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BN-EG-K4 Standard Method for Safety Relief Valve Calculations

1. General2. Formulae3. Instructions for the Use of Safety Relief Valve Calculation Sheet (General) Form BN-K008 104. Instructions for Use of Safety Relief Valve Calculation, Attachment A

(External Fire) Form BN-K0095. Instructions for Use of Safety Relief Valve Calculation, Attachment B

(Heat Exchangers) Form BN-K0106. Instructions for Use of Safety Relief Valve Calculation, Attachment C(Miscellaneous) Form BN-K011

Appendix A - Calculation of Latent Heat of EvapourizationAppendix B - Calculation of the Maximum Duty of a Heat ExchangerAppendix C - Derivation of API-ASME Code FormulaAppendix D - Formulae for Safety Valve Calculation in Accordance with German Rules (TV and AD-Merkblatt A2)Appendix E - Comparation between API-Code Rumanian-Code for the Calculation of Pressure Relieving ValvesAppendix F - Comparation between API-Code and the Italian-Code for the Calculation of Pressure Relieving Valves

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1. General

1.1 Scope

This standard has been developed to standardize the method of safety relief valve calculations in accordance with API,ASME codes and local codes.

1.2 Calculation Sheet

For the calculation of a safety relief valve, the following standard calculation forms can be used:

BN-K008 : Safety relief valve calculation

BN-K009 : Attachment A (External fire)

BN-K010 : Attachment B (Heat exchangers)

BN-K011 : Attachment C (Miscellaneous)

The main safety relief valve calculation sheet, form BN-K008 has been developed to evaluate the governing hazardand to calculate the area of the relief valve.

The attachments have been designed to evaluate the quantity which has to be relieved by the safety relief valve.

1.3 References

1. API Recommended Practice for the design and installation of pressure relieving system in refineries, API RP 520,second edition, Sept. 1960.

2. The design and construction of pressure relieving systems by Nels E. Sylvander and Donald L. Katz, April 1948.

3. Data Handbook on Hydrocarbons by J.B. Maxwell, 5th edition.

4. Engineering Data Book by the National Gasoline Supply Mens Association (NGSMA), 7th edition 1957.

5. Chemical Engineers Handbook, John H. Perry, 3rd edition, 1950.

1.4 Definitions

Safety valve

An automatic pressure relieving device actuated by the static pressure upstream of the valve, and characterized byrapid full opening or pop action. It is used for stream, gas, or vapor service.

Note: This type of valve is normally not specified by Company.

Relief valve

An automatic pressure relieving device actuated by the static pressure upstream of the valve, which opens inproportion to the increase in pressure over the opening pressure. It is used primarily for liquid service.

Note:This type is normally only used in the size 3/4 x 1 or 1 x 1 for thermal expansion.

Safety relief valve

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An automatic pressure actuated relieving device suitable for use as either a safety or relief valve, depending onapplication.

Note:This type is used for general applications.

Set pressure

1. Liquid service:for a relief or safety relief valve on liquid service, the set pressure, in pounds per square inch gauge, is to beconsidered the inlet pressure at which the valve starts to discharge when discharging against atmospheric

conditions.

2. Gas, vapour or steam service:for a safety relief valve on gas, vapour or steam service, the set pressure, in pounds per square inch gauge, is tobe considered the inlet pressure at which the valve pops when discharging against atmospheric conditions.

Popping pressure

Same definition as set pressure for gas, vapour or steam service, when discharging against back pressure. (Pressureat which the valve pops when in use.)

Differential test pressure

The pressure differential in pounds per square inch between the set pressure and the constant superimposed backpressure.

It is applicable only when a conventional type safety relief valve is being used in service against constantsuperimposed back pressure.

Net spring setting

Same definition as for differential test pressure.

Cold differential test pressure

The set pressure or the differential test pressure at which a safety relief valve must be set on cold fluid on a test drumplus a predetermined increase in the cold fluid setting, in pounds per square inch, that will result in the valve openingat the correct set pressure in service when the actual service temperature is higher than that of the cold fluid which isused in the test drum. This is the pressure at which the valve should be set on repair shop testing facilities.

