ENGINEERING DESIGN MANUAL FOR

STORAGE TANK PROTECTION

In the early days, during the development of the oil industry, tank fires were common occurrences. Larger storage tank fires are very complex events and satisfactory extinguishment requires methodical planning and the effective use of resources. But now, there were improvements in codes regarding design and construction of product storage tank. As a result, today, fires in storage tanks are rare events.

NFPA standard provides good recommendation & guidelines for protection of storage tank containing flammable liquids. It recommends foam system for extinguishing fire involving in the fuel tank contains flammable liquids. The foam system can be used for fire prevention, control or direct extinguishment of any flammable or combustible liquid fire within the tank. If the system is engineered, installed and maintained correctly, it will give reliable service for many years. The foam system is classified into two categories Fixed system and Semi -

fixed system. In order to select the correct foam system, it is necessary to understand the above systems:

Fixed System is a complete installation piped from a central foam station, discharging through fixed discharge devices on the hazard being protected. Foam proportioning components are permanently installed.

Semi Fixed System is an installation where the hazard is equipped with fixed discharge device(s) which connect to piping that terminates a safe distance from the hazard (Normally outside the dike wall). Foam producing materials are transported to the scene after the fire starts and are connected to the piping.

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TANK DEFINITIONS:

There are four major types of tanks used to store flammable and combustible liquids.Fixed Conical Roof TankOpen Top Floating Roof TankCovered (Internal ) Floating Roof TankHorizontal Tank

Fixed Conical Roof Storage Tank has vertical sides and is equipped with a fixed cone-shaped roof that is welded to the sides of the tank. Tanks that have been designed shall comply with NFPA 30 have a weak seam at the joint where the roof and sides meet. In the event of an internal explosion, the roof separates and blows off leaving the tank shell intact. This system allows the tank to retain its contents and any resulting fire will involve the full surface of the exposed flammable liquid.

Open Top Floating Roof Storage Tank is similar to the cone roof tank in construction but with the exception that it has no fixed roof. A pontoon type roof floats directly on the flammable

liquid surface. This floating roof has a mechanical shoe or tube

seal attached to its full perimeter. The rim seal covers the space between the floating roof and the tank shell (side wall). Construction shall comply with the requirements set forth in NFPA 30.

Covered (Internal) Floating Roof Storage Tank is a combination of both the cone roof and the

open top floating roof tank. The tank has a cone roof but

with the addition of an internal floating roof or pan that

floats directly on the fuel surface. This type of tank can be

identified by the open vents in the tank side walls just beneath the roof joint.

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If the internal floating roof is other than a steel double deck or pontoon type, the fire protection system should be designed for full surface fires (similar to cone roof tanks.) For the double deck or pontoon internal

roofs of steel construction, a design for seal area protection shall be permitted. Construction shall comply with the requirements set forth in NFPA 30.

Horizontal Storage Tank is a

horizontal cylindrical tank with flanged and

dished heads mounted on structural supports. Since a fire and explosion ruptures this type of tanks, fire protection is not normally applied to the tank except tank cooling sprays. If fire protection is required it is normally for protection of the dike area surrounding the tank

IDENTIFY THE FLAMMABLE LIQUID:

The first step in designing a system is to determine the type of flammable or combustible liquid to be stored in the tank. There are two basic classifications of flammable and combustible liquids:

Hydrocarbon (non water miscible) andPolar Solvent (water miscible)

Hydrocarbons include non water-soluble petroleum liquids such as crude oil, gasoline, jet fuels, fuel oils, kerosene, diesel etc.

Polar solvents include water soluble liquids such as alcohols, ketones, esters etc.

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FLAMMABLE AND COMBUSTIBLE LIQUIDS AS PER NFPA 11:

Flammable Liquid means any liquid with a flash point below 100°F (38°

C) and with a vapor pressure not exceeding 40 pounds per square inch (2.76 bar) absolute at 10°F (38°C). These liquids fall into the following divisions:

Class 1 includes liquids with flash points below 100°

F (38°C). This class falls into these subdivisions:

Class IA includes liquids with flash points below 73°F (23°C) and with a boiling point below 100°F (38°C). For foam application, they require special consideration.

