) and is thus also suitable for pressuring test the BOP.
The valve is loosened and pulled with a special pulling tool, whereby any pressure under the valve can be equalised, either with or without the use of a lubricator. The tubing stack is screwed into the bottom most threaded box. Its tensile stress is transferred to the casing head via a chamber with lip and the cone.
4.
Christmas-tree
4.1.
Summary of the Christmas-tree
The Christmas-tree is the part of the above-ground well completion between the tubing head and the flow pipe. The purpose of the Christmas tree is to provide the option of closing the gas and/or oil flow off from the formation by means of a manually operated valve or by means of an automatically operated valve: the Otis valve. Furthermore, the flow pipe can be opened or closed by means of the flow-arm valve.
Above-ground, gas wells are completed to as great an extent possible with a production cross forged from one piece (solid–block Xmas tree), which consists of one body or housing in which the various valves are installed. Older gas wells are still equipped with a composite production cross (composite Xmas tree). The various cut-off valves are assembled together in a so-called Y-form block.
In comparison to the composite cross, the forged cross has the following advantages:
• Noticeably fewer flange connections, thus less chance of leakage, which above all provides more safety in case of a blowout or a fire in an adjacent well.
• Less high, thus more easily accessible to operating personnel; moreover, a lower well cage will suffice.
A disadvantage of the forged production cross in comparison to the composite production cross is apparent when damaged valves are changed. It may happen that the body is seriously damaged near one of the wing valves (e.g. the flow arm valve) or near the top valve. In that case, the entire Xmas tree must be exchanged and thus the entire well must be killed. In case of a composite production cross, the entire Y piece with wing valves and top valve can be changed, for example, just by closing the two master valves.
For various reasons there are two main valves, the master valve and the Otis (also referred to as the first and second master valves):
• First of all, the gas flow can be shut off by means of the manually operable valve if the Otis valve is not closing for one reason or another, while this is desired.
• Second, statutory safety requirements stipulate that installation components may only be worked on if two closed valves are installed in series, between the pressurised part of the installation and the pressure-free part. This implies that if the flow pipe valve must be repaired, for example, this can take place in safe conditions if both master valves are closed and the pressure-free part, in which the flow pipe valve is installed, has an open connection to the atmosphere.
4.2.
The solid block Christmas tree
The solid –block Christmas tree is used for most gas wells. These consist of a forged steel block, which includes the valves.
The solid block Christmas tree
Top + cap E-top valve C-tubing- casing- connection valve bypass
Scheme Bril flange
Tubing-casing connection
Flow-arm valve Otis operated master valve
Flow line Sandfilter
Master valve
Intermediate flange
Tubing-head spool (type LDO)
Kill pumpline
Alarm switch
Non return valve L zero point for wireline depth gauge
Casinghead spool
Casinghead housing
5” VAM casing as return stack
7” casing 10 3/4” casing
20” stovepipe 16” casing
The Christmas tree consists of the following components:
• the manually operable master valve
• the Otis–operated master valve
• the block in which the valves are installed
• a flow arm valve
• a top valve
• an additional valve on the other side of the flow arm valve
• a top connection with cap (access for well service activities).
The Otis valve is similar to the bottom master valve, with the difference that an automatic operating mechanism is installed instead of a hand wheel. This operating mechanism, usually called the Otis actuator, is activated when the pressure in the Christmas tree drops too low. This may be the case, for example, if a pipe ruptures or if the NTS or flapper valve in the tubing closes.
The Otis actuator also closes the valve if the control air required to operate Otis stops flowing or if a failure occurs in the installation. There is also the option of closing the Otis during regular production from the control room by means of an electrical signal. The Otis valve is of great importance for safety.
The gas supply to the production installation can be closed off by means of the flow arm valve.
The top valve is a vertical access required for a large number of well service activities, such as wireline, perforation and stimulation.
4.3.
