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]]>Nathan’s Fast Facts:
Favorite Color: Purple
Favorite Food: Chicken thighs
Likes: Indy Fuel
Fun Fact: He ran 150 miles when he was 10!
Nathan Blank
Engineer
Email: nblank@westank.com
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]]>The post How To: Quickly Adjust Vessel Precharge Pressure first appeared on Wessels Company.
]]>Fixing a system expansion tank quickly requires some basic understanding of its operation and the principles of compressed gas (air). The expansion tank absorbs the system’s expanded fluid caused by temperature increase. The tank air volume controls the pressure increases in the system. A very large, oversized tank will register a very low pressure increase because the relative air cushion volume change is small. A properly sized expansion tank should show a pressure increase of 2040 psi from empty to “full” of expanded fluid.
The air cushion in an expansion tank follows the principles of Boyle’s law. Boyle’s law states that pressure times volume of a contained gas equals a constant. If the compressed gas is pressurized to twice the starting pressure, the resulting volume of the compressed gas is 1/2 of the original volume. Simply stated, Boyle’s law is:
Air loss over time
Air or nitrogen gas can find its way out of the sealed tank. The points of minute air loss that may not be easily observed through standard bubble testing are threaded connections, the air charge valve, bulkhead fitting (bottom system connection on select tanks), and the bladder itself. The bladder is butyl, which has very low permeability. Higher pressure can drive more air through the bladder by means of diffusion. As the highpressure water is “starved” for oxygen/air, it will pull molecules through the bladder over time.
As air is lost through the tank bladder or threaded connections, the result is the same as underinflated air charge pressure. The tank will back fill with water until the air pressure in the tank equals the system water pressure. This will result in the same issues as an undersized expansion tank.
Annual Bladder Tank Check
Schedule a preventative maintenance event every 12 months. NOTE: there should be a drain valve between the system isolation valve and the tank. If one is not present, have one installed.
Setting the PreCharge “OntheFly”
With the tank in operation, a common question is “Can I adjust the precharge pressure in the tank without draining the tank first completely?” The answer to this question is “Yes,” providing we have working understanding of the compression air cushion of the expansion tank. As we approach the tank, we will notice that the tank is operating and the pressure P1. As we drain water from the isolated expansion tank and measure the captured water in a known volume bucket, a lower pressure P2 should be observed. Armed with these three data points, plus the size of the tank on the job, and the needed precharged pressure for the tank at startup, we can adjust the pressure P2 to ensure that, as the tank empties, it will ramp down to that required precharge pressure.
Let’s look at the variables needed to to be identified to set the precharge pressure with a somewhat full tank:
V_{t } Tank total volume in gallons.
V_{w} Water drawn from the operating expansion tank in gallons.
p_{1} Measured pressure before the water draw in psig.
p_{2} Measured pressure after the water draw in psig.
p_{a} Atmospheric pressure (sea level typically 14.7 psia).
p_{pre} Required tank precharge at system startup in psig.
With these known variables, we will calculate:
PE Potential energy needed in the tank in GAP (gallons absolute pressure in psia).
V_{1} Air volume of the operating expansion tank in gallons.
V_{2} Air volume of the expansion tank after the water draw in gallons.
The expansion tank can be thought of as a container of potential energy of (p_{pre }+ p_{a}) x V_{t} at the startup of the system. Boyle’s law p_{1}V_{1} = p_{2}V_{2 }means that this potential energy should remain the same at any condition of the tank. If we see that the potential energy has dropped, we can increase the air pressure in the tank until that potential energy is met.
We will first calculate the potential energy required by the tank’s air cushion, PE.
PE = (p_{pre }+ p_{a}) x Tv
We will start with Boyle’s Law to help find V_{2} then adjust P_{2} to meet the required potential energy.
(p_{1 }+ p_{a})V_{1} = (p_{2 }+ p_{a})V_{2}
As we draw off water, the resulting air cushion size relationship is:
V_{1} = V_{2}V_{w} (V_{w} is the known draw water volume)
Substituting shows:
(p_{1 }+ p_{a})(V_{2}V_{w}) = (p_{2} + p_{a})V_{2}
Solving for V_{2}:
V_{2} = V_{w}(p_{1} + p_{a})/(p_{1}p_{2})
Because we want the potential energy to match the tank’s empty condition, we solve:
(p_{2(new)} + p_{a})V_{2} = PE
(p_{2(new)} + p_{a}) = PE/V_{2 }
p_{2(new)} = PE/V_{2 }– p_{a}
Example: An NLA800L (211 gallons) at the job site has a pressure reading of 60 psig. After isolating the tank and drawing off 8gallons of water, the pressure reading drops to 52 psig. Atmospheric pressure is at sea level (p_{a} = 14.7 psia). Find the required increase in pressure required after the draw of water to ensure the tank precharge pressure is 12 psig.
