How to Choose A Tank Blanketing Valve
What to consider when choosing a valve.
The vapor space in tanks can be affected in a variety of different ways. The volume of media in the tank will vary due to a variety of 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 liquid is pumped out:
- — Tank vapor space pressure decreases
- — Additional blanketing gas is introduced to compensate for 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 condense
- — Tank pressure decreases
- — Blanketing gas is introduced to compensate for changes
When tank contents heat up:
- — Vapors inside expand
- — Tank pressure increases
- — Blanketing gas is relieved or vented to compensate for changes
Most tanks are not perfectly sealed enclosures and as a result, air and moisture can enter the tank and affect contents.
- — Tank is especially vulnerable during temperature decreases and pump outs
- — This can be prevented with a positive pressure of blanketing gas in the vapor space
Dangerous / volatile emissions can escape from tank
- — Tank is especially vulnerable during temperature increases and pumping in contents
- — This can be prevented with a negative pressure of blanketing gas in the vapor space
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 the padding regulator you choose is suitable to handle the pressure, flow, and set-point required for your application. It is also important to note these requirements might be met by multiple valves with fundamentally different designs.
Proper installation of the blanketing regulator and the sensing line, also referred to as the control line or impulse line, to the tank is the best way to ensure maximum performance in a blanketing regulator.
Sensing Line Installation:
A sensing line must be installed as follows:
● Connect one end of a ¾” pipe to the fitting under the diaphragm.
● Connect the other end to an appropriate fitting on the tank.
● DO NOT locate the control line tap in any location where turbulence or abnormal velocities may occur.
● The control line should be sloped away from the valve.
● Install a pressure gauge to measure pressure in the tank itself, not in the outlet piping or the control line to aid in setting the valve.
It is important to follow these recommendations.
● Keep the regulator as close to the tank as possible and as high as possible.
● Minimize the length of the downstream pipe coming from the valve.
● NEVER reduce the pipe size on the valve outlet to the tank. This line must always be as large as the valve size, or one pipe size larger to assure it does not act as a restriction.
● Any downstream isolation valve after the regulator must be a full port type. The isolation valve cannot act as a restriction.
● The sensing line must be a minimum of ¾” pipe.
● A sensing line isolation valve is recommended. Again, it must be full ported.
● Slope the sensing line to the sensing port on the tank.
● Keep the sensing line as short and straight as possible.
● For each 10 feet of sensing line, increase the line size by one pipe diameter. (Especially important on blanket pressures of less than 5 inches of water column)
● Keep the sensing port on the tank as far removed as possible from the downstream pipe outlet going into the tank.
If these recommendations are followed, your regulator should perform quite well.
API-2000 must be considered when sizing blanketing valves. The tank blanketing valve set point definition is determined by the manufacturer. The capacity requirement of the tank blanketing valve is composed of two components:
1. Inbreathing due to product movement out of the tank
2. Inbreathing due to contraction of the vapors/product due to weather changes
Consider the following example:
Step 1 – Determine Q-displacement
• You will need the maximum pump rate of the tank’s contents, i.e. 15 gpm
• Q-displacement SCFH = maximum pump rate (gpm) x 8
• Q-displacement = 120 SCFH
Step 2 – Determine Q-thermal
• You will need the tank’s capacity, i.e. 84,000 gallons
• An 84,000 gallon tank will require 2,000 SCFH for inbreathing
Step 3 – Q-total=Q-displacement+Q-thermal
• Q-Total= 2,120 SCFH
API-2000 must be considered when sizing blanketing valves. The tank blanketing valve set point definition is determined by the manufacturer. The capacity requirement of the tank blanketing valve is composed of two components:
1. Inbreathing due to product movement out of the tank
2. Inbreathing due to contraction of the vapors/product due to weather changes
Consider the following example:
Step 1 – Determine Q-displacement
• You will need the maximum pump rate of the tank’s contents, i.e. 15 gpm
• Q-displacement SCFH = maximum pump rate (gpm) x 8
• Q-displacement = 120 SCFH
Step 2 – Determine Q-thermal
• You will need the tank’s capacity, i.e. 84,000 gallons
• An 84,000 gallon tank will require 2,000 SCFH for inbreathing
Step 3 – Q-total=Q-displacement+Q-thermal
• Q-Total= 2,120 SCFH