Ball Valve Design Features

BALL VALVE DESIGN FEATURES

The purpose of this post is to understand the various ball valve design features. The points discussed in this post are:

• Applicable codes
• Reduced bore / full bore design
• Soft  seated / Metal seated design
• Side Entry / Top Entry / Welded body design
• Valve ends / Valve operators
• Floating / trunnion mounted design
• SPE / DPE / Body cavity relief features
• DBB / DIB / Extended bonnet / weld overlay
• Anti static / Fire safe design / Sealant Injection
• Seat insert materials / O-rings / Lip seals
• Lifting & supporting requirements

Definitions:

• Uni-directional Valve – Valve designed to block the flow in one direction only
• Bi-directional Valve – Valve designed to block the flow in both upstream & downstream directions
• Bleed – Vent or drain
• Elastomer – Polymer with elasticity (elastic polymer). This is also called as rubber.
• Thermoplastic – Polymer that becomes moldable above a specific temperature, and returns to a solid state upon cooling. It is re-moldable.

Ball Valve – A valve that uses a spherical obstruction (a ball) to stop and start the hydraulic flow. A ball valve is usually rotated 90° to open and close.

Ball Valves Features:

Advantages:

• Quarter turn straight thru valve / fast opening & closing
• Tight Shut off as well as very easy to use
• Application as isolation valve (on and off condition)
• Suitable for Emergency shutdown conditions

Disadvantages:

• Not suitable for throttling
• Fluid trapped in the body cavity
• Limited working temperature range

Applicable Standards for Ball Valve:

• Design Std. – API 6D / ISO 14313 / BS EN 17292
• Testing Std. – API 6D / API 598 / BS 6755 Part I
• Fire testing – As per API 6FA, API 607, ISO 10497 or BS 6755 Part II.
• Dimensional Std. – ASME B16.10 / API 6D

Ball Valves Patterns:

• Short pattern
• Long pattern
• Generally long pattern dimension is selected for ease of connection to pipe flanges.

Ball Valve Internals

Ball Valve Design Types

• Soft seated and Metal seated Design
• Reduced bore and Full bore Design
• Body types – Split, Top entry or welded body
• Floating and Trunnion mounted Design &
• Single / double piston effect design
• DBB / DIB

Reduced Bore (Reduced Port) Design

• Bore dia. 1 size less than pipe dia. for valve size up to 12”NB & 2 size less for 14”NB to 24”NB (and 3 sizes less for sizes above 24”NB).
• Smaller and lighter valves.

• Have lower operating torque, resulting in lower cost actuated valve package.
• Slightly higher pressure drop than full bore valve.
• Prevents pigging
• These valves are normally of one piece – end entry design for smaller sizes (up to 4”-150#) & two / three piece – side entry design for bigger sizes.

These valves are also called as regular port valves.

Full Bore (Full Port) Design

• Bore inside diameter same as pipe inside dia.
• Very less pressure drop
• Of higher weight than reduced bore valve, hence more costly.

Selected for specific process reasons, typically; minimum pressure drop, minimal erosion, pigging requirement and gravity flow (to avoid liquid pocket

Soft Seat Design

• Thermoplastic or Elastomeric seats are inserted in a metallic holder (seat ring) to provide soft seating action
• Provide good sealing ability
• Lower in cost than metal seated valves
• Limited temperature rating
• Should not be used in dirty services, particularly on floating ball valves
• Soft seat materials used are – PTFE, Nylon, Devlon, PEEK, etc
• It is generally accepted a leakage of ISO 5208 Rate A.

Metal Seat Design

• Direct metal to metal contact between seat ring & ball.
• Valves are used for abrasive service and for services where soft seated valves can not be used due to temperature limitations.
• The ball & seat contact surfaces are hard faced to improve resistance to wear & prevent scratching caused by the solid particles contained in the process media.
• Metal sealing may be obtained by tungsten carbide coating (up to 200 deg. C), chromium carbide coating (above 200 deg. C), electroless nickel plating (ENP) or stellite hard facing.
• Acceptable leakage of ISO 5208 Rate D.

Single Piece Body Design

• In the single piece design valve, the body will be cast/forged as one piece. The insertion of the ball will be through the end of the body and is held in position by body insert.
• This design offers the unique advantage of eliminating the possibility of external leakage to the atmosphere through bolted body joints.
• This design restricts the valve to be of reduced port floating design only (for sizes up to 4”NB).

Two piece / Three Piece Design

• Two-piece design complements the single-piece design in sizes of 6” & above for reduced bore and for FB design valves.
• In two piece design, the body is constructed in two pieces and the ball is held in position by body stud. There can be full bore or reduced bore design possible in this construction.

