R.L. Deppmann Company

Expansion and Compression Tanks – Part 9

Norm Hall — Mon, 23 Jan 2012 04:00:18 +0000

Differences in Compression Tank and Expansion Tank Formula

Now we understand the difference between expansion and compression tanks as described in the R. L. Deppmann Monday Morning Minutes of 1-9-12 and 1-16-12. What happens to the formula results when comparing these two types of tanks? In part 1 of this series, we introduced the formula for tank sizing. The denominator of the equation was:

(P_a /P_f) – (P_a /P_o)

P_a = The initial pressure in the tank before any fluid is introduced (Absolute PSIA)— P_f = The cold fill pressure in the tank before heating or cooling (Absolute PSIA)— P_o = The final or maximum pressure your design requires in the tank after heating (Absolute PSIA)

When you use a plain compression tank with no bladder or diaphragm, the initial pressure is 0 PSIG or 14.7 PSIA. When you use a bladder or diaphragm tank, the initial pressure should be equal to the fill pressure at the tank location. This means the tank has to be charged on the air side to the proper fill pressure before being filling the tank. The engineer must schedule the design fill pressure so that the contractor knows what the tank charge needs to be set at prior to filling the system.

Let’s see what happens. In a bladder tank the fill pressure and initial pressure are the same. The denominator of the formula becomes: (1-(P_f/P_o))

Example 1: Let’s assume we have a 5 story building heating system with a system elevation of 60 feet and a design of 180°F with a 30°F ΔT. Let’s assume our maximum pressure at the tank is 50 PSIG or 64.7 PSIA. What is the denominator if we have the tank on the lowest level? Looking back at past MMM, we know this results in a fill pressure of 30 PSIG or 44.7 PSIA.

For a Bladder Tank: (1-(P_f/P_o)) = (1-(44.7/64.7)) = .309

For a Compression Tank: (P_a/ P_f)-( P_a/ P_o) = ((14.7/44.7)-(14.7/64.7)) = .102

Since the number above is in the denominator of the tank sizing formula, the bigger the number, the smaller the tank. The bladder tank will require less tank volume than the compression tank. This could result in a lower cost installation.

Example 2: Let’s use the same system but now put the tank at the top of the system in the penthouse. Now the numbers change. The fill pressure will be 12 PSIG (it could be 4 PSIG at the very top but the fill valve comes standard at 12 PSIG) and let’s make the maximum pressure 20 PSIG so the initial pressure at the bottom of the system remains 50 PSIG. The denominator now becomes:

For a Bladder Tank: (1-(P_f/P_o)) = (1-(26.7/34.7)) = .23

For a Compression Tank: (P_a/ P_f)-( P_a/ P_o) = ((14.7/26.7)-(14.7/34.7)) = .126

Next week we look some selections and costs.

Disclaimer: R. L. Deppmann and it’s affiliates can not be held liable for issues caused by use of the information on this page. While the information comes from many years of experience and can be a valuable tool, it may not take into account special circumstances in your system and we therefore can not take responsibility for actions that result from this information. Please feel free to contact us if you do have any questions.

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Expansion and Compression Tanks – Part 7: Types of Tanks

Norm Hall — Mon, 16 Jan 2012 15:34:03 +0000

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Note: Our editors jumped the gun on last week’s Monday Morning Minutes. Here is Part 7 of Expansion and Compression Tanks.

Bladder tanks, diaphragm tanks, and standard ASME tanks: What is the difference and when should I use them? Let’s start with ASME. In commercial and institutional applications, ASHRAE, as well as most codes require the pressure vessels carry an ASME U stamp. This assures the owner and the owner’s insurance carrier that the vessel was inspected by an independent appraiser and registered with the National Board. Bladder, diaphragm, and standard tanks are all available stamped with the ASME U stamp.

Bladder expansion tanks normally have a membrane inside of them. We can think of it as a balloon inside a tank. The expanded water goes into the balloon and the air is on the outside of the balloon. Bell and Gossett series “B” bladder tanks are ASME constructed and are designed to be supported on the floor. The “balloon” or bladder can expand to the entire volume of the tank so the tank volume and acceptance volume are the same. The separation between the air and water is mechanical.