V_{t } Tank total volume 211 gallons
V_{w} Water drawn from the operating expansion tank 8 gallons.
p_{1} Measured pressure before the water draw 60 psig.
p_{2} Measured pressure after the water draw 52 psig.
p_{a} Atmospheric pressure 14.7 psia.
p_{pre} Required tank precharge at system startup 12 psig.
The potential energy at tank startup should be:
PE = (p_{pre}+p_{a}) x V_{t}
PE = (12+14.7) x 211 = 5,634 GAP (gallons absolute pressure in psia)
Solving for V_{2}:
V_{2} = V_{w}(p_{1} + p_{a})/(p_{1}p_{2})
V_{2} = 8 (60 + 14.7)/(6052)
V2 = 74.7 gallons
We can solve for the current potential energy in the air cushion V_{2} at pressure p_{2} of 52 psig and see that:
PE_{2} = (p_{2}+p_{a}) x V_{2}
PE_{2} = (52+14.7) x 74.7 = 4,982 GAP (gallons absolute pressure in psia)
Now, determine the pressure p2(new) needed to bring the potential energy to the correct value of 5,634 GAP:
p_{2(new)} = PE/V_{2 }– p_{a}
p_{2(new)} = 5,634/74.7 – 14.7
p_{2(new)} = 61 psig
It is important to reiterate that the best way to make sure the precharge pressure is correct is to drain the tank completely of any stored water and measure and adjust the pressure as needed. Often the method of drawing off a small amount of water will expedite the troubleshooting of an expansion tank at the job site. Regardless of findings and calculations, a tank should never be pressurized beyond the ASME working pressure of the tank. If calculations show that a high pressure is needed to reach the necessary potential energy, do not adjust if this could cause a dangerous or lethal situation. It is recommended that the entire tank be drained if this condition occurs.
Sam Fuller
Technical Engineer
Wessels Company
Download PDF Version of “How To: Quickly Adjust Vessel Precharge Pressure”
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]]>The post How To: Select Nitrogen Cylinders to PreCharge a Bladder Tank first appeared on Wessels Company.
]]>Additionally, we need to think in terms of stored potential energy. With the product pV (where p is absolute pressure and V is volume) equal to potential energy in joules (ftlbf), we can calculate the energy in a cylinder of gas and compare it to the potential energy requirement of the bladder tank.
To make the units more conducive to gallons and psia, let’s use pV in terms of GAP (gallons times absolute pressure in psia). Gas cylinders are rated in cubic feet. This is actually the cubic feet of gas at 150 psig (164.7 psia) that, when released, would make up that amount of volume in cubic feet at 0 psia. In terms of potential energy in terms of GAP, the amount of energy in each cylinder is equal to 100 times the “rated” cubic feet of the cylinder. A 20 cubic foot cylinder charged to 2,200 psig yields a potential energy of 20 x 100 = 2,000 GAP.
Some common size cylinders whereas the rating in Cubic Feet based on 150 psig:
Be aware that the rating for the cylinders is referenced above 0 psia. If charging a tank to a pressure greater than 0 psia, which of course is any expansion tank, the potential energy will be somewhat less, but this will yield a close approximation. For example, if the tank at the job site is 250 gallons and needs to be charged from 40 psig to 90 psig, this requires the potential energy to be increased:
250 gallons × (9040) psia=12,500 GAP
Bottle “H” is rated for 244 Cu.Ft. or 24,400 GAP above 0 psig. To recalculate the potential energy above 90 psig, a simple ratio should be used:
The 14.7 factor to convert psig to psia drops out of the equation and yields:
For the above example, the resulting cylinder potential energy above the 90psig set point for the expansion tank calculates thusly:
The potential energy above 90 psig for the “J” 330 cu.ft. is 23,402 GAP and is large enough to handle the tank in this example. An alternative is to take two (2) “Q” tanks which also have the necessary GAP requirement for the example expansion tank.
An excel spreadsheet is available to download for evaluating most potential jobs.
Written by:
Sam Fuller
Technical Sales Engineer
Wessels Company
The post How To: Select Nitrogen Cylinders to PreCharge a Bladder Tank first appeared on Wessels Company.
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