The components of the composite Xmas tree
The composite Xmas tree
Top + cap E-top valve scheme Bypass C-tubing-casing connection valve Y-shape block Flow-arm valve Otis operated master valve pilot solenoid 3 bar air Orifice union Flowline Sand filter Master valve
Tubing-head spool (type LDO) Kill pump line Non return valve L
Zero point for wireline dept gauge Tubing-casing connection Brill flange Alarm switch Casing head spool Casinghead housing 7” casing
5” VAM casing as return stack
10 3/4” casing 16” casing
The composite tree is put together from separate components. This Xmas tree consists of the following components, from top to bottom:
• the manually operable master valve
• the Otis valve (secondary valve)
• the 3-way piece
• the valve for the tubing casing connection
• a flow valve
• a top valve
• a top connection with cap
Various valves are installed on the 3-way piece:
• the Otis valve at the bottom
• three valves from left to right at the top:
o the valve in the tubing casing connection
o the top valve with the top connection on it
o the flow pipe valve
The tubing casing connection serves to release excessive pressure in the annulus to the flow pipe via the tubing casing connection valve. The top valve with connection serves to enable wireline work. The flow line valve can shut off the gas supply to the installation. A choke is installed in some cases past the flow pipe valve. One can make a well produce at different speeds by means of various chokes. In most cases, however, one is liable to find the choke in the installation.
4.4.
The Xmas tree for the production cross for gas lift oil wells
The Xmas tree is built out of the following components:• casing hangers and nipples
• flow line
• gas lift supply pipe
• the usual valves.
Example of a Xms tree for gas lift oil wells
The flow line is equipped with a hand choke, which is used when the well is started. Remote operable valves (ROVs) are installed in the gas lift pipe as well as in the flow line. The ROVs are connected to a number of shut-down actions; the latter depends on the situation (hot well, H2S well, etc.)
Examples of shut-down actions are:
• On ROV in flow line
o HPSD – high pressure shutdown o LPSD - low–pressure shutdown o HTSD – high temperature shutdown
• On ROV in flow line and gas lift pipe
o Leak alarm
o H2S alarm
• Emergency stop from the measuring station
If a corrosion inhibitor pump is installed, it will also stop at a shut-down action. T-piece bullplug Production valve Xmastree cap Adapter flange Top valve or lubricator valve Flow wing of Injection valve Tubing-casing connector Liftgas Casing valve Conductor valve 4 1/2", 3 1/2” or 2 7/8” 7” 10 3/4” 24” T piece Master valve or main valve Adapter flange Tubinghead spool (boll weevil) Casingheadspool Casinghead housing 16”
4.5.
The Xmas tree setup on clusters
As indicated in the figures in the preceding two chapters, all onshore wells have a cellar.
Above-ground well structure of an offshore gas well
The purpose of the cellar is to keep the most frequently used master and operating valves of the Xmas tree at a workable height and to catch any small oil or condensate/servo leaks. In principle, cellars are never made any bigger than strictly necessary to accommodate the part of the above-ground completion, such casing head housing, casing head spools, tubing head spool and annulus connections.
The annulus valves, connection and tapping facilities are not operated on a daily basis. One must descend into the cellar for this, unless extension tools are used. The accumulated rain water is suctioned from the cellar from time to time. The depth of the cellar can be approximately 2.5 metres or more, depending on the number of casings to be sealed off from one another and the type of the well. The immediate environment is also taken into consideration by keeping the tree as low as possible. Some completions are recessed in a cellar for the greater part.
Top cap Top valve (open)
Flow line (injectionline)
Flow-arm valve (open)
Otis-actuated master valve (main valve)
Hand operated master valve
Sealed in open position
Tpiece 1/2 “ kerotest valve Intermediate platform Intermediate deck cellardeck Controlline outlet Tie-line valve Closed and blinded off
No tubing-annulus connector pipe
The Xmas tree setup on platforms
The space available on offshore production platforms is restricted as a result of which the freedom and options during design are also reduced. Platforms have no cellar, but a cellar deck is still referred to. Stairs with an intermediate platform are installed from the cellar deck to operate the valves.
5.
Gate valves
5.1.
Summary of Gate valves
The gas and oil industry makes frequent use of gate valves.
Gate valves are the only valves installed in the Christmas tree. These valves must meet various requirements. We will discuss two sealing principles for this type of valves at this point.
The gas the Christmas tree and the valves are in contact with is at high pressure, can contain H2S and can contain fluid or sand particles. The high speeds of the gas that are generated when the valve is opened and closed require special sealing provisions to limit wear from erosion.
5.2.
The principle structure of a gate valve
In principle, a gate valve consists of a body with exchangeable, parallel seats, in which the gate can move.
