Safety valves and its basic concepts

Pressure Relief Valve:This device is generally fitted on liquid lines like water, oil line. In this, valve the opening is proportional to increase in the line or vessel pressure. Hence the opening of valve is not sudden, but gradual if the pressure is increased gradually. In relief valve valves may not open 100%, as the line pressure reduces valves closes gradually. Pressure relief valves have higher flow capacities
Pressure Safety Valve: It is fitted on compressible fluid or gas lines. For such a valve the opening is sudden. When the set pressure of the valve is reached, the valve opens almost fully.
Pressure safety valve & relief valves are used for system, equipment & man power protection.
Pressure reducing valve: These may be of hydraulic or pneumatic type used for water lines. This valve reduces the pressure of the water that goes through it, and is used to obtaining a regulated and constant value at its outlet.
Pressure control valves: These may be of hydraulic or pneumatic type used for steam lines
Safety valve:
A safety valve must always be sized and able to vent any source of steam so that the pressure within the protected apparatus cannot exceed the maximum allowable accumulated pressure.Here the valves sizing, manufacturing, installation, positioning & setting are more important.
Factors to be considered for selection/design of a pressure safety valve:
  • Connection size and type
  • Operating pressure 
  • Operating Temperature
  • Back pressure
  • Service
  • Required capacity
  • Thermodisc
  • Thermal compensation
  • Blow down & Operating gap
Terminology used in safety valves:

Set pressure: It is the pressure at which safety valve lifts or pops up.It is usually 106-107% of operating pressure.

Reseat pressure: It is the pressure at which Safety valve seats.

Blowdown: It is the Blowdown is the difference between set pressure and reseating pressure of a safety valve expressed as a percentage of set pressure.

Blow down of safety valve = (Set pressure – Reseat pressure) X 100 / Set pressure

Blow down of safety valves is in the range of 2 to 5%.

Chattering: Excessive pressure loss at the inlet of the safety valve will cause extremely rapid opening and closing of the valve, this is called as chattering.

Chattering may result into lowered capacity as well as damage to the seating surface of the valve. Continuous chattering may result into damage to the other parts.

Following recommendation wil assists in eliminating the chattering
  • The area of the inlet nozzle should be equal to the the inlet area of safety valve & that nozzle should be short as possible
  • Inlet nozzle corners must be rounded to a radius of not less than ¼ of the diameter opening
Sonic Vibrations: Flashing, choked flow sudden flow or cut/off of steam in safet valves & related lines may result into  sonic vibration. This velocity is usually reached when the valve pressure drop rises to 50% of the upstream pressure. Vibration of long pipelines can also occur due to mechanical damage.
Precautions to avoid sonic vibrations:
  • Safety valve should be installed at least 8D to 10D of pipe diameter down stream from any bend in steam line.
  • Safety valves should not be installed closer than 8D to 10D to pipe diameters either upstream or down stream from the diverging or converging  “Y” fittings.
  • The safety valve nozzle should never be installed in a steam line in a position directly opposite a branch line of equivalent size.
Accumulating test pressure: The accumulation test is done on boilers to limit the excessive pressure rising while the safety valve is in open. The test is carried on new boilers or new safety valves with full firing condition with MSSV and feed water valves closed. It is conducted as long as water in drum permits generally 7 minutes for water tube boilers.

Consideration for installation of safety valve:
  • Exhaust drain & cover plate vent piping must be installed so that they will not impose under pressure on the safety valve.
         Note: Do not plug the cover plate hole or do not reduce the hole piping size.
  • Discharge pipe of the safety valve should not be supported on the valve body
  • Clearance between the valve exhaust piping and the discharge stack should be sufficient to prevent contact when considering thermal expansion of the boiler valve
  • Steam flowing vertically out of the discharge elbow produces a downward reaction on the elbow, in proportion on the quantity of steam flowing & its velocity.

  • In no case should discharge piping smaller than  the outlet valve

  • For optimum performance safety valves should be serviced regularly
  • Valve assembly should be within 10 vertical alignments.
  • Gaskets fitted should be of correct size, should not close the valve inlet opening

Adjustment of set pressure:
Safety valves are set +/- 1% of set pressure.Set pressure should not be changed without the permission of manufacturing unit.

Before proceeding to check the popping (lift) pressure, ensure the pressure gauges used are calibrated. To adjust the popping pressure, remove the lifting gear, exposing the adjusting bolt lock nut. Loosen the lock nut if the opening pressure is low tighten (turn clockwise) the adjusting bolt, if it is high loosen (turn counter clockwise) the bolt. After each adjustment the lock nut should be securely tightened to prevent loosening of the bolt.

