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Bulletin 74.1:Y693

Principle of Operation

The Type Y693 Gas Blanketing Regulator reduces

a higher-pressure gas to maintain a positive low

pressure of blanket gas over a stored liquid (see

Figure 3). Also when the vessel (or tank) is suddenly

cooled, causing vapors to contract, the regulator

replaces the volume of contracting vapors with a

volume of blanketing gas to prevent the internal vessel

pressure from decreasing. In both cases, a positive

vessel pressure prevents outside air from entering

the vessel and reduces the possibility of atmospheric

pressure collapsing the vessel.

Gas blanketing regulators respond to a slight decrease

in internal vessel pressure (caused by pump out or

atmospheric cooling) by throttling open to increase

theowrateofgasintothevessel.Whenthevessel’s

liquid level has been lowered to the desired point

and the vapor pressure reestablished, the regulator

throttles closed.

When the liquid level drops and vessel pressure

decreases below the setting of the control spring,

the spring force on the diaphragm opens the disk

assemblytosupplytherequiredowofgastothe

vessel.Whenvesselpressurehasbeensatised,

outlet pressure tends to increase slightly, acting on

the diaphragm. When the outlet pressure exceeds the

control spring setting, the diaphragm moves to close

the disk assembly.

Sizing Blanketing Systems

When sizing a gas blanketing regulator system

for a low-pressure application, you must consider

the replacement of blanketing gas required for the

liquid loss during pump out of the vessel plus the

condensation/contraction of vessel vapors during

atmospheric thermal cooling.

UsingtheestablishedproceduresfromAmerican

Petroleum Institute Standard 2000 (API 2000),

determinetheowrateofblanketinggasrequired.

1. Determinethegasowraterequiredtoreplacethe

liquid being pumped out (see Table 4).

2. Determinethegasowratedueto‘‘inbreathing’’

caused by atmospheric thermal cooling (see

Table 5).

3. Add the requirements of 1 and 2 and select regulator

size, based on total capacity required from Table 6.

Figure 3. Typical Type Y693 Installation (Steel or Stainless steel

lower casing version)

BLOCK

VALVE

BLOCK

VALVE

BLOCK

VALVE

VENT

VALVE

VENT

VALVE

VENT

VALVE

TYPE Y693

CONTROL

LINE

GAS BLANKETING PRESSURE

VESSEL / TANK

Sample sizing problem for blanketing applications:

 VesselCapacity ...........210,000 gal. / 795 000 L

 PumpIn/OutCapacity ......... 80 gal/min. / 303 L/m

Inlet (header) Pressure ......40 psig / 2.8 bar Nitrogen

Desired Blanket Setpoint. . . . . . . . 0.5 in. w.c. / 1 mbar

1. MultiplytheowrateconversionfactorfromTable4

bythepumpratetoobtaintheairowrequiredto

replace the volume of liquid pumped out.

8.021x80GPM=642SCFH

2. Determinetheairowrequiredforthermalcooling

from Table 5.

210,000 gal. tank size requires

5000SCFH/134Nm

/h air

Totalrequiredow:642+5000=

5642SCFH/151Nm

/h air

3. Convertairowtonitrogenowbymultiplying

theairowbythesquarerootof1dividedbythe

specicgravityofnitrogen.

5642x1/1.97=5729SCFH/154Nm

/h nitrogen

A6342

1 2 3 4 5 6 7 8 9 10 11 12

Bulletin 74.1:Y693 1

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