Note: The difference in cold and hot setting is the result of change in characteristic of the spring.Cold net spring setting

Same definition as for cold differential test pressure.

Operating pressure

The opening pressure of a vessel is the pressure in pounds per square inch gauge, to which the vessel is usuallysubject in service. A vessel is usually designed for a maximum allowable working pressure, in pounds per square inchgauge, which will provide a suitable margin above the operating pressure in order to prevent any undesirable operationof the relief device. (It is suggested that on processing vessels this margin be approximately 10 percent, or 25 psi -whichever is greater.)

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Note:

1. Take care for reciprocating engines, where peak pressure might reach the set pressure of the valve.2. When setting closer to the operating pressure is required, pilot operated safety valves or 0 ring seal might be

considered.

Design pressure

The gauge pressure at which the vessel has been designed.

Note:Safety relief valves are usually set at this pressure.

Maximum allowable working pressureThe maximum gauge pressure, at the coincidental design temperature, permissible at the top of a vessel in itsoperating position, and which could be the basis for the upper limit in pressure setting of the safety relieving devicesfor any specific operation.

Note:

1. When designing a vessel, the calculations will result in a material thickness. This thickness is rounded off to the firsthigher thickness which is commercially available. Recalculation of the vessel using actual plate thickness will resultin a maximum allowable working pressure which is usually higher than the design pressure.

2. Usually recalculation is not done (difference in plate thickness is low for European standards), in that casemaximum allowable working pressure is equal to the design pressure.

Overpressure

Pressure increase over the set pressure of the primary relieving device is overpressure. It is the same as accumulationonly when the relieving device is set at the maximum allowable working pressure of the vessel.

Accumulation

Pressure increase over the maximum allowable working pressure of the vessel during discharge through the pressurerelief valve, expressed as a percentage of that pressure, or pounds per square inch, is called accumulation.

Notes:

1. For European plants, it is usual to take an accumulation of 10% . For American plants a value of 20% may be takenfor external fire. 25% for liquid relief and in all other cases 10%.

Blowdown

Blowdown is the difference between the set pressure and the reseating pressure of a pressure relief valve, expressedas percentage of the set pressure, or pounds per square inch.

Note:A blowdown between 4% and 5% is usually specified. Care is to be taken that the inlet piping does not have a

total pressure loss in excess of 3%. The total pressure loss shall include the velocity head loss ( 1/2 rV2). Notehowever, that the friction loss should not exceed 1% and the velocity head loss should not exceed 2% of the allowablepressure for capacity relief.

Lift



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The rise of the valve disc in pressure relief valve when the valve disc moves from the closed position is called lift.

Back pressure

Pressure on the discharge side of a pressure relief valve.

1. Constant back pressure:back pressure which does not change appreciably under any condition of operation whether or not the pressurerelief valve is closed or open.

2. Variable back pressure:

back pressure which develops as a result of the conditions outlined below.

a) Built-up back pressure: variable back pressure as a result of flow through discharge piping after a singlepressure relief valve starts to open.b) Superimposed back pressure: variable back pressure that is present before the pressure relief valve starts to

open.

Note:Variable back pressure on conventional safety relief valves shall never exceed 10% of the gauge setpressure.

Variable back pressure on bellow seal safety relief valves shall never exceed 50% of the absolute set pressure asthat is the limit for critical flow, on which the formulae are based.

Coefficient of dischargeCorrection factor for the nozzle area, due to contraction etc., include all deviations from ideal flow through the nozzle.

Note:For full lift safety relief valves, this factor is usually 0.97.For relief valves at 25% accumulation: 0.64.For relief valves at 25% accumulation: 0.64.

These low coefficients correct also for the reduction in area which occurs at lower lifts of the disc above the seat.

General rules for the use of safety relief valves

Following, a few general rules will be given for the application of safety relief valves. These rules are only to be used asa guidance.

