Designing Pressure Relief System

7/31/2019 Designing Pressure Relief System

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DESIGNING PRESSURE RELIEF SYSTEM

Explosion and Fire Destroy Chemical Plants. It would be hard to say how frequently this

phrase has appeared in Newspapers and Safety Bulletins. Certainly more often than anyonewould like. There is no doubt that we would like to eliminate these accidents. Yet despite all our

precautions explosions and fires do occur. The design engineer must make provisions for the

possibility of explosions and fires so as to limit the extent of damage. The consequences of poor

design can be expensive and disastrous.

When deciding the safety requirement of a pressure system, the first approach, although

unrealistic and impractical, is to design the vessel to absorb the maximum internal pressure

which may be encountered under the worst possible conditions. This will result in

overdesigning and increase in cost. The more practical approach to the problem will be to

design the pressure system for normal operating conditions and provide adequate venting

facilities to handle excessive pressure developed by extraordinary circumstances.

SOME DESIGN PARAMETERS:

The pressure relief system design engineers must consider several factors when selecting the

relief systems and designing the discharge header. These are :-

(1)Physical properties of the system constituents;(2)The ASME Code;(3)Relief device operating characteristics and capacities;(4)Design pressure of the operating equipment associated with the relief device; and(5)Line capacities and lengths.

The ASME Code is a safety guide for the processing industries; it establishes the Regulations for

safe practice in design, construction, inspection and repair of pressure vessels handling

petroleum and other hazardous liquids and vapors. In accordance with the ASME Code, all

unfired pressure vessels shall be protected by pressure relieving devices that will prevent the

pressures in the vessels from rising 10 % above the maximum allowable working pressure (i.e.

Design Pressure), except when the excess pressure is caused by fire and or other unexpected

source of heat. In the event of over-pressure of the latter, the pressure relieving devices shallbe capable of preventing the pressure from rising more than 20 % above the maximum

allowable working pressure when all pressure relieving devices are blowing.

Size of the outlet pipe shall be such that any pressure that may exist or develop in the discharge

line will not reduce the relieving capacity of the safety devices below the amount required to

protect the vessel from over-pressures.


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TYPES OF PRESSURE RELIEVING DEVICES:

Relief Valve: - A Relief Valve is an automatic pressure relieving device actuated by the static pressure

upstream of the valve, and which opens further with increase in pressure over the set pressure. It is

used primarily for liquid services and attains rated capacity at 25 % over-pressure.

Safety Valve: - A Safety Valve is an automatic pressure relieving device actuated by the static pressure

upstream of the valve and is characterized by the full opening or POP action upon opening. It is used

for steam, gas or vapor services. Rated capacity is reached at 3 %, 10 % or 20 % over-pressures

depending upon the applicable code.

Safety-Relief Valve:- A Safety-Relief Valve is an automatic pressure relieving device actuated by the

static pressure upstream of the valve and is characterized by an adjustment to allow either a POP or a

NON-POP action and a nozzle type entrance. It is used on steam, gas, vapor and liquids (with

adjustments) and is probably the most general type of valves in Petrochemicals and Chemical Plants.

Rated capacity is reached at 3 % or 10 % over-pressures, depending upon the code and/or process

conditions.

Conventional and Balanced Valves;- The Standard Conventional and Balanced Valves operate

satisfactorily only when there is relatively constant back pressure. Changes in back pressures can

seriously affect its flow capacity. The Balanced Bellow Type Safety Relief Valves operate satisfactorily

under varying back pressures as this has little influence on performances. The normal back pressure

limit for the standard Conventional Type is 10 % of the valve set pressure and that for the Balanced

Bellows Type is 50 % of the valve set pressure.

Rupture Disc; - A Rupture Disc is a thin diaphragm held between flanges and designed to burst at a pre-determined pressure. Each bursting requires the installation of a new disc. The absence of a valving

mechanism or seat makes them particularly advantageous for sticky or gummy and corrosive materials.

