Expansion Vessels and Pressurised Tanks in Data Center Cooling Loops
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Expansion Vessels and Pressurised Tanks in Data Center Cooling Loops
A closed chilled water loop needs a pressure boundary for three reasons that pull in opposite directions: the water expands when it is heated, the pump suction needs a positive head to stay above vapour pressure, and the top of the system needs a relief path before the strongest component reaches its allowable pressure. An expansion vessel discharges into the loop at low temperature to replace the volume the water gives up as it contracts and takes it back as the loop warms. Sizing is two-stage. The first stage is the thermal volume change of the total system water over the design temperature range. The second stage is a pressure question: that volume has to be absorbed between the vessel pre-charge pressure and the maximum allowable pressure, and the ratio between those two numbers, not the temperature range alone, sets the physical size of the vessel. The choice between a diaphragm or bladder vessel and a code-stamped pressure vessel follows from that pressure class and from the pressure equipment regulation in force where the plant operates for expansion tank chilled water
Why a closed loop needs a pressure boundary
An open cooling tower system needs no pressure vessel in the loop, because the tower basin is at atmospheric pressure and the pump develops the whole head. A closed loop removes that reference, so three requirements appear at once and must be met by one pressure level at every point in the circuit for expansion tank chilled water
The first is the expansion of the water, which has nowhere to go in a closed loop. The second is the lower limit: when the plant is cold, or a section drains, the pump suction must stay above the vapour pressure of the water at the lowest expected temperature. The third is the upper limit: a relief path above the allowable pressure of the weakest component. These fix the usable pressure band for expansion tank chilled water
Sizing the expansion vessel from volume and temperature range
The first calculation is a thermal one. Take the total water volume in the system, including the expansion vessel itself, the pipework, the heat exchangers and any connected vessel or basin, and multiply it by the volumetric expansion factor of water over the design range. The factor comes from the specific volume of water at the two temperatures: 1.000 L/kg at 5 C and 1.008 L/kg at 40 C for expansion tank chilled water
Using illustrative inputs only, a system holding 1,000 m3 of water filled at 5 C and designed for 40 C produces an expansion volume of about 8.0 m3, which is the volume the vessel has to accept and give back. A glycol mix has a different specific volume, so a glycol-charged loop uses glycol properties for expansion tank chilled water
Stage | Input | Method | Illustrative result |
Thermal volume change | Total system water volume | Multiply by the water expansion factor | 1,000 m3 x 0.008 = 8.0 m3 |
Pressure band | Pre-charge and maximum allowable pressure | Set the band the vessel works in | 200 kPa abs to 700 kPa abs |
Acceptance factor | The same two pressures | Maximum allowable divided by the difference | 700 / (700 - 200) = 1.40 |
Vessel volume | Expansion volume and acceptance factor | Multiply the two figures | 8.0 x 1.40 = 11.2 m3 |
Pre-charge, system pressure and the acceptance check
The pre-charge is the pressure of the gas cushion in the vessel when it is disconnected from the system and contains no water. It is set from the cold static pressure at the vessel elevation plus a margin, so the vessel is ready to accept the contraction of the loop as it cools rather than being drawn empty at every start. It is never set to the pump discharge pressure for expansion tank chilled water
The acceptance check is a set of readings rather than a calculation, and it is where a mis-set vessel is caught. With the system full, cold and the pumps stopped, the pressure at the vessel should read approximately the pre-charge. Below that, the gas cushion has lost pressure or taken on water; above it, the static pressure is higher than assumed for expansion tank chilled water
Diaphragm, bladder and coded pressure vessels compared
The two non-coded constructions differ in one detail that matters for maintenance: where the flexible membrane is retained. In a diaphragm vessel the membrane is welded to the shell and to the end covers, so it is not a pressure-retaining part and a slow loss of gas does not create a leak path. In a bladder vessel the membrane is a separate bag held by the end covers for expansion tank chilled water
The bladder can be replaced without welding, which is why it is common on larger vessels and where periodic bladder exchange is part of the maintenance plan, but it absorbs water over time at the interface and has to be renewed with the correct pre-charge set. A code-stamped pressure vessel is a different product rather than a better version: it is designed, fabricated, tested, stamped and registered under a pressure vessel code for expansion tank chilled water
When a coded vessel is required and what the documentation must show
The decision is made in two stages. The first is regulatory: the pressure equipment regulation in the jurisdiction where the plant operates is checked against the design pressure, the vessel volume, the fluid classification and any service-specific exemption. Regulators generally define a threshold as a combination of volume and pressure, and a vessel above it falls under design certification for expansion tank chilled water
The second stage is the maximum allowable working pressure of the system, not the nominal pressure: a loop that can see a 900 kPa spike during a pump start may need a coded vessel even where the steady operating pressure is modest. The documentation is then a specific list: for an ASME Section VIII Div 1 vessel, the U stamp with the designator and the authorised inspection agency, the National Board registration, the Form U-1 report, the hydrostatic test at 1.3 times the maximum allowable working pressure, and material traceability records for expansion tank chilled water