Diaphragm expansion tanks have a physical barrier inside the tank which acts to limit the expansion of the diaphragm. Bell and Gossett “D” style diaphragm tanks are ASME constructed and designed for either horizontal or vertical installation. The only difference is the location of the drain fitting. The “diaphragm” is limited in movement and, therefore, the acceptance volume is less than the tank volume. The separation between the air and water is mechanical.

ASME standard or plain steel compression tanks have nothing inside of them but air. These tanks, gallon for gallon, are less expensive than the bladder style tanks. They are mounted above the air separator and normally supported from the ceiling.

Diagrams and tank photographs are taken from the Bell and Gossett A-305G literature.

Next time we continue where to use which tank.

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Expansion and Compression Tanks – Part 8: Types of Tanks – When to Use

Norm Hall — Mon, 09 Jan 2012 14:14:56 +0000

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Choosing to use a Bell and Gossett ASME bladder style expansion tank, ASME diaphragm style tank, or ASME standard compression tank in your hydronic system will depend on several factors. One important consideration in choosing which tank to use depends on the type of “air control” system you design.

If your design is an air elimination system, use a bladder or diaphragm tank. Air elimination systems depend on automatic air vents to continually remove the air from the hydronic system. The only air in the system will be in the tank and outside the bladder. Unless the tank is very small, I recommend our “B” style tank because the bladder is removable. The bladder or diaphragm is a moving part and may fail after a while. “D”: style diaphragm tanks have non-removable internals and would require you to remove and replace the entire tank. With the tight mechanical rooms we have today, that could present a challenge.

From B&G A-307-A Air management literature

If your design is an air control system, use a standard ASME plain steel tank. Air control systems use manual vents. They may have automatic air vents in the system, but have shut off valves which are closed after commissioning. The only air in this type of system is in the compression tank.

You may notice my use of the term; expansion and compression tanks. Be careful because these are often used interchangeably in the HVAC industry but there is a technical difference. Expansion tanks have bladders or diaphragms and the water expands into the tank “balloon”. Compression tanks do not have bladders; the air in the tank compresses as the water expands into the tank. There is not much of a difference in these two definitions so be careful.

Next time we continue with expansion tank sizing.

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Merry Christmas from R.L. Deppmann Company

The Entire R. L. Deppmann Family — Mon, 26 Dec 2011 04:00:33 +0000

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Expansion and Compression Tanks – Part 6 Maximum Pressure (Continued)

Norm Hall — Mon, 19 Dec 2011 04:00:36 +0000

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Maximum pressure in a hydronic system depends on a number of variables. Last week we used an example to introduce the maximum pressure at the expansion tank. Let’s look at a couple more examples.

EXAMPLE TWO: Figure 1 shows a one line diagram of a heating system with a boiler and pumping system. Let’s assume we have a SIX story health care building and the heating system is 90 feet high with a 180 degree supply temperature. The pump and boilers are on the first floor. The BRYAN boilers selected come with 125 PSIG relief valves. Let’s also assume that the pump has a capacity of 800 GPM at 100 feet at design.

Our cold fill pressure will be:
90÷2.31=39 + 4 = 43 PSIG

The owner is interested in a durable pump which will last a long time and with little maintenance or repair in its expected 30 year life. You selected a B&G VSX-VSC-4X6X10.5A with a 30 HP non-overloading motor.

This pump has a design of 100 feet or 43 PSIG and a shutoff of 110 feet or 48 PSIG.

If our maximum design pressure in the system is 125 PSIG, then the maximum pressure we could have at the tank is: 125-48= 77 PSIG

Allowing a 10% safety factor on the maximum pressure gives us a tank maximum of 77 X .9 ∼ 69 PSIG so our tank will be selected for a fill of 43 PSIG and a maximum of 69 PSIG

ONE MORE THOUGHT: Remember, even though the calculations allow a high maximum pressure, you may choose to use a lower pressure. Higher pressures in systems may cause leaks or stress equipment. In addition, if you are using glycol, threaded connections at higher pressures will be tough to seal when filling the system.

Example: Assume you have a chilled water system with a tank fill pressure of 12 PSIG and a possible maximum of 80 PSIG. The tank may be very small anyway, so why not try dropping the pressure and see what happens.

Next week we look at types of tanks.

