psv data book

8/9/2019 Psv Data Book

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PR

SAFETY

TECHNICAL

SSURE

RELIEF

ALVES

ULLETIN 3-I


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TECHNICAL BULLETIN 3-I PRESSURE SAFETY RELIEF VALVES PARCOL

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0. FOREWORD

This technical bulletin is dedicated to thosepressure relieving devices which fall within thedefinition of direct-loaded safety valves, producedby PARCOL since the beginning of the Seventies.

The information contained in this document derivefrom PARCOL experience and internalinstructions and are basically in accordance withthe international standards listed in the followingparagraph.

1. NORMATIVE REFERENCE

API STANDARD 520 Part I (2008), Sizing,Selection and Installation of Pressure-relievingDevices in Refineries Sizing and selection

API RECOMMENDED PRACTICE 520 Part II(2003), Sizing, Selection and Installation ofPressure-relieving Devices in Refineries Installation

API STANDARD 526 (2009), Flanged SteelPressure-relief Valves

API STANDARD 527 (1991), Seat tightness ofPressure-relief Valves

ISO 4126-1 (2004), Safety devices for protectionagainst excessive pressure Safety valves

ISO 4126-7 (2004), Safety devices for protectionagainst excessive pressure Common data

ISO 4126-9 (2008), Safety devices for protectionagainst excessive pressure Application andinstallation of safety devices excluding stand-alone bursting disc safety devices

ISO-WD 4126-11 (2011), Safety devices forprotection against excessive pressure Performance testing

2. TERMS AND DEFINITIONS

Direct loaded safety valveSafety valve in which the loading due to the fluidpressure underneath the valve disc is opposedonly by a direct mechanical loading device such

as a weight, lever and weight, or a spring.

Safety valveAutomatic pressure-relieving device actuated bythe static pressure upstream of the valve andcharacterized by a rapid full opening or popaction.It is normally used for gas or vapour service.

Relief valveAutomatic pressure-relieving device actuated bythe static pressure upstream of the valve.The valve opens in proportion to the increase inpressure over the opening pressure. It is primarilyused for liquid service.

Safety-relief valveAutomatic pressure relieving device actuated bythe static pressure upstream the valve.It is suitable for use as either a safety or reliefvalve, depending on the application.

Max allowable working pressure(MAWP)Maximum allowable working pressure foroperation, in accordance to manufacturing codesand service conditions adopted as the basis fordesign.

AccumulationPressure increase over the MAWP value,expressed as percentage of the MAWP pressure,allowed in the pressurized system. Maximumallowable accumulations are established byapplicable codes for operating and firecontingencies.

Set pressure (pset)Predetermined pressure at which a safety valve

under operating conditions commences to open.It is expressed in gauge units.Three different ways may be used to detect theset pressure depending on the assigneddefinition:- start of opening, which may be checked

measuring the lift or hearing/seeing acontinuous outflow;

- opening pressure, easily recognizable by thesudden movement of the disc (applicable only tocompressible fluids);

- start-to-leak pressure, may be easily detectedas soon as the first bubble or drop comes out

(applicable only to valves having a perfect seal,e.g. with resilient seats).


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PARCOL PRESSURE SAFETY RELIEF VALVES TECHNICAL BULLETIN 3-I

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Cold-differential test pressurePressure at which the valve is adjusted to openon the test bench, including the corrections forservice conditions of back pressure, temperature,or both.

Overpressure (ovp)Pressure increase over the set pressure, at whichthe safety valve attains the lift specified by themanufacturer, usually expressed as a percentageof the set pressure.It is the overpressure used to certify the safetyvalve.

Relieving pressure (p1)Pressure used for the sizing of a safety valvewhich is greater than or equal to the set pressureplus overpressure.

LiftActual travel of the valve disc away from theclosed position.

Coefficient of dischargeValue of actual flowing capacity (from tests, byManufacturer) divided by the theoretical flowingcapacity (from calculation).

Blowdown (bd)Difference between the set pressure and thereseating pressure, usually expressed aspercentage of the set pressure.

Back pressure (pb)Pressure on the discharge side of safety-reliefvalve, due to the pressure existing in thedownstream system.It is the sum of the built-up and of thesuperimposed back pressure.It is expressed as:- percentage of relieving pressure, calculated in

absolute units for compressible fluids and ingauge units for incompressible fluids, accordingto ISO 4126;

- percentage of set pressure, calculated in gaugeunits, according to API.

