pressure relieving

7/29/2019 Pressure Relieving

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Instrumentation Training ProgramEngineers India LtdDelivering excellence through people

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


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


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


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


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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pressure relieving

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