• In case of three piece design, the body has two end pieces and one center piece.
• Three piece design valves are most easily on-line maintainable. By removing the body bolts keeping only one, the body can be swung away using the last bolt as fulcrum, to carry out any installation or maintenance operation on the valve. This feature reduces maintenance downtime to a bare minimum.
• For larger 2 piece or 3 piece valves the dimensions between the body and flange should be checked so that sufficient clearance is available for bolting.
• During vendor drawing review same should be checked.

Top Entry Design

• Maintenance and repair possible in-situ, by removing the top flange. This minimizes the maintenance downtime.
• Limited space is required around the valve for maintenance.
• Available in welded as well as flanged ends, but welded ends are preferred to reduce potential leak paths and minimize valve weight.
• Heaviest and most expensive construction.

Welded Body Design

• Welded body construction eliminates body flanges, reduces potential leak paths and increases resistance to pipeline stresses.
• Minimum number of leak paths, hence beneficial in fugitive emission and vacuum applications.
• Compact and light weight design
• The body draining & venting feature allows the valve maintenance technician to test each seat rings sealing ability with the ball in either the full open or full closed positions.

• Sealant injection fittings access directly to each seat ring. This enables the technician to top-up the quantity of lubricant inside the valves sealant injection system on a periodic basis.
• Valve cleaner can also be injected into these fittings to flush out the old grease in the valve and to clean critical seal faces on the ball.
• Heavier sealants are also injected through the sealant injection fittings during an emergency when a critical seal is required.
• Applications – Oil & Gas pipelines, compressor stations, measuring skids, etc.

Valve Ends

The type of valve ends are as follows:

• Flanged ends with raised face or ring joint face
• Threaded ends
• Socket weld ends
• Butt-weld ends – Soft as well as metal seated butt-welding end valves shall be provided with butt-weld pup pieces.

• This avoids damage to the valve seat as well as soft seal materials due to welding heat.

The pup piece length shall be 200mm for sizes up to 2”NB, 400mm for up to 12”NB size & 800mm above 12”NB sizes.

Valve Operator

• Ball Valves can be operated by a lever, wrench, hand wheel or they can be pneumatic, hydraulic or motor operated.
• A ball valve is rotated in clockwise direction to close & anti-clockwise direction to open.
• The maximum lever length shall not exceed 450mm & max. hand wheel diameter shall not exceed the valve face to face dimension or 800mm whichever is smaller.
• Gear operator is required to be provided for valves as per below criteria:
• 6” & larger for class 150 valves
• 4” & larger for cl. 300 & 600 and
• 3” & larger for cl. 900 onwards

Floating or Seat Supported Design

• Ball valve design in which the ball is not rigidly held on its rotational axis & is free to float between the seat rings.
• In the closed position ball is pushed against the seat by the pressure of the fluid from upstream and hence can pressure seal the downstream of the valve.
• Ball seats on the downstream seat only.
• Seat loading increases at higher pressure and for larger size and becomes excessive, for soft seated valve. Also, the higher the size the heavier the ball, less likely to be moved by pressure. Hence the need of trunnion mounted design comes into picture.

• Floating design valves have lower manufacturing cost.
• Valves of small sizes and lower pressure rating are seat supported (10” for 150#, 6” for 300# & 2” for 600# & above).
• Seat supported design generally needs higher operating torque.
• Metal seated floating ball valves also incorporate spring loaded seats.

Trunnion Mounted Design

• The ball is fixed in position by the stem & the trunnion which are supported in bearings in the body.
• The seat are spring loaded onto the ball, giving reliable sealing at low pressures.
• The key feature is that the ball does not shift as it does in a floating valve to press the ball into the downstream seat. Instead, the line pressure forces the upstream seat onto the ball to cause it to seal.
• As the area on which the pressure acts is much lower, the amount of force exerted on the ball is much less, leading to lower friction values and smaller actuators or gear boxes.
• Seat designs are either single or double piston effect.
• Valves of larger sizes and higher pressure rating are trunnion mounted.
• All standard trunnion mounted ball valves shall be provided with self-relieving seats allowing automatic body cavity relief exceeding 1.33 times the valve pressure rating at 38°C (overpressure due to thermal expansion of trapped fluid).

Body Cavity Relief (Pressure Equalisation)

• Ball valves are double seated valves which incorporate a cavity between the seats.
• Body cavity will get pressurised only when the seats are damaged.
• Cavity relief provision required only for trunnion mounted ball valves. Not required for floating ball valves as the seats are fixed & the ball is floating.
• Where possible, cavity relief shall be to the upstream side of the valve.

Single Piston Effect Seat Design

• Seats are pressed on the ball by means of spring load.
• As the body cavity pressure increases than the spring load, the seats are pushed back and the pressure is released in the line. This is called as single piston effect (the pressure in the body cavity is the only acting parameter)
• Cavity relief to the downstream side, if both valve seats are of single piston effect design.
• Each seat is self-relieving the body cavity overpressure to the line.

