Hydrocarbon Processing - April 2021 - 66

Process Control, Instrumentation and Automation
ing maintains the purity of the product
from contaminants, isolates the tank
from atmospheric air and moisture, and
maintains oxygen levels low enough to
prevent ignition.
Tank blanketing and vapor recovery
(de-pad) regulators, pressure/vacuum relief valves (PVRVs) and emergency relief
vents should be installed on tanks and used
together at staggered setpoints (FIG. 4).
This will ensure protection from conditions above or below the normal pressure
range, either of which could compromise
the tank's contents or its physical integrity.
The environment inside a tank can
be quite complex, which is not necessarily apparent from the outside. Fixedroof tanks at a process facility or storage
terminal can be filled with a wide range
of liquids-from chemicals to finished
products to crude oil-and can contain
volatile vapors under potentially caustic or
hazardous conditions. Therefore, it is best

to think of a tank as an ecosystem and not
as a collection of individual components.
During normal operation, the vapor
space inside a tank may expand or contract due to pumping fluids in or out of the
tank, or as ambient temperature changes.
These tanks are typically blanketed (or
padded) at a slightly positive pressure
with an inert gas. Conversely, the tank
may also be equipped with de-pad or vapor recovery devices to relieve tank pressure when it reaches the upper end of the
operating band.
It is completely normal for tanks to
'breathe' during normal operation, whereby blanketing gas is drawn into the tank
and vented out of the vapor space. During
breathing and other conditions, the tank
blanketing system maintains the tank's
pressure within a desired control band to
ensure tank integrity and the quality of its
contents. Operation of emergency vents
should never be considered a normal
event since these devices are the tank's last
line of defense.
Maintenance of tank pressure is accomplished by regulators, with selection
of the correct type a key factor.

in these applications, and fast speed of
response is required to account for pump
cycling and changes in temperature. These
devices have much tighter control and cycle less frequently than control valves used
for the same purpose, making them the
right choice in these types of applications.
One of the most important design
features of pilot-operated regulators is
pressure amplification by the pilot, called
'gain.' This feature amplifies a small change
in outlet pressure to a much larger change
in the loading pressure of the regulator,
which controls its operation. This improved accuracy at low setpoints results in
a bonus of using less nitrogen or blanketing gas, making the tank less expensive to
operate (FIG. 5).
Purge meters are also recommended in
tank blanketing systems where the process
media is corrosive, volatile or can solidify
in the lines. Purging maintains a low flow
of blanketing gas through the main and
sensing lines to isolate the regulator from
the downstream process fluid, enhancing
the service life of the materials used in the
unit and preventing foreign material from
the tank backing up into the lines.

Pilot-operated advantages and design recommendations. Pilot-operated

Reactive vs. predictive maintenance.

regulators are ideal devices for tank blanketing applications. Accuracy is paramount due to the low pressures needed

To ensure each tank is operating per design,
preventative maintenance and inspections
should be performed for tank blanketing
regulators and other tank top components

FIG. 2. Several layers of protection are
recommended to protect tanks from
overpressure and underpressure conditions.

FIG. 3. Poor control of tank pressure creates
multiple risks. Sources: NYMEX spot price,
July 2020; U.S. EPA and the U.S. Chemical
Safety and Hazard Investigation Board.

66 APRIL 2021 | HydrocarbonProcessing.com

FIG. 4. Full array of devices used together to control pressure of fixed-roof storage tanks.

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Hydrocarbon Processing - April 2021

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