How to Choose A Tank Blanketing Valve

The vapor space in process tanks can be affected in a variety of different ways.  The volume of media in the tank will vary due to several factors including pumping media in and out, temperature changes, air and/or moisture entering the tank and vapors escaping the tank.  As the volume changes, the vapor space in the tank changes as well.

When is it important to have a tank blanketing valve?

When liquid is pumped out:

•   Tank vapor space pressure decreases.
•   Additional blanketing gas is introduced to compensate for the pressure change.

When liquid is pumped in:

•   Tank vapor space pressure increases.
•   Blanketing gas is relieved or vented to  alleviate excess pressure.

When tank contents cool down:

•  Vapors inside the take condense.
•  Tank pressure decreases.
•   Blanketing gas is introduced to compensate for lower pressure.

When tank contents heat up:

• •   Vapors inside expand.
  •   Tank pressure increases.
  •   Blanketing gas is relieved or vented to compensate for increased pressure.

Most tanks are not perfectly sealed enclosures and as a result, air and moisture can enter the tank and affect the tank contents.

• A process tank is especially vulnerable to additional air and moisture during temperature decreases and pump outs
• Seeping in of extra air and moisture can be prevented with a positive pressure of blanketing gas in the tank’s vapor space.

When dangerous or volatile emissions can escape from the tank:

• A process tank is especially vulnerable to volatile emissions during temperature increases and pumping in contents.
• Tank emissions can be prevented by having a negative pressure of blanketing gas in the vapor space.

What types of padding regulators are there?

There are several padding regulator choices available in the marketplace.

Self operated blanketing regulators offer fast response, low initial cost and a simple design.  Self operated regulators can use internal pressure registration which allows easier installation, or, an external sensing line that generally provides greater accuracy.

Self operated regulators are very sensitive and can achieve low set-points due to their large diaphragms and light spring.  For higher inlet pressures and flows, balanced plug and double seated versions are available.

Pilot operated blanketing valves offer higher flows, greater accuracy, and extremely fast lock-up.  Blanketing valves can be internally or externally piloted.

It is extremely important to choose the padding regulator that is suitable to handle the pressure, flow, and set-point required for your application.  It is also important to understand that these process requirements might be met by multiple valves with fundamentally different designs.

How to use API-2000 to choose the right size?

API-2000 must be considered when sizing tank blanketing valves.  The tank blanketing valve set-point definition is determined by the manufacturer.  The capacity requirement of the tank blanketing valve is determined by two factors:

1. Inbreathing due to product movement out of the tank (Q-displacement)
2. Inbreathing due to contraction of the vapors/product due to weather changes (Q-thermal)

Consider the following example for sizing a blanketing valve:

Step 1: Determine Q-displacement

• You will need the maximum pump rate of the tank’s contents during pump-out, i.e. 15 gpm.
• Calculate Q-displacement in standard cubic feet per hour (SCFH) = maximum pump rate in gpm x 8
• In this example, Q-displacement = 15 x 8 = 120 SCFH

Step 2 – Determine Q-thermal

• You will need the tank’s capacity, i.e. 84,000 gallons
• Using the API 2000 sizing table, find the required thermal inbreathing for the matching tank capacity
• In this example, an 84,000 gallon tank will require 2,000 SCFH for inbreathing according to the table

Step 3 – Calculate Q-Total = Q-displacement + Q-thermal

• Q-Total = 2,120 SCFH

In this example, the tank blanketing valve must have a capacity for up to 2,120 standard cubic feet per hour. For more details, read the entire bulletin from Jordan Valve ‘Standard Information Tank Blanketing Regulator Selection’ or Contact Us for help.

Finally, proper installation of the blanketing regulator and the sensing line to the tank, also referred to as the control line or impulse line, is the best way to ensure maximum performance in a blanketing regulator. Read more about how to install a tank blanketing regulator here.