Class IB includes liquids with flash points below 73°F(23°C) and with a boiling point at or above 100°F (38°C).

Class IC includes liquids with flash points at or above 73°F (23°C) and below 100°F (38°C).

Combustible Liquid means any liquid with a flash point at or above 100°F (38°C). These liquids fall into the following divisions:

Class II includes liquids with flash points at or above100°F (38°C) and below 140°F (60°C).

Class IIIA includes liquids with flash points at or above 140°F (60°C) and below 200°F (93°C).

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The next step after finding out liquid type involves considering the type of storage tank. Flammable liquids are stored in tanks of varying designs that depend on storage conditions, characteristics of the flammable liquid and other factors. It may be cone roof tanks, open top floating roof tanks, covered floating roof tanks and horizontal and multiple small tanks.

PROTECTION OF STORAGE TANKS:

A. Fixed Conical Roof Tanks:As per NFPA 11 standard, the following are the three methods used for fire protection of conical roof tanks:

1 Surface application with fixed foam discharge outlet device (Foam Chamber) Method

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Subsurface application Method3

Foam Monitors & Hand lines ( Hose with foam nozzle) method

1. Surface (Foam Chamber) Method:Foam Chambers are air aspirating devices used for the protection of fixed roof flammable liquid storage tanks. This method consists of one or more foam chambers installed on the shell of the tank just below the roof joint. A foam solution

pipe extends from the proportioning source outside the dike wall to the foam maker located upstream of the chamber. A deflector is located inside the tank shell to direct the discharge against the shell that results in a relatively gentle foam application on the fuel surface. The foam chamber has a vapour

seal to prevent the entry of vapour

into the foam chamber and the foam solution pipe. A minimum 3 bar to a maximum of 7bar must be available at the inlet flange into the foam chamber for this discharge device to work efficiently

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SYSTEM DESIGN CONSIDERATIONS

The NFPA 11 has defined certain guidelines for the appropriate design of foam discharge devices which may be briefed as follows:

Number of Foam Chambers:

The number of foam chambers required is determined by the tank diameter. Where two or more foam chambers are required, they shall be spaced equally around the tank periphery and each foam chamber shall be sized to deliver foam at an approximately same rate.

Number of Fixed Discharge Outlets RequiredTanks Containing Hydrocarbons and Polar Solvents:

Table A - 1- 1Tank Diameter Minimum Number

Discharge OutletsMeter Feet

Up to 24 Up to 80 1

Over 24 to 36 Over 80 to 120 2

Over 36 to 42 Over 120 to 140 3

Over 42 to 48 Over 140 to 160 4

Over 48 to 54 Over 160 to 180 5

Over 54 to 60 Over 180 to 200 6

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Minimum Foam Solution Application Rate and Discharge Time: The minimum foam solution application rate is the rate at which the water and foam concentrate in correctly proportioned ratio should be delivered to the surface of storage tank under protection to control and extinguish the fire. For minimum application rate requirement, the guidelines of NFPA has to be followed.

Minimum Discharge Time and Application RateType II Discharge Devices on Fixed Roof Tanks:

Table A-1- 2

Hydrocarbon Type

Minimum Application Rate Minimum Discharge Time

(Min)L/Min /m2 Gpm/ ft2

Flash point between 37.8o

C and 60oC (100oF and 140oF)

4.1 0.1 30

Flash point below 37.8oC (100oF) or liquids heated above their flash points

4.1 0.1 55

Crude Petroleum 4.1 0.1 55

* Although most hydrocarbon products meet the minimum application rate of 0.10 gpm/ft2 (4.1 lpm/m2), there are some hydrocarbons that require higher application rates. For polar solvents, NFPA does not establish a minimum application rate. Although some polar solvents have a minimum application of 0.10 gpm/ft2 (4.1 lpm/m2), the minimum rate can vary drastically for polar solvents.