There are two basic classes of rupture discs- metallic and non-metallic. The metallic discs are formed

into hot shapes and is made of metals like Aluminium, Stainless Steel, Monel, etc. For use in certain

corrosive applications, rupture discs covered on either side with special corrosion resistant coatings like

Teflon are available. When systems operate under vacuum or alternating pressure and vacuum, the

rupture disc will tend to collapse causing premature disc failures. To avoid this, a vacuum support is

used. This is a thicker disc designed to withstand full vacuum and is closely mated to the concave side

(pressure side) of the rupture disc. The vacuum support is either perforated or slit that the covering the

rupture disc is always in contact with the system pressure. Non-metallic discs are usually made ofimpervious graphite, a material well known for corrosion applications in chemical process industries.

Pressure-Vacuum Valves:- Pressure-Vacuum Relief Valves or what is commonly known as Breather Vents

provided to permit escape of vapors as the tank is being filled and emptied or due to change in ambient

conditions. These are provided for above-ground flammable solvent storage. Vents on tanks containing

flammable liquids with a flash point below 110F required to have flame arresters.


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SOME PRESSURE RELIEF TERMS:

SET PRESSURE: - The set pressure in psig, is the inlet pressure at which the safety valve is adjusted to

open.

OVER-PRESSURE:- Pressure increase over the set pressure of the relieving device is the set pressure.

ACCUMULATION: - Pressure increase over the maximum allowable working pressure (Design Pressure)

of the vessel during discharge through the safety or relieving expressed as percent of that pressure is

called accumulation.

BLOW DOWN: - Blow Down is the difference between set pressure and the relieving pressure of a safety

or relieving valve expressed as percent of the set pressure.

BACK PRESSURE; - Back Pressure is the pressure developed on the discharge side of safety valves is back

pressure.

SAFETY RELIEF VALVE DESIGN AND SPECIFICATIONS:The set pressure is usually selected as high as possible but never more than the maximum allowable

working pressure. One widely used rule is to maintain a minimum differential between the set pressure

and the normal operating pressure the system at 15 psig, or more, or 10% of the set pressure whichever

is greater. When the set pressure equals that of maximum allowable working pressure, the system

would require the smallest valve. Safety Valves are identified by the sizes of the inlet, orifice code and

sizes of the outlet. The orifice code identifying the alphabetical letter with orifice area required for

discharge of fluid through the valves:

Orifice letter Area in Sq. Inches Orifice letter Area in sq. Inches

D 0.110 M 3.600

E 0.196 N 4.340

F O.307 P 6.380

G O.503 Q 11.O5

H O.785 R 0r S 16.OO

K 1.287 T 19.64

L 2.853 U 26.OO

SAFETY RELIEF VALVES FOR PRESSURE VESSELS:For vessels with entering streams that are principally vaporous, safety valves must

accommodate the normal vapor quantity including steam and any other vapors entering the

vessel at the normal entry temperature of such vapors, without exceeding the maximum

relieving pressure. Allowance must be made for heat added to the system from a heat source

like reboiler, etc.


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For a vessel with small vapor entering quantity and a large liquid quantity, one of the following

conditions can exist and the valves are to be sized accordingly. First, when the liquid filling time

of the vessel is as short that inadvertent complete filling is an evident possibility, the safety

valve should accommodate both the normal entering vapor and liquid quantities. Second, when

the liquid filling time is so long that no great possibility of complete filling exists, the safetyvalve should relieve normal entering vapor quantity plus an increase in this vapor quantity due

to the vapor displacement caused by the large quantity of entering liquid.

For the discharge of combined liquid and vapor, precise calculations are not generally possible.

When the material to be discharged is nearly all vapors, it is conservative to figure the

estimated liquid content as vapor added to the actual estimated vapor quantity. Where the

discharge is nearly all liquid, the orifice size can be approximated by calculating that required

for the vapor quantity above and adding to the estimated vapor relief requirement assessed for

the liquid alone.

The vapor generated as a result of external fire is calculated assuming that there will be no

material entering the vessel and the valve size is determined only by vapor quantity due to

external fire.

W= 21,000 X FA X 0.82/ L, where

W=Flow in lbs/hr,

L=Latent heat of vaporization,

A=Surface area in sq.ft,

F=Environment factor.