Principle of a direct acting gate valve
A packed screw stem with pressure bearing is installed in the bonnet or cap of the valve. The gate can be opened or closed by turning the hand wheel of the stem, since the gate is moved upward and downward along the thread of the stem. If the gate is in the closed position in the bottom of the body, one uses the term 'direct acting valve'. There are also valves whereby the gate rather seizes against the cap or bonnet in closed position. This type is called 'reverse acting valves' and they are used in Christmas trees in combination with an Otis U (pneumatic and hydraulic) and UX actuator.
Clockwise rotation of the hand wheel closes a direct acting valve.
handwheel Pressure bearing Packing Seal Cap or bonnet House or body Screw spindle Gate Exchangeble seat
The two gate valves we will discuss are both direct-acting valves. They have a nonrising stem. One cannot see from the outside whether the valve is open or closed. Gate valves are open/close valves and not control valves. The valve may never be left in the middle position, thus half open and half closed, for example.
If the hand wheel of an open gate valve (at the top gate position) is turned counter-clockwise, it will seize up because the left-hand thread of the stem comes to the end of the thread in the gate (Cameron type F) or in the lift nut (McEvoy model C). If the gate valve is closed, the hand wheel also seizes against a shut-off valve if turned clockwise any further. For these valves tightening the hand wheel has no effect on sealing, but it does close the sealing valve installed for changing the stem packing.
After the valve has been opened or closed, one must turn the hand wheel back at least a quarter turn to prevent the thread between the stem and the gate as well as the gate itself from seizing. Such seizing up can be aggravated by temperature variations during production and production stops and by the ambient temperature. Turning back a quarter turn also assists with detecting the closed/open position of the valve, namely by first turning the hand wheel clockwise. If the valve is closed, the hand wheel blocks after a quarter turn or half a turn, and in open position one can rotate further (direct acting valves).
5.3.
Requirements set for valves
Valves must above all be safe. This leads to a number of technical requirements, such as:
• Valves must be bubble tight, even though there is dirt in the gas or the valve is slightly damaged. It often happens that valves start leaking anyway after some time, so that pressure is built up past the closed valve. The speed at which the pressure builds up depends on the volume of the space past the valve and the magnitude of the leak.
• The valve must be operable under all conditions, even if the valve separates a large pressure difference, i.e. operable by one person (opening/closing).
• The number of turns required to open and close the valve must be limited.
• If the valve is open, passage must be straight, thereby preventing or limiting turbulence (erosion).
• The valve must be resistant to corrosive and/or erosive components in the oil or gas as well as to weather and wind.
• The valve must be resistant to very high well pressures. The following requirements are set for maintenance work on a valve:
• valves must be of such structure that the packings and the bearings of the gate spindle can be inspected and/or replaced during production without removing the valve or releasing the pressure on it
• valves must make it possible to replace the gate or the seats at the location;
5.4.
The method of sealing gate valves
Gate valves can be sealed in various ways. Different methods are used for the gate valves in use in the gas and oil industry. We will succinctly discuss the two most important methods:
• Metal–to–metal seal method
• Sealing–compound method. Metal–to–metal seal method
If a valve is in the closed position, the valve will be under full pressure, while there will be lower pressure past the valve. This pressure difference results in a force.
Principle of a direct acting gate valve
5.5.
Sealing compound method
The force due to the pressure difference on the valve is often insufficient to guarantee a 100% seal.
Moreover, bigger leaks can be caused by irregularities or scratches.
For these reasons, an additional sealing compound is applied automatically between the gate and the seat to ensure complete sealing. The surfaces of the gate and the seat are then finished normally smooth and parallel.
This sealing method is used in the McEvoy valves.
A packed screw stem with pressure bearing is installed in the bonnet or cap of the valve. The gate can be opened or closed by turning the hand wheel of the stem, since the gate is moved upward and downward along the thread of the stem. If the gate is in the closed position in the bottom of the body, one uses the term 'direct acting valve'. There are also valves whereby the gate rather seizes against the cap or bonnet in closed position. This type is called 'reverse acting valves' and they are used in Christmas trees in combination with an Otis U (pneumatic and hydraulic) and UX actuator.
Clockwise rotation of the hand wheel closes a direct acting valve. handwheel Pressure bearing Packing Seal Cap or bonnet House or body Screw spindle Gate Exchangeble seat
Mc-Evoy gate valve - model C (closed position)
Screwcap Hand wheel
Grease nippel
Seal valve and slanted seat for exchanging packing and bearing
lift nut to move the gate along the spindle
Slide block
Space for water-repellent grease
Exchangeble seats
Pressure bearing Spindle packing Bonnet
Reservoir for sealing compound Nipple with venting plug Lefthand threaded screw spindle Piston
5.6.