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Supplementary Protection:In addition to the primary means of protection, supplementary hose streams are required for protection of fixed roof tanks. They are intended for protection of small spill fires. Approved foam hose stream equipment shall be provided in addition to tank foam installations as supplementary protection for small spill fires.

The minimum number of fixed or portable hose streams required shall be described below as per NFPA 11 Standard

Supplemental Foam Hose Stream RequirementsTable A -1- 3

The equipment for producing each foam stream shall have a solution application rate of at least 189 L/min (50 gpm), with the minimum number of hose streams shown in above table

Operating Times for Supplemental Hose StreamsTable A -1- 4

Diameter of Largest Tank

Minimum Number of Hose Streams Required

Up to 19.5 m (65 ft) 1

19.5 m to 36 m (65 ft to 120 ft) 2

Over 36 m (120 ft) 3

Diameter of Largest Tank Minimum Operating Time

Up to 10.5 m (35 ft) 10 Min

10.5 m to28.5 m (35 ft to 95 ft) 20 Min

Over 28.5 m (95 ft) 30 Min

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System Design:The system design shall be based on the largest single hazard, when more than one tank is protected by the same system.

CRITERIA FOR DESIGNING A FOAM SYSTEM FOR FIXED (CONE) ROOF TANK:Identify the liquid stored in the tank.Determine the best type of foam concentrate to use.Determine the application rate required. This is based on the product stored and the type of foam concentrateDetermine the product surface area to be protected.Determine the solution requirement for protection of the tank. Determine the quantity and size of the foam chambers required. Determine required discharge time for operation to tank.Determine the number of supplementary hose streams

required and minimum operating time.Determine the quantity of foam concentrate required for

operation of the tank and hose streams.Select the proper type of proportioning equipment to

meet the needs of the system.

Note: Foam concentrate is a concentrated liquid foaming agent as received from the manufacturer. It may be either 3% or 6%.

Example:Hazard Details:Tank diameter: 40 metersFoam concentrate to be used: AFFF 3%Pressure at inlet of foam chamber: 3.5 barFlash point of the liquid stored in the tank: -27ºC

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Calculation:

Surface area = Πd2

÷

4 = (3.14 X 402) ÷

4= 1257 Sq. m.

a) Foam solution application rate= * 4.1 LPM /Sq. m X Surface area

= 4.1 X 1257= 5154 LPM

* 4.1 LPM is as per reference of NFPA 11

b) Number of foam chamber required for 40 metersdiameter tank = 3 nos. minimum. (As per NFPA 11)

Capacity of foam chamber required= 5154 ÷3= 1718 LPM

Select a foam chamber that will provide that flow at the available pressure. See NAFFCO Foam data sheets for performance data required for selection the proper size discharge device.

c) Foam Concentrate Required:= Foam solution application rate in LPM x percentage of foam concentrate x discharge time in min= 5154Lpm X 3% (0.03) X 55 Min= 8505 litres

---------a

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d) Supplementary Hose Stream & Discharge time required:

As per NFPA 11, for 40m dia

tank hose stream shall be 3 No. & discharge time shall be 30 min

Foam Concentrate Required:= Foam solution application rate in LPM x percentage of foam concentrate x discharge time in min

= 3Nox189 Lpm X 3% (0.03) X 30 Min

= 511 litres-----------

bTotal Demand

= a + b= 9016 Litres

e) Proportioning System:Suitable proportioning of the foam concentrate is essential to provide the foam solution flow required to protect the tank. The proportioning system shall be sized for operation to the largest tank in the system, plus simultaneous operation to the supplementary hose streams, but should also be capable of operating to the hose streams without discharging the tank system. The proportioning system shall have sufficient pressure to operate against the highest expected residual water pressure as determined by hydraulic calculation of the system piping arrangement.