Venting, air removal and the commissioning sequence
An expansion vessel that has never been properly vented is the most common cause of a system that has pressure and no working expansion vessel. The trapped air sits above the water column inside the vessel, it compresses in exactly the way the gas cushion is supposed to compress, and the vessel then behaves as though it were a fraction of its stated size. The symptom is a slowly rising system pressure with a hot gas bubble for expansion tank chilled water
The commissioning sequence follows the physical order of the system. The loop is filled from the connection point with the high-point vents open, so that air is displaced forward rather than trapped. The vessel is then isolated and bled at the top of the water side to verify the gas-side pressure against the pre-charge and recharged if it has dropped, and the pressure at the vessel is read under cold static conditions for expansion tank chilled water
Where the vessel sits in the loop and how it is isolated
The vessel belongs on the suction side of the pumps, connected to the suction header at the lowest available point. On the suction side it sees the lowest pressure in the system, which is what allows a modest pre-charge to keep the pump protected against vapour formation, and it is where the water it discharges reaches the pump inlet. A vessel on the pump discharge can also be used, but it then sees the full pump head for expansion tank chilled water
The connection details matter more than the position on the drawing. Each vessel needs isolation valves so that it can be removed without draining the loop, a drain valve at the lowest point so the vessel can be emptied and the bladder or diaphragm changed, and a charging and bleed connection on the gas side for setting the pre-charge. The connection also has to be sized so it does not restrict flow into the vessel during expansion for expansion tank chilled water
Water quality, glycol compatibility and corrosion
The fluid in the loop determines several things at once: the expansion factor used in the sizing, the corrosion behaviour of the wetted materials, and the compatibility of the membrane with the fluid inside it. A glycol-charged loop has to be sized with glycol properties, and the membrane rated for that glycol at the design concentration and temperature. Glycol degrades over time and the degradation products are more aggressive, so the inhibitor package and the fluid replacement interval belong to the water quality programme for expansion tank chilled water
The corrosion question is separate from fluid compatibility. In a closed loop, oxygen ingress is the usual initiator, which is why loops are kept pressurised and vented through a device that keeps air out. Where the fill is an open make-up connection, the corrosion allowance and the choice between carbon steel and stainless have to be addressed. For a coated vessel the coating has to be compatible with the fluid and verified before filling for expansion tank chilled water
The specification list and the pre-commissioning checklist
A pressurised vessel specification has to fix the pressure band before it names the vessel, because everything downstream depends on it. The request for quotation should carry the total system water volume including the pipework, the fill and design maximum temperatures, the fluid and its concentration, the static pressure at the vessel elevation at cold fill, the maximum allowable working pressure of the loop, the local regulation and certification route, and the isolation and venting arrangement for expansion tank chilled water
The pre-commissioning checklist then closes the gap between the calculation and the installed vessel, and it is short: the vessel identification and nameplate against the design, the pre-charge recorded before and after connection, the cold fill pressure reading against the pre-charge, the venting of air from the vessel and the high points of the loop, the relief setting confirmed, and the position and locking of every isolation valve for expansion tank chilled water
· Total system water volume, including pipework, heat exchangers and any basin for expansion tank chilled water
· Fill and design maximum temperature, and the fluid with its concentration for expansion tank chilled water
· Static pressure at the vessel elevation at cold fill, used to set the pre-charge for expansion tank chilled water
· Maximum allowable working pressure of the loop, and the relief setting for expansion tank chilled water
· Acceptance factor between the pre-charge and the maximum allowable pressure for expansion tank chilled water
· Certification route: the local regulation and the required stamping for expansion tank chilled water
· Wetted material spec and the membrane material with its fluid and temperature rating for expansion tank chilled water
· Isolation valves, drain valve, charging connection and vent point at the vessel for expansion tank chilled water
· Coating verification records, including the leakage test, before filling for expansion tank chilled water
Project Case Study
Reference delivery for this duty, cited from the verified project library. Match level C. A welded pressure vessel designed to EN 13445, in ASTM A516 Gr.70, with a C5M epoxy coating for a semi-marine environment. It evidences pressure vessel engineering and fabrication capability. It is not an ASME Section VIII certified vessel, so it is not used as an ASME case in this article, as detailed for expansion tank chilled water.
Project ID | Country | Product | Capacity | Tank count | Dimensions | Completion | Supply scope |
MT-PRESS-2.29 | Malta | Pressure Vessel | 2.29 m3 | 2 | ID1240 × 1840 mm and ID1200 × 1665 mm | Not recorded in the available source | Not recorded |
A welded pressure vessel designed to EN 13445, in ASTM A516 Gr.70, with a C5M epoxy coating for a semi-marine environment. It evidences pressure vessel engineering and fabrication capability. It is not an ASME Section VIII certified vessel, so it is not used as an ASME case in this article, as detailed for expansion tank chilled water.
