Vacuum Control
620
Te c h n i c a l
Vacuum Applications
Vacuum regulators and vacuum breakers are widely used in process
plants. Conventional regulators and relief valves might be suitable
for vacuum service if applied correctly. This section provides
fundamentals and examples.
Vacuum Control Devices
Just like there are pressure reducing regulators and pressure relief
valves for positive pressure service, there are also two basic
types of valves for vacuum service. The terms used for each are
sometimes confusing. Therefore, it is sometimes necessary to ask
further questions to determine the required function of the valve.
The terms vacuum regulator and vacuum breaker will be used in
these pages to differentiate between the two types.
Vacuum Regulators
Vacuum regulators maintain a constant vacuum at the regulator
inlet. A loss of this vacuum (increase in absolute pressure) beyond
setpoint registers on the diaphragm and opens the disk. It depends
on the valve as to which side of the diaphragm control pressure is
measured. Opening the valve plug permits a downstream vacuum
of lower absolute pressure than the controlled vacuum to restore
the upstream vacuum to its original setting.
Besides the typical vacuum regulator, a conventional regulator can
be suitable if applied correctly. Any pressure reducing regulator
(spring to open device) that has an external control line connection
and an O-ring stem seal can be used as a vacuum regulator.
Installation requires a control line to connect the vacuum being
controlled and the spring case. The regulator spring range is now a
negative pressure range and the body ow direction is the same as
in conventional pressure reducing service.
Vacuum Breakers (Relief Valves)
Vacuum breakers are used in applications where an increase in
vacuum must be limited. An increase in vacuum (decrease in
absolute pressure) beyond a certain value causes the diaphragm to
move and open the disk. This permits atmospheric pressure or a
positive pressure, or an upstream vacuum that has higher absolute
pressure than the downstream vacuum, to enter the system and
restore the controlled vacuum to its original pressure setting.
A vacuum breaker is a spring-to-close device, meaning that if there
is no pressure on the valve the spring will push the valve plug into
its seat. There are various Fisher® brand products to handle this
application. Some valves are designed as vacuum breakers.
Fisher brand relief valves can also be used as vacuum breakers.
ABSOLUTE
ZERO
-14.7 PSIG (-1,01 bar g),
0 PSIA (0 bar a)
ATMOSPHERIC
5 PSIG (0,34 bar g) VACUUM,
-5 PSIG (-0,34 bar g),
9.7 PSIA (0,67 bar a)
0 PSIG (0 bar g),
14.7 PSIA (1,01 bar a)
5 PSIG (0,34 bar g),
19.7 PSIA (1,36 bar a)
POSITIVE PRESSURE
VACUUM
1 PSIG (0,069 bar) = 27.7-INCHES OF WATER (69 mbar) = 2.036-INCHES OF MERCURY
1kg/cm2 = 10.01 METERS OF WATER = 0.7355 METERS OF MERCURY
Figure 1. Vacuum Terminology
Vacuum Terminology
Engineers use a variety of terms to describe vacuum, which
can cause some confusion. Determine whether the units are in
absolute pressure or gauge pressure (0 psi gauge (0 bar gauge) is
atmospheric pressure).
• 5 psig (0,34 bar g) vacuum is 5 psi (0,34 bar) below
atmospheric pressure.
• -5 psig (-0,34 bar g) is 5 psi (0,34 bar) below
atmospheric pressure.
• 9.7 psia (0,67 bar a) is 9.7 psi (0,67 bar) above absolute zero
or 5 psi (0,34 bar) below atmospheric pressure
(14.7 psia - 5 psi = 9.7 psia (1,01 bar a - 0,34 bar = 0,67 bar a)).
Vacuum Control
621
Te c h n i c a l
A conventional relief valve can be used as a vacuum breaker, as
long as it has a threaded spring case vent so a control line can be
attached. If inlet pressure is atmospheric air, then the internal
pressure registration from body inlet to lower casing admits
atmospheric pressure to the lower casing. If inlet pressure is not
atmospheric, a relief valve in which the lower casing can be vented
to atmosphere when the body inlet is pressurized must be chosen.
In this case, the terminology “blocked throat” and “external
registration with O-ring stem seal” are used for clarity.
UNAVOIDABLE
LEAKAGE
TYPE Y695VR VACUUM REGULATOR
POSITIVE PRESSURE OR
ATMOSPHERE, OR A LESSER VACUUM
THAN THE VACUUM BEING LIMITED
TYPE Y690VB VACUUM BREAKER
Figure 3. Typical Vacuum Breaker
Figure 2. Typical Vacuum Regulator
INLET PRESSURE
CONTROL PRESSURE (VACUUM)
ATMOSPHERIC PRESSURE
A spring that normally has a range of 6 to 11-inches w.c. (15 to 27 mbar)
positive pressure will now have a range of 6 to 11-inches w.c.
(15 to 27 mbar) vacuum (negative pressure). It may be expedient to
bench set the vacuum breaker if the type chosen uses a spring case
closing cap. Removing the closing cap to gain access to the adjusting
screw will admit air into the spring case when in vacuum service.
CONTROL PRESSURE (VACUUM)
OUTLET PRESSURE (VACUUM)
ATMOSPHERIC PRESSURE
VACUUM
BEING CONTROLLED
VACUUM
PUMP
HIGHER
VACUUM SOURCE
VACUUM
PUMP
VACUUM
BEING LIMITED
B2582
B2583
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