Built-up back pressurePressure existing at the outlet of a safety valvecaused by flow through the valve and thedischarge system.

Superimposed back pressurePressure existing at the outlet of a safety valve atthe time when the device is required to operate. Itmay be constant or variable.

Flow areaMinimum cross-sectional flow area between inletand seat used to calculate the theoretical flowcapacity.


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3. DESCRIPTION AND OPERATING CRITERIA

Safety-relief valves may be considered as stopvalves that close using a steady force insistingdirectly on the disc, thus no external actuator isneeded.

When fitted in a pressurized system, they remainnormally closed and open only if the forcegenerated by the pressure is higher than thecorresponding thrust insisting on the disc.Safety valves are conceived to keep the pressureof the system, where they are fitted, under astated value by discharging as much fluid asnecessary for this purpose.The force on the disc may be imposed through: aweight, a weight and a lever, a spring.Weight loaded valves have a lift higher (and thusa higher flow) than spring loaded valves, being allother conditions unchanged, because the latterreacts proportionally to the lift while the formerhas a constant opposite force.On the other hand, weight loaded valves havesome disadvantages due to the inertial effect ofthe moving parts, which limits their use to lightduties with low set pressure.Direct spring loaded safety-relief valves may bedistinguished into two main types:- conventional valves,- balanced valves.

Conventional non-balanced valves

Conventional non-balanced valves are applicablewhen no back pressure exists on the dischargeside or when the back pressure does not alter theset pressure and the performance of the valve

beyond known limits.In practice they may be used without problems incase of atmospheric discharge.They may have an open spring bonnet (i.e.vented) or a closed one, connected to the valvedischarge. The closed type is allowable also forservice on liquids or anyway when the fluid mustnot be spread outsideOn open bonnet types, the influence of backpressure is negligible because they normallyhave a free air discharge.In case of conveyed discharge, the possible backpressure may increase or decrease the set

pressure depending on the valve design.This kind of valve is usually fit on compressed airlines, where external leakages are acceptable, orfar service on boiler or steam lines, where it isnecessary to protect the spring from excessiveheating

Balanced valves

The valve disc balancing is necessary when avariable or unpredictable back pressure exists, asfor instance in petrochemical plants where allprocess fluid discharges must be conveyed for

safety or environmental reasons.

if then

Fig. 1 Principle of balancing of safety-relief valves

In the conventional design the valve body is closed and connected to the valve discharge so that the back pressure

P2 increases the spring force. In the bellows type balanced valve, no pressure exists inside the bellows and thisexcludes the effect of the back pressure P2. In piston type balanced valve, the back pressure P2 downwards isequalized by the pressure on the piston which has the same area as the nozzle seat.


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In conventional valves, if the spring bonnet isvented to the discharge, the back pressure actswith the spring pressure on the whole surface ofthe disc retainer so as to increase the openingpressure (over the set pressure).

Balanced valves are designed in such a way toexclude the unbalanced area from the effect ofthe back pressure and vent it to atmosphere (seefig. 1). This is normally achieved by fitting ametallic bellows between disc retainer and valvebody.The bellows area is fairly equal to the nozzle seatarea.The bellows length is sized to allow the valve liftwithout being compressed too much.Its assembling to the disc retainer is done inseveral manners taking into account that a goodseal must always be performed towards the valvebonnet.The upper part of the bellows has the shape of adisc which is blocked between spring bonnet andvalve body and is nearly adherent to thesupporting guide.In other models the balance of the disc retainer isperformed by a piston solidly connected to thestem and having the same diameter of the nozzleseat.The balancing bellows allows also for theisolation of the spring from the process fluid.Sometimes this model is required for corrosivefluids for which there is no compatibility with thematerial of the spring.The piston model does not allow perfect isolationbecause use of seals on moving parts of thevalve would increase friction. On the other hand,the piston is much more reliable than the bellows,being the bellows subject to rupture under heavyduties (i.e. with steam at high temperature andpressure or due to wrong valve installation onpiping overstressed by vibrations).For the above reasons, PARCOL safety valvescan be supplied with bellows in combination withpiston for maximum reliability. Combination that

PARCOL strongly recommends on heavy dutyapplications.

Relief valve

A relief valve opens in proportion to the increasein pressure over the opening pressure becausethe area where the pressure insists does not varysignificantly with the lift.