The McEvoy gate valve, model C
All McEvoy solid block Christmas trees are equipped with this type.
Mc-Evoy gate valve - model C (closed position)
Screwcap
Seal valve and slanted seat for exchanging packing and bearing
lift nut to move the gate along the spindle
Slide block Exchangeble seats
Space for water-repellent grease
Bonnet Grease nippel
Pressure bearing Spindle packing Bonnet
Nipple with venting plug
Lefthand threaded screw spindle Piston
The gate of the McEvoy gate valve consists of two parallel halves kept apart against the seats by pressure springs. Because the gate is parallel, it cannot seize between the seat rings, and additional tightening of the hand wheel has no effect on the seal. The valves in gas flows can be subject to more serious erosion at the time the gate practically closes the passage. The two so-called sealing surfaces must be very smoothly polished to guarantee complete sealing.
The eventual seal between the gate and the seats in the McEvoy valve is established by a special sealing compound.
This compound is stored in two reservoirs on either side of the gate. The reservoirs can be topped up via external grease nipples, which is very important for correct performance of the valve. The advantage of this type of valve is that if any leakage is detected, it can easily be remedied in a short time by injecting a sealing compound.
The space underneath the gate can be filled up with water-repellent grease to prevent accumulation of fluids, hydrates and dirt.
5.7.
The Cameron gate valve, type F
The Cameron gate valve, type F (see figure). A great deal of research has been done to limit wear of the gate and the seat and to find a good solution for such wear. Gas flows through a very narrow space at high speeds especially when a valve is just being opened and nearly closed. The erosion that occurs at that time is extensive and the wear on the gate and the seat as well as leakage appearing at a later stage will be the result.
The Cameron gate valve is fitted with a seat that can rotate (rotating seat). The intention of this is that a new piece of the seat is exposed to wear by the gas flow each time. This way, the wear of the seat is distributed evenly across the entire circumference.
Cameron gate valve – type F (closed position)
Hand wheel connection piece Shear pin Pressure piece
Grease nipple, also pressure release with non-terun valve
Gate
Seat
Teeth aong the circumference Palm mechanism
Hand wheel Pressure bearings
Screw spindle with left hand thread in gate
Seal valve and slanted seat for sealing when the packing set is exchanged Screw cap
Bonnet flange Packingcap
The valves are mostly not fitted with a rotary seat and a catch mechanism. Problems caused by teeth breaking off were more serious than the advantages.
The valve gate is not shared and moves back and forth between two seats through a threaded stem. The seal is formed by a metal to metal seal. The gate and the seat require a very accurate and smooth finish for this reason. In order to protect the gate, the stem and other internal parts from overload, the force applied to the hand wheel is transferred to the stem via a shear pin. This shear pin is not part of the McEvoy gate valve, model C. The forces on the stem during operation are absorbed by two pressure bearings. One packing set, enclosed by a screwed pressure piece, ensures that no leakage can occur along the stem. Since the packing pressure piece is attached by means of thread, the pressure bearing at the Cameron valve can be exchanged under pressure without other precautions being taken. If the packing set must be changed during production, the space for this above the packing can be closed off from the pipe pressure, just as is the case for the McEvoy valve.
A slanted disc (front) is installed on the stem, which moves up after the valve is closed by turning it further clockwise (due to the left-had thread in the gate) and will seal against the slanted seat in the bonnet flange. The pressure that prevails above the slanted seat can be released via the lubrication nipple with the non-return valve. To do so the ball is pushed from its seat by means of a special tool, whereby the pressure can be released. The screw cap can be safely turned loose and the packing set inspected or replaced only after no more gas is escaping.
Tightening the hand wheel has no effect on the seal, just as is the case for the McEvoy valve. Maintenance of the valve is simple thanks to its structure. The seats and the gate can easily be replaced. Depending on the use, the valve must be lubricated from time to time. One disadvantage of the Cameron valve type F is that, if it turns out during wireline work that the valve leaks, it must first be repaired, while the McEvoy valve, model C, can usually be sealed again by refilling up the sealing compound reservoir.