2. SUBSURFACE INJECTION METHODSubsurface foam injection systems can be used for protection of vertical fixed roof tanks containing hydrocarbons. It is not recommended for use on Class1A liquids or for tanks containing polar solvents. Also, it is not recommended for use on tanks with floating roofs or pans

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NOTE:Subsurface foam systems are not recommended for use with hydrocarbon products having viscosities above 2000 SSU at 60°F (15°C). (As per NFPA 11)

Discharge Device:Subsurface injection uses high back pressure foam makers,

located remote from the discharge outlet, to produce the expanded foam. The high back pressure foam makers used for subsurface injection require high operating pressures. In addition, the design of the downstream piping is critical to prevent excessive back pressure on the discharge line and excessive fuel pick-up if the maximum discharge velocity is exceeded. When designing the system, consideration must be given to the pressure loss down stream of the foam maker.

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Type of Foam ConcentrateFluoroprotein

type foam concentrates were designed for this type of application and are the best choice. Although conventional AFFF type foam concentrates have been used for this application, but they tend to have higher fuel pick-up and less sealability. Standard protein based foams can not be used, as they tend to become saturated with fuel and burn along with the product. Some AR-AFFF’s

may be used but the fluoroprotein

type foam concentrates provide the best performance.

Foam Discharge Outlets:The number of discharge outlets required is determined by the tank diameter. Where two or more discharges are required, they shall be equally spaced around the tank circumference. Each discharge shall be designed to deliver foam at approximately the same rate. Table 2-

1 shows the number of discharge outlets required for various diameter fixed (cone) roof tanks. The maximum velocity of discharge outlet shall not exceed 3m/sec (10 ft/sec) for Class IB products and Crude Oil and 6 m/sec (20 ft/sec) for all others.

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Number of Fixed Discharge Outlets RequiredFor Subsurface Application to Tanks Containing

HydrocarbonsTable A - 2- 1

Tank Diameter Number of Discharge Outlets Required

Flash point below 100oF (38oC)

Flash point 100oF (38oC) or Higher

Up to 24 m Up to 80 ft 1 1

Over 24 –

36 m Over 80 –

120 ft 2 1

Over 36 –

42 m Over 120 –

140 ft 3 2

Over 42 –

48 m Over 140 –

160 ft 4 2

Over 48 –

54m Over 160 –

180 ft 5 2

Over 54 –

60 m Over 180 –

200 ft 6 3

Over 60 m Over 200 ft 6 3

Plus 1 outlet for each additional 465 Sq.m

(5000 sq.ft)

Plus 1 outlet for each additional 697Sq.m

(7500 sq.ft)

Design Example for Subsurface Injection Method:Hazard Details:Tank diameter: 24.38 metersFoam concentrate to be used: AR -AFFF 3%Fuel Stored: GasolineCalculation:Surface area = Πd2 ÷

4 = (3.14 X 24.342) ÷

4= 466.59 Sq. m.

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a) Foam solution application rate= * 4.1 LPM /Sq. m X Surface area= 4.1 X 466.59= 1913 LPM* 4.1 LPM is as per reference of NFPA 11

b) Discharge Device

1 No High Back pressure Foam Maker & Discharge Outlet inside tank –

1 No ( As per NFPA 11)

c) Discharge Duration: 55 Minutes

d) Supplementary Hose Stream Allowance same as Fixed Cone Roof as describe above.

3. FOAM MONITORS & HAND LINES (HOSE WITH FOAM NOZZLE) METHOD:NFPA 11 states that monitors are not to be considered as the primary means of protection for fixed roof tanks over60 ft (18 m) in diameter and Foam handlines are not to be considered as the primary means of protection for tanks over 30ft (9m) in diameter or those over 20 ft(6m)in height. Neither portable or monitor mounted nozzles are considered acceptable for protection of tanks containing polar solvents regardless of size.

Application Rate and Discharge Time:The application rate and discharge time varies for the type of product being protected.

The following chart shows application density and duration for monitors and handlines

on tanks containing hydrocarbons.