Safety valves

A safety valve shows a quick opening of the discachieved through particular devices whichincrease the area of pressure on the disc with the

lift.This effect, together with the reaction of the fluid,produces a sudden lift of the disc which may

reach a high value compared with the diameter ofthe seat (a ratio Iift/nozzle higher than 0,3).The most commonly used means to achieve thiskind of operation (pop action) are an enlargementof the disc and a screwed ring fitted on the nozzle

(see fig. 2) whose position in settable so as tocreate a restriction to the flow.If the fluid is compressible, a pressure isgenerated in the huddling chamber locatedbetween the wing of disc retainer and the settingring. This pressure causes a quick unbalance ofthe disc that increases the lift and therefore theflow of the fluid.As soon as the pressure of the fluid reaches theset value, the disc begins to open.At this point the pressure acts also on the outerside of disc wing. The lifting force increasesquickly and causes a nearly instantaneousopening of the valve.The position of the setting ring determines thepressure gradient on the disc wing during theinitial lift.If the ring is lifted against the disc then the effectof the secondary orifice is amplified: the openingoverpressure decreases and the reclosing gapbecomes greater.The contrary happens if the ring is lowered.The geometry of the disc retainer affects itsdynamic reaction owing to the deviation of theflow and has a great influence on the lift when thedischarged flow is considerable.If the outflow from the disc is downwards thedynamic effect is maximum and its contribution tothe lift is significant.

valve closed

valve partially open

valve fully open

Fig. 2 Operating principle of safety valves for

compressible fluids


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4. VALVE MAIN COMPONENTS ANDCHARACTERISTICS

Body

It is generally angle shaped with threaded,

welding ends or flanged connections. Thedischarge diameter is usually larger than the inletdiameter in order to improve the coefficient ofdischarge and to reduce the built-up backpressure.The dimensions of the body centre-to-face arestandardized by API 526 which, due to its origin,is not always complied with by the Europeanmanufacturers.Bodies generally have different ratings betweeninlet and discharge.In fact back pressure is always lower than inletpressure, thus it would be costly and useless to

adopt a single rating as done on conventionalcontrol valves.As a matter of fact the whole valve is designedaccording to the P/T discharge side values exceptfor the inlet flange, nozzle and disc.All bodies ere provided with drain plug which isuseful in case of conveyed discharge when thedownstream pipe is not duly drained.

Spring bonnet

It is fixed to the body through a flange (or a threadon smaller executions) and tightens the guide ofthe disc retainer.

In case of safety valves it may be made out of anopen structure of two columns. Sometimes theclosed bonnet of safety-relief valves is used withwindows on the outer surface.In the upper part of the bonnet the spring settingscrew is fitted and is accessible only after havingremoved the cap.A Iead seal is performed between bonnet and capwhen required by Official Parties in order to avoidan undue access to the setting screw.The bonnet of safety-relief valves is vented todischarge side through openings in the guide.Some models even have a connecting pipebetween spring bonnet and discharge acting asan ejector to improve the depressurization in thebonnet and to improve the valve opening.

Nozzle

It is usually internally shaped according to aVenturi or equivalent profile that allows highcoefficient of discharge.The nozzle covers the entirely of the valve inletduct and ends with a rib clamped during themounting between body and pipe (or vessel).On cheaper models, the nozzle has reduced

dimensions (semi-nozzle) and is mounted insidethe body quite like the seat of globe valves.

Normally this solution does not allow for highcoefficients of discharge and may cause problemswhen operating at high temperatures (leakageunder the nozzle, loosening).The connection between nozzle and body isusually threaded. Every manufacturer adopt

different solutions about particular coupling andcentring positions of the two pieces.The general rule is that the nozzle should not berigidly fixed to the body so that possible stressesfrom the discharge pipe do not distort the body.The top side of the nozzle has a flat ring wherethe disc retainer lays.This sealing surface must be clean, smooth andhave a little width in order to limit the blowdownand to allow a good repeatability of the setpressure.The internal diameters of the nozzle (orifices)have been standardized by API 526 with thepurpose of making easier the choice and thedesign of the valves.

Disc

The closure member of PARCOL safety valveseries 3-5400 is made out of two pieces: the discand the disc retainer.Other versions have the sealing disc solidlyconnected to the stem which also acts as a guide.Due to the two pieces design (see fig. 3), thesealing part is not affected by thermal distortionswhich easily affect the seal of one piece closure

member.The sealing disc, is in fact designed in such a wayto avoid high thermal gradients when operatingwith high temperature fluids.