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Expansion and Compression Tanks – Part 5 Maximum Pressure

Norm Hall — Mon, 12 Dec 2011 14:56:48 +0000

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The last article, in the R. L. Deppmann Monday Morning Minutes, presented the cold fill pressure calculation needed for the expansion tank calculation. Another piece of information needed is the maximum pressure at the tank.

Maximum pressure in a hydronic system depends on a number of variables. Let’s look at a few examples to make a point. This week we will look at example one.

EXAMPLE ONE: Figure 1 shows a one line diagram of a heating system with a boiler and pumping system. Let’s assume we have a one story building and the heating system is only 15 feet high with a 180 degree supply temperature. Let’s also assume that the pump has a capacity of 100 GPM at 80 feet at design. We selected the Bell and Gossett series 80-2X2X9.5 inline pump with a 5 HP non-overloading motor.

Using last week’s article, we know the minimum cold fill pressure is 10.5 PSIG. Since the B&G B7-12 pressure regulating valve is factory set at 12 PSIG so let’s be smart and set of cold fill pressure at 12 PSIG rather than 10.5.

B&G PRV B7-12

Let’s assume the boiler has a 30 PSIG relief valve. B&G ASME pressure relief valves do not pop open at the set pressure, instead, they “weep” just before the setting switch has the advantage of keeping the water loss to a minimum when we approach the set point pressure. We will take the 30 PSIG relief pressure and subtract 10% to get the maximum pressure for tank sizing, giving us 27 PSIG.

Let’s look at what is happening on the pump discharge. The maximum pressure differential the pump can do is 90 feet or about 39 PSIG at shutoff. If the maximum operating pressure in the tank is 27 PSIG and the maximum the pump can add to that is 39 PSIG, then the maximum pressure in the system is at the pump discharge at 66 PSIG.

Why did we do this last calculation? Most commercial and industrial systems have working pressures of 125 to 150 PSIG. The 66 PSIG is clearly less than those pressures; therefore, it’s fine. What happens if there was a piece of equipment with a maximum pressure of 60 PSIG downstream of the pump? Your 66 PSIG design would exceed the working pressure of the downstream piece of equipment WHICH YOU SPECIFIED OR SELECTED. The time you would find out about this would be long after the equipment was purchased and installed. This is an embarrassing and costly error. I suggest you get in a habit of always calculating and recording the maximum pressure at the pump discharge.

Next week we look at another example to determine the maximum pressure calculation.

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Expansion and Compression Tanks – Part 4

Norm Hall — Mon, 05 Dec 2011 14:25:34 +0000

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Cold fill pressure is defined as the initial pressure required to lift water from the point of the gauge readout to the top of the system plus 4 PSIG for positive venting. This statement holds true for systems from chilled water to heating systems up to 220°F. From 220°F to 250°F, consult the Bell and Gossett Air Management training manual, available from R L Deppmann Company, serving Michigan and Ohio, or from your local B&G representative for other parts of the globe.

We normally define “Cold” as the water temperature entering the hydronic system at the time it is filled. When verifying the proper cold fill pressure in an existing system, allow the water to circulate without heat until it is near room temperature.

To calculate the “Fill Pressure”, take the elevation of the system above the gauge, divide by 2.31, and add 4 PSIG to it. The cold fill pressure in the example below is the setting of the pressure reducing valve (PRV). The “fill” pressure at the compression or expansion tank may be slightly more or less depending on the elevation of the tank, compared to the elevation of the PRV.

Provide a Minimum of 4 PSIG Cold Fill Pressure at the Top of the System.

Note: for temperatures greater than or equal to 220 degrees Fahrenheit, higher pressurization may be necessary to prevent cavitation in the system.

Detail complements of James M. Pleasants Co.

Let’s assume the compression tank, in the example above, is about 9 feet above the pressure reducing valve. Since 9 feet is about 3.9 PSIG, we would say the fill pressure in the tank is 13 PSIG. (17 PSIG minus 3.9 PSIG is about 13 PSIG)

Next Monday Morning Minute will describe the difference between compression and expansion tank systems.

