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SAFETY VALVES

SAFETY VALVES

• To protect systems or vessels from excessive pressure

According to construction

1)     Dead weight

2)     Lever type

3)     Spring

According to operation

1)     Pneumatic

2)     Solenoid

3)     Steam

Safety valves

To provide immediate relief of over pressure by making rapid full opening for compressive fluids.( air, gases and steam)

There should not be any isolation valve before safety valve.

 

Relief valve

To relieve excessive pressure in system containing incompressive fluids.

Generally have gradual lift proportional to the increase in pressure.

( oil & water)  

Safety relief valve

- It is an automatic pressure-relieving device whose open and close movements are fully controlled by source of power –such as air,steam and hydraulic.

- It may be operated manually as desired at any pressure.

 

Non-Reclosing PRV

  - It is designed to remain open after

operation.

Vacuum relief valve

•Designed to admit fluid to prevent an excessive internal vacuum.

•It is designed to re-close and prevent further flow of fluid after normal conditions have been restored.

REQUIREMENT OF SAFETY VALVE

      - Proper functioning

     - Opening at correct set pressure

      - Correct blow down

      - Proper mechanical operation &

        Clean closure without leakage.

 

As per boiler regulations, every boiler shall have the following mountings 

1. Two safety valves

2. Two water level indicator

3. A steam pressure gauge

4. A steam stop valve

5. A feed check valve

6. Two feed pump

7. A Blow down valve

LOCATIONS

•       Drum•       Super heaters•      Reheaters•     Aux. Steam headers•         Deaerators•         Oil heaters•         Soot blower systems

CHATTERING

- Rapid opening and closing of the valve is

known as chattering.

CAUSES

- Insufficient blow down

- More back pressure due to under size

discharge piping

- Insufficient steam flow to the valve

- lifting with water

SIMMERINGThe valve is just about to open

and has indefinite contact.An audible sound or visible

escape of fluid between seat and disc at an inlet static pressure below the popping pressure and at no measurable capacity.

FLUTTER

• Flutter is an abnormal, rapid reciprocating motion of the safety valve stem, in which the disc

does not contact the seat.

Blow down

Opening pressure – Closing pressure

----------------------------------------------- X 100

Opening pressure

Important codes governing safety valves

1)     IBR – 1950

2)     ASME Sec- I Power boilers

3)     ASME Sec – VIII

4)     BS - 756

General conditions for safety valves

- Drum safety valves should be 75% of evaporating capacity

-    Super heater safety valves should be 50% of evaporating capacity

-   Both drum & super heater safety valves discharge capacity should be more than

100% evaporating capacity.

- Each Reheater safety valves shall

discharge more than 20% of evaporation.

- All reheater safety valves combined

should be more than 100% evaporating

capacity.

- Inlet connection should be welded

for high pressure safety valves.

- Safety valves should be full lift

type.

-  Blow down should be 3 to 5 %.

   - Easy maintenance

   - No seat tightness problems

   - No Chatter problems

   - Economy

PARTS OF SAFETY VALVE

      Body , Yoke

    Stem , Seat bushing, Spring,

Disc, spring retainer plates,

Compression screw,

Cross bar,

upper & lower rings

              

DISMANTLING

 

Ensure availability of tools

Availability of spares

Taking measurements

Thorough inspection

Check condition of stem,seat,

spring & disc.

 

OVERHAULING

  Cleaning of all parts

Lapping of seat & disc

  Spindle run out

  Replace defective parts

Painting

ASSEMBLING

Use Molykote lubricant

Assemble as per sequence

Maintain recommended readings

    Tightening of all bolts & Nuts.

 

UPPER RING ADJUSTMENT

-    To shorten the blow down raise

upper ring.

-    To increase the blow down

lower upper ring

LOWER RING ADJUSTMENT

-  To obtain clean popping action

-  To cushion the closing action of the

valve

- Reduces simmering

FILED TESTING

•      Use of hydro set unit•       Full system pressure•       Valve should be set with in 1% of set

pressure •      Valve should be hand popped at

75 to 80% of set pressure

GAGGING

-  Arresting the stem mechanically from

lifting.

-  Do not gag in cold condition

-  Gag at 80% of low set value pressure.

FLOATING

-   Calibrated gauges are to be used.

- Test and achieve within minimum

number of pops.

-   Apply gag load finger tight only

-   Remove gags after floating

- Bring Blow down between 3 and 5 % 

MAINTENANCE

•  The recommended spares and tools

are to be made available.• Flat lapping plate.• High temperature lubricant.• Lapping compound of grit size

• 320 – general, 500- finishing &

1000 for polishing

-  Step height shall be 0.635 MM, if it is

less than 0.254 MM, seat cutting is

to be done.

-  Spindle run out should be with in

0.177 MM.

SAFETY PRECAUTIONS

- Don’t go near discharge side

- Body drain and vent must be piped to safe

area

- Gag the valve during ring adjustment

- Hand pop with 5 to 6 Mts safe rope) 

-  Use ear muffs / hand gloves

-  Adequate access around the valve for

maintenance works.

 

PROCUREMENT OF SAFETY VALVE

      - Inlet & outlet size - Inlet & outlet construction

- Flow     - Medium details        - Blow down  - Set pressure       - Body material    - IBR requirement

ERECTION

-         Flanged

-         Welded – dismantling

-         Preheating & post heating

-         Verticality

-         Vent pipe orientation

-         Accessibility

THANK YOU