Spring

It withstands the pressure and the dynamicstresses of the fluid with the purpose of keepingthe valve in closed position and to reclose it afterdischarge.This device undergoes torsional stresses inelasto-plastic conditions and may be subject torupture, mainly when operating under high

temperatures.The most common effect, caused by severestresses, is the relaxation which decreases theheight of the spring and therefore the force at thebeginning of the lift.This phenomenon is always present but is moresignificant at temperatures above 300 C withclosed spring bonnet.In order to improve the resistance to the stressesand decrease the relaxation, the spring of safetyvalves are set with previous loading cycles justbeyond the torsional elastic threshold.This procedure is carried out at PARCOL facilities

on every spring in cold conditions (cold setting);hot setting is carried out for important and heavyapplications.


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The rupture of the spring is luckily an event whichhappens seldom, being the number of operationsvery low if compared to the valve life.Rupture may be caused by:- defects of the material;- fatigue at low number of cycles;

- corrosion;- hydrogen embrittlement.Material defects are critical for componentssubject to fatigue with high number of cycles.Thus, in the case of safety valves, they are only tobe considered as worsening of other causes.Ruptures due to fatigue at low number of cyclesmay happen on hardened materials under heavystresses and high temperatures (e.g. tungstensteel spring).Ruptures are more frequently caused by:- corrosion, which reduces the effective cross

section of the spring;- embrittlement of the material due to presence of

hydrogen or hydrogen sulphide.Therefore it is important to select a suitablesurface protection and treatment.Chemical protection or galvanizing are not alwaysrecommended because there is a risk ofhydrogen etching.If hydrogen sulphide or its compounds arepresent in the ambient, it is recommended toadopt closed bonnets and to manufacture thespring with suitably resistant materials.

Setting range

PARCOL safety valves are set on the test benchas per Clients requirements.Their regular operation is guaranteed, withoutspring change, within a set pressure range notexceeding the following:- 10% of the set pressure for set pressures up

to 17 bar;- 5% of the set pressure for set pressures

higher than 17 bar.Changes in set pressure shall anyway beevaluated case by case: always contact ParcolTechnical Department for a proper analysis.

The minimum set pressure for conventionalstandard PARCOL valves is 0.5 bar while forbalanced bellows valves this limit is approximatelybetween 2.5 for the smallest orifices and 1 bar forthe largest ones.

Lifting mechanism

If a periodical verification of the functionality isrequired (for instance by ASME Code Sect. VIIIfor operation with air, water or steam over 60 C)the valve is fitted with a lifting lever suitable tocause the discharge of the valve when thepressure in the plant is at least 75% of the set

pressure.The device used on Parcol series 3-5400 isshown on fig. 4 and consists of an eccentric lever

which automatically resets, after operation, to itsinitial position without hindering the opening of thevalve.

Fig. 3 PARCOL conventional safety relief valveseries 3-5400

The off position of the lifting lever shall be fixedand sealed against misoperation.When no leakage of the discharge fluid isallowed, the movement of the lifting lever istransmitted to the internal side of the cap througha sealing stuffing box.If the valve is located in not easily accessiblesites, discharge tests may be carried out through

chain or rope transmissions or by hydraulic,pneumatic or electric actuator.

Fig. 4 Lever for manual lifting

1 cap2 setting screw3 bonnet4 guide5 spring6 disc7 body8 disc ring9 plug10 nozzle


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5. INSTALLATION

Safety valves must be mounted vertically in sucha position to be accessible for maintenance andsetting operations (setting screw, blowdown ringnut, lifting lever).

In case of pressure vessel protection, it isrecommended to install the valve on a nozzledirectly fixed on the upper side of the vessel.Different solutions are allowed only when the fluidinside the tank may be a source of vibrationswhich could affect the valve stability.Almost all codes do not allow for stop valvesbetween the equipment to be protected andsafety valve.When stop valve is tolerated, it must meetparticular requirements: for instance, it shall notrestrict the flow even if fully open.Sometimes more than a safety valve is requiredwhenever:- a high flow rate would require the need of an

enormous valve;- alternate operation is necessary to avoid a plant

shutdown;- the maximum discharge flow rate is very high if

compared with the normal operation of the plant.In the first case it is convenient to install a Ybranch having a flow section not less than thesum of the areas of the two orifices (see fig. 5).If alternate operation is needed, being one valveas back-up to the other, it is necessary to use athree way distribution valve (also called change-over valve) as shown in fig. 6.In the latter case it is suitable to use parallelconnected valves set at slightly different pressureso that only one valve works during normaloperation of the plant.Doing so, the usual inconvenients due to the useof only one oversized valve are avoided.When the conveyed discharge has no adequatedrain or when the position of the valve may causeinternal accumulation of the condensate, then it isnecessary to drain the body using its threadedconnection.

