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.
- 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).
- 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.
Final part will be published soon…