Safety relief valves are required:

1. On all vessels containing fluids which may vapourize when the vessel is subject to fire.2. On all vessels where a closed outlet valve may cause an over pressure.3. On all reciprocating pumps and compressors, if not protected internally.4. On the outlet of turbines, where the design pressure of the low stage casing is lower than the inlet pressure.5. On all distillation and fractionating towers where reflux or cooling water failure may cause excessive vapourization.6. On heaters and furnaces where low flow may cause cooking.7. On heat exchangers or heaters where the cold medium may be blocked.8. On all storage tanks, not vented to atmosphere, where vapour pressure or pumping may cause over or under



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pressure. In this case special combined pressure vacuum relief valves are to be specified. (Breather valves).9. 3. In cases of long lines, containing liquid, which may be blocked when exposed to the sun (for example butane,

propane).10. 4. In all cases where a volatile liquid may get into a vessel under conditions that it vapourizes where the vessel is

not designed for the vapour pressure.11. In cases where non-condensable accumulate in a condenser, decreasing the efficiency, so that a pressure rise

may occur.12. Tube failure in heat exchangers, which may cause vapourization, or excessive pressure on the shell.

13. Control valve failure or instrument failure which either results in one of the hazards 1 through 12, or in any other

hazard as for example a reducing station, failing in the open position.Notes:

1. Where more vessels are connected, so that they cannot be separated by block valves, they are to be consideredas one system. (Except when local codes imply separate valves.)

2. In cases where equipment has to be protected against two or more of the above mentioned hazards, the safetyvalves shall be calculated for the condition causing the highest flow rate to be relieved, thus assuming that onlyone hazard will occur at the same time. (Single risk concept.)

3. It is evident that when a hazard is the result of another hazard, both hazards are to be taken into account.

2. Formulae

Belowa summaryof formulae has been given. Most of these formulae already appear on the calculation sheet, butthis summary also gives some other formulae which are to be used in special cases.

The formulae given on the calculation sheet for evaluation of nozzle area for vapours is a safe approximation: whenaccurate calculations are to be made, formula number 2 has to be used in which the C factor can be evaluated fromformula 2A, or from Binder IV with curves and tables.

Formula 4 gives the nozzle are for steam when the safety valve has to be calculated as per ASME boiler code. Formula10 gives a quantity of thermal expansion when no normal size is taken. See 68, liquid expansion.In each formula for vapours the super compressibility factor (Z) can be included as a multiplier of the absolutetemperature (also under the square root sign).

This will result in a smaller valve size.

Leaving it out always gives a safe approximation.

SUMMARY OF FORMULAEUNITS OF MEASUREMENT



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DESCRIPTION ENGLISH UNITS METRIC UNITS NO.

RELIEF QUANTITY VAPOURS (GENERALPURPOSE)

1

RELIEF QUANTITY VAPOURS (ACCURATE) 2

CONSTANT BASED ON k (C OR C') C'=4.81 x C 2A

RELIEF QUANTITY STEAM (GENERAL PURPOSE) 3

RELIEF QUANTITY STEAM (ASME BOILER CODE) 4

RELIEF QUANTITY LIQUID (INCLUDING VISCOSITYCORRECTION) (NOT FOR BALANSEAL VALVES)

5

VAPOURATION WITH CONDENSATION 6

VAPOURATION WITHOUT CONDENSATION 7

CONSTANT Y 7A

SPLIT TUBE (NO VAPOURIZATION) 8



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SPLIT TUBE (VAPOURIZATION) 9

THERMAL EXPANSION 10

RELIEF QUANTITY LIQUID (INCLUDING VISCOSITYCORRECTION) (FOR BALANSEAL VALVES ONLY)

11

FORMULAE IN METRIC UNITS, GIVING AREA IN CM2

RELIEF QUANTITY VAPOURS (GENERAL PURPOSE) 1A

RELIEF QUANTITY STEAM (GENERAL PURPOSE) 3A

RELIEF QUANTITY LIQUID 5A

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