Being the drain pipe a part of the dischargesystem, it must be subject to the sameprecautions used for the main discharge pipe.Particular care shall be paid to the design ofpiping upstream and downstream the safetyvalve.API RP 520 Part II gives useful indications forcarrying out such connections.The upstream pipe shall have a flow section notlower than the minimum section of the valve andshall be as short as possible to keep the valvenear the protected equipment.Most common standards require a head loss

between protected vessel and valve, at fulldischarge flow, not higher than 3% of the setpressure.

DIMENSIONS

DN D1 40 65 80 100 125 150 200

D2 25 40 50 65 80 100 150

ASeries 150

300170 190 215 240 270 330 452

600 184 208 235 263 296 360 490

BSeries 150

300600

180 205 240 265 290 365 500

The listed ratings refer to the inlet of the safety valvesmounted on the branch.

Fig. 5 PARCOL Y branch series 3-9211

Fig. 6 PARCOL three-way distribution (change-over) valve, series 3-1213


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A higher head loss may in fact reach theblowdown value of the safety valve which causesits sudden closure and subsequent opening(chattering).If the safety valve is installed on the pipe througha 90branch, this one shall be correctly designed

to avoid disturbances to the valve in closedposition.High fluid speed in the pipe may createresonances and subsequent pressure waves inthe branch (till 67 bar peak-to-peak) whichmodify the equilibrium of the closure member.The results may be: unexpected openings,chattering, fluttering, fretting corrosion andleakages through the seal.In order to avoid these inconvenient, theconfiguration of the connection must fall within thelimits shown on fig. 7 taking into account therounding of the branch entrance.The design of the upstream pipe shall alsoconsider the dynamic loads generated by thedischarged fluid.Fig. 8 shows the sizing formulas to this purpose.In case of toxic gases (needing a safe seal) or ofparticular operating conditions which can affectthe performance of the safety valve (if it is incontact with the fluid), the use of rupture discs is

Fig. 7 Proper mounting of safety valves on pipes

To avoid disturbances to the valve in closed position,the ratio AH shall be higher than 2.4 M (being M theMach number of the fluid in the main pipe)

Fig. 8 Discharge reaction forces of a pressure relief valve on gas service

Reaction forces include the effects of momentum and static pressure respectively on inlet valve axis (F vand Fv) andon outlet valve axis (Foand Fo). Terms related to static pressure are in square parenthesis.Reaction forces are calculated assuming steady-state critical flow discharge conditions. In particular, the formula for F0is according to API RP 520 Part II. For correct calculation of Fvit is necessary to know the back pressure pband thelayout of venting pipe. For atmospheric vents consider p20. Term Fvis anyway usually negligible compared to F0.In case of impulsive-state discharge conditions, calculated values shall be doubled.

27.8 1 10 3600

10 10 For practical use: 0.1

A1= area of inlet passage section [cm ]

A2= area of outlet passage section [cm2]

M = molar mass of the flowing fluid [kg/kmol]

p1= relieving pressure [barg]

p2= constant super-imposed back pressure [barg]pb= built-up back pressure [barg]

Qmax= maximum dischargeable flow rate [kg/h]

T1= inlet temperature of fluid [K]

1= specific mass at inlet [kg/m3]

u1= average fluid velocity at inlet connection [m/s]

k = specific heat ratio [-]Note:reaction forces are expressed in Newton


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strongly recommended.A typical installation is shown on fig. 9.If the fluid cannot be discharged into theatmosphere for safety reasons, then the outlet ofthe safety valve is connected to a downstream

pipe leading to a collecting tank or, as oftenhappens in petrochemical plants, to a collectorleading to a torch.The design of the pipe downstream may becritical for the choice and operation of the safetyvalve mainly because of the back pressuredeveloped at the body outlet.When the valve discharges into a collector whosepressure is unknown, a balanced valve shall beused.An unknown back pressure may originate seriousinconvenient, the worst of which is the loss of flowcapacity due to insufficient valve opening or dueto the change of the outflow from critical tosubcritical.The downstream pipe shall be installed in such away to do not transmit heavy stresses to the valvebody.