Frequently Asked Questions
Q1. Why does a closed loop need a pressure boundary?
A closed loop loses the atmospheric reference that an open tower system provides, so three requirements have to be met by the pressure level alone: the water expands as it warms, the pump suction needs positive head to stay above vapour pressure when the plant is cold, and the highest point needs a relief path above the allowable pressure of the weakest component for expansion tank chilled water
Q2. How is an expansion vessel sized?
In two stages. First the thermal expansion of the total system water over the design range is calculated from the water specific volumes, giving the volume the vessel has to accept. Second, that volume has to be absorbed between the pre-charge and the maximum allowable pressure, and the ratio of the two gives the acceptance factor for expansion tank chilled water
Q3. What is the relationship between pre-charge and system pressure?
The pre-charge is set from the cold static pressure at the vessel elevation plus a margin covering static head variation and pipework pressure drop, so the vessel is ready to accept contraction rather than being drawn empty at every start. It is never set to the pump discharge pressure. The acceptance check reads the vessel pressure at cold fill for expansion tank chilled water
Q4. How is a diaphragm vessel chosen over a bladder vessel?
The diaphragm is welded to the shell and end covers, so it is not a pressure-retaining part and a slow loss of gas does not become a leak path. The bladder is a separate closed bag held by the end covers, and it is the pressure-retaining element, which makes it replaceable without welding but means it absorbs for expansion tank chilled water
Q5. When is an ASME coded vessel required?
When the local pressure equipment regulation places the vessel above its certification threshold, normally defined by a combination of design pressure, vessel volume and fluid classification, or when the owner, insurer or client specification requires certification regardless of that threshold. The check is made against the maximum allowable working pressure, not the nominal pressure for expansion tank chilled water
Q6. How are venting and air removal commissioned?
The loop is filled from the connection point with the high-point vents open so that air is displaced forward. The vessel is then isolated and bled at the top to compare the gas-side pressure with the pre-charge and recharged if it has dropped, and the pressure at the vessel is read under cold static conditions for expansion tank chilled water
Q7. Where does the vessel sit in the loop and how is it isolated?
On the suction side of the pumps, connected to the suction header at the lowest available point, so that it sees the lowest pressure in the circuit and can protect the pump against vapour formation at low pre-charge. Each vessel needs isolation valves, a drain valve for bladder exchange, and a charging and bleed connection on the gas side for expansion tank chilled water
Q8. What do water quality and corrosion require?
The fluid determines the expansion factor used in sizing, the corrosion behaviour of the wetted materials and the compatibility and temperature rating of the membrane. Glycol loops are sized with glycol properties, and the inhibitor package and fluid replacement interval belong in the water quality programme, as detailed for expansion tank chilled water.
The design pressure and the design temperature have to be fixed before any expansion vessel is chosen, because the pressure band between the pre-charge and the maximum allowable pressure determines the physical size of the vessel, and the local pressure equipment regulation determines whether the vessel is a stamped pressure vessel or a non-coded pressure part. That ordering is the whole content of this topic: a vessel specified on temperature range alone will be the wrong size, and one specified on a nominal pressure will be the wrong certification class. The two stages of the calculation, the pre-charge acceptance check and the commissioning sequence are what turn the specification into a vessel that can be verified as doing the job it was bought for for expansion tank chilled water
Talk to an Engineer
For an expansion vessel or pressurised tank in a chilled water loop, send the total system water volume including pipework and heat exchangers, the fill temperature and design maximum temperature, the fluid and its concentration, the static pressure at the vessel elevation at cold fill, the maximum allowable working pressure of the loop and the intended relief setting, the local pressure equipment regulation and the certification route required, and the isolation and venting arrangement. We will return a sizing study showing the thermal expansion volume, the acceptance factor, the resulting vessel volume, the pre-charge to be set and the acceptance readings to be recorded, the material and membrane recommendation, and the pre-commissioning checklist for the site for expansion tank chilled water
About Center Enamel
Shijiazhuang Zhengzhong Technology Co., Ltd (Center Enamel) is a professional manufacturer dedicated to the design and fabrication of bolted storage tanks since 2008. The product range includes Glass-Fused-to-Steel (GFS) tanks, Fusion-Bonded Epoxy (FBE) tanks, stainless steel tanks, galvanized steel tanks and aluminum geodesic dome roofs, together with EPC technical support and tank ancillaries. Production runs under ISO 9001, ISO 14001 and ISO 45001, and the product range is certified to NSF/ANSI 61, WRAS, CE and EN 1090, ISO 28765, AWWA D103-09 and NFPA where the application requires it. The company holds close to 200 enameling patents, runs a 150,000 m2 production base, produces about 300,000 enamel-coated steel plates a year, has completed more than 30,000 installed projects and has shipped tanks to more than 100 countries. The engineering design capability covers tank volumes up to 60,000 m3, as detailed for expansion tank chilled water.
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