Values of F:

(1)Bare Metal=1.0;(2) Insulated Vessels:

a. 4 inches thick insulation=0.075;b. 2 inches thick insulation=0.15;c. 1 inch thick insulation=0.30.

(3)Water application facilities on bare vessels in process area=1.0;(4)Depressurizing and emptying facilities=1.0;(5)Underground Storage=0.0;(6)Earth covered storage aboveground=0.03.


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The surface area exposed to fire (A in the above Formula) is normally restricted to exposed area

up to a height of 25 feet aboveground.

PUMPS AND COMPRESSORS: When the maximum discharge pressures can exceed the casing

design pressures, relief valves should accommodate the maximum, discharge quantities

without exceeding the casing design pressure.

HEAT EXCHANGERS: The relief valve size is calculated fewer than one of the following

conditions and under external fire and the larger value of the two is selected:

(1)Vaporization in an exchanger: If the vapor pressure of the cold medium at atemperature equal to the normal inlet temperature of the medium on the hot side is

more than 1.3 times the design pressure on the cold side, a relief valve is needed to

protect against vaporization:

W=Q (t - t) / L (t - t), where

W=Vapor generated when block valve is closed, lbs/hr;

Q=Normal heat Duty of heat exchanger, Btu/hr;

L=Latent Heat of Vaporization, Btu/lb;

t=the normal inlet temperature of the medium, F;

t=the cold side boiling temperature at pressure prevailing, when block valve is

closed, F; and

t=the cold side temperature at normal conditions i.e. average of inlet and

outlet, F.

(2)Liquid Expansion:W=BH /60C A=W /196 1.5 PD S, where

A=Effective nozzle area of relief valve, sq.inches;

W=Liquid to be relieved, lbs/min.;

H=Normal Heat Exchanger Duty, Btu/hr;

C=Specific Heat of cold Medium;

S=Specific Gravity of cold Medium;

PD=Design Pressure of cold side, psig; and

B=Coefficient of Expansion of cold medium.

Some values of B are: Oil 3 to 35 API=0.0004;

Oil 35 to 51 API=0.0005;

Oil 51 to 64 API=0.0006;

Oil 64 to 79 API=0.0007;

Oil 79 to 89 API=0.0008;

Oil 89 and upAPI=0.00085 and

Water =0.0001.


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RUPTURE DISC DESIGN AND SPECIFICATION:

Metal Disc Manufacturers recommended that for general applications, the maximum normaloperating pressure of the vessel or system should be held to 67 % of the disc rating. Operating

pressures for non-metallic discs are restricted to 75 % of the rated burst pressure.

Manufacturers print tables listing capacity of their product under various conditions. Or, given

the venting capacity required, they will recommend a size and type for the job. The Design

Engineers first problem which the manufacturer is generally not in a position to work out is

determining the specific venting requirement. Calculation of venting requirement would follow

the procedure outlined under Safety Relief Valves.

When ordering Rupture Discs, the following specifications should be given to the manufacturer:

(1)Net inside Diameter of opening leading to the flange or holding arrangement for thedisc, inches: or the cubic feet of vapor at stated conditions of bursting pressure;

(2) Preferred material of construction or state service;(3)Type of hold-down arrangement, flanged, screwed or special;(4)Material of construction of hold-down arrangement;(5)Temperatures for (a) continuous operation and (b) at burst pressure and(6)Required burst pressure in vessel and the back pressure on the disc, if any.

BREATHER VENT SIZE:

The recommendation of the Factory Insurance Association for Breather Vent Sizes is given below:

Tank Capacity in Gallons Breather Vent in inches Volume vented in cu.ft./hr at

60F, One inch water pressure

1000 to 2000 1 1920

2000 to 10000 2 2800

10000 to 50000 3 6200

50000 to 150000 4 1250015000 to 400000 4 & 3 used together 18100

Emergency venting of some sort is also required to relieve over-pressure that may develop from

exposure to fire. Factory Insurance Association has recommended emergency vent sizes. For tanks

normally at atmospheric pressure, emergency venting can be provided by:


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(1) A standard emergency vent of the size recommended by FIA;(2) A cover of adequate size, weighted in manner that would allow it to open at a pressure of 2

ounces/sq.inch. A convenient solution would be to use a hinged manhole cover as an emergency

vent. This would be supplementary to a breather vent; and

(3) A specially designed seam. On large tanks, the head seam can be designed to be weaker thanother joints on the tanks so that it will rupture on pressure build up.