Sliding supports shall be provided and longstraight pipes shall not be directly connected tothe valve discharge.For gases and vapours the downstream pipe shallbe oriented upwards and be provided with drainholes.For liquids it is recommended to have a dischargepipe downwards to avoid the flooding of the valvebody.

Fig. 9 Typical assembly of rupture disk upstreama safety valve

No pressure shall exist between the disk and valve, anexcess flow valve and a bleed valve are thus foreseen.The disk shall be replaced as soon as a flow of fluid

gets through it.


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6. SETTING TEST

The setting test of safety-relief valves may beperformed with different procedures dependingon:- how the set pressure is defined (refer to

paragraph Terms and Definitions);- the kind of fluid;- the available test equipment;- the test location (bench or line).The procedure usually followed by PARCOL forthe setting test on the stand is to increase theupstream pressure till continuous flow isdischarged; for gases this is brought to evidenceby a well known noise or whistle while for liquidsthis is shown by an uninterrupted stream.In the USA it is common practice for gases to takeas set pressure the popping pressure, which iseasily detectable and may coincide with theopening pressure if a proper setting of theblowdown ring is made.The maximum deviation of the set pressureversus the required value is:- 0.15 bar for set pressure lower than 5 bar;- 3 % for set pressure higher than 5 bar.The set pressure (differential pressure at coldcondition) shall take into account the operatingback pressure and temperature.For non balanced valves with closed springbonnet, the back pressure, if constant, isdeducted from the set pressure (i.e. the spring isless compressed).The effect of the operating temperature is takeninto account increasing the setting as listed herebelow:

Operating temperature[C]

Set pressure increase

up to 100 0101 250 2%

251 500 3%over 500 5%

Correction for temperature shall anyway beevaluated case by case, according to effective

service conditions and Clients requests.

7. TIGHTNESS TEST

The tightness test is performed closing the valvedischarge with a 6 x 8 mm flanged pipe, 90bentand immersed for a depth of 13 mm (see fig. 10).The leakage is evaluated counting the bubbles

outgoing the pipe and keeping the upstream airpressure at 90% of the set pressure. Valves withsetting lower than 3.5 bar are tested with anupstream pressure 0.35 lower than the setpressure.The time duration of the test shall be at least:- 1 minute for valves with 2 DN and less;- 2 minute for valves till 4 DN;- 5 minute for valves with higher DN.Acceptance criteria (as per API 527) aresummarized on the diagrams of fig. 11.

Fig. 10 Tightness test

Fig. 11 Maximum allowable leakage during test incold conditions


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8. NOISE LEVEL

The discharge noise of a safety valve may reachintensities dangerous to the hearing whenoperating with high pressure gas or vapours.Luckily, the number and duration of discharges is

very low, therefore the acoustic problem is notconsidered critical and some standards allow for anoise level up to 135 db in proximity to thedischarge.lf the position of the discharge is high, the noisespreading is spherical, which means a 6 dbdecrease for every doubling of the distance fromthe source, so that, if the distance increases from1 to 30 m the noise level decreases byapproximately 30 db.To accept 135 db at 1 m means to accept 105 dbat a distance of 30 m: though high, this noiselevel is tolerable for very short periods.The solution of noise problems is anyway noteasy because silencers are expensive and causeback pressures which may not be tolerated byvalve and plant.The only remedies are:- to install the discharge as high as possible (so

to have a better spreading) and as far aspossible from personnel;

- acoustical insulation of valve and discharge pipe(if present) for at least 1015 m;

- a reasonable advantage may be reached bydividing the discharge (in case of large dimen-sions) into few ducts duly shaped and sized withregard to each other.

A simple way is to insert a drilled disc in the outletduct which must be enlarged like a diffuser (seefig.12).This solution leads, on the other hand, to backpressure problems and its reducing effect is nothigh (less than 15 db).

Fig. 12 Drilled disc device mounted on thedischarge of a safety valve to achieve noisereduction

No pressure shall exist between the disk and valve, anexcess flow valve and a bleed valve are thus foreseen.The disk shall be replaced as soon as a flow of fluidgets through it.