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Table A- 3- 1

Hydrocarbon Type Minimum Application Rate Minimum Discharge Time (Min)

L/Min /m2 Gpm/ ft2

Flash point between 37.8o

C and 60oC (100oF and 140oF)

6.5 0.16 50

Flash point below 37.8oC (100oF) or liquids heated above their flash points

6.5 0.16 65

Crude Petroleum 6.5 0.16 65

B. Open Top Floating Roof Tank:The seal area of open top floating roof shall be protected by using following fire protection methods:

i. Fixed Discharge Outletii. Foam Hand linesiii. Foam Monitors

1. Top Seal Protection Using Fixed Discharge Outlet:There are two methods of applying foam to the seal area of the roof using fixed discharge outlet. In the first method, fixed discharge devices are mounted around the circumference of the tank and connected to a ring main installed around the circumference of the tank which is supplied by a single lateral.

The discharge devices may have an integral deflector designed to direct the foam down the inside of the tank shell to the seal

area of the roof. The second application method uses discharge devices mounted on the floating roof. The discharge devices are typically connected to a ring main installed near the edge of the roof.

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As per NFPA 11 standard, the Design parameters for Top seal protection of Open Top Floating roof as follows:

Table B- 1- 1

Seal TypeApplicable Illustration

Detail

Minimum Application Rate

m

Maximum Spacing Between Discharge outlets with

L/min/m2 gpm/ft2

305 mm (12”) Foam dam

610 mm (24”) Foam dam

ft m ft

Mechanical Shoe seal A 12.2 0.3 20 12.2 40 24.4 80

Tube Seal with metal weather shield

B 12.2 0.3 20 12.2 40 24.4 80

Fully or partly combustible secondary seal

C 12.2 0.3 20 12.2 40 24.4 80

All metal secondary seal

D 12.2 0.3 20 12.2 40 24.4 80

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Note:The number of foam discharge devices required is based on the circumference of the tank and the height of the foam dam. The rate of application and supply of foam concentrate shall be calculated using the area of the annular ring between the circular dam and the tank shell.

CRITERIA FOR DESIGNING A FOAM SYSTEM FOR OPEN TOP FLOATING ROOF TANK:Identify the liquid stored in the tank.Determine the best type of foam concentrate to use.Determine the application rate required. This is based on the product stored and the type of foam concentrateDetermine the foam dam size (annular area) to be protected.Determine the solution requirement for protection of the tank. Determine the quantity and size of the foam discharge device required. Determine required discharge time for operation to tank.Determine the number of supplementary hose streams required and minimum operating time.Determine the quantity of foam concentrate required for operation of the tank and hose streams.Select the proper type of proportioning equipment to meet the needs of the system.

Design Example:Hazard Details:Tank diameter

: 42 metersFoam Dam Size

: 0.6 m from shell: 0.6 m height

Product Stored

: Hydrocarbon

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a) Area of Dam = pi /4 (d12 -

d22)] =3.14/4 [(422 -

40.82)= 78.03 Sq.m.

b)Minimum solution application rate= Area of dam X 12.2 Lpm

/sq.m. ( As per NFPA 11) = 78.03 X 12.2 Lpm

/ Sq.m= 951.96 LPMc) Number of Foam Discharge Device (Foam Makers):Circumference of tank = pi x d

= 3.14 X 42= 131.95 mtrs.

As per NFPA 11, Maximum spacing permissible with 0.6 meters Foam dam height shall be 24.4 metersNumber of Foam Makers = 131.95 ÷

24.4= 5.40= 6 Unit

Note: The foam discharge outlet device size shall depend on flow & inlet pressure as per manufacturer standard

d) Minimum Foam Concentrate Required:= Foam solution application rate in LPM x percentage of

foam concentrate x discharge time in min= 951.96 x 0.03x 20* (* As per NFPA 11)= 517.17+5% = 543 Litres

-------

a

e) Supplementary Hose Stream & Discharge time required:

As per NFPA 11, for 42m dia

tank hose stream shall be 3 No. & discharge time shall be 30 min

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Foam Concentrate Required:= Foam solution application rate in LPM x percentage of foam concentrate x discharge time in min

= 3Nox189 Lpm X 3% (0.03) X 30 Min= 511 litres-----------

bTotal Demand

= a + b= 1054 Litres

f) Proportioning System:Suitable proportioning of the foam concentrate is essential to provide the foam solution flow required to protect the tank. The proportioning system shall be sized for operation to the largest tank in the system, plus simultaneous operation to the supplementary hose streams, but should also be capable of operating to the hose streams without discharging the tank system. The proportioning system shall have sufficient pressure to operate against the highest expected residual water pressure as determined by hydraulic calculation of the system piping arrangement.