Emergency venting requirements will be reduced if some method is provided to reduce the heat

input to the tank contents. Means frequently employed are the use of an automatic water spray

system or insulating the tank. The insulation must be material that is not likely to be damaged by

flames or hose streams.

INSTALLATION:

To obtain the advantage of both the rupture disc and the safety valve, the two devices are frequently

used in series. The rupture disc will prevent the process material from coming into contact with the

moving parts of the safety valve. Such an installation is resorted to in case of clogging and highly

corrosive services. Whenever the Rupture Disc is installed in series with the safety valve, the interspaces

between the devices should be vented to the atmosphere to prevent build up process material in the

interspaces. This is done with the help of an Excess Flow Valve. The Excess Flow Valve is similar in

construction to a ball type check valve. When the rupture disc ruptures due to pressure build up, the

ball in Excess Flow Valve is pushed against its outlet port, thus preventing the escape of large amount of

hazardous process material to the atmospheres. When a small leak from the rupture disc is noticed by

the vapors coming out of the Excess Flow Valve, the disc should be replaced at the earliest opportunity.

The rupture disc and safety valve can be installed in parallel; in some installation either two safety valves

or a safety valve and rupture disc are provided in parallel on a common inlet spool piece with a three-

way valve in between. In continuous processes such installations permit maintenance work to be carried

out on the pressure relieving device without shut-down. The inlet piping size should be same as that of

the valve or disc inlet. The pressure drop in the inlet spool piece should not be more than 3 % of the set

pressure. The discharge from the safety devices are to be routed through a common blow-down header.

The simultaneous discharge of different valves into a common header might create too great a back

pressure on the low pressure valve. When using standard safety-relief valves, the maximum back

pressure at the valve discharge should be limited to 10 % of the set pressure. The hazardous vapors are

vented to the atmospheres from the blow-down header through a water seal drum and a vent stack.

The height of the vent stack will be decided upon by the permitted toxicity levels of the gases

discharged. In some installations, the gases discharged are routed to a flare system. Safe purge rates of

either steam or hydrocarbon gases are maintained in the vent or flare stack to maintain the oxygen

levels in the stack at a certain specified minimum level.


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In conclusion, the following set of rules is presented as GOOD OPERATING PRACTICE FOR PRESSURE

RELIEF SYSTEM DESIGN:

(1) Pressure relieving devices shall be installed as close to the vessel as possible;

(2) To avoid valve chatter, inlet piping shall be designed for a maximum drop of 3 % of the ratedpressure

(3) No stop valve shall intervene between the vessel and its pressure relieving device;(4) The opening through all pipe and fittings between a vessel and its relieving device shall have at

least the area of the inlet of pressure relieving device;

(5) Pressure relieving devices shall be installed so that the proper functioning will not be hinderedby the nature of the vessels contents;

(6) Safety and Relief Valves shall be installed in a vertical, up-right position;(7) Discharge piping shall be designed so that back pressure will not exceed 10 % of the set pressure

of the relieving device;

(8) Discharge Lines must be braced in accordance with the standard piping practice, to compensatefor pressure shock and to eliminate unnecessary strain on the pressure relieving device;

(9) Discharge Lines shall be at least the size of the safety valve outlet or burst area of the rupturedisc;

(10)Discharge Lines shall lead to a safe point of release;(11)On rupture disc installations, the axis of discharge piping shall be perpendicular to the plane of

the rupture disc throughout the entire length; and

(12)Discharge Lines from pressure relieving devices shall be designed to facilitate drainage, or shallbe fitted to with a means of draining, to prevent the liquid from loading on the discharge line.

Designing Pressure Relief System

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