Calculation of noiseThe following formulas are valid to evaluate noiselevel from valve only when sonic conditions arereached at the outlet.Noise level at 1 meter from valve is calculates as:

86 10 where:- Lp(A) is the noise level [db(A)];- Qmis the mass flow rate [kg/h];- k is the isentropic coefficient [-];- T2is the discharge temperature;- M is the molar mass [kg/kmol].

For distances higher than 1 meter and dischargefairly near to the soil, the noise level is:

20 3where L is the distance in meters betweenmeasuring point and discharge point.For distances higher than 1 meter and dischargehigh over the soil, the noise level is instead: 20


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9. VALVE SIZING

PARCOL pressure safety relief valves are usuallysized as follows:- according to ISO 4126-1 for gas, vapour, steam,

liquid or alternate discharge of gas and liquid;

- according to API 520-1 Annex C for two-phasemixtures.

Alternative sizing can be performed, on request,according to other recognized internationalstandards or according to Clients specifications.

Sizing for gas and vapour serviceAccording to ISO 4126-1, the formula to be usedis:

0.9 100

where:- Ag is the minimum required area [cm

2];

- Qmis the mass flow rate [kg/h];- p1is the relieving pressure [bar abs.];- T1is the relieving temperature [K];- Z is the compressibility factor [-];- M is the molar mass [kg/kmol];- C is function of the specific heat ratio k [-];- Kd is the certified coefficient of discharge (from

test, by Manufacturer) [-];- Kbis the theoretical capacity correction factor for

subcritical flow [-].

The subcritical flow occurs when the followinginequality is verified: 2 1/where k is the specific heat ratio of the fluid.In this case, Kb is calculated according to thefollowing equation:

2 1 / / 2 1/ In case of critical flow, Kbis equal to 1.

The coefficient C is calculated as follows:

3.948 2 1/

Example 1Calculate the minimum flow area to dischargecompressed air at the following conditions:- pset = 8 barg;- T1 = 50 C- Qm = 10 000 kg/h;

- ovp = 10%;- pb = 2.5 barg;

First of all, let us verify the flow type (critical orsubcritical).Being:- p1 = pset + ovp + 1 = 9.8 bar;- k = 1.4;the inequality is not verified.3.5

9.8 0.357 22.4

./.

0.528

The flow is critical and Kbis equal to 1.The coefficient C is equal to:

3.9481.422.4./. 2.703Under the hypothesis to supply a valve having acertified coefficient Kdequal to 0.835 at 35.7% ofback pressure (always refer to Manufactures forproprietary values), the minimum required area is:

10 0000.9 100 9.8 2.703 0.835323 128.964 16.78

Fig. 13 PARCOL safety relief valve during

discharge test at Fluid-dynamics of Turbo-machines Laboratories (LFM) at Politecnico diMilano University (air test bench)


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Sizing for liquid serviceAccording to ISO 4126-1, the formula to be usedis:

0.9 100 1.61

where:- Al is the minimum required area [cm2];- Qmis the mass flow rate [kg/h];- p1is the relieving pressure [bar abs.];- v1 is the specific volume at relieving conditions

[m3/kg];

- pbis the back pressure [bar abs.];- Kd is the certified coefficient of discharge (from

test, by Manufacturer) [-];- Kvis the viscosity correction factor [-].

The viscosity correction factor is function ofReynolds number, according to the followingformula:

0.9935 2.878. 342.75. .Reynolds number is calculated as follows:

31.3 where:- 0 is the specific mass of water at 20 C, equal

to 1 kg/dm3;

- is the dynamic viscosity [cp].Being the Reynolds number function of Al, aniterative calculation is required.

Example 2Calculate the minimum flow area to dischargepressurized water at the following conditions:- pset = 30 barg;- T1 = 20 C;- Qm = 85 000 kg/h;- ovp = 10%;- pb = atmospheric.

The relieving pressure is:- p1 = pset + ovp + 1 = 34 bar;

From water data:- the specific volume is 0.0010 m

3/kg;

- water viscosity is negligible, then Kvis equal to 1and no iterative calculation is required.

Under the hypothesis to supply a valve having acertified coefficient Kdequal to 0.740 without backpressure (in percentage terms, the back pressureis null), the minimum required area is:

85 0000.9 100 1.61 0.740 0.00103 4 1 4.36Always remember to refer to Manufactures forproprietary certified coefficient of discharge.

Fig. 14 PARCOL safety relief valve duringdischarge test at Fluid-dynamics of Turbo-

machines Laboratories (LFM) at Politecnico diMilano University (water test bench)


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