Double Piston Effect Seat Design

• In this seat design, medium pressure as well as the body cavity pressure creates a resultant thrust that pushes the seat rings against the ball. This is called as double piston effect (the pressure in pipe & that in the body cavity both are acting parameters)
• Valves with this design requires a cavity pressure relief device to reduce the body cavity pressure.
• DPE is synonymous with “bi-directional”, and SPE is synonymous with “uni-directional” as defined by API 6D/ISO 14313.

DPE – External pressure relief

• When the body cavity pressure increases above the net spring load of the pressure relief valve, the cavity pressure is vented through Pressure Relief Valve.
• The RV outlet line can be vented to atms / connected to vent system or back to the upstream piping.

Combination Seats

• In some cases, single piston effect seat is used for upstream side and double piston effect seat is used for downstream side.
• This enables the cavity overpressure to release to the valve upstream side and also don’t require an external relief valve.
• These valves are unidirectional and flow direction is clearly marked on the valve body.

Seat Design for Export Line

• This seat configuration gives a single barrier against normal flow condition and a double barrier against reverse flow coming from downstream pipeline.
• For ESD/PSD valve, reverse configuration is required than that shown here. ESD valves requires SPE for upstream seat and DPE for downstream seat.

Double Block & Bleed (DBB) feature

• When the valve is in fully closed or fully open position, each seat seals off the process medium independently at the same time between the up/down stream and body cavity; it allows bleeding of the cavity pressure through drain or vent valve.
• This DBB feature permits in-line periodic inspection of the valves and the checking of sealing integrity when the valve is installed in line.
• This feature is available with self-relieving seat (SPE) configuration.

DBB Vs DIB

• If a ball valve has both seats as unidirectional (SPE) seats, it is called as Double Block & Bleed (DBB).
• If a ball valve has one or both bidirectional (DPE) seats, it is called as Double Isolation & Bleed (DIB).
• In the DBB valve, the downstream seat pushes away from the valve once the body cavity pressure is higher than the downstream pressure, allowing fluid to flow downstream past the closed valve. In the DIB valve the downstream seat seals and prevents the upstream pressure from reaching the downstream piping.

Blow Out Proof Stem Design

• When the valve is in the open / closed position, pressure is always acting upon the bottom of the stem, trying to push the stem up.
• The stem is sealed by O-rings and graphite packing rings.
• The stem is held in position by the stem housing, which is bolted to the body.
• The graphite packing rings are compressed and held in position by the gland flange, which is bolted to the stem housing.

• Therefore, when the gland flange is removed to replace the graphite packing rings, the stem is still held securely, by the stem housing.
• That means the blow-out proof stem feature ensures that the top graphite packing rings can be replaced while the valve is under pressure, without the stem being pushed out (blown out).

Anti-Static Design

• Build-up of static electricity can occur as a result of constant rubbing of the ball against the PTFE seats. This can be a potential fire hazard, especially while handling flammable fluids.
• In the anti-static feature, spring loaded balls are provided between the ball & stem and stem & body which provides electrical continuity.

Fire Safe Design

1. Internal Leakage Prevention (from pipeline to body cavity)

• When non-metal resilient seats are destroyed in a fire, the upstream medium pressure push the ball into the downstream metal seat lip to cut-off the line fluid and prevent the internal leakage due to a secondary metal-to-metal seals.
• Another fire safe packing is provided at the seat ring for internal leakage prevention to body cavity.
• Graphite is normally used as a fire safe packing material, because the melting point of graphite is 1000 deg.C.

2. External leakage prevention (from body/stem joints to atms)

• All the possible external leakage points between stem & gland flange, gland flange & body and body & adapter are sealed with primary O-ring then secondary graphite gasket. When fire burned out the primary O-ring seal, the secondary graphite gasket seal can prevent the process medium from external leakage.
• Fire safe seals are generally not designed for fugitive emission performance (fugitive emission – emissions of gases or vapors from pressurized equipment due to leaks).
• The fire testing of valves is carried out as per API 6FA, API 607, ISO 10497 or BS 6755 Part II.

Fire Safe Vs Fire Tested Design

• Fire safe design is a design that by the nature of it’s features and materials is capable of passing a fire test.
• It is capable of passing a fire test with specified limits on leakage to the atmosphere and downstream after being closed subsequent to fire exposure.
• A fire tested design is a design subjected successfully to fire testing as per the applicable testing standard.
• That means the fire safe valves are not necessarily be fire tested by the manufacturer.