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Bell and Gossett, a Xylem Brand

Norm Hall — Mon, 31 Oct 2011 04:00:49 +0000

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The name Xylem (zy-lem) is derived from classical Greek and is the tissue that transports life giving and life improving water in plants. Effective today, Xylem is also the PARENT COMPANY of the Bell and Gossett, McDonnell-Miller, Domestic, Hoffman, Gould, and Centripro brands that we represent and or distribute. The Board of Directors of the former parent company, ITT determined that breaking up into three focused businesses provides more value to the shareholders and the world. The three companies: ITT will be a manufacturer of critical components in energy, infrastructure, electronics, & transportation; ITT Exelis will a manufacturer in the defense, aerospace, and information systems areas; Xylem (NYSE XYL) will focus on finding solutions to the world’s most challenging water and wastewater problems. In addition to pumping applications, Xylem will be a leading global water technology provider, enabling customers to transport, treat, test and efficiently use water in public utility, residential and commercial building services, industrial and agricultural settings.

BELL AND GOSSETT

MCDONNELL & MILLER

DOMESTIC PUMP

HOFFMAN SPECIALTY

GOULDS WATER TECHNOLOGY

and CENTRIPRO

NEXT WEEK WE CONTINUE THE Monday Morning Minutes Expansion Tank Series.

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Expansion and Compression Tanks – Part 3

Norm Hall — Mon, 17 Oct 2011 04:00:51 +0000

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In part 2 of the R L Deppmann Monday Morning Minutes we determined the initial and final temperatures of the hydronic system in order to calculate the expansion tank selection. When we heat the water in a hydronic system, the water expands. This expansion is expressed as Ew in the formula:

Ew – Ep = The expansion of the water minus the expansion of the pipe.

This expansion is calculated for you in the Bell and Gossett ESP-PLUS selection program when you enter the initial and final temperatures and the fluid. The term Ew is not a mystery. It is expressed as:

Ew = [((specific volume at hot Temp.)/ (specific volume at cold Temp.))-1]

The calculation of the required tank acceptance volume is (Ef – Ep) X system volume. Let’s look at an example and assume the system has 50% Ethylene Glycol but you did not change the default from water.: Assume you have a closed hydronic heating system with a maximum temperature of 200°F and a 20°F ΔT. Assume the volume is 3000 gallons. What happens to the acceptance volume? The acceptance volume required in this example using 50%.


The acceptance volume for water is 93.6 gallons	The acceptance volume for 50% E.G. is 125.4 gallons

As you can see in the example, forgetting to change the fluid type default results in a tank which is 25% undersized. The result of this might be weeping pressure relief valves at the elevated temperatures.

Next week we look at pressures in a hydronic system.

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Expansion and Compression Tanks – Part 2

Norm Hall — Mon, 10 Oct 2011 13:15:29 +0000

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Last week the Monday Morning Minute introduced the expansion and compression tank formula. We described the numerator as the tank acceptance. The formula is (Ef – Ep) X Vs where Vs is the system volume and (Ef – Ep) is the expansion of the fluid minus the expansion of pipe. Let’s review some of the required data.

The volume of the system is the most difficult item to calculate. Some drawing design programs will calculate the system volume for you. Sometimes you have the difficult task of measuring the pipe lengths and the need for a spreadsheet. Bell and Gossett offers a quick selection program in the ESP-PLUS. This program offers a handy calculator to help with the manual calculations saving time and money.


click to enlarge	TEH 1195A

The expansion of the fluid and pipe is calculated from the fill temperature to the average maximum temperature of the system. This is often miss-understood. It is not the delta T of the system or the return temperature to the supply temperature. Example: Assume you have a closed heating system in Cleveland with a supply temperature of 180°F and a return temperature of 150°F. The average of the design water temperature is 165°F. In a worst case, in the winter, the fill temperature could be 40°F. So the initial temperature is 40°F and the maximum average temperature is 165°F.

	Showing 180°F Supply and 30°F ΔT so the average temperature is 165°F	Showing 180°F Supply and 2°F ΔT so the average temperature of the design is 179°F
If the reset system does not work there could be times when the delta T is very low. Often the engineer might select the maximum temperature very near the design supply temperature. This will result in a bit of oversizing of the tank but also provides a measure of safety factor. Here are snapshots of the ESP-PLUS program showing both methods.

Next week we will continue to examine important calculation parameters.

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