pressure relieving
TRANSCRIPT
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WELCOME TO THE PRESENTATION
ON
INTERFACE PLAN AND
COORDINATION WITH OTHER
DEPARTMENT
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INTERFACE PLAN
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SAFETY & PRESSURE
RELIEVING DEVICES
SAFETY OR PRESSURE RELIEF DEVICES ARE USED FORPROTECTION OF LINES AND EQUIPMENTS IN CLOSEDPRESSURISED SYSTEM AGAINST OVER- PRESSURE.
Causes of Over Pressure :
Blocked Discharge.
Cooling water failure
Failure of upstream equipment
Expansion due to external heat source
Thermal stresses.
Each equipment is designed keeping in mind that the workingpressure never exceeds the Maximum Allowable working pressure.Safety or pressure relief devices ensure the same by opening/burstingand relieving the excess pressure into a closed system or atmosphere.
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SAFETY & PRESSURE
RELIEVING DEVICES (Cont.)
Safety devices are basically of two types :
* self closing type Spring loaded Safety or pressure
relief devices
* Non closing type Rupture Discs.
Operational characteristics vis-vis system and device pressure is given in Figure 3
enclosed
Safety valves: These are spring loaded pressure relieving devices , designed to
provide full opening with minimum overpressure. The valves are designed to open
rapidly in POP action and when the operating pressure falls to below the set
pressure by a pre set margin the valve reseats.
Relief Valves: These devices are similar to safety valve except that they are
designed for liquid service and display proportional lift against POP action.
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SAFETY & PRESSURE
RELIEVING DEVICES (Cont.)
Type of Pressure Relief valves :
* Conventional used primarily when the valvedischarges to open atmosphere or into a low pressure
closed system having negligible effect on the operationof the valve. Conventional valves are used when theback pressure is either constant or variable but within10 % of set pressure.
*Balanced - used when the valve discharges to closedsystem with variable pressure. This type of valveincorporates means of reducing the effect of backpressure such as Bellows.
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SAFETY & PRESSURE
RELIEVING DEVICES (Cont.)
Pilot operated. These valves are a combination of Piston operated
main valve operated through an external pilot. These valves are used
when the operating pressure is very close to the set pressure or the
back pressure is very high compared to set pressure (more than 90 %)
Rupture Discs:
Rupture discs are non closing devices which burst when the pre
assigned pressure is reached. A rupture disc assembly generally
consist of a Disc, enclosed with in a pair of disc holder which can be
mounted between line flanges and a tell tale assembly comprising of a
pressure gauge or pressure switch for detection of disc rupture orfailure.
These devices can be used as standalone pressure relieving devices
or in combination with pressure relieving valves where they are
installed upstream of the valve
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SAFETY & PRESSURE
RELIEVING DEVICES (Cont.)
Rupture Discs (Cont.):
Rupture discs can also be used for isolating the congealing,
toxic or corrosive fluids from The pressure relief valves when
used in combination with them.
Types of Rupture disc assemblies:
Many type of rupture disc assemblies are available Plain,Scored type, knife edge type, Reverse buckling type etc. but
Scored reverse buckling type is preffered as it is immune to
metal fatigue and does not produce fragments on bursting.
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SAFETY & PRESSURE
RELIEVING DEVICES (Cont.)
Governing Codes and Standards:
API RP 520 Sizing Selection and Installation
of Pressure Relieving devices. API 526 Flanged Steel Safety Relief Valves.
API 527 Seat tightness of Pressure Relief
valves
ASME Boiler and Pressure Vessel Code Sec I
and Sec VIII.
IBR regulations
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SAFETY VALVE SIZING
Procedure for safety valve sizing
Step :1 Selection of valve type based on Service
Process data and sizing code.Step : 2 Calculation of minimum area based on
formulas as per selected code.
Step:3 Selection of number of valves based on
Minimum required area, Standard valve andpressure temp rating available for that
given size.
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SAFETY VALVE SIZING (Cont.)
Sizing of safety valves will be covered in class room lectures. However
following salient features should be noted.
For multi phase relieving requirement, areas of each
phase is calculated separately and these are added to getthe final area.
If API assigned discharge coefficients are used then for
valve selection API designations and API area needs to be
considered. If ASME certified coefficients are used then
manufacturers effective orifice area can be used. In case rupture disc assembly is installed upstream of the
safety valve, the valve area needs to be de-rated by a factor of
0.90
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SAFETY VALVE SIZING (Cont.)
Special requirements of steam Boiler / Generator :
Valves certifies for the IBR/ ASME Sec Irequirements only to be used.
Valves to be conventional type , with open bonnet andprovided with lifting lever
Over pressure requirements as per the governingcode to be followed.
In case of multiple valves each valve shall beprovided with individual Vessel nozzle.
Vessel nozzle ID shall be equal to or more than theValve nozzle ID.
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