Valve Fire Testing Criteria

• One test valve may be used to qualify valves larger than the test valve, not exceeding twice the size of the test valve.
• A 16” size valve will qualify all larger sizes.
• One test valve may be used to qualify valves with higher pressure ratings but no greater than twice the pressure rating of the test valve.
• The above criteria is acceptable for valves of same basic design as the test valve & same non-metallic materials.

 

Sealant Injection System

• Valves are to be equipped with sealant & lubricant injection connections located at stem and seats area, if specified by purchaser.
• The valve design & material selection should negate the need for such connection.
• If specified, this injection connection is integrated with check valve to provide backup sealing, also a check valve is equipped at front of seat sealant injection to avoid blowing out in case of wrong operation.
• When the soft sealing materials (seat inserts and o-rings) are damaged and leakage happened by fire or other accident, the sealant can be injected through the injection fittings.
• The sealant injection system through the seat up to the ball contact circle may provide temporary sealing until it is possible to restore the primary seal.
• No seat sealant injection shall be provided for ESD valves.

Extended Bonnet

• The integrity of stem seals at very low temperatures (-30 deg.C& below) is the major hurdle that must be overcome.
• Specially designed extended bonnets installed to valves offers a safe & efficient method to accomplish stem seal integrity.
• The bonnet extension provides a gas column that allows the gas to vaporise from contact with the warm ambient temperature outside the service line. This vapour column insulates the stem seal and maintain the seal integrity.
• Bonnet extension also helps for thermal insulation installation.

Weld Overlay

• Sealing areas & other wetted parts of ball valve can be cladded in case of corrosive service.
• More frequent used materials for the overlay process are stainless steel, DSS & high nickel alloys.
• This technology is cost effective for ball valves in highly corrosive or erosive services.
• Considerable cost saving without sacrifice to service life or performance.
• Can be done cost effectively for size 8” and larger.
• Welding is performed in accordance with ASME BPV section 9.

Seat Insert Materials

•  Thermoplastic seat / seal inserts

DevlonV : Temp. Range -100 deg. C to 150 deg. C

• Elastomeric seat / seal inserts

Seat Insert Materials

• Zero leakage is easier obtained by softer seals (elastomeric), while the resistance to scratches and other factors (temp., pressure, erosion) is obtained by harder seals (thermoplastic).
• PTFE is generally not recommended for high pressure (cl. 900 & higher) while it is suitable for a wide range of temperature and resistant to many fluids.
• Nylon 12G is more suitable than PTFE for higher pressure, but has a limited range in temperature.
• Nylon 6 should not be used as it absorbs humidity.
• Devlon V is similar to Nylon 12G, but with a wider range of temperature application (lower & higher)
• PEEK is recommended for high temperature (up to 260 deg.C) but it is very hard compared to other non metallic materials.
• Kel-F is specially recommended for cryogenic service.

O-Rings (Elastomeric)

O-Rings are used for below applications:

• Stem seals
• Seals between seat and body/closure
• Seals between body and bonnet/closure

Materials are generally as follows:

• Viton (fluor elastomer)
• NBR (nitrilicbutadine rubber)
• HNBR

As per latest PDO SP-2154 (Valve technical specification) O-rings are not allowed in the seat ring-body joint as well as for body-bonnet joint. The ball valve seat ring shall have primary lip seal with a fire safe graphite ring.

At stem side, if the seal material specified in requisition as thermoplastic, it shall be of lip seal type with Inconel 718 spring. If the seal material is specified as elastomeric, it shall be of AED type.

Lip Seal

• For applications where elastomeric O-rings are not reliable, lip seals are used (for body & stem sealing).
• Lip seals are self energised seal systems, made of a Teflon cover and a spring (Inconel 718 material).
• The spring provides the initial load (due to the low elasticity of Teflon), while the fluid pressure provides the load to force the lips on the sealing surfaces.
• Lip seal housing on CS valves shall be SS316 weld overlayed (3mm thk).

Valve lifting & Supporting Provision

 

• Valves of sizes 8”NB and above or 250 Kg & heavier shall be equipped with lifting lugs.
• Tapped holes & eye bolts are not acceptable.

Valve shall have support lugs for valve weighing more than 750Kg  and supports should be designed to take care of the vertical & lateral loads of valves. The support height shall be as minimum as possible

Other Requirements

Drain & Vent connections

• Shall be drilled & threaded for ball valves up to 900# pressure class & for sizes less than 6” –FB & 8”-RB. The connections shall be fitted with threaded plug.
• The plug shall be suitable locked by locking ring to prevent loosening .
• The drain & vent connections for ball valves above 900# pressure class & 6” –FB / 8” -RB & above sizes shall be fully welded  flanged type, fitted with a blind flange.
• If drain / vent / sealant injection is asked, ensure orientation of the connections is accessible at site. During vendor drawing review same should be checked.