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Continued from part 1 and part 2……..

Body Cavity Relief (Pressure Equalisation)

Ball valves are do uble seated valv es which

incorporate a cav ity betwe en the seats.

Body cav ity will get pressurised only when the seats

are damaged.

Cavity relief prov ision required only for trunnion

mounted ball valv es. Not required for floating ball

 valv es as the se ats are fixed & the ball is floating.

 Where possib le, cav ity relief shall be to the upstream

side of the valv e.

What is Piping An attempt to ex plain pr oc ess piping engineering basic s in detail to help all the pro ce ss piping engineering

professionals around the world.

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20th July 2014    wa nt 2lea rn   interview     1 Comment

BALL VALVE DESIGN FEATURES: A Literature-

Part-3 of 4

21 1

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Body cav ity Relief 

Single Piston Effect Seat Design

Seats are pressed on the ball by means of spring load.

 As the body cav ity pressure inc rease s than the spring load, the seats are pushed back and the p ressure is

released in the line. This is called as single piston effect (the pressure in the body cav ity is the o nly ac ting

parameter)

Cavity relief to the downstream side, if both valv e seats are of single piston effect design.

Each seat is self relieving the body cav ity o verpressure to the line.

Singl e Piston effect Seat Design

Double Piston Effect Seat Design

In this seat design, medium pressure as well as the body cav ity pressure c reates 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 c avity both are acting parameters)

 Valv es with this de sign requires a c av ity pressure relief dev ice to reduc e the b ody cavity pressure.

DPE is synonymous with “bi-directional”, and SPE is synonymous with “uni-directional” as defined by 

 API 6D/ISO 14313.

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Double Piston Effect Sea t Design

DPE – External pressure relief 

 When the b ody cavity pressure inc rease s abov e the net spr ing load of the pre ssure relief valv e, the

cav ity pressure is vented through Pressure Relief Valve.

The RV o utlet line can be v ented to atms / connected to vent sy stem or back to the upstream piping.

DPE – External pressure relief 

Com bination 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 ov erpressure to release to the v alve upstream side and also don’t require an

external relief valv e.

These valves are unidirectional and flow direction is clearly marked on the valv e body .

Seat Design for Export Line

This seat configuration give s a single barrier against normal flow condition and a double barrier against

reverse flow coming from downstream pipeline.

For ESD/PSD v alv e, rev erse c onfiguration is required than that sho wn here. ESD valve s require s SPE for

upstream se at and DPE for downstre am seat.

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Seat Design for Export Line

Double Blo ck & Bleed (DBB) feature

 When the v alv e is in fully closed o r fully open positio n, eac h seat seals o ff the pro ce ss medium

independently at the same time between the up/down stream and body cav ity; it allows bleeding of the

cavity pressure through drain or vent valve.

This DBB feature permits in-line periodic inspect ion of the valv es and the chec king of sealing integrity  when the v alv e is installed in line .

This feature is av ailable with self reliev ing seat (SPE) co nfiguration.

DBB Vs DIB

If a ball v alv e has bo th seats as unidirectio nal (SPE) seats, it is called as Double Block & Bleed (DBB).

If a ball va lve has one o r both bidirect ional (DPE) seats, it is called as Double I solation & Bleed (DIB).

In the DBB valve, the downstream seat pushes away from the valv e once the b ody cav ity pressure is

higher than the downstream pressure, allowing fluid to flow downstream past the closed v alve. I n the

DIB valv e the do wnstream seat seals and prevents the upstream pressure from reaching the downstream

piping.

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Double Block & Bleed (DBB) featu re

Blow Out Proof Stem Design

 When the v alv e is in the open / c losed po sition, pr essure is alway s ac ting upo n the botto m of the ste m,

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 ho using.

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 re placed

 while the v alv e is under pr essure, without the stem being pushe d out (blo wn o ut).

3-Way Pig Valveshillsflow.com

HFC 3-way pig valves are design for oil and gas industries.

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Blow Out Proof Stem Design

 

 Ant i-Stat ic Design

Build-up of static electricity can oc cur 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 prov ided between the ball & stem and stem & body 

 whic h pr ov ides elec trical c ontinuity .

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 An ti Stat ic Stem Design

Fire Safe Design

1) Internal Leakage Prevention (from pipeline to body cav ity)

 When no n-me tal resilie nt seats are destr oy ed in a fire, the upstream medium pressur e push the b all into

the downstream metal seat lip to c ut-off the line fluid and prevent the internal leakage due to a

secondary metal-to-metal seals.

 Anothe r fire safe packing is prov ided at the se at r ing fo r internal leakage prev ent ion to bo dy cav ity .

Graphite is normally used as a fire safe packing material, because the melting point of graphite is 1000

deg.C.

 

Fire Safe Design

2) External leakage prev ention (from bo dy/ stem joints to atms)

 All the p ossible e xternal leakage points b etwee n stem & gland flange, g land flange & bo dy and bo dy &

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if specified by purchaser.

The valve design & material selection should negate the need for such c onnection.

If specified, this injection connection is integrated with check v alve to prov ide backup sealing, also a

check v alve is equipped at front of seat sealant injection to avo id blowing out in case of wrong operation.

 When the soft sealing materials (seat inse rts and o-rings) ar e damaged and leakage happened by fire o r

other acc ident, the sealant can be injected through the injection fittings.

Sealant Injection Sy stem

 

The sealant injection system thro ugh 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…

Click h ere to read part 1

Click h ere to read part 2

 

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sciric

Wednesday 23 July 2014, 12:35 pm

Is it possible to give us example of modeling Air Cooler Haeders and Nozles in CaesarII?

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