Adjustment of blowdown:

If the blow down is not as desired when the set pressure has been obtained, it is must to adjust the rings. The guide (adjusting) ring is the principal blow down control ring. To change its position, remove the guide set screw on the back of the valve body. Insert a screw driver or similar tool and engage one of the notches (these can be seen through set screw hole). The ring can then be turned to the right or left as desired. Turning the guide (upper) ring to the right raises it up and reduces the blow down. Turning the guide (upper) ring to the left lowers it and increases the blow down. After each adjustment always replace and tighten the set screw being careful that its point engages a notch and does not rest on the top of the tooth.

Note: Do not attempt to adjust blow down with lower ring

Factors which cause safety valve to damage or failure:

  • Quantity & quality of the steam
  • Discharge piping stress and back pressure
  • Variation in ambient temperature
  • Improper gagging
  • Improper bolting of flanges
  • Foreign material in the steam
  • Improper method of assembly & disassembly

Guidelines for Boiler safety valve setting:

Preliminary checks
  • Ensure calibrated pressure & temperature gauges are fitted.
  • Gauges for each individual valves should be fitted
  • Discharge piping has to be inspected for binding on the valves,supports and welds on piping.
  • A rope appx. 6-7 meters with a hook one end should be attached to the valve lifting lever before starting the pressure rise. It will help in operating the lever to avoid chattering & over pressure
  • Have the correct tooling available
  • Establish the good communication system
  • If the unit has Electromatic safety valve, this valve should be in operation firts for more safety of the unit.
  • Drum valves to should be tested first: Possibilities of valve part damage because of GIRL BLASTING are grater on superheated valves in contrast to the drum valves .If super heater valve is gagged after seat damage while testing other valves, the total valve damage will increase.

  • Boiler temperature increases during the testing cycle of the Drum valves. Consequently higher temperature steam will be available for super heater steam valves and produce accurate results
  • Keep water level low as possible, if drum level is high the safety valves may slugged with water causing long blow down & also may result damages to seat & disc.
  • Maintain pressure rising in the range of 2-3 kg/cm2 per minute, slow pressure rising may result into simmering of the valve.
  • If fuel feeding system fails at nearer set pressure, then reduce the boiler pressure at least 10% & raise again. Holding the boiler pressure nearer to set pressure for long time may result into simmering & valve lift erratically.
  • If a valve has to be lifted several times, cooling off period is very must. Cooling period is around 20-30 minutes.
  • If valves have not been tested with hydro test prior to the steam condition, it is recommended to hand lift before steam actuation.

Safety valve floating procedure:
  • Normally the highest set pressure valve is the valve floated first. While setting this valve other safety valves are gagged.
  • Start the boiler as per cold start up procedure by modulating the firing.                
  • When the drum pressure reaches about 60–70% of operating pressure gently tighten gage on other safety valve. 
  • Raise pressure slowly by throttling start up vent valve. When 80% of popping up pressure is reached manually operate the safety valve under test. This will blow off any debris or dust left over in the valve internals.
  • Raise the boiler pressure by modulating the firing
  • When the pressure reaches nearer to the set pressure close the start up vent. While the safety valve pops (lift), open the start up vent valve and note down the lifting/set pressure value.When the valve sits back, note down the reset pressure
  • Control of drum level is important to avoid possibility of water carry over from drum to the super heater.
  • The set pressure is adjusted by either tightening or loosening the adjusting nut. Tightening the nut increases the set pressure and vice versa
  •  Blow down is adjusted by upper rings adjustment.
  • After setting the set pressure and blow down, bring down the boiler pressure to operating level.
Examples-1: A boiler steam drum safety valve lifts at 125 kg/cm2 and reseats at 120 kg/cm2, then calculate its blow down percentage?
                                   BD% = (125 – 120) X 100/125

Example-2: A boiler super heater safety valve has blow down 3% & has been set at 70 kg/cm2, calculate the reseat pressure.

3% = (70-P2) X 100/70

Reseat pressure P2 =67.90 kg/cm2

Precautions shall be taken during Super heater safety valve set at lower operating temperature than actual:

Safety valves blow down should be set more than required, as blow down percentage decreases as the steam temperature increases. An approximate rule is to add 0.5% of set pressure to the blow down for each 56.5 °C rise in SH steam temperature.

If a Super heater safety valve lifts at 189.5 kg/cm2 & reseats at 180 kg/cm2 at the temperature of 400 deg c, then calculate the blowdown calculation at 540 deg c

We have,

Lift pressure        = 189.5 kg/cm2

Reseat pressure =180 kg/cm2

Difference           =9.5 kg/cm2

Difference in temperature =540-400 = 140 Deg C

Asper above condition for every 55.6 deg c  rise in steam temperature blow down percentage increases by 0.5% of set pressure

140/55.6  X 0.5 X 189.5/100

=2.385 kg/cm2

Hence, blow down  at 540 deg c =9.5-2.385 =7.115 kg/cm2

Available @ Flipcart/Amazon/Notion press

No comments:

Post a Comment

Hi all,
This article is written based on practical experience..If liked, share with others, or any suggestions leave in comment box.

15-Emergencies in power plant operation

Most visited posts