2. Below Seal Protection:In this application the foam is discharged directly under

the seal or secondary seal. As per NFPA 11 standard, the Design parameters for Top seal protection of Open Top Floating roof as follows:

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Table B- 2- 1

* Metal secondary seal is equivalent to a foam dam

Note:The number of foam discharge devices required is based on

the circumference of the tank and the type of seal. The rate of application and supply of foam concentrate shall be calculated using the area of the annular ring between the edge of the pontoon and the tank shell. In applications requiring a foam dam, it shall be calculated on the annular area between the foam dam and the tank

shell.

Seal TypeApplicable Illustration

Detail

Minimum Application Rate

Minimum Discharge Time (Min)

Maximum Spacing Between Discharge outlets

L/min/m2 gpm/ft2

Mechanical Shoe seal A 20.4 0.5 10 39m (130 ft)-

Foam dam not required

Tube Seal with more than 152mm (6”) between top of tube and top of pontoon

B 20.4 0.5 10 18 m (60 ft)-

Foam dam not required

Tube seal with less than 152mm (6”) between top of tube and top of pontoon

C 20.4 0.5 10 18 m (60 ft)-

Foam dam required

Tube seal with foam discharge below metal secondary seal*

D 20.4 0.5 10 18 m (60 ft)-

Foam dam not required

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ii) Foam Hand line Method:

The hand line method allows fire fighters to apply foam directly to the fire area, The Foam hand lines shall be permitted to use either from the wind girder or directly from the roof.

iii) Foam Monitor Method:

As per NFPA 11, Monitors are not recommended as the primary means of floating roof seal fire extinguishment because of the difficulty of directing foam into the annular space and the possibility of sinking the roof.

C. Covered (Internal) Floating Roof Tank:

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It is a combination of both the cone roof and the

open top floating roof tank. The NFPA 11 has established the following guideline to protect the above tank.

i

In case full surface fire protection is required, the covered (internal) floating roof tank shall be considered equivalent to a fixed cone roof and it’s design guidelines has to be followed.

ii If seal surface fire protection is required, the

covered (internal) floating roof tank shall be considered equivalent to an open top floating roof and it’s design guidelines has to be followed

DIKE AREA PROTECTION:As per NFPA 11 standard, the dike area protection can be achieved by using either low level fixed foam discharge outlets, portable or fixed monitors or foam hose lines.

PROPORTIONING SYSTEM:There are three different type proportioning systems are used in the foam application systemi Vacuum inducing type or Venturiii Pressure Proportioner

typeiii Balanced Pressure type

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i) Venturi type

A venturi

type device also called a line proportioner

(an inductor or an eductor) manipulates the incoming water velocity. In turn this function creates a vacuum within the device. By connecting a foam concentrate supply to this vacuum chamber, concentrate is draw into and mixed with the water that passes through the venturi

device.

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ii) Pressure proportioner type:

A pressure proportioner

type is a device that takes advantage of the incoming water and uses a small volume of that water. It forces foam concentrate

contained inside a pressure vessel into the flowing water stream. A standard pressure proportioner

type has no physical separation between the stored concentrate and the incoming water.On the other hand, the bladder type, also known as a diaphragm or bladder tank, has a physical separation between the pressurized water and the foam

concentrate.

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iii) Balanced Pressure type:

A balanced pressure proportioner

is a device that proportions correctly when the foam concentrate and the water pressure are identical. This system requires a pumped source of foam concentrate. It covers not only standard balanced or back pressure (BP) controlled units but in-line balanced pressure (ILBP) units also.

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